There are 74 participants who submitted an abstract.
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Name: Arabsalmani, Maryam
Title: HI 21 cm mapping of a GRB host galaxy: the solution of a long standing mystery
Abstract:
I would present the first HI 21 cm mapping of a galaxy hosting a Gamma Ray Burst (GRB). The birth place of the GRB in this galaxy is a rare star forming region with a high specific star formation rate, which dominates the radiation of the galaxy across various wavelengths. Although this region has been the subject of many studies, the cause of its rare properties has not yet been resolved. I would discuss how the HI study of this galaxy resolves the long standing mysteries of the GRB's birth place. |
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Name: Bagetakos, Ioannis
Title: An overview of MHONGOOSE
Abstract:
MHONGOOSE (MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters) is a 6000 hr survey of 30 nearby galaxies selected from the SINGG/SUNGG sample over a uniform range in log(MHI). The aim of the survey is to study the connection between star formation, HI, dynamics and to directly detect the effects of cold accretion from the intergalactic medium. Each galaxy will be observed for ~200 hr allowing us to reach column densities of 1.2 x 10^19 cm^-2 (at 5σ) at 30" resolution and 5 x 10^17 cm^-2 at 90" sufficiently deep to detect the connection of the gas with the cosmic web. MHONGOOSE will be complemented by a large set of observational data at other wavelengths, which will enable us to accurately model the various mass components in these galaxies and study the effects of environment, dark matter and other galactic properties. We give an overview of our sample of galaxies, selected uniformly over logMHI that cover a wide range of inclinations, star formation rates and galaxy types. |
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Name: Benjamin, Robert
Title: Models of the Kinematics of Gas in the Disk-Halo Interface: A Galactic Perspective
Abstract:
Do galaxies produce their own gaseous halos or do gaseous halos produce their own galaxies?
If galaxies are responsible for their surrounding halos, these are created by outflows of gas, either in a gravitationally unbound galactic wind or in a gravitationally bound galactic fountain. For decades, this has been the viewpoint of researchers working on the Galactic interstellar system, producing a body of research focused on questions of hydrostatic support, superbubbles, chimneys, fountains and winds.
If gaseous halos are responsible for making galaxies, there must be inflows of intergalactic gas, either in the form of a shock-heated accretion flow, cold streams, cold condensed halo clouds, or mixtures of these. Then the gaseous halo gradually feeds the galaxy with the raw material necessary for successive generations of star formation. This has been the viewpoint of recent cosmological research on the development of structure in the universe and the process of galaxy formation, a process that continues to the present day.
The truth appears to be a mix of these viewpoints. I will discuss how different models for the kinematics of circum-galactic gas, both infalling and outflowing, fare when compared to data from the Milky Way Galaxy and then suggest how observations of other galaxies may provide complementary tests of these models. |
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Name: Besla, Gurtina
Title: The Evolution of the Gaseous Magellanic System
Abstract:
I will provide an overview of our current understanding of the gaseous Magellanic System, including the origin of the 150 degree long Magellanic Stream, the Magellanic Bridge connecting the Large and Small Magellanic Clouds, and the evolution of the gaseous disk of the LMC. These structures have been created/modified by the interactions between the LMC and its companion the Small Magellanic Cloud, and also by ram pressure stripping as the Clouds move through the gaseous halo of the Milky Way. In particular, the structure of the LMC's gaseous disk allows us to tightly constrain the gas density of the Milky Way's circumgalactic medium at distances of ~50 kpc. |
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Name: bidaran, bahar
Title: Searching for a cool neutral medium in ultra-compact high velocity clouds
Abstract:
Ultra-compact high velocity clouds (UCHVCs) could potentially represent low mass, gas rich, but (nearly) starless galaxies located in our own Local Group. A sample of the best galaxy candidates have been imaged with the Westerbork Synthesis Radio Telescope. These data have the spectral resolution to distinguish the presence of a two-component medium containing both a warm and cool neutral medium component; the presence of a cool neutral medium component (CNM) has important implications for the potential of these systems to form stars. We present preliminary results of a search for a CNM component in the UCHVCs.
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Name: Bomans, Dominik
Title: 3D Spectroscopy of Galactic Outflows
Abstract:
Galactic outflows and winds are key ingredients for the birth and live of galaxies in the intergalactic medium. Via outflows the halos of galaxies are structured, heated and enriched with heavy elements. Using integral field spectroscopy data (mostly from the CALIFA survey using
the CAHA 3.5m plus PPAK and some early data from MUSE) we will explore the properties and frequency of outflows/winds based on a large, complete sample of intermediate and
high mass galaxies in the local universe.
Additional observational information for the low mass end (using Fabry-Perot 3d-spectrocopy) will be presented to further explore the parameter space for the occurance and properties of outflows and winds, and therefore the resulting sphere of influence of these galactic feedback processes. |
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Name: Bonnell, Ian
Title: Spiral Shocks and the triggering of star formation
Abstract:
I will present ongoing work on how galactic scale flows due to spiral arms can lead to the formation of dense molecular clouds and star formation. Star formation occurs due to a combination of the large scale convergent flows coupled with thermal instabilities and self-gravity once the gas reaches higher densities. Internal turbulence arises primarily due to the large scale motions coupled with smaller scale structures and instabilities. Resulting star formation rates scale with the surface density of gas to the 3/2 power and linearly with the surface density of dense molecular gas. Stellar feedback often appears to have a minimal affect on halting star formation but could have greater impact in removing any external boundary layer and thus unbinding the molecular clouds. These simulations provide a means of understanding global star formation laws and of providing self-consistent initial conditions to study star formation
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Name: Braun, Robert
Title: Local Group Clues to the Galaxy Gas Life Cycle
Abstract:
Both the densest and most diffuse phases of the atomic ISM provide some surprising insights into galaxy evolution. The proximity of the Local Group galaxies, the LMC, M31 and M33, makes them ideal to study both these regimes with maximum sensitivity. In this talk we'll consider both the cold atomic phase in the inner disks and its role in priming star formation as well as the warm atomic outskirts and the lessons they hold for galaxy group dynamics. |
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Name: Burkhart, Blakesley
Title: The Origins and Implications of Turbulence in Galaxies
Abstract:
Magnetic fields and turbulence are vital components in galactic processes, including cosmic ray transport, ISM structure formation and star formation. However turbulence is difficult to measure observationally and the role of simulations is vital for both testing theories of ISM turbulence and gauging observational diagnostics via synthetic observations. In this talk I will discuss the origins of turbulence in galaxies, and its connection to the star formation process, both from observations and the Illustris AREPO cosmological simulation. I will also highlight how turbulence can be measured in spectral line observations of molecular clouds and diffuse gas in galaxies in order to constrain and test simulations as well as obtain important properties of turbulence such as the injection scale, spectral index and Mach number. |
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Name: Busekool, Eva
Title: HI properties of galaxies in the Ursa Major and Perseus-Pisces environments.
Abstract:
Blind HI imaging surveys have been carried out of the Ursa Major region and the Perseus-Pisces filament. The goals of these surveys are to investigate the low-mass end of the HI Mass Function (HIMF) as well as the detailed HI morphologies and kinematics of the galaxies in these two
environments.
The low-mass end of the HIMF in the Ursa Major region is measured with the VLA-D down to HI masses of 2x10^7 Msun and its slope (α ~ -0.95) is quite different from HIPASS (α ~-1.37) and ALFALFA (α ~ -1.33). The Ursa Major environment is very dynamical: 40 out of 58 galaxies show
disturbed HI morphologies and/or kinematics. The most striking cases are the three brightest lenticulars with huge HI tails, indicating tidal encounters with other galaxies.
The Perseus-Pisces survey consists of a mosaic of 44 VLA-C pointings and spans eight square degree on the sky. Within this volume two lenticulars with very extended HI disks are discovered. The detailed HI properties of galaxies in the Perseus-Pisces filament will be discussed and compared to those of galaxies in the Ursa Major environment. |
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Name: Cluver, Michelle
Title: Atomic and Molecular Gastronomy in a remarkable HI-massive Galaxy
Abstract:
In recent years targeted and large surveys have made huge strides towards uncovering the properties of the star-forming population at redshifts 1 − 2. However, the devil is in the details, and we continue to rely on spatially-resolved studies of nearby galaxies in order to formulate an understanding of the crucial interplay between gas, dust, and the feedback from star formation. The role of nearby galaxies, and even regions of the Milky Way, as local analogues to the interstellar chemistry of more distant systems should not be underestimated in their value. Lying partially hidden by the Southern Milky Way, at a redshift of z~0.036, HIZOA J0836-43 has shown itself to be an ideal laboratory for studying the kinematics and interstellar medium chemistry of inside-out disk building. This luminous infrared galaxy has one of the largest known HI reservoirs, is undergoing elevated star formation spread across the inner third of its ∼130 kpc neutral gas disk, and exhibits PDR (photodissociation region)-dominated emission. Photometry and spectroscopy from Spitzer, WISE and Herschel, combined with high spatial and velocity resolution ATCA HI observations, provide fundamental insight into the mechanism of star formation in large, gas-rich star-forming disks, with direct relevance to z∼1 galaxy evolution. |
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Name: de Blok, Erwin
Title: The HALOGAS survey
Abstract:
We present an overview of the HALOGAS (Hydrogen Accretion in LOcal GAlaxieS) Survey, which is the deepest systematic investigation of cold gas accretion in nearby spiral galaxies to date. Using the deep HI data that form the core of the survey, we are able to detect neutral hydrogen down to a typical column density limit of about 10^19 cm−2 and thereby characterize the low surface brightness extra-planar and anomalous-velocity neutral gas in nearby galaxies with excellent spatial and velocity resolution. Through comparison with sophisticated kinematic modeling, our 3D HALOGAS data also allow us to investigate the disk structure and dynamics in unprecedented detail for a sample of this size. Key scientific results from HALOGAS include new insight into the connection between the star formation properties of galaxies and their extended gaseous media, while the developing HALOGAS catalogue of cold gas clouds and streams provides important insight into the accretion history of nearby spirals. We conclude by motivating some of the unresolved questions to be addressed using forthcoming 3D surveys with the modern generation of radio telescopes. |
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Name: Denes, Helga
Title: Dark gas in the Milky Way
Abstract:
To gain a better understanding of star formation it is important to know how the inter stellar medium (ISM) transitions from the atomic gas into molecular clouds and how much molecular gas is there in our Galaxy. Based on gamma ray and dust temperature measurements, we know that there is some hidden 'dark gas' in the Milky Way, that can not be traced by HI or CO emission. This gas could be optically thick atomic hydrogen (HI), molecular hydrogen (H2) or a combination of these to. To investigate the nature of the dark gas, I will present new, more accurate measurements of the molecular hydrogen content of the Galaxy. |
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Name: Dettmar, Ralf-Juergen
Title: Magnetic fields and cosmic rays in the disk-halo interface of spiral galaxies
Abstract:
The disk-halo interface provides a diagnostics for the global state of the dynamical ISM in star-forming disk
galaxies. Special emphasis is given to the role of cosmic rays and magnetic fi elds since these components contribute signi cantly to the pressure of the ISM. Results for the cosmic ray transport in the galaxies NGC 7090 and NGC 7642 are presented showing that convection and diffusion will result in different total power and spectral index distributions. An outlook on results from the "Continuum HAlos in Nearby Galaxies - an EVLA Survey (CHANG-ES)" of 35 edge-on galaxies will be given. |
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Name: Donovan Meyer, Jennifer
Title: Star Formation in HI Dominated Environments
Abstract:
I will discuss recent work on star forming, HI-dominated environments from nearby dwarf galaxies to nearby, extended ultraviolet (XUV) disks. Both environments are forming stars at low levels, requiring interstellar media capable of doing so, but the gas column densities and molecular fractions in these regions differ substantially from those in the main disks of normal, star forming galaxies. With well-resolved observations of dwarfs and XUV disks, we present comparisons of the gas available for stars to form between these very different environments. |
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Name: Dopcke, Gustavo
Title: A novel model for the stellar IMF
Abstract:
The mass distribution of stars at their birth, the initial mass function (IMF), has implications on many properties of astrophysical objects. Also, determining whether the IMF is universal or instead depends on the environment is critical to understand how stars form. Such understanding requires models that can explain the shape of the observed IMF and its small variations. Furthermore, there are some indications of a non-standard IMF, which the present models for the IMF are unable to explain.
This work aims to build a theoretical model that can explain the shape of the IMF, and also make room for alternative distributions. Moreover, we based the model on simple physical properties of star forming clusters, without claiming a specific astrophysical process that drives star formation. Also, we intent to develop method that can explain stellar mass distribution in different times of the cluster evolution.
In order to create such method, we started by examining some basic processes involved in the formation of stellar clusters. More specifically, we based the model in the facts that new stars are formed, they accrete the surrounding gas, and share its content. Thus, we based the model in those simple premisses, leading to the variables: initial stellar mass (M_i), total cluster mass accretion (dM/dt), fragmentation rate (dn/dt), and the accretion power (alpha). Such variables can evolve on time, leading to objects with different masses and accretion histories.
We applied this model to data from observations and simulations. From the simulations data, we calculated the model parameters by averaging the values over time. As for the observations, we calculated the average cluster mass accretion and fragmentation rates from the state at the time of the observation. This means that we simply divided the total mass and number of stars by the cluster age, in order to calculate the mass accretion and fragmentation rates, respectively.
The results point to a good agreement between the modelled cluster, and the values from simulations and observations. Also, we used the model to evolve simulations beyond its final stage, resulting in a cluster with a mass distribution close to the canonical IMF.
Finally, we concluded that the model developed in this work is a good option to study star formation in stellar clusters, being adapted to simulations and observations. |
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Name: Egorov, Oleg
Title: Multiwavelength view on stellar feedback and gas kinematics in nearby dwarf galaxies
Abstract:
The structure and gas kinematics of interstellar medium in irregular galaxies are regulated mostly by stellar feedback. Gas distribution in those galaxies represents a large amount of expanding shells and supershells in star formation regions. We performed the observations at 6-m SAO RAS telescope with scanning Fabry-Perot interferometer in H-alpha emission line and used the archival observations in HI 21 cm line in order to study the gas kinematics in individual star forming regions in several nearby dwarf irregular galaxies. We used available data in UV, optical and IR ranges to compare the gas structure and kinematics, the dust distribution and composition, and the properties of young stellar population in the regions studied. Here we present most interesting results of our investigations. |
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Name: Elmegreen, Bruce
Title: The role of HI in star formation
Abstract:
Star formation requires cold and dense gas, which is generally molecular, but HI still plays an important role in the star formation process. HI is the pervasive component of the ISM, filling the volume between molecular clouds and extending to a large scale height and radius. Atomic gas dominates molecular gas and even stars in the outer parts of spirals and in most dwarf Irregulars. Energy from star formation feedback goes primarily into this pervasive HI component. The turbulence generated determines the scale height of the disk, and through this, the midplane density and pressure, which affect the large-scale star formation rate per unit gas mass. The exponential radial profile of star formation hardly changes when entering the HI dominated regime in the outer part, but the slope of the KS relation increases from ~1 in the inner parts to ~2 in the outer parts. This change occurs when the gas begins to dominate stars and the disk thickness increases at relatively constant HI velocity dispersion. In dIrrs, the average molecular fraction in the disk decreases with radius in proportion to the HI column density. This decrease could be the result of a changing mass in the form of diffuse molecular clouds, which are associated with HI clouds that shield themselves and each other from UV light. Large-scale gas dynamics in spiral waves also occurs in HI-rich gas because of its high filling factor, and this is the dynamics that forms molecular clouds in the densest regions by compression and self-gravity. |
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Name: Faerman, Yakov
Title: Hot Galactic Halos
Abstract:
Collaborators: Amiel Sternberg (TAU), Chris McKee (UCB)
X-ray absorption studies of QSO sightlines provide evidence for a hot gas (~1e6 K) halo around the Milky Way, in the form of highly ionized metals (OVII and OVIII). Extended structures of hot gas around galaxies are also suggested by galaxy formation theory and numerical simulations. Recent observations of OVI absorption in the UV show the existence of hot halos around L* galaxies in the low redshift universe.
Due to the challenging nature of the observations, many questions remain open. Where exactly does the gas reside and what are its properties? What is the extent and the total mass of the halo? These questions are important to our understanding of galaxy evolution and are tightly linked to other topics such as gas accretion onto galaxies, star formation and stellar feedback.
We construct a simple model for the hot halo which successfully reproduces a wide range of observations (UV, X-ray and other). In our model, the halo is a large-scale structure, extending to the virial radius of the MW, and has a significant gas mass (~1e11 M_sun). If such halos are common, as suggested by the COS-Halos survey, they can provide a solution to the missing baryons problem in the Local Universe. |
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Name: Falgarone, Edith
Title: The ISM
Abstract:
Since its discovery, almost a century ago, the ISM has kept challenging theorists and observers.
I will introduce the planned discussion on the various phases of the ISM by reviewing (1) the many existing observations that are still unsolved puzzles, and (2) recent theoretical and observational progresses on the characteristics, interactions and origins of the various ISM phases. |
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Name: Forbes, John
Title: How to suppress star formation in dwarf galaxies
Abstract:
I will present the results of 3D adaptive mesh refinement hydrodynamical simulations of isolated dwarf galaxies. In these simulations we have included a detailed supernova feedback model with high enough resolution to resolve the cooling radius of supernova remnants, obviating the overcooling problem. We find that in these galaxies, contrary to various theoretical expectations, the supernovae play a secondary role in terms of regulating star formation. Heating from the grain photoelectric effect immediately sterilizes large swaths of the galaxy upon the birth of high mass stars. This may explain the nearly-starless galaxies recently discovered via HI. |
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Name: Forgan, Duncan
Title: Detecting Structure in Smoothed Particle Hydrodynamics Simulations via Tensor Classification
Abstract:
Inspired by the classification of N Body simulations of the cosmic web, I will present results from our recent work in attempting to characterise structure in the density field of smoothed particle hydrodynamics simulations (SPH) using the tidal and velocity shear tensors.
I will outline the wide range of applications of this technique, including the detection of filaments and sheets in molecular clouds, and spiral arms in discs.
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Name: Frank, Bradley
Title: Dynamics of Molecular Gas in Nearby Galaxies
Abstract:
Molecular gas typically traces the star formation in galaxies, but also provides an unique tracer of the kinematics in the inner regions - where the neutral hydrogen column density may be too low to provide a reliable view.
In my talk I will present recent results which compare the CO and HI content and kinematics of a sample of nearby galaxies using the HERACLES and THINGS surveys, respectively.
I will present a comprehensive comparison of the distribution and kinematics of the CO and HI in these galaxies, by comparing emission line profiles, velocity fields and rotation curves.
Finally, I will present mass models in which the molecular gas has been explicitly added, showing that the molecular gas can be an important contributor to the dynamics of the galaxies in our sample. |
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Name: Fraternali, Filippo
Title: Will we ever see gas accretion?
Abstract:
Several pieces of evidence indicate that disc galaxies must continuously accrete gas from the intergalactic environment to fuel their star formation. However, the direct observation of gas accretion has proven extremely elusive. Several mechanisms are expected to bring gas into galaxies including cosmological filaments, cooling of the hot coronae and minor mergers. Which of these is really taking place in the local Universe remains unknown. The gas can come in the form of clouds, filamentary structures or drizzle and it could be detected directly as for the Galactic high-velocity clouds or through disturbances of the structure and/or the kinematics of the gaseous discs. Have we already detect gas accretion? Can we hope to see it with future HI facilities? Or should we give it up entirely and concentrate on different kinds of observations, e.g. absorption towards background AGNs? I will review the current status for the quest of gas accretion into galaxies and discuss future prospects. |
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Name: Fujimoto, Yusuke
Title: Birthing star forming clouds in the grand design
Abstract:
We present results of giant molecular clouds formed in a hydrodynamical model of the barred spiral galaxy, M83. To include the formation, evolution, and destruction processes of the giant molecular clouds with the global gas dynamics taken into account, we perform whole galactic disc simulations with thermal stellar feedback. Our global model resolves down to 1.5 pc and explores the properties of the clouds forming in the bar, spiral and disc environments. The property distributions of the GMCs show only a small dependence based on their environment, however we notice three distinct different cloud types emerging, whose relative number is environment dependent. These cloud types consist of typical clouds (ones whose mass and radius agrees well with observations), massive clouds (with radii larger than 30 pc and mass above 10^7 Msun) and transient clouds who are typically unbound with short life times. This presentation discusses the environment dependence of these cloud types and effects of the thermal feedback on the clouds. |
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Name: Gazol, Adriana
Title: The Atomic Gas Segregation at Different Radii in the Galactic Plane
Abstract:
In recent years it has been recognized that the interactions between the segregation of the neutral atomic gas, due to thermal instability (TI), and other relevant ingredients in the ISM have a direct effect on the star formation process. On the other hand, in the Galactic Plane the net thermal balance leading to TI depends on the galactocentric distance. However, observations seem to
indicate that the relative mass fraction of warm and cold atomic gas remains approximately constant for radii between ~10 and ~20 kpc. In this work we present results from numerical experiments aimed to study the segregation process of the atomic gas at different galactocentric radii. To this end we use hydrodynamic models with initial conditions and cooling functions appropriate to describe thermal conditions at 8.5, 11, 15, and 18 kpc and forced turbulence at different intensities. We study the behaviour of the relative mass fraction of cold gas as well as that of the pressure and density distributions. |
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Name: Girichidis, Philipp
Title: SILCC: Chemical and dynamical evolution of the SN-driven ISM and the launching of outflows
Abstract:
Within the SILCC (SImulating the Life Cycle of molecular Clouds) project we present three-dimensional (magneto)hydrodynamic simulations of the interstellar medium (ISM) in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of H+, H, H2, C+, and CO and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, SN clustering and different positions of the SNe, in particular SNe in density peaks and at random positions. Molecular hydrogen forms in dense filaments and clumps and contributes 20% - 40% to the total mass, whereas most of the mass (55% - 75%) is in atomic hydrogen. The ionised gas contributes < 10%. For high SN rates (0.5 dex above Kennicutt-Schmidt) as well as for SN in density peaks the formation of H2 is strongly suppressed. Also without self-gravity the H2 fraction is significantly lower (~ 5%). Most of the volume is filled with hot gas (~ 90% within ±2 kpc). We highlight that individual chemical species, in particular atomic hydrogen, populate different ISM phases and cannot be accurately accounted for by simple temperature-/density-based phase cut-offs. The position of the SNe has a major impact on the turbulent dynamics of the ISM. Placing the SNe at density peaks leads to the formation of compact discs with low velocity dispersions and no outflows. In contrast, random SNe positions result in more realistic velocity dispersions (σHI~σ300-8000K-10km/s, σHα~σ8000-3e5K~20km/s), and they launch strong outflows with mass-loading factors of up to 10. The outflows do not contain any molecular gas but are composed of H and H+ in roughly equal proportions. The bulk of the outflowing mass is dense (n~1cm-3) and slow (v~20-40 km/s) but there is a high-velocity tail of up to v~500km/s at n~1e-3cm-3. |
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Name: Habe, Asao
Title: Role of Cloud Cloud Collisions in High Mass Star Formation
Abstract:
Massive star formation has the very important role in evolution of
the interstellar matter, the chemical evolution and the galaxy evolution.
We study cloud cloud collisions (CCCs) by numerical simulation, motivated
by several recent reports of observational evidence of CCCs that suggest
triggering of massive star formation by CCCs and frequent occurences of
CCCs in global galaxy scale simulations of GMCs formation and evolution.
Our numerical results show that CCCs of turbulent clouds result in formation
of massive dense cores that are expected to form high mass stars.
We examine several cases of colllision velocities, cloud sizes, and masses of
colliding clouds and show how these parameters affect shape of the core
mass function.
We study star formation by CCCs by using star particle model.
We discuss the role of CCCs in the high mass star formation in Galaxy.
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Name: Heyer, Mark
Title: Discussion on the Role of Star Formation in Gas Cycling
Abstract:
The production of stars within the molecular ISM of galaxies plays a
critical role in the cycling of gas through different phases.
I will lead a discussion on this role with an emphasis on the transition
from atomic to molecular gas, the regulation of star formation within
molecular clouds, and energy feedback processes -- mostly from an observational perspective.
To frame the discussion, a series of questions will be posed based on the session presentations.
Two key questions related to these topics are:
1) What is the typical lifetime of molecular clouds and does this vary
with galactic scale environment?
2) How can the observations quantify the role of feedback on molecular cloud evolution?
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Name: Hughes, Annie
Title: GMC Populations and Star Formation in Nearby Galaxies
Abstract:
The observed ~kpc scaling relations between molecular gas and star formation in galaxies must ultimately be due to highly localised star formation activity that is occurring within individual giant molecular clouds (GMCs). In this talk, I will summarise what we have learnt about the dynamical properties and evolution of GMCs from recent high resolution surveys of CO emission in nearby galaxies (e.g. PAWS, MAGMA), which provide a unique view of the relationship between a GMC population and its host galaxy. Contrary to the view that GMCs are universal, I will highlight new work showing (i) that key properties of the GMC population such as the mass distribution, average surface density and characteristic cloud lifetime each vary with galactic environment, and (ii) that ~kpc scale estimates of the molecular gas depletion time in galaxies reflect variations in the physical properties of the underlying GMC ensemble.
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Name: Hunter, Deidre
Title: HI and Star Formation in Dwarf Irregular Galaxies
Abstract:
Dwarf Irregular (dIrr) galaxies are forming stars in spite of models that
predict that they should have no star formation. Furthermore, they appear
to be forming stars in their outer disks, an even more extreme environment
for star formation, with exponential stellar disks extending to many disk
scale lengths. I will summarize what is currently known of HI disks, star
formation, and their relationship in dIrr galaxies, with an emphasis on
outstanding issues. |
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Name: Imara, Nia
Title: Atomic Hydrogen and the Origin of Molecular Clouds
Abstract:
Observations of Galactic and extragalactic giant molecular clouds (GMCs) show that these star-forming structures are often associated with high-density atomic hydrogen (HI). In this presentation, I will discuss what recent observational and theoretical studies tells us about how GMCs may form from HI and what these studies say about the HI-to-H2 transition in molecular clouds. Other observations, at millimeter wavelengths, indicate that many GMCs have systematic linear velocity gradients. In order to address the question of the origin of the gradients and, therefore, the question of GMC evolution, I will present a detailed analysis comparing the velocity fields of molecular clouds and the HI surrounding them. I have used CO and 21-cm observations to make first-moment velocity maps of the molecular clouds and associated HI. For GMCs in the Galaxy and M33, my key finding is that the gradient position angles of HI and the associated GMCs are generally not aligned. And if, as some authors have argued, the linear velocity gradients in GMCs are caused by rotation, their angular momentum is much less than that predicted by formation scenarios in which GMCs collapse from the surrounding interstellar medium. Furthermore, in M33 I find that the atomic gas associated with GMCs generally has higher velocity gradients than does high-density atomic gas in which GMCs are not observed. |
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Name: Jaskot, Anne
Title: Tracing HI Gas Cycles and Global Star Formation with the ALFALFA H-alpha Survey
Abstract:
The atomic gas supply of galaxies fuels star formation and traces the cycles of gas flows that shape galaxy evolution. With data from the ALFALFA H-alpha survey, we investigate the relationship between atomic gas content and global star formation in a statistically uniform, HI-selected sample of 565 low-redshift galaxies. This sample reveals the role of HI gas in driving high star formation rates, the factors that promote efficient star formation, and the cycling of HI gas throughout starburst episodes. In contrast to previous studies, we find that galaxies' specific star formation rates are only weakly linked with their HI gas fractions. Instead, we show that dust generates the tight observed trend between HI gas fraction and galaxy color. This dust-HI relation results from the regulation of HI, metallicity, and star formation by gas inflows and outflows and from the effect of dust on HI to H2 conversion. Galaxy structure may also affect HI conversion efficiency, as demonstrated by the tight correlation between stellar surface density and HI consumption time in disk galaxies. Although spheroid galaxies deviate from this trend, we find that their bulge strength does not affect their star formation efficiency. We confirm previous results that more HI-rich galaxies tend to have higher SFRs, which may indicate the active role of HI accretion in fueling star formation at low redshift. On the other hand, starburst galaxies do not appear unusually HI-rich or HI-depleted relative to similar mass non-starbursts. Feedback and self-regulation maintain a relatively stable HI content throughout a starburst, allowing galaxies to preserve HI fuel for future star formation episodes. Mergers appear to cause the enhanced star formation efficiency in some of the starbursts, while supernova feedback in the lowest mass galaxies may drive cyclical bursts, similar to expectations for high-redshift galaxies. |
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Name: Józsa, Gyula I. G.
Title: UGCA 105: A dwarf with a beard
Abstract:
Some galaxies exhibit "beards", a specific kinematic signature observed in their (neutral) gaseous component. With increasing height above the mid-plane the gas rotates slower than expected, such that spectra taken on positions along the major axis exhibit a faint tail towards the galaxies systemic velocity, and the spectra become more elongated than a simple fountain model would predict. These features have been brought into connection with gas infall, which could remove angular momentum from the high-latitude gas and could hence lead to the observed slowing. While studies of such anomaleous gas have hitherto concentrated on large spiral galaxies, we have started to study the vertical kinematical structure of gas-rich dwarf galaxies, to investigate how it compares to the one of large (gas-rich) spirals. We present WSRT HI observations of UGCA 105, a dwarf galaxy of Magellanic type, showing very characteristic beard emission, and discuss a kinematical analysis of its HI disk. |
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Name: Juette, Eva
Title: The search for isolated HI clouds in the environment of HALOGAS galaxies
Abstract:
Accretion of cold neutral gas has been suggested as an important element of galaxy evolution, in particular for the ongoing star formation in present-day galaxies. So far, however, a full census of accreting material has not been identified by observations. We make use of the very sensitive HI data provided by the HALOGAS Survey to search for isolated clouds within the environment of the 22 nearby target galaxies. Here, we present first results of this ongoing study. We describe the techniques employed to identify and characterise the clouds, and describe the effective detection limits of the HALOGAS observations. The preliminary catalog will be presented, and we will discuss the implications of the observed prevalence of detected neutral gas. |
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Name: Kasparova, Anastasia
Title: Can molecular clouds live long?
Abstract:
Anatoly Zasov (1,2), Anastasia Kasparova 1)
1. Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Moscow, Russia
2. Department of Physics, Moscow M.V. Lomonosov State University, Moscow, Russia
It is generally accepted that the lifetime of molecular clouds does not exceed 3x10^7 yr due to disruption by stellar feedback. We put together some arguments giving evidence that a substantial fraction of molecular clouds (primarily in the outer regions of a disc) may avoid destruction process for at least 10^8 yr or even longer. A molecular cloud can live long if massive stars are rare or absent. Massive stars capable to destroy a cloud may not form for a long time if a cloud is low massive, or stellar initial mass function is top-light, or if there is a delay of the beginning of active star formation. A long duration of the inactive phase of clouds may be reconciled with the low amount of the observed starless giant molecular clouds if to propose that they were preceded by slowly contraction phase of the magnetized dark gas, non-detected in CO-lines.
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Name: Konstandin, Lukas
Title: Density structures caused by supersonic turbulent motions
Abstract:
Turbulence is an extremely important phenomenon in astrophysical media on nearly all scales. It occurs in accretion discs, supernova remnants, star forming molecular clouds, and the diffuse phases of the interstellar medium. A thorough understanding of the density structures caused by supersonic turbulent motions is of great importance, as it has a direct influence on quantities like, e.g. the star formation rate, star formation efficiency, initial mass function, and core mass function. We model driven, compressible, isothermal, turbulence with Mach numbers ranging from the subsonic (M ≈ 0.65) to the highly supersonic regime (M ≈ 16) and preform a systematic analysis of the probability distribution function of the mass density and the density power spectrum. We discuss a modified relation between the standard deviation of the density and the Mach number of the turbulent medium, as well as a model describing the density power spectrum depending on the Mach number of the flow. We link the scaling properties of the density filed with the fractal dimension of the medium and its space filling factor. Our measurements suggest an overall fractal dimension in the range D≈2−3
in agreement with the observations of interstellar clouds. |
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Name: Kruijssen, Diederik
Title: The multi-scale nature of galactic star formation in discs and nuclei across cosmic time
Abstract:
A wide range of recent work shows that galactic star formation (SF) relations between the gas mass (density) and the star formation rate (density) develop substantial scatter or even change form when considered below a certain spatial scale. In this talk, I will show how this multi-scale behaviour of galactic SF relations can be exploited to determine ill-constrained, cloud-scale quantities such as the cloud lifetime, SF/feedback time-scales, SF efficiencies, and gravitational instability lengths, using galaxy-scale observations. The method is validated using high-resolution numerical simulations of SF in disc galaxies. I will then present the first results of applying the formalism to observations, providing statistically representative measurements of the molecular cloud lifetime in the Galactic Centre, M33, M51 and M31, as well as using high-resolution ALMA Cycle 2 observations of the nearby flocculent spiral NGC 300. These lifetimes are used to put direct constraints on the star formation efficiency in molecular clouds and the emergence of the galaxy-scale SF relation, supplementing our earlier estimates for the star formation cycle in the Galactic Centre.
In the ALMA era, our new technique will enable the detailed characterisation of the SF process on the cloud scale in galaxies out to z = 4, i.e. across a cosmologically representative part of the galaxy population rather than the limited sample of Local Group galaxies where these measurements were previously possible. This enables the systematic study of SF physics as a function of the cosmic environment. |
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Name: Lee, Cheoljong
Title: I_CO-A_V in the Local Group: Towards a Prescription for the CO Emission as a Function of Metallicity
Abstract:
I will discuss a physically motivated but still observational approach to understand CO emission from galaxies at different metallicities. We consider CO emission from clouds as a separable problem with the PDF of column densities, the dust-to-gas ratio, and the dependence of CO emission on extinction each a distinct problem. The last of these: the dependence of CO intensity on dust shielding, is the key to this approach and the least well studied. I will present new, recently published, measurements of the relationship between CO intensity ($I_{\rm CO}$) and visual extinction (`$A_V$', traced by infrared emission) in the Large Magellanic Cloud, Small Magellanic Cloud, the Milky Way, and the Andromeda galaxy on 10 pc scales. We show that these systems all have similar CO intensity at a given $A_V$ despite the difference in metallicity, supporting the theoretical picture that dust shielding primarily determines the location of bright CO emission within a cloud. Leveraging this observation, along with Milky Way work on the gas column density PDF, we make empirically motivated predictions for how the CO-to-H$_2$ conversion factor ($X_{\rm CO}$) varies as a function of metallicity. We show that in very low metallicity systems this calculation - and indeed the whole idea of a CO-to-H$_2$ conversion factor - is intrinsically very unstable and that this has a sensible physical basis.
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Name: Lee, Min-Young
Title: The impact of the optically thick HI on the HI-to-H2 transition in the ISM
Abstract:
Recent observations of nearby galaxies have shown that the HI surface density sharply saturates at ~10 solar mass/pc2 on kpc scales. This holds even on sub-pc scales, as shown by Lee et al. (2012) for the Perseus molecular cloud. The three-dimensional H2 formation model by Krumholz et al. (2009) explains this saturation as the minimum HI surface density required for shielding H2 against photodissociation. While the predicted HI surface density of ~10 solar mass/pc2 for solar metallicity is consistent with the observations, however, the HI saturation could alternatively result from a large amount of the optically thick HI.
We investigate the impact of high optical depth on the observed HI saturation in Perseus by using Arecibo HI emission and absorption measurements toward 26 radio continuum sources (Stanimirovic et al. 2014; Lee et al. 2015). The spin temperature and optical depth are estimated for individual HI components along each line of sight, enabling us to directly derive the correction factor for the high optical depth HI. The pixel-by-pixel correction to the optically thin HI column density image results in only a ~10% increase in the total HI mass and the HI surface density is still uniform with ~7-9 solar mass/pc2, suggesting that H2 formation is mainly responsible for the HI saturation in Perseus. We also compare the optically thick HI with the observed "CO-dark" gas and find that the optically thick HI only accounts for ~20% of the "CO-dark" gas in Perseus.
In the coming era of Apertif, MeerKAT, and ASKAP, a large number of HI emission and absorption measurements will be available for a wide range of ISM environments. These data sets will be essential to build a comprehensive picture of phase transitions in galaxies (warm neutral medium <--> cold neutral medium <--> "CO-dark" H2 <--> "CO-bright" H2) and our study is the first step toward achieving such a goal. |
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Name: Lenz, Daniel
Title: A global view on the correlation of dust and gas
Abstract:
The IRAS discovery of infrared cirrus clouds was the starting point of correlation studies of neutral atomic hydrogen and dust far-infrared emission. These yield important insights into the gas and dust physics, the accretion history of the Milky Way, the X_CO factor, and the distribution of the CO-dark molecular gas. So far, full-sky analyses were limited by the angular resolution of the HI data of about one degree. The recently finished Effelsberg-Bonn HI Survey (EBHIS) improved this limit by an order of magnitude. Here, we aim to consistently quantify the relation between dust and gas, using the most accurate full-sky data sets. Moreover, a Bayesian framework and state-of-the-art methods of inference and image analyses allow us to deduce all-sky maps of e.g. the X_CO factor, dust emissivity and the (CO-dark) molecular gas. We present the first results of this work and compare our results with targeted observations. |
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Name: Lisenfeld, Ute
Title: The star formation law in Tidal Dwarf Galaxies
Abstract:
Tidal Dwarf Galaxies (TDGs), formed from recycled gas in
galaxy interaction, have similar properties as
ordinary dwarf galaxy. One important difference
is their higher metallicity since they are made from
recycled gas. This property makes CO a good tracer
for the molecular gas content, in contrary to ordinary
dwarf galaxies, and allows us therefore to study
the relation of star formation and molecular gas in this
special environment directly and with only minor uncertainties
about the X-factor.
I will show the results for VCC 2062, a very nearby TDG in the Virgo cluster.
Based on interferometric CO data from Plateau de Bure and a large set
of additional ancillary data we are able to study the star formation, its history
and its relation to the molecular gas content.
We find that the SFR is lower than what is expected from its molecular
gas content and I will discuss possible reasons.
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Name: Lockman, Felix
Title: Neutral Gas Clouds in the Local Group
Abstract:
There are three types of neutral gas clouds that are detected outside of galactic disks in the Local Group: high velocity clouds, remnants of interaction like the Magellanic Stream, and the material found between M31 and M33 whose origin is not known. I will present new GBT observations that have detected ~10 discrete neutral clouds each with a size of a few kpc and HI mass of 10^5 in the region between M31 and M33. They do not seem to be part of M31's circumgalactic medium. The largest of these clouds is not too different from the Smith Cloud, a high-velocity cloud that may be a dark-matter dominated subhalo currently being accreted by the Milky Way. |
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Name: Marasco, Antonino
Title: Disk-corona gas cycle in simulated Milky Way-like galaxies.
Abstract:
We present the analysis of a set of N-body+SPH simulations of a Milky Way-like disc galaxy produced by the radiative cooling of a spinning hot corona embedded in a dark matter halo.
Our systems evolve in isolation for 10 Gyrs, so that their growth is driven by the disc-corona gas cycle alone.
We show that the star formation history, the mass distribution and the kinematics of our systems are in good agreement with those determined for the Milky Way.
We found that stellar feedback lifts HI above the disc and produces a layer of extra-planar gas whose kinematics is in remarkable agreement with that inferred by HALOGAS observations. We show evidence that such kinematics is produced by the disc-corona interplay.
In contrast with COS observations, however, gas at temperature below 2e5 K is
absent in the circumgalactic regions of our simulated systems, suggesting that absorption features in the halo of galaxies are evidence for either ongoing filamentary cold accretion or previous merging events. |
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Name: Martel, Hugo
Title: The Connection between Star Formation and Metallicity Evolution in Barred Spiral Galaxies
Abstract:
We simulate the evolution of a barred disc galaxy, to determine the physical processes responsible for the increase in the central gas-phase metallicity and central star formation rate (SFR) observed in the SDSS. We define a 2 kpc diameter central aperture to approximate the integrated spectroscopic fibre measurements from the SDSS. At t ~ 0.5 Gyr, a strong bar forms, whose length is considerably longer than the 2 kpc aperture. The stars and gas follow elongated orbits that cause an intense mixing of the gas between the central region and its surroundings. During the next 1.5 Gyr, the orbits of the gas contract significantly until the entire gas bar is contained inside the 2 kpc
aperture, resulting in a net flux of metal-enriched gas into the central region. Our main result is that the observed enrichment in the centers of barred galaxies is not caused primarily by stars formed in the center. Rather, star formation occurs along the full length of the bar, much of which occurs initially outside the 2 kpc aperture. We conclude that there is no direct connection between central SFR and central metallicity. The central metallicity does not originate exclusively from central stars. Instead, the global SFR (especially along the bar) and the large-scale flow of enriched gas play a major role. |
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Name: McClure-Griffiths, Naomi
Title: Think Global, Act Local: Atomic Hydrogen in the Milky Way
Abstract:
In our on-going efforts to understand the global cycle of gas within and around galaxies, our local Milky Way and Magellanic System provide us with the closest laboratories for studying the life-cycle of gas. These systems offer us the chance to study how galaxies acquire fresh gas to fuel their continuing star formation, how they circulate gas from one part of the galaxy to another and how they turn warm, diffuse gas into molecular gas and ultimately, stars. The dominant constituent of the interstellar gas, hydrogen, traverses an amazing cycle from the circum-galactic medium, through its various warm, unstable and cold states on its way to becoming molecular star-forming gas - and then reversing back through all those phases, states and locations. A significant portion of the gas cycle in galaxies is traced by HI emission and absorption, which together give critical information about the density, temperature and dynamics of interstellar gas in the Milky Way. In this talk I will give an overview of what atomic hydrogen is telling us about the hydrogen cycle within the Milky Way and Magellanic system. |
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Name: Moiseev, Alexey
Title: Extended gaseous discs in early-type galaxies: 3D spectroscopy of the ionized gas
Abstract:
We have studied the origin of gas in a sample of early-type (S0-S0/a) galaxies possessing extended (more than a few kpc in size) ionized-gas discs and/or external UV-rings. The large-scale ionized gas velocity fields were constructed from 3D spectroscopic observations undertaken with a scanning Fabry-Perot interferometer of the SAO RAS 6-m telescope. Analysis of these data together with available archival information on the central regions (e.g. the SAURON kinematical maps) and on the external HI gas behaviours provides evidences for gas accretion from outside in the most of the observed objects. |
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Name: Murray, Claire
Title: Deciphering local, multi-phase HI with 21-SPONGE and artificial intelligence
Abstract:
The properties of interstellar HI, including the cold neutral medium (CNM) and warm neutral medium (WNM), are crucial to understanding the conversion between atomic and molecular gas at the heart of star formation. Here we present results from 21-SPONGE, the largest, most sensitive study of Galactic HI absorption to date at the Karl G. Jansky Very Large Array (VLA). After 600 hours at the VLA, and in combination with HI emission from the Arecibo telescope, 21-SPONGE is uniquely sensitive to measuring all CNM and WNM spin temperatures from 10-10^4 K and column densities from 10^16-10^21 cm^-2. Despite our unprecedented sensitivity, the maximum spin temperature we detect so far for an individual spectral line is Ts~1500 K, which lies below theoretical expectations for the WNM (Ts~2000-4000 K). In addition, we detect much less thermally unstable HI (20%) than expected from previous observations. Understanding the radiative and dynamic processes responsible for these results is best done by directly comparing observations with theory. To facilitate these comparisons, we have developed an open-source, Python-based method for analyzing 1-D spectra with Gaussian functions called Autonomous Gaussian Decomposition (AGD). By applying AGD to both 21-SPONGE and 1000s of synthetic HI emission and absorption spectra from 3-D numerical ISM simulations, we correct our measurements of CNM and WNM spin temperature and column density for completeness and observational biases. We find that simulations cannot reproduce observed "tiny scale atomic structure" clouds with very small optical depths and line widths, and that simulated WNM clouds have larger optical depths than observed WNM clouds. These discrepancies likely probe the strength and topology of the Lyman alpha radiation field and its importance to HI excitation, which are complex and poorly-constrained, yet important for understanding the energy balance of the ISM. Our results are among the first to statistically quantify the success of numerical simulations in reproducing observed HI properties, and represent crucial steps towards understanding the role of HI in star formation. |
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Name: Omar, Amitesh
Title: Star formation and tidal interactions in Wolf Rayet galaxies
Abstract:
The star formation in Wolf-Rayet (WR) galaxies is observed in the youngest phase, of the order of a few Million year old. The star formation triggers in galaxies, therefore, can possibly be easily and accurately traced in such galaxies. Our recent optical (H-alpha) and radio (HI 21cm-line) study comprising of about 30 WR galaxies selected from the SDSS, aims at constraining star formation rates, and finding direct signatures of recent tidal interactions. Several WR galaxies studied here are dwarf systems. Majority of galaxies show signs of tidal interactions with nearby, previously un-identified dwarf companions or HI gas clouds. Misaligned central H-alpha disks are also found in some galaxies. We also identified some galaxies, which are bright in H-alpha but have inadequate non-thermal radio continuum. We find that these galaxies follow radio-FIR correlation, however, the ‘q’ parameter shows a correlation with the star formation rate constrained using the H-alpha and the GALEX UV data. The results from this ongoing survey will be discussed in a context of trigger of star formation in galaxies through acquisition of fresh gas after a recent tidal interaction. |
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Name: Parrish, Ian
Title: The Life Cycle of Gas in Galaxy Clusters
Abstract:
Galaxy clusters, and in particular their Brightest Cluster Galaxies (BCG's), have fascinating similarities and differences to field galaxies in the life cycle of their gas. I will discuss the process by which gas can precipitate from the Intracluster Medium (ICM) and cool onto the central BCG. Accretion and star formation in galaxy clusters can also have interesting differences to field galaxies due to the high pressure confining environment and differing levels of turbulence. I will highlight some recent puzzling ALMA observations that reveal massive amounts of molecular gas (10^10 solar masses) that is not in disks in some systems. In addition to these observations and comparison with Herschel and Spitzer observations, I will also discuss some interesting anomalies in the dust to gas ratios of these BCG's and speculate on the causes. |
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Name: Pearson, Sarah
Title: Gaseous Dwarf-Dwarf Interactions in the Local Universe
Abstract:
Tidal interactions between massive galaxies have been studied extensively, but galaxy interactions between low mass, dwarf galaxies, the most frequent types of mergers in the Universe, remain a mystery. Stierwalt et al. (2015) studied ~100 dwarf galaxy pairs and found their star formation rates increase with decreasing pair separation. Despite this elevation in star formation, isolated dwarf pairs were not gas depleted. Here I will present resolved HI observations of 11 local dwarf pairs to examine the properties of the gas in these systems. The pairs are located at different distances from massive galaxies and are captured in various interaction stages, allowing us to disentangle the role of environment and interaction stage in removing gas from these systems. This study allows us to uniquely quantify the role of hierarchical processes, such as mergers and tidal interactions, in regulating the baryon cycle of dwarf galaxies. |
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Name: Pidopryhora, Yurii
Title: High-Resolution Images of Diffuse Neutral Clouds in the Milky Way
Abstract:
A set of diffuse interstellar clouds in the inner Galaxy within a few hundred pc of the Galactic plane has been observed at a resolution of ~1 arcmin/~2-3 pc combining data from the NRAO Green Bank Telescope and the Very Large Array. These clouds have been selected to be somewhat out of the Galactic plane and are thus not confused with unrelated emission, but in other respects they are a Galactic population. They are located near the tangent points in the inner Galaxy, and thus at a quantifiable distance. These diffuse neutral HI clouds may constitute a considerable fraction of the ISM. One cloud of the observed set may lie within the hot wind from the nucleus of the Galaxy, and at least four clouds show distinct evidence of two thermal phases as would be expected from equilibrium models of the interstellar medium. The clouds seem to be dynamic entities whose properties must reflect their history as well as conditions at their current locations. The appearance of many of the disk-halo clouds suggests that they are interacting with their local environment producing the observed sharp ridges or extended tails. |
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Name: Pingel, Nickolas
Title: A GBT Survey of the HALOGAS Galaxies: Revealing the Complete HI Environment Around Spirals.
Abstract:
High resolution neutral hydrogen (HI) observations undertaken over the past decade have revealed the presence of large amounts of extraplanar gas around numerous nearby galaxies. Exposing the significance of this extraplanar gas and its connection to the continuous gas accretion that must be taking place to fuel star formation is a major goal of large projects like the HALOGAS survey. In order to fully understand gas accretion we must have a comprehensive picture of the overall HI environment. The HI environment is best studied by supplementing high resolution data from interferometers with total power observations from a large, extremely sensitive single dish with well behaved sidelobes. The Robert C. Byrd Green Bank Telescope (GBT) is the only single dish telescope in the world that meets all of these requirements. I present preliminary results from deep (~10^18 cm^-2) HI observations performed with the GBT of each source in the HALOGAS survey. On average we detect about 10% more HI for each source than the original WSRT data. We also see the areal coverage of low column density HI is larger for the GBT data. The excess HI corresponds spatially to anomalous HI features seen in the high resolution data and is most likely the diffuse component of these features. By using the GBT and WSRT data in conjunction we have created some of the deepest HI maps of nearby galaxies to date. The initial results I present here will provide the framework to achieve the main GBT survey goals of relating the intrinsic properties of each HALOGAS source to its overall HI environment and searching for possible signatures of cold mode accretion at low column densities. I will also share progress made by teams at BYU, NRAO Green Bank, and WVU to develop a phased array feed (PAF) for use on the GBT. This instrument will reduce telescope time of similar large surveys by a potential factor of five by forming multiple beams on the sky. |
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Name: Popping, Attila
Title: How gas feeds galaxies with a phased array feed.
Abstract:
In order to understand the gas properties of galaxies and to study their evolution, it is crucial to have large samples of galaxies describing the full mass range in different environments. In addition we also need to probe the intergalactic medium that lies outside the galaxies and holds the reservoir of gas inflows and outflows. Both these arguments justify the need for a new generation of survey instruments that perform large surveys over extended areas on the sky. The Australian SKA Pathfinder (ASKAP) is currently in it’s early science and commissioning phase, observing with six telescopes that are equipped with phased array feeds (PAF) in which 9 beams are formed simultaneously. The wide field of view makes ASKAP and excellent survey instrument to observe the gas properties of a large number of galaxies as well as intergalactic gas clouds that play a crucial role within galaxy formation. In this talk I will present recent early science results from ASKAP, highlighting detections of neutral hydrogen gas both in the local Universe by observing nearby groups of galaxies and more distant measurements of neutral hydrogen by applying stacking techniques. |
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Name: Ramon-Fox, Gerardo
Title: Simulations of Star Formation in Flocculent Spiral Galaxies
Abstract:
F. G. Ramón-Fox & I. A. Bonnell
University of St. Andrews
Star formation is an important mechanism affecting the evolution of galaxies and the dynamics of the ISM as it determines the rate at which gas is converted into stars. It is important to understand gas flows from a galactic perspective and their role in feeding regions of star formation. Observations show a broad range of morphologies in spiral galaxies: from barred galaxies and prominent grand designs, to flocculent discs with less prominent arms. These morphological features may have an effect on gas flows and star formation activity. The present work explores gas flows and star formation in a galaxy with a flocculent morphology using global galaxy-scale high-resolution N-body with Smoothed Particle Hydrodynamics simulations of a disc galaxy, and preliminary results are presented. |
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Name: Rathborne, Jill
Title: Star formation in high pressure environments
Abstract:
Galaxies at redshifts z>2, when the Universe was only a few billion years old, are vigorously forming stars at rates up to several 1000 solar masses per year. Theoretical efforts aimed at explaining this rapid conversion of gas into stars have thus far extrapolated theories of supersonically turbulent, isothermal media describing the density structure of low-pressure (P/k<10^5 K cm-3), nearby molecular clouds. However, it is unknown if these theories extend to clouds in extreme, high-pressure (P/k>10^7 K cm-3) environments. In this talk I will describe new ALMA observations of a cloud immersed in the high-pressure Galactic Centre environment, which represents the best local-Universe analogue to clouds in z>2 galaxies. Our analysis shows that the mean and dispersion of its normalised column density PDF very closely matches the predictions of theoretical models of supersonic turbulence in gas of such high density and turbulence. This is the first confirmation of these models in such an extreme, high-pressure environment. Moreover, our observations are consistent with the theoretically predicted environmentally-dependent threshold for star formation which may provide a natural explanation for the low star formation rate in the Galactic Centre environment. Our results provide the first empirical evidence that the current theoretical understanding of molecular cloud structure derived from clouds in the solar neighbourhood also holds in extreme, high-pressure environments, allowing its application to rapidly star-forming galaxies in the early Universe.
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Name: Roy, Arpita
Title: Forming molecules in a violent environment: outflows in starburst nuclei
Abstract:
Authors: Arpita Roy, Biman B. Nath, Yuri Shchekinov, Prateek Sharma.
Recent observations of nearby starburst galaxies have found molecular outflows in their central regions. These shells with molecular mass of order 10^7-10^8 solar mass are observed to move with speeds <= 100 km/s at distances ~ 100 pc. It is not yet clear how molecules can survive behind shocks driven by stellar wind/supernovae in the central region. It is also unclear if molecules can form {\it in situ}. We outline a scenario in which molecules can form in dense shells behind the shock of a superbubble driven by OB associations in the centre. We also speculate upon how these molecular shells evolve into large scale outflows dominated by atoms/ions. |
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Name: Roychowdhury, Sambit
Title: The spatially resolved Kennicutt-Schmidt relation in the HI dominated regime
Abstract:
I would present the results from a study of the Kennicutt–-Schmidt relation between average star formation rate and average cold gas surface density in the HI dominated ISM of nearby spiral and dwarf irregular galaxies. We divide galaxies into grid cells varying from sub-kpc to tens of kpc in size. Grid-cell measurements of low SFRs using H-alpha emission can be biased and scatter may be introduced because of non-uniform sampling of the IMF or because of stochastically varying star formation. In order to alleviate these issues, we use far-ultraviolet emission to trace SFR, and we sum up the fluxes from different bins with the same gas surface density to calculate the average SFR surface density at a given value of gas surface density. We study the Kennicutt–-Schmidt relation in 400 pc, 1 kpc and 10 kpc scale grids in nearby massive spirals and in 400 pc scale grids in nearby faint dwarf irregulars. We find a relation with a power law slope of ~1.5 in the HI-dominated regions for both kinds of galaxies. The relation is offset towards longer gas consumption timescales compared to the molecular hydrogen dominated centres of spirals, but the offset is an order-of-magnitude less than that quoted by earlier studies. Our results lead to the surprising conclusion that conversion of gas to stars is independent of metallicity in the HI dominated regions of star-forming galaxies. Our observed relations are better fit by a model of star formation based on thermal and hydrostatic equilibrium in the ISM, in which feedback driven turbulence sets the thermal pressure. |
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Name: Saburova, Anna
Title: Dark gas in the disc of the giant low surface brightness galaxy Malin 2
Abstract:
The low surface brightness disc galaxy Malin2 challenges the standard theory of galaxy evolution by its enormous total mass ~2 10^12 Ms which must have been formed without recent major merger events. We performed the Malin2 mass modelling, estimated the contribution of the host dark halo and found that it had acquired its low central density and the huge isothermal sphere core radius before the disc subsystem was formed. One of the unique properties of Malin2 turned to be the apparent imbalance of ISM: the molecular gas is in excess with respect to the atomic gas for given values of the gas equilibrium turbulent pressure. We explain this imbalance by the presence of a significant portion of the dark gas not observable in CO and the 21 cm lines. We also show that the depletion time of the observed molecular gas traced by CO is nearly the same as in normal galaxies. |
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Name: Saintonge, Amelie
Title: The link between HI, H2 and star formation in the local Universe: the COLD GASS perspective
Abstract:
Decades of careful studies have given us a vast network of scaling relations relating the optical properties of galaxies (such as masses, star formation rates, colours, morphologies, and metallicities). To translate this phenomenological framework into a real physical ab initio model for galaxy formation, we must now learn more about the full cycling of gas in and out of galaxies, by studying statistically large samples that are complete and representative of the entire galaxy population at a given epoch. In this talk, I will review the main results from COLD GASS (and its recently completed extension), the first survey giving us such a complete view of the molecular and atomic gas contents of local galaxies with log(M*/Msun)>9. Using both Arecibo HI and IRAM-30m CO spectra for a representative sample of 500 SDSS-selected galaxies, we are able to robustly determine how the gas contents of galaxies scale with their global properties, and what determines the efficiency of the star formation process. For example, quantifying how the HI and H2 contents of galaxies vary across the entire SFR-M* plane allows us to identify where (1) inflow of gas is suppressed, (2) the atomic-to-molecular conversion is most efficient and (3) the efficiency of star formation out of the available H2 is enhanced/suppressed. Because of the large size of the sample and the wide mass range covered, we can quantify the impact of a range of mechanisms on the availability and processing of gas, such as environment, AGN, stellar bars, and interactions. |
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Name: Salak, Dragan
Title: Molecular gas in the starburst galaxy NGC 1808 revealed with ALMA
Abstract:
NGC 1808 is a nearby starburst galaxy with a galactic wind. In order to study the evolution of molecular gas and dust in the dynamic starburst region and wide galactic disk we have carried out observations with ALMA. We present the results from cycle 1 that include large-scale (7.5 x 7.5 kpc^2) imaging of molecular gas traced with CO(1-0). A circumnuclear disk was discovered surrounded by a 500-pc ring. Farther out, molecular gas is detected in the galactic bar, spiral arms, and there is evidence of an outflow from the central region. |
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Name: Schruba, Andreas
Title: Properties of the Molecular Gas and Star Formation Process in Andromeda
Abstract:
I summarize results from a large CARMA survey to image CO(1-0) emission at high sensitivity and resolution (20pc, 1km/s) in the nearby Andromeda galaxy (M31). This survey augments a new extensive set of multi-wavelength data including Galex, Hubble, Spitzer, Herschel, JVLA photometry and spectroscopy that provide an unprecedented knowledge of the stars, dust, and gas in M31. Combined with IRAM 30m single dish mapping, this survey provides the most detailed view on the molecular ISM in an early-type spiral galaxy to date. We find the molecular ISM to be tenuous, clustered, and surprisingly diffuse. Significant CO emission originates from an extended, low signal-to-noise molecular gas component that is well mixed with the atomic gas. On the other hand, the molecular gas is also highly clustered (clumpy), with 2/3 of CO emission falling within the tidal radius (~50pc) of identified molecular clouds. For a sample of 500 molecular clouds with masses as low as 1e4 Msun (similar to Perseus or Taurus) we perform a statistical study of GMC properties across a wide range of environments that we compare to other nearby galaxies. The molecular gas is M31 is directly connected to other phases of the ISM and the local potential, while the molecular clouds are in pressure equilibrium rather than isolated self-gravitating entities. However, the molecular gas also appears closely associated with star formation, although it is not the majority of the gas reservoir in the main 10kpc star-forming ring--so this picture is a portrait of a star-forming medium, not a contaminant. Finally, by comparing the molecular cloud catalog with young clusters detected in Hubble data, we can measure molecular cloud lifetimes, and determine the timescale and efficiency of feedback. These observations provide essential benchmarks for models of ISM phase balance, GMC and star formation a massive spiral galaxies before star formation completely diminishes. |
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Name: Serra, Paolo
Title: Kinematical misalignments in the EAGLE simulations
Abstract:
The kinematical misalignment between gas and stars is a powerful probe of the gas accretion history of galaxies as a function of environment density, galaxy mass and morphology. In early-type galaxies the molecular/atomic/ionised gas kinematics is misaligned relative to the stellar kinematics in as many as half of the objects outside clusters but in just 10 percent of the objects inside clusters. In spirals one may expect misalignments to be less common -- something that future radio and IFU surveys will be able to test in detail.
Kinematical misalignments should arise naturally in hydrodynamical LCDM simulations because they incorporate the stochastic and anisotropic accretion of gas-rich satellites and gas from the medium. Yet, a set of simulations that I have analysed recently produces almost exclusively aligned galaxies (Serra et al. 2014). Is this a general problem of all LCDM hydro simulations?
In this talk I will present an investigation of the kinematical misalignments in the 100-Mpc volume of the new, state-of-the-art EAGLE simulations. EAGLE succeeds in producing misaligned galaxies over a range of galaxy properties. I will show the distribution of misalignment angles as a function of galaxy mass, galaxy type (star-forming, quiescent, slow- and fast rotating), gas content and environment density -- essentially a prediction for the future combination of radio and IFU surveys. By "observing" the simulated galaxies I will also address the observational biases in studying misalignments due to, e.g., the limited field of view of IFUs. Finally, I will attempt to establish whether the misalignments in the simulations are primarily caused by gas accreted in the form of satellites or from the medium. |
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Name: Shetty, Rahul
Title: Diffuse molecular gas and variations in star formation scaling relationships in local galaxies
Abstract:
Infrared and sub-millimeter observations provide information on the star formation rate and molecular gas content, and thereby constrain the Kennicutt-Schmidt (KS) relationship. Using recent survey data, I discuss variations in the KS relationship in local galaxies. Hierarchical and Bayesian statistical methods allow for a robust treatment of uncertainties, both in the assumed conversion factors and observational noise, and can reveal intrinsic variations between the individual galaxies within a sample. I show that the KS relationship varies between galaxies, and that most galaxies portray a sublinear KS trend. This is indicative of an increasing gas depletion time with increasing molecular content. These results indicate that CO observations do not solely trace star forming clouds, but also a diffuse molecular component. Other recent molecular line observations also reveal a significant diffuse gas fraction comprising at least 30% of the total molecular content. |
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Name: Sorgho, Amidou
Title: HI observations of galaxies in southern filament of Virgo Cluster with the KAT-7 and the WSRT
Abstract:
The HI distribution of galaxies in a region located south of the Virgo cluster was mapped using the Karoo Array Telescope (KAT-7) and the Westerbork Synthesis Radio Telescope (WSRT). Despite the different beam size of the two telescopes, we reach similar column density sensitivities of NHI ~10^18 cm^-2 over 16.5 km/s. We develop a new approach to combine the observations and take advantage of their sensitivity to both the large and small scale structures. We detect, out to an unprecedented extent, an HI tail of ~60 kpc being stripped off NGC 4424, a peculiar spiral galaxy. The properties of the galaxy, together with the shape of the tail, suggest that NGC 4424 is a post-merger galaxy undergoing a ram pressure stripping as it falls into the centre of the Virgo Cluster along a filamentary structure. We present an HI catalog of the mapped region. |
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Name: Sternberg, Amiel
Title: HI-to-H_2 Transitions in Galaxy Star-Forming Regions
Abstract:
I will describe new analytic theory for the atomic to molecular (HI-to-H_2)
transitions, and the build-up of atomic-hydrogen (HI) gas columns in star-forming clouds, based on fundamental physics principles. I will discuss my general-purpose formula for the total HI columns produced by photodissociation in optically thick media, valid for beamed or isotropic radiation fields, the weak- to strong-field limits, gradual to sharp HI-to-H_2 transitions, and for arbitrary metallicity. The analytic theory is validated with detailed numerical radiative transfer computations. The general theory may be broadly used for interpreting 21 cm studies of individual Galactic
sources and global galaxy properties, and may also be incorporated into
hydrodynamics simulations. |
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Name: van Zee, Liese
Title: Gas and Stars in Nearby Dwarf Galaxies with Extended Neutral Hydrogen Disks
Abstract:
As a class, dwarf galaxies are notable for their high gas mass fractions, blue colors, and low metallicities. However, even within this gas-rich class, there are a few extreme examples of galaxies with unusual gaseous properties, such as extremely high gas mass-to-luminosity ratios or unusually extended (relative to the optical) gaseous disks. These extreme examples provide unique test cases for our understanding of the connections between gas distributions, kinematics, and star formation activity. I will present the results of a recent study of the baryonic components of a small sample of nearby dwarf galaxies with extended neutral hydrogen disks. The selected sample includes 5 galaxies with well-ordered HI disks that extend more than 5 times their optical size. In all cases, deep infrared imaging observations with the Spitzer Space Telescope trace their stellar disks to larger radii than ground-based optical images, but the stellar distribution still falls-off much more rapidly than the gaseous disk. The majority of these galaxies have only modest current star formation rates despite their relatively large (for their luminosity) gaseous component. However, as is typical for low mass galaxies, their atomic gas surface densities fall well below the Toomre criterion throughout their gaseous disk, indicating that these galaxies are relatively stable against cloud collapse and subsequent star formation activity. We discuss these results in the context of galaxy evolution and the growth of stellar disks in nearby galaxies.
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Name: Verheijen, Marc
Title: HI imaging surveys with Apertif
Abstract:
The Westerbork radio telescope is currently being transformed into an efficient 21cm survey facility, taking advantage of the innovative Apertif receiver system consisting of phased-array feeds, thereby doubling the bandwidth and increasing the field-of-view by a factor 30. I will present the technical capabilities of this system, discuss the main HI science cases to be addressed with Apertif and present the HI imaging surveys that are foreseen to be carried out in 2017-2020. I will highlight current insights from existing HI imaging studies and project these to what can be expected from the Apertif surveys.
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Name: Wang, Jing
Title: Star forming islands in the huge HI disk of ESO215-G?009
Abstract:
We present a multi-wavelength study of the star formation in an extreme case of dwarf galaxy, ESO215-G?009. This galaxy has a very high HI mass to stellar mass ratio (M_HI/M_*=6.7) and very large HI to optical size ratio (R1_HI/R26_F606W=12). Patchy star formation is detected throughout the large neutral hydrogen disk, with little optical light. We investigate why star formation appears to be inhibited in this galaxy, and whether the star-forming clumps indicate a stochastic way of star formation in the galaxy. |
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Name: Williamson, David
Title: Production, diffusion, and tidal stripping of metals in SPH simulations
Abstract:
We present chemodynamical SPH simulations of dwarf galaxies under tidal fields to quantify the effects on metal production (i.e. increased star formation from tidal stirring), and metal depletion (i.e. tidal striping). We examine numerical methods for star formation, as well as metal production and diffusion within this context. |
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Name: Zafar, Tayyaba
Title: On the non-evolution of neutral gas mass density
Abstract:
Damped Lyman-alpha absorbers (DLAs) seen along the line of sights of luminious quasars are a unique probe to select neutral hydrogen rich galaxies. These galaxies allow to estimate neutral gas mass over cosmological scales, which is a possible indicator of gas consumption as the star formation proceeds. The DLAs and sub-DLAs are believed to contain a large fraction of neutral gas mass in the Universe. A search for DLAs and sub-DLAs is made in the reduced archival Ultraviolet Visual Echelle Spectrograph (UVES) dataset of 250 quasars. Because of a chosen redshift window a statistical analysis on 195 quasars is performed. For better statistics, the dataset of archival UVES quasars is analyzed in conjunction with other DLA and sub-DLA samples from the literature. Using the sample, redshift evolution of the number density and the line density are derived for DLAs and sub-DLAs and compared with the Lyman limit systems (LLSs) from the literature. Furthermore, the column density distribution down to the sub-DLA limit is determined. The redshift evolution of column density distribution is also determined, indicating presence of more sub-DLAs at high redshift as compared to low redshift. The HI column density distribution is further used to determine the HI gas mass density between 1.5 < z < 5.0. The complete sample shows that sub-DLAs contribute 10–20% to the total neutral hydrogen gas mass. In agreement with previous studies, no evolution of neutral gas mass is seen from low redshift to high redshift, suggesting that star formation solely cannot explain this non-evolution and replenishment of gas, and/or recombination of ionized gas is needed. |
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Name: Zasov, Anatoly
Title: Total mass of HI and dynamical parameters of starforming galaxies
Abstract:
We consider the idea that a total mass of interstellar gas (mostly HI) within the optical radius R of starforming galaxies is closely linked to the specific angular momenta VR of their discs as the result of the nearly-marginal gravitational stability of their gaseous layers within a large interval of radial distances. The most tight relationship between HI and VR is found for isolated galaxies and for thin edge-on galaxies, which represent the most homogeneous sample of late-type galaxies. The accretion of the outer gas onto a disc and nearly constant efficiency of star formation is needed to prevent this dependence from blurring during the lifetime of galaxies. |
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Name: Zheng, Yong
Title: Evidence of gas accretion at the disk-halo interface of M33
Abstract:
The disk-halo interface represents a transition region where inflowing halo gas and outflowing disk gas mix. Gas accretion through this interface may provide an answer to the gas consumption problem which states the material in the disk is not sufficient to sustain a galaxy's SFR; this has been an issue for both the Milky Way and external galaxies. Replenishment of the fuel through gas accretion is suggested by numerical simulations, however, it is not observationally clear how it occurs. Here I will report direct observational evidence of gas accretion at the disk-halo interface of M33, which is an efficient star former and may cease its star formation rapidly without additional fuel. I will show new COS observations of a set of O type stars in the disk of M33, from which absorption features from accreting gas at the disk-halo interface are found both in weakly and highly ionized species. I will also present a suite of warm/hot gas accretion models and discuss how the observed accreting phenomena fit into these scenarios. This work provides insight into the velocity structure and radial scale of gas accretion at the disk-halo interface, and clarifies the role of gas accretion in sustaining a galaxy's star forming activities.
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Name: Zschaechner, Laura
Title: AGN Feedback in Action: The Molecular Outflow in Circinus Galaxy
Abstract:
Galactic winds/outflows are poorly understood although they are essential to feedback processes that quench star formation and limit the total mass of large galaxies in today's universe. Thus, insufficient understanding of feedback associated with them - in particular of the molecular phase, which likely dominates the mass budget - is one of the greatest shortcomings in our knowledge of galaxy evolution. Multiphase outflows associated with galactic winds fueled by AGN have been well-studied at a range of wavelengths, but observations of the molecular phase are only now becoming feasible with new instruments such as ALMA. We present ALMA observations and kinematic models of the molecular outflow in the well-known Circinus galaxy. Using these data, we constrain the molecular mass of the wind and outflow rate - both crucial to future star formation in Circinus. Additionally, we compare the AGN-driven molecular wind in Circinus to the starburst-driven wind in NGC 253 (Bolatto et al. 2013) and note key differences in the ways each type of wind impacts star formation. |
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