Contributions for HI absorption 2017


Abstracts for the HI absorption 2017 meeting (12-16 June)

There are 24 participants who will submit an abstract.

Name Contribution Title
Abstract
Allison, James Talk Title: ASKAP Status Update / A statistical method for measuring the HI spin temperature
Abstract:
ASKAP Status Update

I will give an update of the current status of the Australian SKA Pathfinder, including recent examples from commissioning and early science observations. I will discuss how these results inform us of the technical challenges and advantages of using phased array feeds for 21-cm line absorption surveys.

A statistical method for measuring the HI spin temperature

The HI spin temperature measured in 21-cm line observations of DLAs can be used to make inferences about the physical state of the neutral interstellar medium in gas-rich galaxies at high redshift. I will present a statistical method that can be used to infer the harmonic-mean spin temperature as a function of redshift solely using the detection yields from future large 21-cm surveys. No ancillary data is required!
Borthakur, Sanchayeeta Talk Title: Tracing the coolest neutral gas component of galaxies
Abstract:
Understanding the distribution and kinematics of the cold neutral medium is key to connecting gas physics to other processes in the broad scheme of galaxy evolution.

21cm HI absorbers present us with a unique way to trace cold (T<300K) neutral gas in galaxies. I will discuss our work on the distribution (radial profile) and kinematics of cold HI as traced by 21cm absorbers. I will focus on what these measurements tell us about the physical conditions and connect them to the broad picture of galaxy evolution. Finally, I will end by summarizing the knowledge we gained from our GBT-VLA programs on the requirements for designing future successful absorption programs. The future radio telescopes will be the next frontier. Observing programs that can effectively detect these systems will be the key to furthering our understanding of galaxy evolution via this technique.
Chandola, Yogesh Talk Title: HI absorption towards radio AGNs and FAST radio telescope.
Abstract:
In radio AGNs, studying the gaseous components in their environments as well as in the nuclear regions could provide insights towards understanding the triggering of radio activity, fuelling processes and probing jet-cloud interactions. HI absorption towards radio AGNs is a good probe for neutral gas. I will present our recent work in which we find HI absorption detection rates towards radio AGNs significantly depend on WISE W2-W3 colour. Sources with W2-W3 > 2 have significantly higher HI absorption detection rate than those with W2-W3 < 2. Five hundred metre Aperture Spherical radio Telescope (FAST) will be one of the sensitive radio instrument for HI absorption studies. I will also talk about current status of FAST and its utility for HI absorption studies in future.
Curran, Stephen Talk Title: The spin temperature of damped Lyman-alpha absorption systems and star formation history
Abstract:
Damped Lyman-alpha absorption systems (DLAs) may account for 80% of the neutral gas density and are hypothesised to be the sites of star formation in the early Universe. However, while the star formation rate density (SFR) exhibits a steep evolution, peaking at redshifts of z ~ 2 - 3, the mass density of neutral hydrogen remains essentially flat with redshift, particularly at z > 1, where most of the data are from DLAs.

By comparing the line-strength of the 21-cm absorption with the total neutral hydrogen column density, we verify the well documented increase in the spin temperature, degenerate with the covering factor (T/f), with redshift. However, upon correcting this for the geometry effects of an expanding Universe, we find this increase to apply only at redshifts of z > 2, with a decrease in T/f over 0 < z < 2. Using high resolution radio imaging to break the spin temperature/covering factor degeneracy, we find that the DLA extent, normalised by the spin temperature, also exhibits a peak at z ~ 2- 3, increasing by an order of magnitude over its value at z = 0. This suggests that, although the total hydrogen column density shows little relation to the SFR, the fraction of the cold neutral medium may. Therefore, further efforts to link the neutral gas with the star formation history should also consider the cool component of the gas.
Davis, Julie Talk Title: HI Absorption with CHILES
Abstract:
The COSMOS HI Large Extragalactic Survey (CHILES) is a project currently underway at the Jansky Very Large Array (VLA). The CHILES collaboration has been granted 1000 hours of B-array observing time (of which 600 hours have been completed) to image neutral hydrogen (HI) in a 30' x 30' cone centered on the COSMOS field and covering a redshift range of z = 0 to z = 0.45. The CHILES continuum data will be one of the most sensitive radio continuum images yet, and thus an ideal dataset for HI absorption analysis. In the survey field, there are 430 (~15 per square arcminute) continuum sources brighter than 100 uJy, 50 of which are brighter than 1 mJy. With a per channel sensitivity of 70 uJy per beam (measured from the first 178 hours integration), CHILES will have a S/N of 100 and a column density sensitivity toward the 1 mJy sources of < 10^19 cm^-2 at 5 sigma. A variety of opportunities for stacking in redshift or along multiple sightlines for a given galaxy will potentially achieve even deeper column density sensitivity (< 10^18 cm^-2). Given a predicted 300 HI galaxy detections representing a wide range of environments, the CHILES dataset offers a rich array of absorption analysis possibilities.
Dutta, Rajeshwari Talk Title: Distribution of cold gas around galaxies
Abstract:
During the last few years we have been studying the distribution of cold
gas around galaxies at both low and high redshifts using HI 21-cm absorption.
At low redshifts (z<0.4), we have used the technique of searching for absorption
around quasar-galaxy pairs to characterize the strength and covering factor of
HI 21-cm absorbers as a function of various geometrical and physical parameters
of galaxies. At high redshifts (0.5 to select sightlines that have high probability of harbouring cold dense gas
close to galaxies and to study the dependence of such cold gas on metal and dust
content. The main results from our above studies will be presented in this talk.
Edge, Alastair Talk Title: HI and CO absorption against the cores of Brightest Cluster Galaxies
Abstract:
I will present an overview of the observations of HI and CO absorption in BCGs with particular attention to the two systems where we have joint detections of HI and CO in A2597 and Hydra-A. With ALMA in full operations we will be entering an era when tens to hundreds of systems can be studied in CO absorption and I'll review the potential for these observations and the science they will reveal.
Emig, Kimberly Talk Title: Unveiling the physical conditions of neutral gas with radio recombination lines
Abstract:
HI absorption has provided the primary means of tracing cold, neutral gas across redshift. However, it does not constrain well the gas physical properties. No longer stifled by instrument capabilities, low-frequency (< 500 MHz) recombination lines provide a complementary way to determine gas conditions such as temperature, density, and cloud size.

Using the Low Frequency Array, we have achieved the first detections of radio recombination lines in AGN by initially targeting bright sources with known HI absorption features. In this talk, we present these first results, thereby demonstrating (1) the capacity of RRLs to probe cold, diffuse gas clouds at z > 1, and (2) the complementary nature of RRL and HI-absorption observations.
Emonts, Bjorn Talk Title: Atomic gas in the halo of a massive proto-cluster galaxy: prospects for high-z HI studies
Abstract:
In this talk, I will present the first results of an HI 21cm absorption study of neutral gas in the halo environment of the massive Spiderweb radio galaxy at z~2, using experimental P-band spectroscopy at the Very Large Array. Previously, we discovered that the Spiderweb galaxy contains a giant halo of cold molecular CO-emitting gas that fuels in-situ star formation across almost 100 kpc! Based on low-surface-brightness millimeter techniques, new ALMA data reveal that also atomic carbon, [CI], is abundant across the halo. Our VLA HI absorption study will complement our millimeter work by tracing the neutral hydrogen gas in the halo along the extended, bright radio source. This will further reveal the multi-phase nature of the circum-galactic medium, and show how massive galaxies may have condensed out of cold reservoirs of molecular and atomic gas at high redshifts. This work shows the great prospects for studying the cold neutral and molecular medium in the Early Universe, using both mm and cm telescopes like ALMA, (Next-Generation) VLA, LOFAR and SKA.
Frank, Bradley Talk Title: MeerKAT
Abstract:
The MeerKAT telescope is currently under advanced construction; the 16-dish subarray of MeerKAT has already released first light images. I will present an update on the progress of MeerKAT, and an overview of the facilities available for science data processing and collaboration on the large survey projects.
Gupta, Neeraj Talk Title: Blind HI and OH absorption line search: status update on MALS and early results from uGMRT
Abstract:
Deep galaxy surveys have revealed that the global star formation rate (SFR) density in the universe peaks at 1< z <2 and sharply declines towards z = 0. The AGN activity also peaks during the same epoch. A clear picture of underlying processes that drive the evolution of star formation and AGN activity is yet to emerge. The MeerKAT Absorption Line Survey (MALS) will use MeerKAT’s L- and UHF-band receivers to carry out a sensitive dust-unbiased search of HI 21-cm and OH 18-cm absorption lines at 0
JNHS, Aditya Talk Title: Cold Gas in High Redshift Active Galaxies
Abstract:
We have conducted a survey using GMRT to search for "associated" HI-21 cm absorption from 74 active galactic nuclei (AGNs), selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. Out of 63 sources which have usable data 17 sources are at 0 < z < 0.5, 39 are at 1.1 < z < 1.5 and 7 are at z \sim 3.5. We have obtained 4 new detections of HI-21 cm absorption.
Out of these, the tentative detection towards TXS 0604+728 at z = 3.53, if confirmed, would be at the highest redshift at which HI-21 cm absorption has been detected till date. Including 29 CJF sources with searches for associated HI-21 cm absorption in the literature, we construct a sample of 92 uniformly selected flat-spectrum sources. A Peto-Prentice test for censored data finds (at \approx 3\sigma significance) that the strength of HI-21 cm absorption is weaker in the high-z sample than in the low-z sample; this is the first statistically significant evidence for redshift evolution in the strength of HI-21 cm absorption in a uniformly selected AGN sample (ref Aditya et al. 2016).

Upon adding 27 Gigahertz Peaked Spectrum (GPS) sources to the full sample of CJF sources, forming a sample of 119 compact AGNs, the null hypothesis that the low-z and high-z sub-samples of HI-21 cm optical depths are drawn from a similar distribution is rejected at \approx 4.1 \sigma.

However, the two-sample test also finds that the HI-21 cm absorption strength is higher in AGNs with low ultraviolet/radio luminosities, at \approx 3.4\sigma. The fact that the higher-luminosity AGNs of the sample typically lie at high redshifts implies that it is currently not possible to break the degeneracy between AGN luminosity and redshift evolution as the primary cause of the low HI-21 cm opacities in high-redshift, high-luminosity AGNs.
Maccagni, Filippo Talk Title: The Last Survey of the old WSRT: tools and results for the future HI absorption surveys
Abstract:
Absorption of neutral hydrogen (HI) in radio AGN is a powerful tool to identify cold gas flowing in and out of the active nucleus and to determine the HI content in early-type galaxies. Just before Apertif was installed on the WSRT, our research group observed 250 radio sources, detecting HI absorption in 30% of them. In this talk, I show how the HI content in radio-AGN varies according to different properties of their nuclei and their host galaxy. I argue stacking experiments suggest early-type galaxies either have HI with column densities of NHI ≥1020 cm-2 either they have HI at very low column densities (NHI ≤1019 cm-2). I focus on the predictions this survey allows us to make about which HI absorption lines and in which kind of radio sources we will detect with the upcoming dedicated HI absorption surveys on Apertif (SHARP), MeerKAT (MALS) and ASKAP (FLASH). In this talk, I also illustrate a set of data reduction and analysis tools that we developed for this survey and that will be useful for the upcoming HI absorption surveys. In particular, I present a software that identifies the most likely distribution of cold gas reproducing the observed integrated absorption line and separates features of the lines (e.g. asymmetries with respect to the systemic velocities, broad wings, double peaks, etc.) that may trace gas flowing in or out of the AGN.
Mahony, Elizabeth Talk Title: Searching for HI absorption in the brightest southern radio galaxies
Abstract:
Detections of HI absorption in distant radio galaxies can provide a powerful tool in understanding the role that cold gas plays in the formation and evolution of radio-loud AGN. Using the ASKAP-BETA telescope we have searched for HI absorption against sources selected from the 2-Jansky sample; a sample of southern radio galaxies with flux densities above 2 Jy at 2.7 GHz and redshifts less than 0.7. In this talk I will present early results from this survey including a new detection of HI absorption towards a powerful FRII radio galaxy at z=0.7. Results obtained from this pilot study will provide valuable insight into what we can expect to detect in the upcoming ASKAP, MeerKAT and AperTIF absorption line surveys.
Morganti, Raffaella Talk Title: SHARP: the HI absorption survey with Apetif
Abstract:
I will present the SHARP survey, the the HI absorption survey with Apetif. I will describe the goals of the survey and the new possibilities offered.
I will summarise the results obtained from the work done in preparation of the survey as well as the tools developed. The talk by Filippo Maccagni will give more details about some of these tools. I will also discuss how this survey complements the other HI absorption surveys planned by other pathfinders, in particular FLASH and MALS.
Moss, Vanessa Talk Title: Connecting HI and soft X-ray absorption in distant AGN with next-generation telescopes
Abstract:
Recent studies of young radio galaxies have revealed the presence of dense and dusty gas near the active nucleus traced by both 21cm HI absorption and soft X-ray absorption (e.g. Ostorero et al. 2010, Siemiginowska et al. 2016, Glowacki et al. 2017, Moss et al. in prep), offering new insight into the physical nature of the circumnuclear medium in these distant galaxies. Using a catalogue of known HI absorbers/non-absorbers, we compare HI and X-ray properties to expose telling trends in this population of radio galaxies across redshift. In order to further investigate the correlation between HI absorption and X-ray absorption, we conducted pilot observations during commissioning with ASKAP of bright radio galaxies selected on the basis of their archival XMM-Newton X-ray data. Of the five galaxies observed so far, we obtained a new detection of HI absorption towards PKS 1657-298. I will detail the multi-wavelength properties of this galaxy and the significance of our result, as well as future work to be conducted as part of ASKAP Early Science and beyond. I will also highlight how these pilot observations reflect the future prospects of all-sky radio/X-ray complementarity in this space, particularly in the context of synergy between telescopes such as ASKAP, eRosita, Athena and the Square Kilometre Array.
Oonk, Raymond Talk Title: The role of the cold atomic gas in our Galaxy and beyond.
Abstract:
The interstellar medium (ISM) is a key factor in the evolution of galaxies over cosmic time. Cold, atomic clouds are a key component of the ISM, but so far this phase has been difficult to study, because its main tracer, the HI 21cm line, does not constrain the basic physical information of the gas (e.g., temperature, density) well. Low-frequency telescopes enable studies of this ISM component through a complementary tracer in the form of low-frequency carbon radio recombination lines (CRRL). These CRRLs provide a sensitive probe of the physical conditions in cold, diffuse clouds (e.g. Oonk et al. 2015; Salgado et al. 2017a,b). In combination with HI absorption studies of the same clouds this will allow us to study the chemical enrichment and cosmic ray ionization rate of the ISM. In this talk I will focus on our recent LOFAR studies of the CRRL emitting cold clouds towards Cassiopeia A (Oonk et al. 2017; Salas et al. 2017). Here we obtain the first self-consistent physical CRRL model fit for these clouds and show that they are best fit with a temperature 85 K and an electron density 300 cm-3. Both are constrained to within 15%. I will argue that much of the CRRLs arise in the CO-dark surface layers of cold clouds where most of the carbon is ionized but hydrogen is near the transition from atomic to molecular. I will then highlight the latest results from our ongoing LOFAR Galactic plane CRRL survey and relate these larger-scale measurements to our detailed Cassiopeia A study. If time allows then at the end I will shift topics to briefly discuss the role of cold atomic gas in extragalactic sources. In particular I will review what we have learned from HI absorption about the multi-phase gas flows in cool-core clusters.
Popping, Attila Talk Title: Linking absorption and emission measurement of the CGM with IMAGINE
Abstract:
The interaction of galaxies with their direct environment is a crucial aspect of galaxy formation. How gas is distributed around galaxies, and how it is accreted onto them to support star formation, is one of the most fundamental questions in galaxy evolution. In this talk I will present first results of the IMAGINE survey (Imaging Galaxies’ Intergalactic and Nearby Environment). IMAGINE is a Legacy Project on the Australia Telescope Compact Array of over 2500 hours to study the HI properties of gas around 28 nearby galaxies at column densities log(NHI) < 18. These surface brightness levels start to match the HI column densities found in recent absorption studies with the Cosmic Origins Spectrograph (COS) on the HST. These absorption studies measure gas and their metallicities indirectly along a number of line of sights in order to understand the distribution and history of gas in the CGM. IMAGINE will complement such observations by observing the entire CGM in emission, giving a much more detailed understanding of the distribution and kinematics of this gas.
Sadler, Elaine Talk Title: A successful search for intervening 21 cm HI absorption at 0.4 < z < 1 with ASKAP
Abstract:
I will present and discuss the first detections of intervening 21cm HI absorption made with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope. We used the first 6-12 dishes of ASKAP in a pilot study to search for redshifted 21 cm HI absorption intervening along the line of sight to 53 bright background continuum sources. The total redshift path length covered was dz ~ 20. Three detections of intervening lines were made, two of them new. These detections allow us to make the first 21 cm-based estimate of the DLA number density at redshift z~0.6. While the current sample is small, it represents an important proof of concept for the much larger 21 cm HI absorption surveys to be carried out with the full 36-antenna ASKAP telescope once it comes into full operation.
Schulz, Robert Talk Title: Probing HI outflows on parsec scales in radio galaxies with VLBI
Abstract:
Observations of fast outlfows of HI provide important insight on the impact of the radio jets on the host galaxy's gas distribution. They have been detected in absorption in several radio galaxies, but so far mostly with insufficient angular resolution to localise the outflow with respect to the jets. However, this can be achieved by Very Long Baseline Interferometry (VLBI) as demonstrated for the first time in 4C12.50. Based on this, we have been conducting a study to locate and characterise the outflow of neutral gas on parsec scales in a small sample of young and recently restarted radio galaxies. We will present results from this ongoing study, in particular on the restarted giant radio galaxy 3C236, in which for the first time, we recover part of the outflow over a velocity range of about 600 km/s. This has been achieved due to improved sensitivity and larger bandwidth compared to previous observations. Thus, we will also discuss the feasibility of such VLBI observations in the context of this sample and future larger sample studies.
Sharma, Ravi Talk Title: Automated data processing for uGMRT and MeerKAT absorption line surveys
Abstract:
The next generation telescopes will clearly make it impossible to process and analyze the large volumes of data through conventional techniques that require frequent manual interventions.To address this, we are developing an Automated Radio Telescope Image Processing Pipeline (ARTIP) to automate the entire process of flagging, calibration and imaging for the absorption line surveys with uGMRT and MeerKAT. The pipeline is written in Python using CASA and other standard libraries. In this talk, we will present the overall design of our pipeline and discuss the strategy based on various statistical methods to identify and flag bad antennas and baselines. We are testing performance of the pipeline on narrow and wideband (>200 MHz) datasets from uGMRT on sequential and distributed computing systems. We will present these results and plans to make it scalable for ten times larger datasets from the MeerKAT Absorption Line Survey.
Staveley-Smith, Lister Talk Title: Evolution of Cosmic Gas Density
Abstract:
HI absorption line measurements give information about gas column density and temperature at all redshifts and may help bridge the gap in our understanding of the cosmic gas content of the Universe at low redshifts, as measured by HI emission experiments, and measurements from Damped Lyman alpha lines at high redshifts. I will review existing work (and present new work) at low and intermediate redshifts from HI emission and intensity mapping experiments and relate to existing high-redshift CO emission and HI absorption measurements in the context of theoretical predictions.
Wilcots, Eric Talk Title: Using HI Absorption to Probe the CGM and IGM in Galaxy Groups
Abstract:
Galaxy groups are one of the most common environments in which galaxies reside. To develop a deeper understanding of the role of environment in galaxy evolution, it is imperative to form a more complete picture of how galaxies accrete gas, transform it into stars, and eject it back into the surrounding circumgalactic and intergalactic medium (CGM/IGM). Given that a large portion of the CGM and IGM is very diffuse and thus difficult to measure in emission, many observational studies have focused primarily on viewing the CGM in absorption, using quasar absorption lines in neutral hydrogen (HI), Mg II, O VI, and a variety of other species to probe different gas regimes. While we cannot detect in emission the lower (10^13 to 10^18 cm^-2) column densities reached by these ultraviolet and optical absorption studies, the new generation of radio observations with sufficiently long integration times and high sensitivities offer the ability to probe neutral hydrogen in absorption in the CGM and IGM to scientifically interesting column density limits. This contribution presents an overview of our current understanding of the CGM and IGM, particularly in galaxy groups, and how new HI absorption line surveys may enhance that understanding.
Zwaan, Martin Talk Title: Parsec-scale HI absorption structure in a low-redshift DLA galaxy
Abstract:
We present global VLBI observations of the 21-cm transition of atomic hydrogen seen in absorption against the radio source J0855+5751. The foreground absorber (SDSS J085519.05+575140.7) is a dwarf galaxy at z = 0.026. As the background source is heavily resolved by VLBI, the data allow us to map the properties of the foreground HI gas with a spatial resolution of 2 pc. The absorbing gas corresponds to a single coherent structure with an extent >35 pc, but we also detect significant and coherent variations, including a change in the HI optical depth by a factor of 5 across a distance of <6 pc. The large size of the structure provides support for the Heiles & Troland model of the interstellar medium, as well as its applicability to external galaxies. The large variations in HI optical depth also suggest that caution should be applied when interpreting T_S measurements from radio-detected DLAs.

 

 

 

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