Summer student programme 2015

PRIDE (Planetary Interferometry and Doppler Experiment), a project led by the Joint Institute for VLBI – ERIC (JIVE), aims for ultra-precise determination of spacecraft state vectors (i.e. position, velocity, etc). By state vectors determination, PRIDE has already produced results on the atmospheres of Titan, Venus, and Mars

My summer student project focused on PRIDE observations of ESA’s Mars EXpress (MEX) spacecraft during a close (58 km) flyby of Phobos, one of the two small moons of the planet Mars. We used the bright radio source J1232-0224 as a phase reference, applying its calibration solutions to MEX. In this manner, we produced some 200 images of the spacecraf.

We fitted Gaussians to these images to find the position of MEX as a function of time. All of the processing has been done in AIPS (Astronomical Image Processing System) controlled via ParselTongue, a python wrapper for AIPS created at JIVE.

The project focused on the technical aspect of the experiment. Pascal Rosenblatt (PI of the observations) will include our data in his dynamical modelling of the Phobos flyby. Using this model, one can constrain the structure of Phobos’ gravitational field. The latter will give information about the moon’s interior. By determining the interior composition we hope to constrain the possible origin scenarios, and determine whether Phobos was formed in situ, or whether it is a captured asteroid.

My name is Zhigang Wen, and I am a Ph.D. student from Xinjiang Astronomical Observatory, China. This summer, I worked at JIVE on Fast Radio Bursts (FRBs) searching and localization with the European VLBI Network (EVN). I was honored very much to be under instruction of five supervisors: Zsolt Paragi, Aard Keimpema, Andrew Siemion, Sander ter Veen and Mike Garrett.

FRBs are intense bursts of radio emission that have durations of milliseconds and exhibit the characteristic dispersion sweep of radio pulsars. The origin and progenitor of FRBs are unknown. They are generally thought to be extragalactic because of the very high observed dispersion measure. The EVN would be a powerful instrument to prove their extragalactic origin by precise localization, to ensure that FRBs can be used as cosmological probes.

During my summer internship, I reduced data of six pulsars and two Rotating Radio Transients (RRATs) obtained from phase-referencing VLBI observations. The positions were determined by imaging the single pulses. The pulsar test data showed that individual pulses could be localized at the few tens to 100 milliarcsecond accuracy level. We then imaged the brightest of the 7 pulses we found for RRAT J1819-1458. For the first time, we determined its position well below arcsecond accuracy using radio interferometry data. Our summer work provided technical and methodical preparation for FRBs searching and localization with EVN.

Radio-loud galaxies are a subset of AGN that show enormous regions of radio emission outside the visible extent of the host galaxy. In the nearby universe, these double-lobed radio emissions have almost invariably been associated with elliptical galaxies. It has long been theorized (and supported by observations) that elliptical galaxies are formed by the merger of smaller galaxies and that such mergers provide the mechanism for the infall of gas into the accreting regions of the nucleus. It has also been proposed that such mergers are what result in the launching of kiloparsec-scale radio-loud jets from the central AGN by spinning up the supermassive black hole.

0313-192 was the first confirmed double-lobed radio source hosted by a late-type galaxy and is the archetype for the Spiral DRAGN (Double Radio sources associated with Active Galactic Nuclei) phenomenon. The image above shows the preliminary results from the reduction of S-band (13cm) data obtained from the VLBA (Very Large Baseline Array, USA). It confirms beyond doubt that the radio source associated with 0313-192 is not a chance alignment. The angle of the milli-arcsecond scale jet seen matches (within error) with previously obtained images from the VLA and VLBA (3.6cm, X-band). A comparison with the X-band image yields a core spectral index of alpha = -0.46 confirming that the core is optically thick (very dense) and that synchrotron self-absorption is taking place at these frequencies. The next step for the team is to reduce L-band (21 cm) data from the VLBA to study the HI absorption in the galaxy.

The atomic hydrogen gas (HI) in late-type galaxies usually extends well beyond their stellar disk. However, due to the low surface brightness of the gas in the external regions of the galaxies, many HI observations fail to map their full extent.

The present galaxy is NGC 7424, a ~10^10 Msun galaxy located ~13 Mpc away and observable from the southern hemisphere. It is a candidate of the upcoming MHONGOOSE (MeerKAT HI Observations of the Nearby Galactic Objects: Observing Southern Emitters) survey, a survey that intends to make use of the upcoming MeerKAT telescope in South Africa to study galaxy evolution. This galaxy, along with 29 other nearby galaxies, will be observed to low column densities. The galaxy was observed using the seven-dish Karoo Array Telescope (KAT-7) in South Africa. Because of its short baselines, KAT-7 is an ideal tool to observe low column density gas in galaxies.

The central part of the map shows a disturbed distribution of the gas in the inner region of the galaxy, consistent with previous observations with the australian compact array (ATCA). The gas extends out to more than twice the optical size of the galaxy.

Over the summer development began on applying holography to improve LOFAR’s station calibration. Current calibration methods require 24 hours of all-sky imaging followed by an equal period of post-processing, making station calibrations a costly ordeal. So far initial holography trials have already proven to be highly efficient, allowing one to both observe and reduce the data for the entire LOFAR array within a day, a significant improvement over current methods. At this point we are working on applying the derived solutions produced by our holographic observations to LOFAR’s calibration tables.

For most frequency sub-bands the beam model and residues show a good fit reaching a couple percent error. However, for some sub-bands both the observations and model fail, notably 220 MHz and 227 MHz that coincide with digital radio broadcasts.

The lack of observed dwarf galaxies compared to predictions for numerous low-mass dark matter halos is a well-known cosmological problem, often referred to as the missing satellites problem. Ultra-compact high velocity clouds (UCHVCs) identified in the ALFALFA HI survey have been presented as potential low mass galaxy candidates in the Local Group due to their single-dish HI properties. We observed some of the UCHVCs with the Westerbork Synthesis Radio Telescope (WSRT) to understand them as potential galaxies. By using the HI kinematics from the WSRT observations, we can constrain the underlying mass distribution, addressing the question of whether these systems represent gas in dark matter halos. We can also study the state of the interstellar medium (ISM) on spatially resolved scales, including searching for the presence of a cool neutral medium (CNM) component, addressing the potential of these systems to form stars and be recognized as ‘ bona-fid ‘ galaxies.

To accomplish this aim we have written a code to retrieve kinematical information by fitting both single and double component Gaussians to the resolved HI gas. As a test case, we focused on AGC 249525, which is interesting for showing ordered velocity motion, similar to another good galaxy candidate AGC 198606 (see Daily Image from 19-12-2014). We produced WSRT HI data cube at a resolution of 210″ (matched to the Arecibo beam) with a velocity resolution of 4 km/s. We fit Gaussians to the data cube, to produce velocity fields. This velocity field reveals ordered velocity motion with a gradient of ~15 km/s. This can be interpreted as rotation with amplitude of ~12 km/s.

In addition, studying the velocity dispersion on spatially resolved scales allows us to understand the star formation (SF) potential of these systems. The velocity dispersion traces the temperature structure of the gas. We are searching for a cool neutral medium (CNM) component that has the potential to collapse and form stars. A single Gaussian fit (or traditional moment 2 map) reveals no evidence for a CNM component. We are involved in an ongoing work to fit 2 Gaussian components to search robustly for a CNM component.

During my visit to ASTRON, I met great people, had so much fun, and learned a lot about radio astronomy. Thanks to Dr. Kelley Hess’ guidance, I successfully reduced the HCG 44 data from the KAT-7 telescope and produced a cube showing HI abundance on HCG 44 compact group. Understanding the gas content and the impact of the group environment on galaxies is important for understanding their overall evolution. We detected that the tail is even more massive and more extended than the previously reported observations. We also found more large scale gas within and between the three compact group galaxies that make up HCG 44.