4 Aug 2015
Next-generation radio telescopes will make it possible to conduct the first large-scale HI absorption-line surveys, which will enable us to study the evolution of neutral gas in galaxies over a large range of cosmic time. However, we don’t currently have the understanding to derive physical galaxy properties from absorption-line data alone.
To gain this understanding, we need to start by knowing the expected detection rate of intervening HI absorption. Previous studies have suggested that the detection rate is around 50% for sightlines bypassing the galaxy at distances of 20 kpc or less. However, these studies have typically targeted sightlines to quasars which provide very bright, compact radio sources ideal for detecting HI absorption against. Since only around 10% of all radio sources are quasars, it is therefore possible that such studies will have over-estimated the detection rate, compared to what future blind surveys might expect to find.
In a new study, CAASTRO researcher Sarah Reeves (University of Sydney) and colleagues have investigated the detection rate of intervening absorption in an unbiased sample of radio sources. Importantly, they also obtained HI emission-line data, allowing them to map the distribution of HI gas in the target galaxies. This means that where they did not detect an absorption-line, they were able to pin-point the reason for the non-detection, i.e. whether the lack of absorption was due to the sightline not intersecting the HI disk of the galaxy or due to the properties of the background radio source (e.g. too dim) – or some other reason.
This publication presents observations and results from the pilot sample (six of an eventual 16 sources). In this pilot sample, no intervening absorption-lines were detected. While observations for the full sample are required to better establish the detection rate, this preliminary result suggests that the detection rate is considerably lower than estimated by previous studies – perhaps around 5-10%. The team found that most of their sightlines did intersect the HI disk of the target galaxies, meaning that the low detection rate must be due to properties of the background sources. They found that many of the background sources resolved into multiple components at higher resolution, lowering the flux and reducing the absorption-line sensitivity. These results show that source type and structure can significantly affect the detection rate of absorption-line surveys, and help astronomers to better prepare for future large surveys, such as FLASH (‘The First Large Absorption Survey in HI’).
S. N. Reeves, E. M. Sadler, J. R. Allison, B. S. Koribalski, S. J. Curran, and M. B. Pracy in MNRAS (2015) HI emission and absorption in nearby, gas-rich galaxies
The First Large Absorption Survey in HI (FLASH) is a wide-field ASKAP survey that will provide world-class science through the provision of new measurements of the amount and distribution of HI in distant galaxies, allowing us for the first time to investigate the relationship between HI gas supply and star formation rate in individual galaxies at z>0.5.
ASKAP will be an array of 36 antennas each 12m in diameter, capable of high dynamic range imaging and using wide-field-of-view phased array feeds. ASKAP is intended to be a world-class telescope in its own right as well as a pathfinder instrument for the Square Kilometre Array.
ASKAP’s large spectral bandwidth (300 MHz bandwidth over the frequency range 700-1800 MHz) and wide field of view (30 square degrees) will open up a completely new parameter space for large, blind HI absorption-line surveys using background radio continuum sources. Since the detection limit for such surveys is independent of redshift, ASKAP-FLASH will allow us to learn about the neutral gas content of galaxies in the poorly-explored redshift range 0.5 < z < 1.0, where the HI emission line is too weak to be detectable in even the deepest ASKAP surveys. The FLASH survey aims to detect and measure several hundred HI absorption lines (from both intervening and associated absorbers). This will provide a unique dataset for studies of galaxy evolution as well as a new estimate of the HI mass density at intermediate redshifts. The FLASH data will also be used for HI emission-line stacking experiments in combination with large-area optical redshift surveys like WiggleZ and GAMA.
Professor Elaine Sadler (Project Leader)
CAASTRO Member Node
Professor Lister Staveley-Smith University of Western Australia
Associate Professor Martin Meyer University of Western Australia
Dr. James Allison University of Sydney
Dr. Stephen Curran University of Sydney
Ms. Sarah Reeves University of Sydney
Mr. Marcin Glowacki University of Sydney
Associate Professor Chris Blake Swinburne University
Professor Matthew Colless Australian National University
See the full article here.
Please help promote STEM in your local schools.
Astronomy is entering a golden age, in which we seek to understand the complete evolution of the Universe and its constituents. But the key unsolved questions in astronomy demand entirely new approaches that require enormous data sets covering the entire sky.
In the last few years, Australia has invested more than $400 million both in innovative wide-field telescopes and in the powerful computers needed to process the resulting torrents of data. Using these new tools, Australia now has the chance to establish itself at the vanguard of the upcoming information revolution centred on all-sky astrophysics.
CAASTRO has assembled the world-class team who will now lead the flagship scientific experiments on these new wide-field facilities. We will deliver transformational new science by bringing together unique expertise in radio astronomy, optical astronomy, theoretical astrophysics and computation and by coupling all these capabilities to the powerful technology in which Australia has recently invested.
The University of Sydney
The University of Western Australia
The University of Melbourne
Swinburne University of Technology
The Australian National University
University of Queensland