Pilot study data prepare astronomers for future blind HI surveys
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”