Study detects new hydrogen absorption system in nearby galaxy

Next-generation radio telescopes will allow us to map out the evolution of gas in galaxies up to a redshift of z = 1.0, revealing what galaxies were like when the Universe was just half its current age. Neutral hydrogen gas (HI), along with stars, forms one of the two main components of spiral galaxies and reveals evidence of past or ongoing galaxy interactions (in the form of tidal tails and bridges), which often cannot be seen in stellar light. The study of hydrogen gas in and around galaxies is therefore a critical part of understanding how galaxies grow and evolve over cosmic time.

HI absorption occurs when hydrogen gas is located along the sightline to a bright background radio source. The detectability is independent of redshift, depending only on the brightness of the background source, such that absorption is detectable at much higher redshifts than HI emission. FLASH (‘the First Large Absorption Survey in HI’) will search for HI absorption along 150,000 sightlines at 0.5<z<1.0, using the Australian Square Kilometre Array Pathfinder telescope (ASKAP) in Western Australia.

In a recent study, CAASTRO researcher Sarah Reeves (University of Sydney), together with CAASTRO Director Prof Elaine Sadler (University of Sydney) and other colleagues, searched for HI absorption in a sample of nearby, gas-rich galaxies. This work builds on the results of a previous paper, completing the full sample of 16 galaxies and 23 individual sightlines. The team detected one new absorption-line system, giving an overall detection rate of 4.3%. This is much lower than in previous studies, but the radio source sample much more closely matches that of future blind surveys such as FLASH and therefore provides a more realistic estimate of the expected detection rate. From this, they determined that the detection rate of intervening absorption is probably not greater than about 10% for sightlines passing within 20 kpc (or less of the centre of the galaxy) and radio sources brighter than 50 mJy.

Their most exciting result, however, is the detection of a new intervening absorption-line system in the galaxy NGC5156, at a redshift of z = 0.01. The absorption-line arises at a distance of 19 kpc from the galaxy’s centre, making it the second furthest-out ever detected. High-resolution images taken with the Australia Telescope Compact Array at 5 and 8 GHz reveal that the background source is resolved into two components, with a separation of 2.6 arcseconds, corresponding to a distance of 500 kpc. The researchers’ analysis suggests that the absorption arises against a single component, which helps place constraints on the physical size of the absorbing cloud.

Publication details:

Sarah Reeves, Elaine Sadler, James Allison et al. in the Monthly Notices of the Royal Astronomical Society (2016) “HI emission and absorption in nearby, gas-rich galaxies II. – sample completion and detection of intervening absorption in NGC5156”