A fourth HI 21-cm absorption system in the sight-line of MG J0414+0534: a record for intervening absorbers

TL;DR

This paper reports the discovery of a record four HI 21-cm absorption systems along a single sight-line to a quasar, suggesting a high prevalence of gas-rich galaxies in obscured environments and implications for future surveys.

Contribution

It presents the first detection of four intervening HI 21-cm absorbers along one sight-line, exceeding previous records and indicating a potential link to the source's red color.

Findings

Detected four HI 21-cm absorption systems along one sight-line.

No molecular gas detected despite multiple atomic absorbers.

Possible correlation between atomic line strength and optical-near-infrared color.

Abstract

We report the detection of a strong HI 21-cm absorption system at z=0.5344, as well as a candidate system at z=0.3389, in the sight-line towards the z=2.64 quasar MG J0414+0534. This, in addition to the absorption at the host redshift and the other two intervening absorbers, takes the total to four (possibly five). The previous maximum number of 21-cm absorbers detected along a single sight-line is two and so we suspect that this number of gas-rich absorbers is in some way related to the very red colour of the background source. Despite this, no molecular gas (through OH absorption) has yet been detected at any of the 21-cm redshifts, although, from the population of 21-cm absorbers as a whole, there is evidence for a weak correlation between the atomic line strength and the optical--near-infrared colour. In either case, the fact that so many gas-rich galaxies (likely to be damped Lyman-alpha absorption systems) have been found along a single sight-line towards a highly obscured source may have far reaching implications for the population of faint galaxies not detected in optical surveys, a possibility which could be addressed through future wide-field absorption line surveys with the Square Kilometre Array.