A third HI 21-cm absorption system in the sight-line of MG J0414+0534: A redshift for Object X?

TL;DR

This paper reports the discovery of a third HI 21-cm absorption system towards quasar MG J0414+0534, potentially linked to a gas-rich galaxy or active nucleus, with implications for understanding intervening absorbers and galaxy environments.

Contribution

First detection of a third HI 21-cm absorber in this sight-line, identifying a new potential galaxy or active nucleus at z=0.38.

Findings

Strong HI 21-cm absorption at z=0.38

No detection of OH lines at the same redshift

Possible association with an active galaxy or star-forming gas-rich galaxy

Abstract

We report the detection of a third HI 21-cm absorber in the sight-line towards the z=2.64 quasar MG J0414+0534 (4C +05.19). In addition to the absorption at the host redshift and in the z=0.96 gravitational lens, we find, through a decimetre-wave spectral scan towards this source, strong absorption at z=0.38. We believe this may be associated with "Object X", an additional feature apparent in the field of the lensing galaxy and lensed images, on the basis of its close proximity to the quasar images and the possible detection of the [OIII] doublet in a published optical spectrum. If real, the strength of the [OIII] emission would suggest the presence of an active galactic nucleus, or a gas-rich galaxy undergoing rapid star formation, either of which is consistent with the strong outflows apparent in the 21-cm spectrum. Although this is the strongest intervening 21-cm absorber yet found, simultaneous observations failed to detect any of the 18-cm OH lines at the 21-cm redshift. This suggests that, as for the lensing galaxy, this is not the primary location of the intervening material responsible for the very red colour of MG J0414+0534.