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We present results from MUSE observations of a 21-cm HI absorption systemdetected with the Australian Square Kilometre Array Pathfinder radio telescopeat redshift $z = 0.4503$ towards the $z = 1.71$ quasar PKS 1610-771. Weidentify four galaxies (A, B, X and Y) at the same redshift as the 21-cm H IDamped Lyman-{\alpha} (DLA) absorption system, with impact parameters rangingfrom less than 10 kpc to almost 200 kpc from the quasar sightline. Ca II and NaI absorption is seen in the MUSE spectrum of the background QSO, withvelocities coinciding with the initial HI 21-cm detection, but tracing lessdense and warmer gas. This metal-line component aligns with the rotatingionised disc of galaxy B (impact parameter 18 kpc from the QSO) and appears tobe co-rotating with the galaxy disc. In contrast, the 21-cm HI absorber isblueshifted relative to the galaxies nearest the absorber and has the oppositesign to the velocity field of galaxy B. Since galaxies A and B are separated byonly 17 kpc on the sky and $70$ km s$^{-1}$ in velocity, it appears likely thatthe 21-cm detection traces extragalactic clouds of gas formed from theirinteraction. This system reveals that the cold 100 K neutral gas critical forstar formation can be associated with complex structures beyond the galaxydisc, and is a first case study made in preparation for future large 21-cmabsorption surveys like the ASKAP First Large Absorption Survey in HI.

Observations of cold extragalactic gas clouds at $z = 0.45$ towards PKS 1610-771