Detection of broad 21-cm absorption at z = 0.656 in the complex sight-line towards 3C336

TL;DR

This paper reports the first detection of broad 21-cm hydrogen absorption at redshift 0.656 towards 3C336, suggesting the presence of cold gas in a galaxy or galaxy group, with implications for understanding DLA systems.

Contribution

It presents the discovery of an unusually broad 21-cm absorption line at intermediate redshift, indicating cold gas in a galaxy or group, and compares it to known DLA properties.

Findings

Detected broad 21-cm absorption at z=0.656

Derived a very low spin temperature (<60 K)

Absorption likely occurs in a galaxy disk or group

Abstract

We report the detection of 21-cm absorption at z = 0.656 towards 1622+238 (3C336). The line is very broad with a Full-Width Half Maximum (FWHM) of 235 km/s, giving a velocity integrated optical depth of 2.2 km/s. The centroid of the line is offset from that of the known damped Lyman-alpha absorption (DLA) system by 50 km/s, and if the Lyman-alpha and 21-cm absorption are due to the same gas, we derive a spin temperature of < 60 K, which would be the lowest yet in a DLA. The wide profile, which is over four times wider than that of any other DLA, supports the hypothesis that the hydrogen absorption is occurring either in the disk of a large underluminous spiral or a group of dim unidentified galaxies, associated with the single object which has been optically identified at this redshift.