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

Abstract

We report the detection of 21-cm absorption at z_abs = 0.656 towards 1622 + 238 (3C336). The line is very broad with a full width to half-maximum of 235 km s^-1, giving a velocity-integrated optical depth of . The centroid of the line is offset from that of the known damped Lyman α absorption system (DLA) by 50 km s^-1, and if the Lyman α and 21-cm absorption are due to the same gas, we derive a spin temperature of T_s <= 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 disc of a large underluminous spiral or in a group of dim unidentified galaxies, associated with the single object which has been optically identified at this redshift.