Letter to the Editor

HI absorption in a gravitational lens at z ~ 0.7645

¹ Kapteyn Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
² RSAA, The Australian National University, Mount Stromlo Observatory, Cotter Road, ACT 2611, Australia

Corresponding authors: N. Kanekar, nissim@astro.rug.nl fbriggs@mso.anu.edu.au

Received: 7 October 2003
Accepted: 31 October 2003

Abstract

We have used the Westerbork Synthesis Radio Telescope to detect HI 21 cm absorption at in the gravitational lens system towards PMN J0134-0931. The 21 cm profile has two broad components, with peak optical depths of and , at heliocentric redshifts and , respectively. The redshift of the stronger component matches that of CaII H and K absorption detected earlier. The absorption has a total velocity width of ~500 km s^-1 (between nulls) and an equivalent width of  km s^-1. This would imply a total HI column density of  cm^-2, for a spin temperature of 200 K and a covering factor of unity. The high estimated HI column density is consistent with the presence of large amounts of dust at the lens redshift; the intervening dust could be responsible for the extremely red colour of the background quasar.