HI absorption in a gravitational lens atz~ 0.7645

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.