2003A&A...412L..29K

We have used the Westerbork Synthesis Radio Telescope to detect HI 21 cm absorption at z∼0.7645 in the gravitational lens system towards PMN J0134-0931. The 21cm profile has two broad components, with peak optical depths of 0.047±0.007 and 0.039±0.007, at heliocentric redshifts 0.76470±0.00006 and 0.76348±0.00006, 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 ∼500km/s (between nulls) and an equivalent width of 7.1±0.08km/s. This would imply a total HI column density of 2.6±0.3x10²¹/cm2, for a spin temperature of 200K 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.