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W. M. Lane, F. H. Briggs (Kapteyn Astronomical Institute)
We present results from an extensive search for HI 21cm absorption at moderate redshifts. The survey was limited to systems with Mg II absorption at redshifts in the range 0.2 < z < 1.0, which lie in front of bright radio quasars (S > 0.5 Jy at 1400 MHz). Narrow band observations centered at the metal-line redshifts were made with the UHF-high system and, when possible, the new DZB correlator at the WSRT. Three new absorbers have been confirmed and several candidate absorbers identified. Optical depth limits of a few percent at velocity resolutions of about 6 km s-1 are obtained for most of the non-detections. The incidence of low redshift 21cm absorbers is calculated.
The detection rate is lower than that found in a comparable UV survey for damped Ly\alpha absorption lines in MgII absorbers (Rao and Turnshek, astro-ph/9909164). The 21cm search is complementary to the damped Ly\alpha survey, and by combining information from both we are able to better understand physical conditions in these high neutral column density absorbers. We find that the 21cm objects represent a cold subset of high neutral column density absorbers, but are not necessarily the highest column density systems. Observations in the optical, infrared, and radio are used to identify and characterize the galaxies which give rise to the absorption.
Support for W. M. Lane is provided by an Ubbo Emmius fellowship for graduate study at the Rijksuniversiteit Groningen.