Detection of 21 Centimeter H i Absorption at z = 0.78 in a Survey of Radio Continuum Sources

Abstract

We report the detection of a deep broad H I 21 cm absorption system at z = 0.78 toward the radio source (HB89) 2351+456 (4C+45.51) at z = 1.992. The H I absorption was identified in a blind spectral line survey conducted at the Green Bank Telescope spanning 0.63 8.5 K, this system is by definition a damped Lyabsorption system (NHI � 2 × 10 20 cm −2 ). The line is unusually broad, with a FWHM of 53 km s −1 and a full span of 163 km s −1 , suggesting a physically extended H I gas structure. Radio surveys identify damped Lysystems in a manner that bypasses many of the selection effects present in optical/UV surveys, including dust extinction and the atmospheric cutoff for z < 1.65. The smooth broad profile of this H I 21 cm absorption system is similar to the z = 0.89 H I absorption toward PKS 1830 211, which suggests that the absorber toward (HB89) 2351+456 is also a gravitational lens and a molecular absorption system. However, very long baseline interferometry and Hubble Space Telescope observations show little evidence for gravitational lensing, and BIMA millimeter observations show no HCO + (1 ! 2) or HCN (1 ! 2) absorption down to � = 0.15 (3�) in 5 km s −1 channels. Although this radio damped Lyselection technique would include dusty, molecule-rich systems, (HB89) 2351+456 appears to be a vanilla H I 21 cm absorber. Subject headings: galaxies:ISM — quasars:absorption lines — quasars: individual((HB89) 2351+456)