2004ApJ...613L.101D

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<z<1.10 toward a large sample of radio continuum sources. The H I column density is N_HI_=2.35x1019^(T_s/f)/cm2, where T_sis the spin temperature and f is the continuum covering factor of the absorbing gas. For T_s/f>8.5 K, this system is by definition a damped Lyα absorption system (N_HI_≥2x10²⁰/cm2). The line is unusually broad, with an FWHM of 53 km/s and a full span of 163 km/s, suggesting a physically extended H I gas structure. Radio surveys identify damped Lyα systems 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 τ=0.15 (3 σ) in 5 km/s channels. Although this radio damped Lyα selection technique would include dusty, molecule-rich systems, [HB89] 2351+456 appears to be a ``vanilla'' H I 21 cm absorber.