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 < z < 1.10 toward a large sample of radio continuum sources. The H I column density is NHI = 2.35 × 1019(TS/f) cm-2, where Ts is the spin temperature and f is the continuum covering factor of the absorbing gas. For Ts/f > 8.5 K, this system is by definition a damped Lyα absorption system (NHI ≥ 2 × 1020 cm -2). The line is unusually broad, with an 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 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-1 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.