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Title:
Detection of 21 Centimeter H I Absorption at z=0.78 in a Survey of Radio Continuum Sources
Authors:
Darling, Jeremy; Giovanelli, Riccardo; Haynes, Martha P.; Bolatto, Alberto D.; Bower, Geoffrey C.
Affiliation:
AA(Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101; ), AB(Department of Astronomy and Space Sciences, Cornell University, Ithaca, NY 14853; , ), AC(Department of Astronomy and Space Sciences, Cornell University, Ithaca, NY 14853; , ), AD(Department of Astronomy, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411 , ), AE(Department of Astronomy, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411 , )
Publication:
The Astrophysical Journal, Volume 613, Issue 2, pp. L101-L104. (ApJL Homepage)
Publication Date:
10/2004
Origin:
UCP
ApJ Keywords:
Galaxies: ISM, Galaxies: Quasars: Absorption Lines, Galaxies: Quasars: Individual: Alphanumeric: [HB89] 2351+456
DOI:
10.1086/425143
Bibliographic Code:
2004ApJ...613L.101D

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.
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