Abstract
We present the results of a survey for intervening HI 21-cm absorbers at intermediate and low redshift (0<z<1.2). For our total sample of 24 systems, we obtained high quality data for 17 systems, the other seven being severely affected by radio frequency interference (RFI). Five of our targets are low redshift (z<0.17) optical galaxies with small impact parameters (<20 kpc) toward radio-bright background sources. Two of these were detected in 21-cm absorption, showing narrow, high optical depth absorption profiles, the narrowest having a velocity dispersion of only 1.5 km/s, which puts an upper limit on the kinetic temperature of T_k<270 K. Combining our observations with results from the literature, we measure a weak anti-correlation between impact parameter and integral optical depth in local (z<0.5) 21-cm absorbers. Of eleven CaII and MgII systems searched, two were detected in 21-cm absorption, and six were affected by RFI to a level that precludes a detection. For these two systems at z~0.6 we measure spin temperatures of T_s=(65+/-17) K and T_s>180 K. A subset of our systems were also searched for OH absorption, but no detections were made.
Highlights
Observing atomic hydrogen in absorption is an extremely powerful probe of galaxies at all redshifts: the detection sensitivity is not dependent on many of the properties of the system under study but depends rather on the unrelated characteristics of the background source
We present the combined results of several Green Bank Telescope1 (GBT) surveys for intervening H I and OH absorption at low and intermediate redshifts (0 < z < 1.2)
The total sample of observed systems consists of Ca II absorbers, Mg II absorbers, apparent quasar–galaxy pairs with small impact parameters at low redshift, and Mg II and Ly α absorbers with identified host galaxies
Summary
Observing atomic hydrogen in absorption is an extremely powerful probe of galaxies at all redshifts: the detection sensitivity is not dependent on many of the properties of the system under study but depends rather on the unrelated characteristics of the background source. A more complete sampling of low-redshift H I absorbers will eventually help in accurately determining the evolution of the cosmic mass density of neutral hydrogen H I over half the age of the Universe, especially when the results of large-scale 21-cm absorption line surveys from the Square Kilometre Array and its pathfinders will become available. The systems selected to be observed in the OH 1667 MHz line are a subset of those observed in the 21-cm line, concentrating on the galaxies with small impact parameters to background radio sources The purpose of this survey was to identify molecular absorption lines in the centres of galaxies, where it is known that the ratio of atomic to molecular gas density is higher (e.g. Leroy et al 2009).
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