科研之友
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摘要
We report results from a deep search for redshifted H I 21 cm absorption in 55 strong Mg II lambda 2796 absorbers (having W(0)(lambda 2796) > 0.5 angstrom) at intermediate redshifts, 0.58 < z(abs) < 1.70, with the Green Bank Telescope (GBT) and the Giant Metrewave Radio Telescope (GMRT). Nine detections of H I 21 cm absorption were obtained, all at 1.17 < z(abs) < 1.68, including three systems reported earlier by Gupta et al. (2007). Absorption was not detected at > 3 sigma significance in 32 other Mg II absorbers, with 26 of these providing strong upper limits to the H I 21 cm optical depth, tau(3 sigma) < 0.013 per similar to 10 km s(-1). For the latter 26 systems, the spin temperature T(s) of the absorber must be > [800 x f] K (where f is the covering factor), if the H I column density is >= 2 x 10(20) cm(-2), i.e. if the absorber is a damped Lyman alpha system (DLA). Data on the remaining 13 systems of the sample were affected by radio frequency interference and were hence not useful. Two of the Mg II absorbers, at z(abs) similar to 1.4106 towards 2003-025 and at z(abs) similar to 0.9115 towards 2149+212, are known DLAs. We detect H I 21 cm absorption towards 2003-025 with the GMRT and estimate the spin temperature of the DLA to be T(s) = [(905 +/- 380) x f] K. Conversely, the GBT observations of 2149+212 resulted in a non-detection of H I 21 cm absorption, yielding the 3 sigma limit T(s) > [2700 x f] K. Excluding 'associated' systems (within 3000 km s(-1) of the quasar redshift), the detection rate of H I 21 cm absorption in strong Mg II lambda 2796 absorbers is x(21),(Mg) (II)((z) over bar = 1.1) = 25(-8)(+11) percent, at a 3 sigma optical depth sensitivity of similar to 0.013 per 10 km s(-1). Comparing the detection rates of H I 21 cm and damped Lyman alpha absorption in strong Mg II absorber samples yields a detection rate of H I 21 cm absorption in DLAs of x(21,) (DLA)((z) over bar = 1.1) = (73 +/- 27) per cent, consistent with the detection rate in low-z DLAs. Since H I 21 cm absorption arises in cold neutral gas, this indicates that most gas-rich galaxies contain significant fractions of cold H I by z similar to 1. Finally, we use the observed detection rate of H I 21 cm absorption in Mg II absorbers to infer the cosmological mass density of neutral gas in DLAs, assuming that (1) the average HI column density in our HI 21 cm absorber sample is the same as that measured by Rao, Turnshek & Nestor in their DLA sample and (2) the detection rate of HI 21 cm absorption in the DLAs of our MgII sample is the same as that in known DLAs at 0.09 < z(abs) < 3.45. We obtain Omega(GAS) similar to (0.55(-0.22)(+0.42)) x 10(-3), at (z) over bar similar to 1.1, slightly lower than, but consistent with, the value obtained by Rao, Turnshek & Nestor from their DLA survey at similar redshifts.
- 出版日期2009-6-11
