2009MNRAS.396..385K

We report results from a deep search for redshifted Hi 21cm absorption in 55 strong Mgiiλ2796 absorbers (having W^λ2796₀ > 0.5Å) 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 Hi 21cm 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 >3σ significance in 32 other Mgii absorbers, with 26 of these providing strong upper limits to the Hi 21cm optical depth, τ_3σ< 0.013 per ∼10km/s. For the latter 26 systems, the spin temperature T_s of the absorber must be >[800 xf] K (where f is the covering factor), if the Hi column density is ≥2x10²⁰/cm2, i.e. if the absorber is a damped Lyman α 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 Mgii absorbers, at z_abs∼ 1.4106 towards 2003-025 and at z_abs∼ 0.9115 towards 2149+212, are known DLAs. We detect Hi 21cm absorption towards 2003-025 with the GMRT and estimate the spin temperature of the DLA to be T_s = [(905±380)xf] K. Conversely, the GBT observations of 2149+212 resulted in a non-detection of Hi 21cm absorption, yielding the 3σ limit T_s> [2700xf] K.

Excluding `associated' systems (within 3000km/s of the quasar redshift), the detection rate of Hi 21cm absorption in strong Mgiiλ2796 absorbers is per cent, at a 3σ optical depth sensitivity of ∼0.013 per 10km/s. Comparing the detection rates of Hi 21cm and damped Lyman α absorption in strong Mgii absorber samples yields a detection rate of Hi 21cm absorption in DLAs of per cent, consistent with the detection rate in low-z DLAs. Since Hi 21cm absorption arises in cold neutral gas, this indicates that most gas-rich galaxies contain significant fractions of cold Hi by z ∼ 1.

Finally, we use the observed detection rate of Hi 21cm absorption in Mgii absorbers to infer the cosmological mass density of neutral gas in DLAs, assuming that (1) the average Hi column density in our Hi 21cm absorber sample is the same as that measured by Rao, Turnshek & Nestor in their DLA sample and (2) the detection rate of Hi 21cm 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 Ω_GAS∼ (0.55^+0.42_–0.22) x10^–3, at, slightly lower than, but consistent with, the value obtained by Rao, Turnshek & Nestor from their DLA survey at similar redshifts.