A search for H i 21 cm absorption in strong Mg ii absorbers in the redshift desert.

We report results from a deep search for redshifted H i 21 cm absorption in 55 strong Mg iiλ2796 absorbers (having ) at intermediate redshifts, , 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 , including three systems reported earlier by Gupta et al. (2007) . Absorption was not detected at significance in 32 other Mg ii absorbers, with 26 of these providing strong upper limits to the H i 21 cm optical depth, per . For the latter 26 systems, the spin temperature T_s of the absorber must be (where f is the covering factor), if the H i column density is , 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 Mg ii absorbers, at towards and at towards 2149+212, are known DLAs. We detect H i 21 cm absorption towards with the GMRT and estimate the spin temperature of the DLA to be . Conversely, the GBT observations of 2149+212 resulted in a non-detection of H i 21 cm absorption, yielding the 3σ limit . Excluding ‘associated’ systems (within of the quasar redshift), the detection rate of H i 21 cm absorption in strong Mg iiλ2796 absorbers is per cent, at a 3σ optical depth sensitivity of ∼0.013 per . Comparing the detection rates of H i 21 cm and damped Lyman α absorption in strong Mg ii absorber samples yields a detection rate of H i 21 cm absorption in DLAs of 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 . 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 H i column density in our H i 21 cm absorber sample is the same as that measured by Rao, Turnshek & Nestor in their DLA sample and (2) the detection rate of H i 21 cm absorption in the DLAs of our Mg ii sample is the same as that in known DLAs at . We obtain , at , slightly lower than, but consistent with, the value obtained by Rao, Turnshek & Nestor from their DLA survey at similar redshifts. [ABSTRACT FROM AUTHOR]