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

Abstract

We report results from a deep search for redshifted H I 21 cm absorption in 55 strong Mg IIλ2796 absorbers (having Wλ27960 > 0.5 Å) at intermediate redshifts, 0.58 < zabs < 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 < zabs < 1.68, including three systems reported earlier by Gupta et al. (2007). Absorption was not detected at >3σ significance in 32 other Mg II absorbers, with 26 of these providing strong upper limits to the H I 21 cm optical depth, τ3σ < 0.013 per ∼10 km s^-1. For the latter 26 systems, the spin temperature Ts of the absorber must be >[800 ×f] K (where f is the covering factor), if the H I column density is ≥2 × 1020 cm^-2, 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.