2009MNRAS.398..201G

We present the results of a systematic Giant Metrewave Radio Telescope (GMRT) survey of 21-cm absorption in a representative and unbiased sample of 35 strong Mgii systems in the redshift range: z_abs∼ 1.10-1.45, 33 of which have W_r≥ 1Å. The survey using ∼400h of telescope time has resulted in nine new 21-cm detections and stringent 21-cm optical depth upper limits (median 3σ optical depth per 10km/s of 0.017) for the remaining 26 systems. This is by far the largest number of 21-cm detections from any single survey of intervening absorbers. Prior to our survey, no intervening 21-cm system was known in the above redshift range, and only one system was known in the redshift range 0.7 ≤ z ≤ 1.5. We discuss the relation between the detectability of 21-cm absorption and various properties of UV absorption lines. We show that if Mgii systems are selected with the following criteria, Mgii doublet ratio ≤1.3 and W_r(Mgi)/W_r(Mgii) ≥ 0.3, then a detection rate of 21-cm absorption up to 90 per cent can be achieved. We estimate n₂₁, the number per unit redshift of 21-cm absorbers with W_r(Mgii)>W_o and integrated optical depth, and show that n₂₁ decreases with increasing redshift. In particular, for W_o= 1.0Å and falls by a factor of 4 from <z> = 0.5 to <z> = 1.3. The evolution seems to be stronger for stronger Mgii systems. Using a subsample of systems for which high-frequency Very Long Baseline Array (VLBA) images are available, we show that the effect is not related to the structure of the background radio sources and is most probably due to the evolution of the cold neutral medium filling factor in Mgii systems. We find no correlation between the velocity spread of the 21-cm absorption feature and W_r(Mgii) at z ∼ 1.3.