2017MNRAS.468.1029D

We have searched for H I 21-cm absorption in 11 strong Mg II systems (W_r(Mg II λ2796) >= 1 Å) at 0.3 < z < 0.5 using the Giant Metrewave Radio Telescope. We have detected H I 21-cm absorption in two of these systems. From the integrated optical depth ([?]τdv), we estimate N(H  I) = 43 ± 2 and 9 ± 2 in units of 10¹⁹ cm^–2 for the absorbers towards J1428+2103 (z_abs = 0.3940) and J1551+0713 (z_abs = 0.3289), respectively, assuming spin temperature T_s = 100 K and gas covering factor C_f = 1. The velocity width of the H I absorption towards J1428+2103 and J1551+0713 indicates that the gas temperature is <1600 and <350 K, respectively. The 3σ upper limits on [?]τdv in the case of the H I 21-cm non-detections indicate that these Mg II absorbers are likely to arise from sub-damped Lyman α systems, when we assume T_s = 100 K and C_f = 1. This is verified for one of the systems which has N(H  I) measurement using Lyman α absorption detected in the ultraviolet spectrum. We estimate the detection rate of H I 21-cm absorption in strong Mg II systems in our sample to be 0.18^+0.24_–0.12 at z ∼ 0.35, for an integrated optical depth sensitivity of <=0.3 km s^–1. Comparing with the results of H I 21-cm absorption surveys in strong Mg II systems at higher redshifts from the literature, we do not find any significant evolution in the incidence and number density per unit redshift of H I 21-cm absorbers in strong Mg II systems over 0.3 < z < 1.5.