HI 21-cm absorption from $z\sim0.35$ strong MgII absorbers
  We have searched for HI 21-cm absorption in 11 strong MgII systems ($W_{\rm
r}$(MgII $\lambda 2796$) $\ge 1$ \AA) at $0.3<z<0.5$ using the Giant Metrewave
Radio Telescope. We have detected HI 21-cm absorption in two of these. From the
integrated optical depth ($\int\tau~dv$) we estimate $N$(HI) = $43 \pm 2$ and
$9 \pm 2$ in units of $10^{19}$ 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 HI absorption towards J1428+2103 and J1551+0713
indicate that the gas temperature is $<1600$ K and $<350$ K, respectively. The
$3\sigma$ upper limits on $\int\tau~dv$ in case of the HI 21-cm non-detections
indicate that these MgII absorbers are likely to arise from sub-damped
Lyman-$\alpha$ systems, when we assume $T_s = 100$ K and $C_f = 1$. This is
verified for one of the systems which has $N$(HI) measurement using
Lyman-$\alpha$ absorption detected in the ultraviolet spectrum. We estimate the
detection rate of HI 21-cm absorption in strong MgII systems in our sample to
be $0.18^{+0.24}_{-0.12}$ at $z\sim0.35$, for an integrated optical depth
sensitivity of $\le 0.3$ km s$^{-1}$. Comparing with the results of HI 21-cm
absorption surveys in strong MgII systems at higher redshifts from the
literature, we do not find any significant evolution in the incidence and
number density per unit redshift of HI 21-cm absorbers in strong MgII systems
over $0.3<z<1.5$.
