# HI 21-cm absorption from $z\sim0.35$ strong MgII absorbers

## TL;DR

This study searches for HI 21-cm absorption in strong MgII absorbers at redshift 0.3-0.5, detecting two cases and estimating their properties, revealing no significant evolution in absorber incidence over 0.3<z<1.5.

## Contribution

First detection of HI 21-cm absorption in strong MgII systems at z~0.35, providing insights into gas properties and evolution of such absorbers.

## Key findings

- Detected HI 21-cm absorption in 2 out of 11 systems.
- Estimated HI column densities and gas temperatures for detected systems.
- No significant evolution in HI 21-cm absorber incidence from z=0.3 to 1.5.

## Abstract

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 $0.3<z<1.5$.