2022MNRAS.516.1339S

We report the emergence of a new H I 21-cm absorption at z_abs = 1.172 635 in the damped Lyα absorber (DLA) towards the γ-ray blazar PKS 2355-106 (z_em∼1.639) using science verification observations (2020 June) from the MeerKAT Absorption Line Survey (MALS). Since 2006, this DLA is known to show a narrow H I 21-cm absorption at z_abs = 1.173019 coinciding with a distinct metal absorption-line component. We do not detect significant H I 21-cm optical depth variations from this known H I component. A high-resolution optical spectrum (2010 August) shows a distinct Mg I absorption at the redshift of the new H I 21-cm absorber. However, this component is not evident in the profiles of singly ionized species. We measure the metallicity ([Zn/H] = -(0.77 ± 0.11) and [Si/H]= -(0.96 ± 0.11)) and depletion ([Fe/Zn] = -(0.63 ± 0.16)) for the full system. Using the apparent column density profiles of Si II, Fe II, and Mg I, we show that the depletion and the N(Mg I)/N(Si II) column density ratio systematically vary across the velocity range. The region with high depletion tends to have a slightly larger N(Mg I)/N(Si II) ratio. The two H I 21-cm absorbers belong to this velocity range. The emergence of z_abs = 1.172 635 can be understood if there is a large optical depth gradient over a length-scale of ∼0.35 pc. However, the gas producing the z_abs = 1.173 019 component must be nearly uniform over the same scale. Systematic uncertainties introduced by the absorption-line variability has to be accounted for in experiments measuring the variations of fundamental constants and cosmic acceleration even when the radio emission is apparently compact as in PKS 2355-106.