Emergence of a new H i 21-cm absorption component at z ∼ 1.1726 towards the γ-ray blazar PKS 2355-106

• Published in: Monthly notices of the Royal Astronomical Society Vol. 516; no. 1; pp. 1339 - 1346
• Main Authors: Srianand, Raghunathan, Gupta, Neeraj, Petitjean, Patrick, Momjian, Emmanuel, Balashev, Sergei A, Combes, Françoise, Chen, Hsiao-Wen, Krogager, Jens-Kristian, Noterdaeme, Pasquier, Rahmani, Hadi, Baker, Andrew J, Emig, Kimberly L, Józsa, Gyula I G, Kloeckner, Hans-Rainer, Moodley, Kavilan
• Format: Journal Article
• Language: English
• Published: Oxford University Press 05.09.2022; Oxford University Press (OUP): Policy P - Oxford Open Option A
• Subjects: Sciences of the Universe; quasars: absorption lines; galaxies: ISM; quasars: individual PKS 2355-106; quasars:individual PKS 2355-106; Astrophysics - Astrophysics of Galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stac1877

ABSTRACT We report the emergence of a new H i 21-cm absorption at zabs = 1.172 635 in the damped Lyα absorber (DLA) towards the γ-ray blazar PKS 2355-106 (zem∼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 zabs = 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 zabs = 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 zabs = 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.