Emergence of a new H i 21-cm absorption component at z ∼ 1.1726 towards the γ-ray blazar PKS 2355-106. Issue 1 (22nd July 2022)

• Record Type: Journal Article
• 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
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 516:Issue 1(2022)
• Page Start: 1339
• Page End: 1346
• Publication Date: 2022-07-22
• Subjects: galaxies: ISM -- quasars: absorption lines -- quasars: individual PKS 2355-106; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac1877
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We report the emergence of a new H i 21-cm absorption at z abs = 1.172 635 in the damped Ly α 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.