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

Raghunathan Srianand, Neeraj Gupta, Patrick Petitjean, Emmanuel Momjian, Sergei A. Balashev, Françoise Combes, Hsiao Wen Chen, Jens Kristian Krogager, Pasquier Noterdaeme, Hadi Rahmani, Andrew J. Baker, Kimberly L. Emig, Gyula I.G. Józsa, Hans Rainer Kloeckner, Kavilan Moodley

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Pages (from-to)1339-1346
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Oct 1 2022

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • galaxies: ISM
  • quasars: absorption lines
  • quasars: individual PKS 2355-106


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