The ESO UVES Advanced Data Products quasar sample - IV. On the deficiency of argon in DLA systems

In this work, we study argon abundances in the interstellar medium of high-redshift galaxies (2.0 ≤ zabs ≤ 4.2) detected as damped Lyα absorbers (DLA) in the spectra of background quasars. We use high-resolution quasar spectra obtained from the European Southern Observatory Ultraviolet-Visual Echelle Spectrograph Advanced Data Products data base. We present three new measurements and five upper limits of Ar I. We further compiled DLAs/sub-DLA data from the literature with measurements available of argon and α-capture elements (S or Si), making up a total of 37 systems, i.e. the largest DLA argon sample investigated so far. We confirm that argon is generally deficient in DLAs, with a mean value [Ar/α] ≃ −0.4 ± 0.06 dex (standard error of the mean). The [Ar/α] ratios show a weak, positive trend with increasing H I column density and increasing absorption redshift, and a weak, negative trend with dust-free metallicity, [S/H]. Detailed analysis of the abundance ratios indicates that Ar I ionization, rather than dust depletion or nucleosynthetic evolution, is responsible for the argon deficiency. Altogether, the observational evidence is consistent with a scenario of argon ionization dominated by quasar metagalactic radiation modulated by local H I self-shielding inside the DLA host galaxies. Our measurements and limits of argon abundances suggest that the cosmic reionization of He II is completed above z ∼ 3, but more measurements at zabs > 3.5 are required to probe the final stages of this process of cosmic reionization.