The ESO UVES Advanced Data Products Quasar Sample - IV. On the deficiency of Argon in DLA systems

TL;DR

This study investigates argon deficiency in high-redshift DLA systems using high-resolution spectra, revealing ionisation as the primary cause and providing insights into cosmic reionisation processes.

Contribution

It presents the largest sample of argon measurements in DLAs, confirming argon deficiency and identifying ionisation effects as the main driver, advancing understanding of interstellar medium conditions at high redshift.

Findings

Argon is generally deficient in DLAs with a mean [Ar/alpha] of -0.4 dex.

Argon deficiency correlates weakly with NHI, redshift, and metallicity.

Ionisation, not dust or nucleosynthesis, explains argon deficiency.

Abstract

In this work, we study argon abundances in the interstellar medium of high-redshift galaxies (2<z(abs)<4.2) detected as Damped Lya absorbers (DLA) in the spectra of background quasars. We use high-resolution quasar spectra obtained from the ESO-UVES advanced data products (EUADP) database. We present 3 new measurements and 5 upper limits of ArI. We further compiled DLAs/sub-DLA data from the literature with measurements available of argon and alpha-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/alpha]= -0.4+/-0.06dex (standard error of the mean). The [Ar/alpha] ratios show a weak, positive trend with increasing NHI and increasing absorption redshift, and a weak, negative trend with dust-free metallicity, [S/H]. Detailed analysis of the abundance ratios indicates that ArI ionisation, rather than dust depletion or nucleosynthetic evolution, is responsible for the argon deficiency. Altogether, the observational evidence is consistent with a scenario of argon ionisation dominated by quasar metagalactic radiation modulated by local HI self-shielding inside the DLA host galaxies. Our measurements and limits of argon abundances suggest that the cosmic reionisation of HeII is completed above z=3, but more measurements at z(abs)>3.5 are required to probe the final stages of this process of cosmic reionisation.