$\ion{Ar}{i}$ as a tracer of ionization evolution

Abstract

Abstract. We present a study of Ar abundances in 15 damped Ly α systems (DLAs) in the redshift interval 2.3 ≤ z ≤ 3.4. The sample includes 4 UVES/VLT measurements of Ar  column densities presented here for the first time, together with 6 measurements and 5 upper/lower limits collected from the literature (UVES/VLT and HIRES/Keck observations). The majority of DLAs show significant underabundances of Ar relative to other α-capture elements with common nucleosynthetic origin. We show that neither dust depletion nor intervening H  regions inside DLAs offer a viable justification to these underabundances. A natural explanation is found in the framework of photoionization models of H  regions embedded in an ionizing continuum with varying spectral distribution. At z ∼ 2.5 the observed Ar deficiencies are large, [Ar/α] −0.6/ − 0.8 dex, suggestive of a hard, QSO-dominated spectrum. At z > ∼ 3 the deficiencies are instead small, suggestive of a soft, stellar-type spectrum, though more data are needed to generalize this high-z result. Should the change of Ar abundances with redshift be governed by the evolution of the UV stellar emission internal to DLAs, a synchronization of the star formation in DLAs would be required, with a strong stellar emission at z > 3, but weak at z < 3. This variation seems inconsistent with the weak signal of evolution indicated by abundance studies of DLAs. More likely, the change of Ar abundances is induced by the evolution of the UV metagalactic continuum, in which case the UV emission internal to DLAs must be small (i.e. DLAs should have modest star formation rates) and the external background must become softer at z > 3. The former requirement is consistent with the modest evolution of DLAs abundances and the lack of Ly α and H α emissions associated with DLAs. The latter requirement is consistent with the observed evolution of Si /C  ratios in the IGM, the claims of high escape fraction of UV photons from Ly-break galaxies at z > ∼ 3, and the recent finding that the He  re-ionization seems to occur between z ∼ 3.4 and z ∼ 3. Comparison with results from local interstellar studies indicates that Ar abundances can be used to trace the evolution of the ionization history of the universe down to z = 0, where [Ar/α] ∼ −0.2 dex. We predict a rise of Ar abundances in the redshift range from z 2.3 to z = 0, at the epoch at which the metagalactic field of galaxies overcomes that of quasars.

A new comprehensive set of elemental abundances in DLAs

“…The derived H i column densities, log N(H i) = 20.99 ± 0.08 for the first DLA and log N(H i) = 20.78 ± 0.08 for the second DLA, are in a very good agreement with the values from Centurión et al (2003) and Vladilo et al (2003).…”

“…However, our column density measurements of Fe ii, Zn ii, S ii, and Ar i differ by almost 0.1 dex from those of Centurión et al (2003) and Vladilo et al (2003). This discrepancy is likely to be the result of the lower signal-to-noise ratio in their UVES/VLT spectra and an underestimation of their uncertainties.…”

“…We also derived lower limits to the column densities of C ii and Al ii from saturated lines, and upper limits to the column densities of C ii * , Fe iii, N ii, and S iii. Discrepancies between 0.05 and 0.1 dex are observed between our column density measurements and those obtained by Prochaska & Wolfe (1999), Prochaska et al (2001), Centurión et al (2003), and Vladilo et al (2003). In the majority of cases, we used a larger number of metal-line transitions to measure the column density of a given ion, which appreciably increases the accuracy of the measurement, especially in those cases where the transitions lie outside the Lyα forest.…”

“…More recently, high resolution spectra obtained with HIRES/Keck and UVES/VLT were analyzed by Prochaska & Wolfe (1999), Prochaska et al (2001), Centurión et al (2003), Ledoux et al (2003), and Vladilo et al (2003).…”

Physical conditions in high-redshift GRB-DLA absorbers observed with VLT/UVES: implications for molecular hydrogen searches

“…The former ratio is indicative of a high nitrogen enrichment dominated by intermediate-mass stars (4−8 M ), before significant oxygen production by supernovae, and a past low star-formation efficiency (e.g., Henry & Prochaska 2007;Petitjean et al 2008). The latter ratio is a tracer of the radiation field in H 0 regions (Vladilo et al 2003). The observed high abundance of argon, typical of the local ISM, is consistent with the expectation for a neutral medium embedded in a soft ionizing continuum.…”