C, N, O abundances in the most metal-poor damped Lyman alpha systems

Abstract

This study focuses on some of the most metal-poor damped Lyα (DLA) absorbers known in the spectra of high-redshift QSOs, using new and archival observations obtained with ultravioletsensitive echelle spectrographs on the Keck and VLT telescopes. The weakness and simple velocity structure of the absorption lines in these systems allow us to measure the abundances of several elements, and in particular those of C, N and O, a group that is difficult to study in DLAs of more typical metallicities. We find that when the oxygen abundance is less than ∼1/100 of solar, the C/O ratio in high-redshift DLAs and sub-DLAs matches that of halo stars of similar metallicity and shows higher values than expected from galactic chemical evolution models based on conventional stellar yields. Furthermore, there are indications that at these low metallicities the N/O ratio may also be above simple expectations and may exhibit a minimum value, as proposed by Centurión and her collaborators in 2003. Both results can be interpreted as evidence for enhanced production of C and N by massive stars in the first few episodes of star formation, in our Galaxy and in the distant protogalaxies seen as QSO absorbers. The higher stellar yields implied may have an origin in stellar rotation which promotes mixing in the stars' interiors, as considered in some recent model calculations. We briefly discuss the relevance of these results to current ideas on the origin of metals in the intergalactic medium and the universality of the stellar initial mass function.

THE PRIMORDIAL DEUTERIUM ABUNDANCE OF THE MOST METAL-POOR DAMPED Lyα SYSTEM∗

“…We also note that the [N/O] ratio is well-determined in this DLA, since both N I and O I trace the H I bearing gas due to charge transfer reactions (Field & Steigman 1971;Steigman et al 1971). Our value of [N/O] is consistent with, or slightly lower than, the primary N/ O plateau (Izotov & Thuan 1999;Centurión et al 2003;van Zee & Haynes 2006;Petitjean et al 2008;Pettini et al 2008;Pettini & Cooke 2012b;Zafar et al 2014).…”

The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars

“…7, for two different choices of S H , together with those for higher-metallicity disc stars obtained by Bensby & Feltzing (2006) We have included in Fig. 7 the [C/O] measurements in metalpoor DLAs by Pettini et al (2008) which seem to match well the values deduced here in halo stars of similar [O/H]. The good agreement in the [C/O] ratios measured in different astrophysical environments and at different epochs strengthens the interpretation that carbon was somehow overproduced in early stages of galactic chemical evolution.…”

“…For example, Erni et al (2006) found the chemical abundances in a DLA with [O/H] ∼ −2.5 to be consistent with enrichment from a single starburst of massive (10−50 M ) zero-metallicity stars. Pettini et al (2008) very recently derived near-solar [C/O] ratios in a small sample of the most metal-poor DLAs/subDLAs known. Becker et al (2006) studied absorption toward high-redshift (4.9 < z < 6.4) quasars, inferring a mean [C/O] ∼ −0.3 in the Lyman alpha forest clouds which trace the low-density and low-metallicity intergalactic medium (IGM).…”

The low-metallicity QSO HE 2158 − 0107: a massive galaxy growing by accretion of nearly pristine gas from its environment?

“…Nitrogen can be produced similarly as a primary element, but it can also have a secondary origin in intermediate mass (old) stars generated from the original metals of the star (e.g. Matteucci 1986;Pilyugin et al 2003;Pettini et al 2008). In this simple picture the ratio of N/O is constant if nitrogen has a primary origin, while it depends on O/H if it has a secondary origin.…”