Towards Understanding the Mass-Metallicity relation of Quasar Absorbers: Evidence for bimodality and consequences

TL;DR

This study investigates the metallicity properties of HI-selected galaxies and quasar absorbers, revealing a bimodal distribution linked to different physical origins and challenging the interpretation of the mass-metallicity relation.

Contribution

It introduces a bivariate metallicity distribution dependent on HI column density and MgII equivalent width, highlighting the heterogeneity of DLA samples and proposing physical origins for absorber bimodality.

Findings

Metallicity depends on HI column density and MgII equivalent width.

DLA samples are intrinsically heterogeneous due to selection effects.

Bimodality in metallicity distribution suggests different physical origins of absorbers.

Abstract

One way to characterize and understand HI-selected galaxies is to study their metallicity properties. In particular, we show that the metallicity of absorbers is a bivariate function of the HI column density (NHI) and the MgII equivalent width (Wr(2796)). Thus, a selection upon Wr(2796) is not equivalent to a HI selection for intervening absorbers. A direct consequence for damped absorbers with NHI>20.3 from the bivariate metallicity distribution is that any correlation between the metallicity [X/H] and velocity width (or Wr(2796)) arises from the HI cut and therefore can not be interpreted as a signature of the mass-metallicity relation akin to normal field galaxies. Thus DLA samples are intrinsically heterogeneous. On the other hand, a sample of MgII-selected absorbers, which are statistically dominated by lowest NHI systems (sub-DLAs), are found to have a more uniform metallicity distribution. We postulate that the bivariate metallicity distribution can be explained by two different physical origins of absorbers, namely sight-lines through the ISM of small galaxies and sight-lines through out-flowing material. Several published results follow from this bivariate [X/H] distribution such as (a) the two classes of DLAs, reported by Wolfe et al., and (b) the constant dust-to-gas ratio for MgII-absorbers.