Towards understanding the mass–metallicity relation of quasar absorbers: evidence for bimodality and consequences

Abstract

Abstract One way to characterize and understand H i-selected galaxies is to study their metallicity properties. In particular, we show that the metallicity of absorbers is a bivariate function of the H i column density (NH i) and the Mg ii equivalent width (Wλ2796r). Thus, a selection upon Wλ2796r is not equivalent to a H i selection for intervening absorbers. A direct consequence for damped absorbers with log NH i > 20.3 that falls from the bivariate metallicity distribution is that any correlation between the metallicity [X/H] and velocity width (using Wλ2796r as a proxy) cannot be interpreted as a signature of the mass–metallicity relation akin to normal field galaxies. In other words, damped Lyα absorber (DLA) samples are intrinsically heterogeneous and the [X/H]–Wλ2796r or [X/H]–Δv correlation reported in the literature arises from the H i cut. On the other hand, a sample of Mg ii-selected absorbers, which are statistically dominated by lowest NH i systems (sub-DLAs) at each Wλ2796r, is found to have a more uniform metallicity distribution. We postulate that the bivariate distribution [[X/H](NH i, Wλ2796r)] can be explained by two different physical origins of absorbers, namely sight lines through the interstellar medium of small galaxies and sight lines through the out-flowing material. Several published results follow from the bivariate [X/H] distribution, namely (i) the properties of the two classes of DLAs, reported by Wolfe et al. and (ii) the constant dust-to-gas ratio for Mg ii absorbers.