Towards understanding the mass-metallicity relation of quasar absorbers: Evidence for bimodality and consequences

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 (Wλ2796r ). Thus, a selection upon Wλ2796r is not equivalent to a HI selection for intervening absorbers. A direct consequence for damped absorbers with log NHI > 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]-Δυ correlation reported in the literature arises from the HI cut. On the other hand, a sample of MgII-selected absorbers, which are statistically dominated by lowest NHI systems (sub-DLAs) at each Wλ2796r, is found to have a more uniform metallicity distribution. We postulate that the bivariate distribution [[X/H](NHI, 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 MgII absorbers. © 2008 The Author. Journal compilation © 2008 RAS.