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We consider the questions of whether the damped Lyman-alpha (DLA) and sub-DLAabsorbers in quasar spectra differ intrinsically in metallicity, and whetherthey could arise in galaxies of different masses. Using the recent measurementsof the robust metallicity indicators Zn and S in DLAs and sub-DLAs, we confirmthat sub-DLAs have higher mean metallicities than DLAs, especially at $z\lesssim 2$. We find that the intercept of the metallicity-redshift relationderived from Zn and S is higher than that derived from Fe by 0.5-0.6 dex. Wealso show that, while there is a correlation between the metallicity and therest equivalent width of Mg II $\lambda 2796$ or Fe II $\lambda 2599$ for DLAs,no correlation is seen for sub-DLAs. Given this, and the similar Mg II or Fe IIselection criteria employed in the discovery of both types of systems at lowerredshifts, the difference between metallicities of DLAs and sub-DLAs appears tobe real and not an artefact of selection. This conclusion is supported by oursimulations of Mg II $\lambda 2796$ and Fe II $\lambda 2599$ lines for a widerange of physical conditions. On examining the velocity spreads of theabsorbers, we find that sub-DLAs show somewhat higher mean and median velocityspreads ($\Delta v$), and an excess of systems with $\Delta v > 150$ kms$^{-1}$, than DLAs. Compared to DLAs, the [Mn/Fe] vs. [Zn/H] trend forsub-DLAs appears to be steeper and closer to the trend for Galactic bulge andthick disk stars, possibly suggesting different stellar populations. Theabsorber data appear to be consistent with galaxy down-sizing. The data arealso consistent with the relative number densities of low-mass and high-massgalaxies. It is thus plausible that sub-DLAs arise in more massive galaxies onaverage than DLAs.

Do Damped and Sub-damped Lyman-alpha Absorbers Arise in Galaxies of Different Masses?