Revisiting the origin of the high metallicities of sub-damped Lyman-alpha systems

Sub-damped Lyman- alpha systems (sub-DLAs) have previously been found to exhibit a steeper metallicity evolution than the classical damped Lyman- alpha systems (DLAs), evolving to close to solar metallicity by z similar to 1. From new high-resolution spectra of 17 sub-DLAs, we have increased the number of measurements of [Fe/H] at z < 1.7 by 25 per cent and compiled the most complete literature sample of sub-DLA and DLA abundances to date. We find that sub-DLAs are indeed significantly more metal-rich than DLAs, but only at z < 1.7; the metallicity distributions of sub-DLAs and DLAs at z > 1.7 are statistically consistent. We also present the first evidence that sub-DLAs follow a velocity width-metallicity correlation over the same velocity range as DLAs, but the relation is offset to higher metallicities than the DLA relation. On the basis of these results, we revisit the previous explanation that the systematically higher metallicities observed in sub-DLAs are indicative of higher host galaxy masses. We discuss the various problems that this interpretation encounters and conclude that in general sub-DLAs are not uniquely synonymous with massive galaxies. We rule out physically related sources of bias (dust, environment, ionization effects) and examine systematics associated with the selection and analysis of low-redshift sub- DLAs. We propose that the high metallicities of sub- DLAs at z < 1.7 that drives an apparently steep evolution may be due to the selection of most low-redshift sub- DLAs based on their high MgII equivalent widths.