DLA surveys (Sara Ellison)

Our survey of MgII systems will also provide much needed insight into the nature of DLAs and sub-DLAs, the main reservoirs of neutral gas in the Universe. The strong Mg II systems found in this survey will essentially all be associated with either DLAs (log N(HI) >20.3) or sub-DLAs (19 < log N(HI) < 20.3). Peroux et al. (2007) have shown that several sub-DLAs have super-solar metallicity and that they exhibit a much steeper metallicity evolution than the DLAs. This has led to the conclusion that the sub-DLAs are the most massive galaxies of their epoch and thus an important population for study in their own right. However, Dessauges-Zavadsky et al. (2009) have shown that the difference in DLA versus sub-DLA metallicity evolution occurs only at z < 1.7 where the sub-DLAs are mostly found as a by-product of DLA searches based on strong Mg II absorption (Fig. 3). Since the metal line EW of saturated transitions correlates with metallicity, Dessauges-Zavadsky et al. suggested that the apparent steep rise of sub-DLA metallicity at low z was due to a selection effect, and also raise other objections that challenge the interpretation that sub-DLAs are the most massive galaxies. A blind search for low z sub-DLAs would settle the issue of Mg II selection bias, but would require a prohibitive HST allocation, as the Ly-alpha line is in the UV at z < 1.7. A similar test can be done from the ground only if both Ly-alpha AND Mg II 2976, 2803 A are covered, requiring optical+IR coverage - a criterion that only X-shooter can achieve. X-shooter can additionally yield metallicities from Fe II and Si II lines, filling the gap in metallicity evolution between high and low redshift regimes where DLAs and sub-DLAs seem to diverge.