2004MNRAS.352.1291P

The damped Lyα systems (DLAs), seen in absorption in the spectrum of background quasars, are believed to contain a large fraction of the neutral gas in the Universe. Paradoxically, these systems are more difficult to observe at z_abs< 1.7, since they are rare and their HI feature then falls in UV spectra, thus requiring the use of space-borne facilities. In order to overcome this observational difficulty, Rao & Turnshek pioneered a method based on MgII-selected DLAs; that is, absorbers discovered thanks to our knowledge of their MgII feature in optical spectra. In the present work, we use new observations undertaken at the TNG as well as a careful literature and archival search to build samples of low-redshift absorbers classified according to the technique used for their discovery. We successfully recover N_HIand equivalent widths of FeII 2600, MgII 2796, MgII 2803 and MgI 2852 for a sample of 36 absorbers, 21 of which are MgII-selected. We find that the MgII-selected sample contains a marginally larger fraction of absorbers with log N_HI> 21.0 than seen otherwise at low redshift. If confirmed, this property will in turn affect estimates of Ω_HI, which is dominated by the highest HI column densities. We investigate the source of the potential discrepancy and find that log N_HIdoes not correlate significantly with metal equivalent widths. Similarly, we find no evidence that gravitational lensing, the fraction of associated systems or redshift evolution affect the absorber samples in a different way. We conclude that the hint of discrepancies in N_HIdistributions between the MgII-selected DLAs and the others probably arises from small-number statistics. Therefore, further observations based on both HI and MgII selection techniques are required in order to clarify the impact on estimates of Ω_HIat low redshifts.