| Abstract
| - Abstract. 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 zabs< 1.7, since they are rare and their H I 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 Mg II-selected DLAs; that is, absorbers discovered thanks to our knowledge of their Mg II 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 NH I and equivalent widths of Fe II 2600, Mg II 2796, Mg II 2803 and Mg I 2852 for a sample of 36 absorbers, 21 of which are Mg II-selected. We find that the Mg II-selected sample contains a marginally larger fraction of absorbers with log NH I> 21.0 than seen otherwise at low redshift. If confirmed, this property will in turn affect estimates of ΩH I, which is dominated by the highest H I column densities. We investigate the source of the potential discrepancy and find that log NH I does 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 NH I distributions between the Mg II-selected DLAs and the others probably arises from small-number statistics. Therefore, further observations based on both H I and Mg II selection techniques are required in order to clarify the impact on estimates of ΩH I at low redshifts.
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