MgiiAbsorber Number Density atz ≃ 0.05: Implications for ΩDLAEvolution

An unbiased sample of 147 quasar/AGN spectra, obtained with the FOS/HST, hasbeen searched for intervening MgII absorbers over the redshift range 0=0.06,yielding a redshift number density dN/dz=0.22(+0.12)(-0.09) for absorbers withW_r(2796)>0.6 Ang. This is consistent with the value expected if these systemsdo not evolve from higher redshifts (z=2.2). [2] No systems with W_r(2796)<0.6Ang were found. It is a 2-sigma result to have a null detection of smallerW_r(2796) systems. If this implies a turnover in the low W_r(2796) region ofthe equivalent width distribution at z~0, then there is at least a 25%reduction in the average galaxy gas cross section from z<0.2 galaxies. [3]These systems have strong FeII absorption and are good candidates for dampedLy-alpha absorbers DLAs (see Rao & Turnshek 2000, ApJS, 130, 1). Thistranslates to a redshift number density of dN/dz=0.08(+0.09)(-0.05) for DLAs atz~0. In tandem with the data analyzed by Rao & Turnshek, these results indicatethat the redshift number density of DLAs does not evolve from z~4 to z~0. Ifthe HI mass function does not evolve from z~0.5 to z~0, then the cosmologicalHI mass density is also deduced to not evolve from z~4 to z~0. These z~0results for MgII absorption-selected DLAs are at odds with those based upon21-cm emission from HI galaxies by a factor of five to six.Comment: 23 pages, 7 Figures, accepted to ApJ. Replaced version includes additional figures and tables and substantial modifications to the tex