Mg II Absorber Number Density at z~0.05: Implications for Omega_DLA Evolution

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Abstract
[Abridged] A 147 quasar/AGN spectra, obtained with FOS/HST, have been searched for Mg II absorbers for 0 < z < 0.15. The total redshift path is 18.8, being 80% complete to a 5-sigma rest-frame equivalent width, W_r, of 0.6 Ang and 20% complete to 0.3 Ang. (1) Four systems were found, with a mean redshift of = 0.06, yielding dN/dz = 0.22(+0.12)(-0.09) for absorbers with W_r > 0.6 Ang. This is consistent with the value expected if these systems do not evolve from higher redshifts (z = 2.2). (2) No systems with W_r < 0.6 Ang were found. Accounting for the survey completeness, it is a 2-sigma result to have a null detection of smaller W_r systems. At face value, this implies a 40% reduction in the average galaxy gas cross section from z > 0.2 galaxies. (3) Three systems are candidates for damped Lyman-alpha absorbers (DLAs). Based upon the results of Rao & Turnshek (2000, ApJS, 130, 1), this translates to dN/dz = 0.08(+0.09)(-0.05) for DLAs at z ~ 0. This would suggest that dN/dz for DLAs does not evolve from z = 4 to z = 0. However, because of the distribution of H I mass in 21-cm selected galaxies, the Rao & Turnshek finding that the cosmological H I mass density, Omega_DLA, decreases either rapidly from z = 0.5 to z = 0, or more gradually from z = 1.5, still holds.

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