Mg ii Absorber Number Density atz ≃ 0.05: Implications for ΩDLAEvolution

An unbiased sample of 147 quasar/active galactic nuclei spectra, obtained with the Faint Object Spectrograph on board the Hubble Space Telescope , has been searched for intervening Mg II λλ2796, 2803 absorbers over the redshift range 0 < z ≤ 0.15. The total redshift path searched is 18.8, with the survey being 80% complete to a 5 σ rest-frame equivalent width W r (λ2796) of 0.6 Å and 20% complete to W r (λ2796) = 0.3 Å. Main results of this work are the following: (1) Four systems were found, with a mean redshift of ⟨ z ⟩ = 0.06, yielding a redshift number density dN / dz = 0.22 for absorbers with W r (λ2796) ≥ 0.6 Å. 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 (λ2796) < 0.6 Å were found. Based on no evolution expectations and accounting for the survey completeness, it is a 2 σ result to have a null detection of smaller W r (λ2796) systems. If this implies a turnover in the low W r (λ2796) region of the 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 Fe II absorption and, based on the results of Rao & Turnshek, are good candidates for damped Lyα absorbers (DLAs). This translates to a redshift number density of dN / dz = 0.08 for DLAs at z ~ 0. In tandem with the data analyzed by Rao & Turnshek, these results indicate that the redshift number density of DLAs does not evolve from z ≃ 4 to z ≃ 0. If the H I mass function does not evolve from z ≃ 0.5 to z ≃ 0, then the cosmological H I mass density Ω DLA is also deduced so as to not evolve from z ≃ 4 to z ≃ 0. These z ≃ 0 results for Mg II absorption-selected DLAs are at odds with those based on 21 cm emission from H I galaxies by a factor of 5-6.