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

Abstract

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^+0.12_-0.09 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^+0.09_-0.05 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. Based in part on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the STScI for the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.