Field Galaxy Evolution Since Z~1 from a Sample of QSO Absorption-Selected Galaxies

We present some of the results of a large survey aimed at establishing the properties of galaxies selected by their having produced detectable Mg~II $\lambda \lambda 2796$, 2803 absorption in the spectra of background QSOs. The present sample covers the redshift range $0.2\le z\le 1.0$, with $\langle z \rangle = 0.65$. From an extensive program of optical and near-IR imaging and optical spectroscopy, we find that the galaxies appear to be similar to normal galaxies at the present epoch, ranging from late–type spiral galaxies to those whose spectra and colors resemble present–day ellipticals. Contrary to some faint field galaxy samples selected using different criteria, over the redshift range observed we find no evidence for significant evolution in rest–frame $B-K$ color, space density, or (rest–frame $B$ or $K$) luminosity. The average'' Mg~II absorbing galaxy appears to be consistent with a normal 0.7$L_B^{\ast}$ Sb galaxy having a roughly constant star formation rate since $z \sim 1$, although galaxies spanning a range of a factor of $\sim 70$ in luminosity are found in the absorber sample. The diffuse gas cross-section selection imposed by studies of this kind appears to be biased {\it against} the relatively underluminous, blue galaxies which apparently dominate the number counts at faint magnitudes. However, essentially all normal’’ field galaxies, independent of spectroscopic type, appear to be potential QSO absorbers.