Evolution of quasar absorption-selected galaxies

Wepresent the results of a survey of galaxies selected by their gas cross-section which give rise to Mg ii absorption lines in the spectra of background quasars. The sample comprises 11 galaxies covering the redshift range 0.7 < z < 1.2 with 〈z〉=0.9, and is combined with a lower redshift (z < 0.7) sample of 15 Mg ii absorbing galaxies. We first describe the properties of these two samples of galaxies whose spectra range from present-day elliptical to irregular galaxies with a dominance of Sbc and Scd types. The strong correlation found previously at lower redshift between the halo radius and the galaxy luminosity still holds at z ' 1, which implies no significant evolution in redshift of the gaseous halo sizes. We find no evidence of a correlation between the rest-frame equivalent width of the [O ii]λ3727 emission line and the rest-frame luminosity MAB(B). This could be a consequence of the very blue B−K color of intrinsically faint absorbing galaxies in the infrared, thus optically bright as found in other surveys. For the low and high redshift (z <> 0.7) samples, we have built template spectra by combining individual galaxy spectra. The lack of detection of the [N ii] emission line in the low-redshift template, [N ii]λ6568/Hα <0.2, togetherwith the values obtained for empirical abundance indicators, suggests a O/H abundance in the disk ofMg ii absorbinggalaxies in the range [O/H]=−0.6,−0.1. The spectral energy distribution (SED) of these templates at λr >3646Åcan be fitted by present-day Sbc or Sc galaxies, but the UV excess at λr <3500Å follows the SED of Scd and Im present-day galaxies at 〈z〉=0.4 and 0.9 respectively. This blueing of the SED with redshift is associated with an increase of wr([O ii]λ3727) by about 45%. To investigate the past history of star formation of the Mg ii absorbing galaxies, we have used the spectral evolution models of stellar populations developed by Bruzual & Charlot as well as red and blue absorber subsamples (individual galaxies with a Ba + Ca ii break <> 1.55). The red subtemplates are well fitted by a Sb galaxy and the derived ages, 4.5 and 8.3 Gyr at z=0.9 and 0.4 respectively, implies a similar galaxy formation epoch for the 〈z〉=0.4 and 0.9 Mg ii absorbing galaxies. Results for the blue subtemplates using Sc galaxy models, especially that at 〈z〉=0.9, suggests that intense Send offprint requests to: J. Bergeron ([email protected]) ? Based onobservationsmade at theEuropeanSouthernObservatory, La Silla, Chile (programme 1-012-43K) star formation activity is occurring at z '1, as also found for larger field galaxy samples at similar redshifts.