On the Observed W_MgII--L_[OII] Correlation in SDSS QSO Spectra
  This paper investigates the effect of differential aperture loss with SDSS
fibers and examines whether such selection bias would result in the observed
correlation between rest-frame absorption equivalent width of MgII absorbers,
Wr(2796), and mean associated [OII] luminosity, L_[OII], in SDSS QSO spectra.
We demonstrate based on a Monte Carlo simulation that the observed Wr(2796) vs.
L_[OII] correlation of MgII absorbers can be well-reproduced, if all galaxies
found in deep surveys possess extended MgII halos and if the extent of MgII
halos scales proportionally with galaxy mass as shown in previous studies. The
observed correlation can be explained by a combination of (1) the known
Wr(2796) vs. rho anti-correlation in galaxy and MgII absorber pairs and (2) an
increasing aperture loss in the 3" diameter SDSS fiber for galaxies at larger
rho. Galaxies at larger projected distances produce on average weaker MgII
absorbers and weaker (or zero) L_[OII] in SDSS QSO spectra. We show that such
correlation diminishes when larger fibers are adopted and is therefore not
physical. While under a simple halo model the majority of MgII absorbers do not
directly probe star-forming disks, they trace photo-ionized halo gas associated
with galaxies. We show that because of the scaling relation between extended
gas cross-section and galaxy mass, the number density evolution of the MgII
absorber population as a whole provides a good measure of the cosmic star
formation history.
