2007ApJ...658..161Z

We present a statistical analysis of the photometric properties and spatial distribution of more than 2800 Mg II absorbers with 0.37<z<1 and rest equivalent width W₀(λ2796)>0.8 Å detected in SDSS quasar spectra. Using an improved image-stacking technique, we measure the cross-correlation between Mg II gas and light (in the g, r, i and z bands) from 10 to 200 kpc and infer the light-weighted impact parameter distribution of Mg II absorbers. This quantity is well described by a power law with an index that strongly depends on absorption rest equivalent width W₀, ranging from ~-1 for W₀≲1 Å to ~-2 for W₀≳1.5 Å. At redshift 0.37<z_abs≤0.55, we find the average luminosity enclosed within 100 kpc around Mg II absorbers to be M_g=-20.65±0.11 mag, which is ∼0.5L^*_g. The global luminosity-weighted colors are typical of present-day intermediate-type galaxies. We then investigate these colors as a function of Mg II rest equivalent width and find that they follow the track between spiral and elliptical galaxies in color space; while the light of weaker absorbers originates mostly from red passive galaxies, stronger systems display the colors of blue star-forming galaxies. We argue that the origin of strong Mg II absorber systems might be better explained by models of metal-enriched gas outflows from star-forming/bursting galaxies. No significant redshift dependence for both impact parameter and rest-frame colors is observed up to z=1. However, we do observe a brightening of the absorbers' related light at high redshift (∼50% from z_abs∼0.4 to 1). We argue that Mg II absorbers are a phenomenon typical of a given evolutionary phase that more massive galaxies experience earlier than less massive ones, in a ``downsizing'' fashion.