Optical properties and spatial distribution of MgII absorbers from SDSS image stacking

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 W-0(lambda 2796) > 0.8 angstrom 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 W-0, ranging from similar to-1 for W-0 less than or similar to 1 angstrom to similar to-2 for W-0 greater than or similar to 1.5 angstrom. 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 similar to 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 (similar to 50% from zabs similar to 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.