Constraining the Photometric Properties of Mg II-absorbing Galaxies with the Sloan Digital Sky Survey

Using a sample of nearly 700 quasars with strong [W0(2796) > 0.8 Å] Mg II absorption lines detected in the Early Data Release of the SDSS, we demonstrate the feasibility of measuring the photometric properties of the absorber systems by stacking SDSS imaging data. As Mg II lines can be observed in the range 0.37 < zabs < 2.2, the absorbing galaxies are in general not identified in SDSS images, but they produce systematic light excesses around QSOs that can be detected with a statistical analysis. Here we present a 6 σ detection of this effect over the whole sample in the i band, rising to 9.4 σ for a low-redshift subsample with 0.37 < zabs ≤ 0.82. We use a control sample of QSOs without strong Mg II absorption lines to quantify and remove systematics with typical 10%-20% accuracy. The signal varies as expected as a function of absorber redshift. For the low-zabs subsample, we can reliably estimate the average luminosities per Mg II absorber system in the g, r, and i bands, and we find them to be compatible with a stellar population a few hundred million years old at Mr ~ -21 in the rest frame. The colors are also consistent with typical absorbing galaxies resembling local Sb-c spirals. Our technique does not require any spectroscopic follow-up and does not suffer from confusion with other galaxies along the line of sight. It will be applied to larger samples and other line species in upcoming studies.