  Using a sample of nearly 700 quasars with strong (W_0(2796)>0.8 Angstrom)
MgII 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 MgII lines can be observed
in the range 0.37<z_abs<2.2, the absorbing galaxies are in general not
identified in SDSS images, but they produce systematic light excesses around
QSOs which can be detected with a statistical analysis. In this Letter we
present a 6-sigma detection of this effect over the whole sample in i-band,
rising to 9.4-sigma for a low-redshift subsample with 0.37<z_abs<=0.82. We use
a control sample of QSOs without strong MgII 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 z_abs subsample we can reliably
estimate the average luminosities per MgII absorber system in the g, r, and i
bands and find them to be compatible with a few-hundred-Myr old stellar
population of M_r ~ -21 in the rest frame. 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 arising along the line-of-sight. It will be applied to larger
samples and other line species in upcoming studies.
