  We estimate the absolute magnitude distribution of galaxies which lie within
about a Mpc of Mg II absorption systems. The absorption systems themselves lie
along 1880 lines of sight to QSOs from the Sloan Digital Sky Survey Data
Release 3, have rest equivalent widths greater than 0.88 Angstroms, and
redshifts between 0.37 < z < 0.82. Our measurement is based on all galaxies
which lie within a projected distance of about 900 kpc/h of each QSO
demonstrating absorption. The redshifts of these projected neighbors are not
available, so we use a background subtraction technique to estimate the
absolute magnitude distribution of true neighbors. (Our method exploits the
fact that, although we do not know the redshifts of the neighbors, we do know
the redshift of the absorbers.) The absolute magnitude distribution we find is
well described by a bell-shaped curve peaking at about rest-frame M_B = -20,
corresponding to L/L* = 1.4. A comparison of this observed distribution to ones
in the literature suggests that it is unlikely to be drawn from a population
dominated by late-type galaxies. However, the strong equivalent width systems
may be associated with later galaxy types. Finally we use the absolute
magnitude distribution, along with the observed covering fraction of about 8
percent, to estimate the extent of the MgII absorbing gas around a galaxy. For
an L* galaxy, this scale is about 70 kpc/h. We provide an analytic description
of our method, which is generally applicable to any dataset in which redshifts
are only available for a small sub-sample. Hence, we expect it to aid in the
analysis of galaxy scaling relations from photometric redshift datasets.
