  Spectroscopic galaxy-galaxy lens searches are presently the most prolific
method of identifying strong lens systems in large data sets. We study the
probabilities associated with these lens searches, namely the probability of
identifying a candidate with rogue [OII] emission lines in a galaxy's spectrum,
and the probability that the candidate will show features of strong lensing in
follow-up photometric observations. We include selection effects unique to
spectroscopic data, and apply them to the Sloan Lens ACS (SLACS) survey (Bolton
et al. 2006). The most significant selection effect is the finite size of the
spectroscopic fiber which selects against large separation lenses and results
in a non-monotonic dependence of the rogue line probability on velocity
dispersion. For example, with the 3 arcsec diameter SDSS fiber and 2 arcsec
FWHM seeing, we find that the probability that a given LRG has a rogue [OII]
line in its spectrum decreases with velocity dispersion from 150 km/s to 300
km/s and then increases up to 400 km/s for a given source size. The total
probability for observing a rogue line in a single survey spectrum is
~0.9-3.0%, and the total lensing rate is ~0.5-1.3%. The range is due to
uncertainties in the physical size of [OII] emission regions, and in the
evolution of the [OII] luminosity function. Our estimates are a factor of ~5
higher than the results of the SLACS survey, a discrepancy which we attribute
to the SLACS requirement that multiple rogue lines be observed simultaneously.

Astrophysics