Lensing probabilities for spectroscopically selected galaxy-galaxy strong lenses

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 [O II] 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. The most significant selection effect is the finite size of the spectroscopic fiber which selects against large separation lenses and results in a nonmonotonic dependence of the rogue line probability on velocity dispersion. For example, with the 3 '' diameter SDSS fiber and 2 '' FWHM seeing, we find that the probability that a given LRG has a rogue [O II] line in its spectrum decreases with velocity dispersion from 150 to 300 km s(-1) and then increases up to 400 km s(-1) for a given source size. The total probability for observing a rogue line in a single survey spectrum is similar to 0.9%-3.0%, and the total lensing rate is similar to 0.5%-1.3%. The range is due to uncertainties in the physical size of [O II] emission regions, and in the evolution of the [O II] luminosity function. Our estimates are a factor of similar to 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.