Is the Optically Unidentified Radio Source FIRST J121839.7+295325 a Dark Lens?

We present evidence that the optically unidentified radio source FIRST J121839.7+295325 may be strongly lensing a background galaxy. We estimate the redshift of the assumed gravitational arc, discovered in parallel imaging with HST, from MMT-Blue Channel spectroscopy to be [image]. We present lens models with an Einstein radius of [image] which contains a mass of [image] M sub([image]), where the uncertainty reflects the range of possible lens redshifts. The putative lens is not detected to [image] mag and [image] mag in our MMT-SWIRC imaging. Using the flux limits from WFPC2 and SWIRC, we estimate that the dynamical mass-to-light ratio of J121839.7+295325 is [image] M sub([image]) [image] for [image] mag, and this lower limit could be as high as 30 M sub([image]) [image] for [image] mag. Since the radio source is optically unidentified ([image] mag) and has a radio flux of [image] mJy, it is likely a massive early-type galaxy which hosts a radio- loud AGN at [image]. However, the present data cannot uniquely determine the mass-to-light ratio of the lensing galaxy, and hence the possibility that this system may be a reasonably dark lens is not ruled out.