2008ApJ...688...43R

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 z_arc=2.48^+0.14_–0.05. We present lens models with an Einstein radius of R_E=1.3" which contains a mass of M_dyn=10^12±0.5 M_☉, where the uncertainty reflects the range of possible lens redshifts. The putative lens is not detected to J_lim=22.0 mag and H_lim=20.7 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 M_dyn/L_B≳10 M_☉L^–1_☉ for A_V=1 mag, and this lower limit could be as high as 30 M_☉ L^–1_☉ for A_V=0 mag. Since the radio source is optically unidentified (V_lim=25.5 mag) and has a radio flux of S_1.4GHz=33 mJy, it is likely a massive early-type galaxy which hosts a radio-loud AGN at 0.8≲z_radio≲1.5. 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.