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 z_arc = 2.48 _-0.05^+0.14. We present lens models with an Einstein radius of RE = 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 gsim; 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_{1.4 GHz} = 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.