Is the Optically Unidentified Radio Source, FIRST J121839.7+295325, a Dark Lens?
Abstract
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 zarc=2.48-0.05+0.14. We present lens models with an Einstein radius of RE=1.3" which contains a mass of Mdyn=1012 +- 0.5 Msol, where the uncertainty reflects the range of possible lens redshifts. The putative lens is not detected to Jlim=22.0 mag and Hlim=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 Mdyn/LB >~ 10 Msol/Lsol for AV=1 mag, and this lower limit could be as high as 30 Msol/Lsol for AV=0 mag. Since the radio source is optically unidentified (Vlim=25.5 mag) and has a radio flux of S1.4 GHz=33 mJy, it is likely a massive early-type galaxy which hosts a radio-loud AGN at 0.8<z<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.