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

TL;DR

This paper investigates whether the radio source FIRST J121839.7+295325 is a dark lens by analyzing its potential gravitational lensing effects, mass-to-light ratio, and optical properties, suggesting it could be a massive, optically faint galaxy with a radio-loud AGN.

Contribution

The study provides the first lens model and mass estimate for FIRST J121839.7+295325, exploring its nature as a potentially dark lens with high mass-to-light ratio.

Findings

Estimated redshift of background arc: z=2.48

Lens Einstein radius: 1.3 arcseconds

Mass-to-light ratio lower limit: ~10 to 30 M_sun/L_sun

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 z_{arc}=2.48_-0.05^+0.14. 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_{sol}, 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_sol/L_sol for A_V=1 mag, and this lower limit could be as high as 30 M_sol/L_sol 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<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.