B2045+265: A new four-image gravitational lens from CLASS

We have discovered a new gravitational lens in the Cosmic Lens All-Sky Surv ey (CLASS). The lens B2045+265 is a four-image system with a maximum separa tion of 1".9. A fifth radio component is detected, but its radio spectrum a nd its positional coincidence with infrared emission from the lensing galax y strongly suggest that it is the radio core of the lensing galaxy. This im plies that the B2045+265 lens system consists of a flat-spectrum radio sour ce that is being lensed by another flat-spectrum radio source. Infrared ima ges taken with the Hubble Space Telescope and the Keck I Telescope detect t he lensed images of the background source and the lensing galaxy. The lense d images have relative positions and flux densities that are consistent wit h those seen at radio wavelengths. The lensing galaxy has magnitudes of J = 19.2, m(F160W) = 18.8, and K = 17.6 mag in a 1".9 diameter aperture, which corresponds to the size of the Einstein ring of the lens. Spectra of the s ystem taken with the Keck I Telescope reveal a lens redshift of z(l) = 0.86 73 and a source redshift of z(s) = 1.28. The lens spectrum is typical of an Sa galaxy. The image splitting and system redshifts imply that the project ed mass inside the Einstein radius of the lensing galaxy is M-E = 4.7 x 10( 11) h(-1) M-.. An estimate of the light emitted inside the Einstein radius from the K magnitude gives a mass-to-light ratio in the rest-frame B band o f (M/L-B)(E) = 20 h (M/L-B)(.). Both the mass and mass-to-light ratio are h igher than what is seen in nearby Sa galaxies. In fact, the implied rotatio n velocity for the lensing galaxy is 2-3 times higher than what is seen in nearby spiral galaxies. The large projected mass inside the Einstein ring r adius may be the result of a significant amount of dark matter in the syste m, perhaps from a compact group of galaxies associated with the primary len sing galaxy; however, it may also arise from a misidentification of the sou rce redshift. A simple model of the gravitational potential of the lens rep roduces the image positions well, but further modeling is required to satis fy the constraints from the image flux density ratios. With further observa tions and modeling, this lens may yield an estimate of H-o.