B2045+265: A New Four-Image Gravitational Lens from CLASS

We have discovered a new gravitational lens in the Cosmic Lens All-Sky Survey (CLASS). The lens B2045+265 is a four-image system with a maximum separation of 1.”9. A fifth radio component is detected, but its radio spectrum and its positional coincidence with infrared emission from the lensing galaxy strongly suggest that it is the radio core of the lensing galaxy. This implies that the B2045+265 lens system consists of a flat-spectrum radio source that is being lensed by another flat-spectrum radio source. Infrared images taken with the Hubble Space Telescope and the Keck I Telescope detect the lensed images of the background source and the lensing galaxy. The lensed images have relative positions and flux densities that are consistent with 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 system taken with the Keck I Telescope reveal a lens redshift of z_l = 0.8673 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 projected mass inside the Einstein radius of the lensing galaxy is M_E = 4.7 × 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 of (M/L_B)_E = 20 h (M/L_B)⊙. Both the mass and mass-to-light ratio are higher than what is seen in nearby Sa galaxies. In fact, the implied rotation 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 radius may be the result of a significant amount of dark matter in the system, perhaps from a compact group of galaxies associated with the primary lensing galaxy; however, it may also arise from a misidentification of the source redshift. A simple model of the gravitational potential of the lens reproduces the image positions well, but further modeling is required to satisfy the constraints from the image flux density ratios. With further observations and modeling, this lens may yield an estimate of H_0