The redshift of the Einstein ring in MG 1549+305

A deep spectrum taken with the Echelle Spectrograph and Imager (ESI) at the Keck II Telescope as part of the Lenses Structure and Dynamics (LSD) Survey reveals the redshifts of the extremely red source of the radio Einstein ring in the gravitational lens system MG 1549+305 (zs= 1.170 ± 0.001) and an intermediate-redshift lensed spiral galaxy (zG2= 0.604 ± 0.001). The source redshift allows us to determine the mass of the SB0 lens galaxy enclosed by the Einstein radius (RE= 1.15 ± 0.05 arcsec): ME≡M(<RE) = 8.4 ± 0.7 × 1010h−165 M⊙. This corresponds to a singular isothermal ellipsoid (SIE) velocity dispersion σSIE= 214 ± 5 km s−1, in good agreement with the measured stellar velocity dispersion σ= 227 ± 18 km s−1. The mass-to-light ratio within the Einstein radius (∼1.4 effective radii) is 10 ± 1 h65 M⊙/LB, ⊙. This is only marginally larger than typical stellar mass-to-light ratios of local early-type galaxies, indicating that dark matter is not likely to be dominant inside the Einstein radius.