Evidence for dark matter in MG 1654+134

The radio ring lens MG 1654+134 strongly constrains the structure and mass of the 19 mag lens galaxy at its center. The best-fit effective radius for a de Vaucouleurs model of the lens galaxy is Rₑ_ = 2. 0" (1. 5" < Rₑ_ < 3. 0") marginally consistent with fits to the observed lens galaxy. The B band mass-to-light ratio interior to the ring that the galaxy would have today is (M/L) B_ = (20. 4 +/- 2. 8) (fₑ_/1. 4) h, where a passively evolving early type galaxy fades by a factor of fₑ_~ 1. 4 between the lens redshift and the present. This (M/L) B_ is inconsistent with stellar dynamical models of nearby ellipticals, but it is consistent with estimates from the statistics of gravitational lens numbers and separations. Isothermal models fit the ring with a halo velocity dispersion of σDM_ = (216+/-2) km s^-1^, consistent with the fundamental plane, the stellar dynamics of ellipticals in dark matter halos predicted dispersion (212+/-12) (fₑ_/14) ⁰. 29^ km s^-1^, and the statistics of gravitational lenses. This strongly supports the conclusion from lens statistics that early type galaxies do not have constant mass- to-light ratios even on scales comparable to the effective radius. Density distributions with ρ is proportional to (r²^ +s²^) ^ (α - 3) /2^ fit the data only in the restricted range 0. 9 <~ α <~ 1. 1 where α = 1 is an isothermal model. Centrally concentrated models such as the Plummer or modified Hubble models require large, finite core radii (800 h^-1^ and 600 h^-1^ pc, respectively) but fail to fit the structure of the ring. Isothermal models only have upper limits of order 100 h^-1^ pc on the core radius. The major axis of the mass distribution is 7ᵈeg^ + 10ᵈeg^ from the major axis of the luminosity distribution. The models clearly distinguish between different angular structures for the lens model, and a quadrupole expansion of an ellipsoidal density distribution fits the data better than a similar monopole in an external shear.