The Einstein Cross: Constraint on Dark Matter from Stellar Dynamics and Gravitational Lensing
Abstract
We present twodimensional lineofsight stellar kinematics of the lens galaxy in the Einstein Cross, obtained with the GEMINI 8 m telescope, using the GMOS integralfield spectrograph. The stellar kinematics extend to a radius of 4'' (with 0farcs2 spaxels), covering about twothirds of the effective (or halflight) radius R_{e} ~= 6'' of this earlytype spiral galaxy at redshift z_{l} ~= 0.04, of which the bulge is lensing a background quasar at redshift z_{s} ~= 1.7. The velocity map shows regular rotation up to ~100 km s^{1} around the minor axis of the bulge, consistent with axisymmetry. The velocity dispersion map shows a weak gradient increasing toward a central (R < 1'') value of σ_{0} = 170 ± 9 km s^{1}. We deproject the observed surface brightness from Hubble Space Telescope imaging to obtain a realistic luminosity density of the lens galaxy, which in turn is used to build axisymmetric dynamical models that fit the observed kinematic maps. We also construct a gravitational lens model that accurately fits the positions and relative fluxes of the four quasar images. We combine these independent constraints from stellar dynamics and gravitational lensing to study the total mass distribution in the inner parts of the lens galaxy. We find that the resulting luminous and total mass distribution are nearly identical around the Einstein radius R_{E} = 0farcs89, with a slope that is close to isothermal, but which becomes shallower toward the center if indeed mass follows light. The dynamical model fits to the observed kinematic maps result in a total masstolight ratio Upsilon_{dyn} = 3.7 ± 0.5 Upsilon_{sun,I } (in the I band). This is consistent with the Einstein mass M_{E} = 1.54 × 10^{10} M _{sun} divided by the (projected) luminosity within R_{E} , which yields a total masstolight ratio of Upsilon_{ E } = 3.4 Upsilon_{sun,I }, with an error of at most a few percent. We estimate from stellar population model fits to colors of the lens galaxy a stellar masstolight ratio Upsilon_{sstarf} from 2.8 to 4.1 Upsilon_{sun,I }. Although a constant dark matter fraction of 20% is not excluded, dark matter may play no significant role in the bulge of this ~L _{sstarf} earlytype spiral galaxy.
 Publication:

The Astrophysical Journal
 Pub Date:
 August 2010
 DOI:
 10.1088/0004637X/719/2/1481
 arXiv:
 arXiv:0807.4175
 Bibcode:
 2010ApJ...719.1481V
 Keywords:

 galaxies: kinematics and dynamics;
 galaxies: photometry;
 galaxies: structure;
 gravitational lensing: strong;
 Astrophysics;
 Astrophysics  Cosmology and Extragalactic Astrophysics
 EPrint:
 18 pages, 8 figures, published in ApJ, 2010, 719, 1481