We present the first results from the X-Shooter Lens Survey ( XLENS ) : an analysis of the massive early-type galaxy SDSS J1148+1930 at redshift z = 0.444 .
We combine its extended kinematic profile – derived from spectra obtained with X-Shooter on ESO Very Large Telescope – with strong gravitational lensing and multi-color information derived from Sloan Digital Sky Survey ( SDSS ) images .
Our main results are ( i ) the luminosity-weighted stellar velocity dispersion is \langle \sigma _ { * } \rangle ( \la R _ { eff } ) = 352 \pm 10 \pm 16 \mathrm { km s ^ { -1 } } , extracted from a rectangular aperture of 1 \aas@@fstack { \prime \prime } 8 \times 1 \aas@@fstack { \prime \prime } 6 centered on the galaxy , more accurate and considerably lower than a previously published value of \sim 450 \mathrm { km s ^ { -1 } } ; ( ii ) a single-component ( stellar plus dark ) mass model of the lens galaxy yields a logarithmic total-density slope of \gamma ^ { \prime } = 1.72 ^ { +0.05 } _ { -0.06 } ( 68 per cent CL ; \rho _ { tot } \propto r ^ { - \gamma ^ { \prime } } ) within a projected radius of \sim 2 \aas@@fstack { \prime \prime } 16 ; ( iii ) the projected stellar mass fraction , derived solely from the lensing and dynamical data , is f _ { * } ( < R _ { E } ) = 0.19 ^ { +0.04 } _ { -0.09 } ( 68 per cent CL ) inside the Einstein radius for a Hernquist stellar profile and no anisotropy .
The dark-matter fraction inside the effective radius f _ { DM } ( < R _ { eff } ) = 0.60 ^ { +0.15 } _ { -0.06 } \pm 0.1 ( 68 per cent CL ) , where the latter error is systematic ; ( iv ) based on the SDSS colors , we find f _ { *, Salp } ( < R _ { E } ) = 0.17 \pm 0.06 for a Salpeter Initial Mass Function ( IMF ) and f _ { *, Chab } ( < R _ { E } ) = 0.07 \pm 0.02 for a Chabrier IMF .
The lensing and dynamics constraints on the stellar mass fraction agree well with those independently derived from the SDSS colors for a Salpeter IMF , which is preferred over a Chabrier IMF at variance with standard results for lower mass galaxies .
Dwarf-rich IMFs in the lower mass range of 0.1 – 0.7 M _ { \odot } , with \alpha \geq 3 ( with dN / dM \propto M ^ { - \alpha } ) – such as that recently suggested for massive early-type galaxies with \alpha = 3 in the mass range 0.1 - 1 M _ { \odot } – are excluded at the > 90 per cent C.L .
and in some cases violate the total lensing-derived mass limit .
We conclude that this very massive early-type galaxy is dark-matter dominated inside one effective radius , consistent with the trend recently found from massive SLACS ( Sloan Lens ACS ) galaxies , with a total density slope shallower than isothermal and a IMF normalization consistent with Salpeter .