  A recently discovered quadruply-imaged QSO, SDSS J1004+4112 (Inada et al.
2003; Oguri et al. 2004) in the core of a $z=0.68$ galaxy cluster has an
unprecedented image separation of ~13''. This lens gives us a unique
opportunity to study the detailed mass distribution in the central regions of
this cluster. We present free-form reconstructions of the lens using recently
developed methods. The projected mass within 100 kpc is well-constrained as
5+/-1 x 10^{13} M_solar, consistent with previous simpler models. Unlike
previous work, however, we are able to detect structures in the lens associated
with cluster galaxies. We estimate the mass associated with these galaxies, and
show that they contribute not more than about 10% of the total cluster mass
within 100 kpc. Typical galaxy masses, combined with typical luminosities yield
a rough estimate of their mass-to-light ratio, which is ~<10, implying that
these galaxies consist mostly of stars, and possess little dark matter.

Astrophysics