We measure the stellar mass surface densities of early type galaxies by
observing the micro-lensing of macro-lensed quasars caused by individual stars,
including stellar remnants, brown dwarfs and red dwarfs too faint to produce
photometric or spectroscopic signatures. Instead of observing multiple
micro-lensing events in a single system, we combine single epoch X-ray
snapshots of ten quadruple systems, and compare the measured relative
magnifications for the images with those computed from macro-models. We use
these to normalize a stellar mass fundamental plane constructed using a
Salpeter IMF with a low mass cutoff of 0.1 solar mass and treat the zeropoint
of the surface mass density as a free parameter. Our method measures the
graininess of the gravitational potential produced by individual stars, in
contrast to methods that decompose a smooth total gravitational potential into
two smooth components, one stellar and one dark. We find the median likelihood
value for the normalization factor F by which the Salpeter stellar masses must
be multiplied is 1.23, with a one sigma confidence range, dominated by small
number statistics, of 0.77 < F < 2.10Comment: Revised in response to referee's suggestions and re-submitted to ApJ;
changes to the adopted effective radii propagate to a new value of the factor
F (by which Salpeter stellar masses must be multiplied) of 1.2
