Central mass-to-light ratios and dark matter fractions in early-type
  galaxies
  Dynamical studies of local ETGs and the Fundamental Plane point to a strong
dependence of M/L ratio on luminosity (and stellar mass) with a relation of the
form $M/L \propto L^{\gamma}$. The "tilt" $\gamma$ may be caused by various
factors, including stellar population properties, IMF, rotational support,
luminosity profile non-homology and dark matter (DM) fraction. We evaluate the
impact of all these factors using a large uniform dataset of local ETGs from
Prugniel & Simien (1997). We take particular care in estimating the stellar
masses, using a general star formation history, and comparing different
population synthesis models. We find that the stellar M/L contributes little to
the tilt. We estimate the total M/L using simple Jeans dynamical models, and
find that adopting accurate luminosity profiles is important but does not
remove the need for an additional tilt component, which we ascribe to DM. We
survey trends of the DM fraction within one effective radius, finding it to be
roughly constant for galaxies fainter than $M_B \sim -20.5$, and increasing
with luminosity for the brighter galaxies; we detect no significant differences
among S0s and fast- and slow-rotating ellipticals. We construct simplified
cosmological mass models and find general consistency, where the DM transition
point is caused by a change in the relation between luminosity and effective
radius. A more refined model with varying galaxy star formation efficiency
suggests a transition from total mass profiles (including DM) of faint galaxies
distributed similarly to the light, to near-isothermal profiles for the bright
galaxies. These conclusions are sensitive to various systematic uncertainties
which we investigate in detail, but are consistent with the results of dynamics
studies at larger radii.
