The quantity of dark matter in early-type galaxies and its relation to
  the environment
  We study the behavior of the dynamical and stellar mass inside the effective
radius of early-type galaxies (ETGs) as a function of environment considering
Newtonian dynamics, different surface--brightness profiles, different initial
mass functions (IMF) and different redshift ranges. We use several samples of
ETGs --ranging from 19,000 to 98,000 objects-- from the ninth data release of
the Sloan Digital Sky Survey. We assume that any difference between the
dynamical and stellar mass is due to dark matter and/or a non-universal IMF.
The main results, considering samples in the redshift range 0.0024
$\leq\;z\;\leq$ 0.35 are: (i) the amount of dark matter inside ETGs depends on
the environment; (ii) ETGs in low density environments span a wider dark matter
range than ETGs in dense environments; (iii) the amount of dark matter inside
ETGs in the most dense environments will be less than approximately 55\%--75\%
of the dynamical mass; (iv) the accurate value of this upper limit depends on
the impact of the IMF on the stellar mass estimation; (v) in the case of an
ETGs sample which is approximately complete for log$({\bf M_{Virial}}/{\bf
M_{Sun}}) > 10.5$ and in the redshift range 0.04 $\leq\;z\;\leq$ 0.08 we find
that the amount of dark matter in the most dense environments will be less than
approximately 60\%--65\% of the dynamical mass.
