The stellar populations of early-type galaxies -- II. The effects of
  environment and mass
  The degree of influence that environment and mass have on the stellar
populations of early-type galaxies is uncertain. In this paper we present the
results of a spectroscopic analysis of the stellar populations of early-type
galaxies aimed at addressing this question. The sample of galaxies is drawn
from four clusters, with <z>=0.04, and their surrounding structure extending to
~10R_{vir}. We find that the distributions of the absorption-line strengths and
the stellar population parameters age, metallicity and alpha-element abundance
ratio do not differ significantly between the clusters and their outskirts, but
the tight correlations found between these quantities and velocity dispersion
within the clusters are weaker in their outskirts. All three stellar population
parameters of cluster galaxies are positively correlated with velocity
dispersion. Galaxies in clusters form a homogeneous class of objects that have
similar distributions of line-strengths and stellar population parameters, and
follow similar scaling relations regardless of cluster richness or morphology.
We estimate the intrinsic scatter of the Gaussian distribution of metallicities
to be 0.3 dex, while that of the alpha-element abundance ratio is 0.07 dex. The
e-folding time of the exponential distribution of galaxy ages is estimated to
be 900 Myr. The intrinsic scatters of the metallicity and alpha-element
abundance ratio distributions can almost entirely be accounted for by the
correlations with velocity dispersion and the intrinsic scatter about these
relations. This implies that a galaxies mass plays the major role in
determining its stellar population.
