Revisiting the original Morphology-Density Relation
  In light of recent findings from the kinematic morphology-density relation,
we investigate whether the same trends exist in the original morphology density
relation, using the same data as Dressler. In addition to Dressler's canonical
relations, we find that further refinements are possible when considering the
average local projected density of galaxies in a cluster. Firstly, the
distribution of ellipticals in a cluster depends on the relative local density
of galaxies in that cluster: equivalent rises in the elliptical fraction occur
at higher local densities for clusters with higher average local densities.
This is not true for the late-type fraction, where the variation with local
density within a cluster is independent of the average local density of
galaxies in that cluster, and is as Dressler originally found. Furthermore, the
overall ratio of ellipticals to early-types in a cluster does not depend on the
average density of galaxies in that cluster (unlike the ratio of lenticulars to
disk systems), and is fixed at around 30%. In the paradigm of fast and slow
rotators, we show that such an elliptical fraction in the early-type population
is consistent with a slow rotator fraction of 15% in the early-type population,
using the statistics of the ATLAS3D survey. We also find the scatter in the
overall ratio of ellipticals to early-types is greatest for clusters with lower
average densities, such that clusters with the highest elliptical fractions
have the lowest average local densities. Finally, we show that average local
projected density correlates well with global projected density, but the latter
has difficulty in accurately characterising the density of irregular cluster
morphologies.
