More than just halo mass: Modelling how the red galaxy fraction depends
  on multiscale density in a HOD framework
  The fraction of galaxies with red colours depends sensitively on environment,
and on the way in which environment is measured. To distinguish competing
theories for the quenching of star formation, a robust and complete description
of environment is required, to be applied to a large sample of galaxies. The
environment of galaxies can be described using the density field of neighbours
on multiple scales - the multiscale density field. We are using the Millennium
simulation and a simple HOD prescription which describes the multiscale density
field of Sloan Digital Sky Survey DR7 galaxies to investigate the dependence of
the fraction of red galaxies on the environment. Using a volume limited sample
where we have sufficient galaxies in narrow density bins, we have more dynamic
range in halo mass and density for satellite galaxies than for central
galaxies. Therefore we model the red fraction of central galaxies as a constant
while we use a functional form to describe the red fraction of satellites as a
function of halo mass which allows us to distinguish a sharp from a gradual
transition. While it is clear that the data can only be explained by a gradual
transition, an analysis of the multiscale density field on different scales
suggests that colour segregation within the haloes is needed to explain the
results. We also rule out a sharp transition for central galaxies, within the
halo mass range sampled.
