A multiscale approach to environment and its influence on the colour
  distribution of galaxies
  We present a multiscale approach to measurements of galaxy density, applied
to a volume-limited sample constructed from SDSS DR5. We populate a rich
parameter space by obtaining independent measurements of density on different
scales for each galaxy, avoiding the implicit assumptions involved, e.g., in
the construction of group catalogues. As the first application of this method,
we study how the bimodality in galaxy colour distribution (u-r) depends on
multiscale density. The u-r galaxy colour distribution is described as the sum
of two gaussians (red and blue) with five parameters: the fraction of red
galaxies (f_r) and the position and width of the red and blue peaks (mu_r,
mu_b, sigma_r and sigma_b). Galaxies mostly react to their smallest scale (<
0.5 Mpc) environments: in denser environments red galaxies are more common
(larger f_r), redder (larger mu_r) and with a narrower distribution (smaller
sigma_r), while blue galaxies are redder (larger mu_b) but with a broader
distribution (larger sigma_b). There are residual correlations of f_r and mu_b
with 0.5 - 1 Mpc scale density, which imply that total or partial truncation of
star formation can relate to a galaxy's environment on these scales. Beyond 1
Mpc (0.5 Mpc for mu_r) there are no positive correlations with density. However
f_r (mu_r) anti-correlates with density on >2 (1) Mpc scales at fixed density
on smaller scales. We examine these trends qualitatively in the context of the
halo model, utilizing the properties of haloes within which the galaxies are
embedded, derived by Yang et al, 2007 and applied to a group catalogue. This
yields an excellent description of the trends with multiscale density,
including the anti-correlations on large scales, which map the region of
accretion onto massive haloes. Thus we conclude that galaxies become red only
once they have been accreted onto haloes of a certain mass.
