CFHT Legacy Ultraviolet Extension (CLUE): Witnessing Galaxy
  Transformations up to 7 Mpc from Rich Cluster Cores
  Using the optical data from the Wide component of the CFHT Legacy Survey, and
new ultraviolet data from GALEX, we study the colours and specific star
formation rates (SSFR) of ~100 galaxy clusters at 0.16<z<0.36, over areas
extending out to radii of r~7Mpc. We use a multicolour, statistical background
subtraction method to study the galaxy population at this radius; thus our
results pertain to those galaxies which constitute an excess over the average
field density. We find that the average SSFR, and its distribution, of the
star-forming galaxies (with SFR>0.7 M_sun/yr at z~0.2 and SFR>1.2 M_sun/yr at
z~0.3) have no measurable dependence on the cluster-centric radius, and are
consistent with the field values. However, the fraction of galaxies with SFR
above these thresholds, and the fraction of optically blue galaxies, are lower
for the overdense galaxy population in the cluster outskirts compared with the
average field value, at all stellar masses M*>10^{9.8} M_sun and at all radii
out to at least 7Mpc. Most interestingly, the fraction of blue galaxies that
are forming stars at a rate below our UV detection limit is much higher in all
radial bins around our cluster sample, compared with the general field value.
This is most noticeable for massive galaxies M*>10^{10.7} M_sun; while almost
all blue field galaxies of this mass have detectable star formation, this is
true for less than 20% of the blue cluster galaxies, even at 7Mpc from the
cluster centre. Our results support a scenario where galaxies are pre-processed
in locally overdense regions, in a way that reduces their SFR below our UV
detection limit, but not to zero.
