Submillimetre observations of galaxy clusters with BLAST: the
  star-formation activity in Abell 3112
  We present observations at 250, 350, and 500 um of the nearby galaxy cluster
Abell 3112 (z=0.075) carried out with BLAST, the Balloon-borne Large Aperture
Submillimeter Telescope. Five cluster members are individually detected as
bright submillimetre sources. Their far-infrared SEDs and optical colours
identify them as normal star-forming galaxies of high mass, with globally
evolved stellar populations. They all have B-R colours of 1.38+/-0.08,
transitional between the blue, active population and the red, evolved galaxies
that dominate the cluster core. We stack to determine the mean submillimetre
emission from all cluster members, which is determined to be 16.6+/-2.5,
6.1+/-1.9, and 1.5+/-1.3 mJy at 250, 350, and 500 um, respectively. Stacking
analyses of the submillimetre emission of cluster members reveal trends in the
mean far-infrared luminosity with respect to cluster-centric radius and Ks-band
magnitude. We find that a large fraction of submillimetre emission comes from
the boundary of the inner, virialized region of the cluster, at cluster-centric
distances around R_500. Stacking also shows that the bulk of the submillimetre
emission arises in intermediate-mass galaxies (L<L*), with Ks magnitude ~1 mag
fainter than the giant ellipticals. The results and constraints obtained in
this work will provide a useful reference for the forthcoming surveys to be
conducted on galaxy clusters by Herschel.
