  We explore the amount of obscured star-formation as a function of environment
in the A901/902 supercluster at z=0.165 in conjunction with a field sample
drawn from the A901 and CDFS fields, imaged with HST as part of the STAGES and
GEMS surveys. We combine the COMBO-17 near-UV/optical SED with Spitzer 24um
photometry to estimate both the unobscured and obscured star formation in
galaxies with Mstar>10^{10}Msun. We find that the star formation activity in
massive galaxies is suppressed in dense environments, in agreement with
previous studies. Yet, nearly 40% of the star-forming galaxies have red optical
colors at intermediate and high densities. These red systems are not
starbursting; they have star formation rates per unit stellar mass similar to
or lower than blue star-forming galaxies. More than half of the red
star-forming galaxies have low IR-to-UV luminosity ratios, relatively high
Sersic indices and they are equally abundant at all densities. They might be
gradually quenching their star-formation, possibly but not necessarily under
the influence of gas-removing environmental processes. The other >40% of the
red star-forming galaxies have high IR-to-UV luminosity ratios, indicative of
high dust obscuration. They have relatively high specific star formation rates
and are more abundant at intermediate densities. Our results indicate that
while there is an overall suppression in the star-forming galaxy fraction with
density, the small amount of star formation surviving the cluster environment
is to a large extent obscured, suggesting that environmental interactions
trigger a phase of obscured star formation, before complete quenching.

Astrophysics