OMEGA -- OSIRIS Mapping of Emission-line Galaxies in A901/2: II. --
  Environmental influence on integrated star formation properties and AGN
  activity
  We present a study of the star formation and AGN activity for galaxies in the
Abell 901/2 multi-cluster system at z~0.167 as part of the OMEGA survey. Using
Tuneable Filter data obtained with the OSIRIS instrument at the GTC we produce
spectra covering the Halpha and [N II] spectral lines for more than 400
galaxies. Using optical emission-line diagnostics, we identify a significant
number of galaxies hosting AGN, which tend to have high masses and a broad
range of morphologies. Moreover, within the environmental densities probed by
our study, we find no environmental dependence on the fraction of galaxies
hosting AGN. The analysis of the integrated Halpha emission shows that the
specific star formation rates (SSFRs) of a majority of the cluster galaxies are
below the field values for a given stellar mass. We interpret this result as
evidence for a slow decrease in the star formation activity of star-forming
galaxies as they fall into higher-density regions, contrary to some previous
studies which suggested a rapid truncation of star formation. We find that most
of the intermediate- and high-mass spiral galaxies go through a phase in which
their star formation is suppressed but still retain significant star-formation
activity. During this phase, these galaxies tend to retain their spiral
morphology while their colours become redder. The presence of this type of
galaxies in high density regions indicates that the physical mechanism
responsible for suppressing star-formation affects mainly the gas component of
the galaxies, suggesting that ram-pressure stripping or starvation are
potentially responsible.
