The nature of H-alpha star-forming galaxies at z~0.4 in and around Cl
  0939+4713: the environment matters
  Cluster star-forming galaxies are found to have an excess of Far-Infrared
emission relative to H-alpha (Ha), when compared to those in the field, which
could be caused by intense AGN activity, dust and/or declining star formation
histories. Here we present spectroscopic observations of Ha emitters in the Cl
0939+4713 (Abell 851) super-cluster at z=0.41, using AF2+WYFFOS on the WHT. We
measure [OII], Hbeta (Hb), [OIII], Ha and [NII] for a sample of 119 Ha emitters
in and around the cluster. We find that 17+-5% of the Ha emitters are AGN,
irrespective of environment. For star-forming galaxies, we obtain Balmer
decrements, metallicities and ionisation parameters with different methods,
individually and by stacking. We find a strong mass-metallicity relation at all
environments, with no significant dependence on environment. The ionisation
parameter declines with increasing stellar mass for low-mass galaxies. Ha
emitters residing in intermediate environments show the highest ionisation
parameters (along with high [OIII]/Ha and high [OIII]/[OII] line ratios,
typically twice as large as in the highest and lowest densities), which decline
with increasing environmental density. Dust extinction (A$_{H\alpha}$)
correlates strongly with stellar mass, but also with environmental density.
Star-forming galaxies in the densest environments are found to be significantly
dustier (A$_{H\alpha}$~1.5-1.6) than those residing in the lowest density
environments (A$_{H\alpha}$~0.6), deviating significantly from what would be
predicted given their stellar masses.
