Star forming galaxies in the Hercules cluster: Halpha imaging of A2151
  This paper presents the first results of an Halpha imaging survey of galaxies
in the central regions of the A2151 cluster. A total of 50 sources were
detected in Halpha. The morphologies of the 43 H$\alpha$ selected galaxies
range from grand design spirals and interacting galaxies to blue compacts and
tidal dwarfs or isolated extragalactic HII regions, spanning a range of
magnitudes of -21 <= MB <= -12.5 mag. A comparison with the clusters Coma and
A1367 and a sample of field galaxies has shown the presence of cluster galaxies
with L(Halpha) lower than expected for their MB, a consecuence of the cluster
environment. This fact results in differences in the
  L(Halpha) vs. EW(Halpha) and L(H\alpha) distributions of the clusters with
respect to the field, and in cluster to cluster variations of these quantities,
which we propose are driven by a global cluster property as the total mass.
Overall, we conclude that both, the global cluster environment as well as the
cluster merging history play a non negligible role in the integral star
formation properties of clusters of galaxies.
