Star-Forming Galaxies in the Hercules Cluster: Hα Imaging of A2151

This paper presents the first results of an H? imaging survey of galaxies in the central regions of the A2151 cluster. A total of 50 sources were detected in H?, from which 41 were classified as secure members of the cluster and 2 as likely members based on spectroscopic and photometric redshift considerations. The remaining seven galaxies were classified as background contaminants and thus excluded from our study on the H? properties of the cluster. The morphologies of the 43 H? selected galaxies range from grand design spirals and interacting galaxies to blue compacts and tidal dwarfs or isolated extragalactic H?II regions, spanning a range of magnitudes of ?21 ? MB ? ?12.5 mag. From these 43 galaxies, 7 have been classified as active galactic nucleus (AGN) candidates. These AGN candidates follow the L(H?) versus MB relationship of the normal galaxies, implying that the emission associated with the nuclear engine has a rather secondary impact on the total H? emission of these galaxies. A comparison with the clusters Coma and A1367 and a sample of field galaxies has shown the presence of cluster galaxies with L(H?) lower than expected for their MB , a consequence of the cluster environment. This fact results in differences in the L(H?) versus EW(H?) and L(H?) 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. In addition, the cluster H? emitting galaxies tend to avoid the central regions of the clusters, again with different intensity depending on the cluster total mass. For the particular case of A2151, we find that most H? emitting galaxies are located close to the regions with the higher galaxy density, offset from the main X-ray peak. Overall, we conclude that both the global cluster environment and the cluster merging history play a non-negligible role in the integral star formation properties of clusters of galaxies.