An Ha ¯ i story of galaxies in Abell 2626 and beyond

Context. To study the effects of environment on galaxies, we use Ha ¯I observations of galaxies in and around the cluster Abell 2626 (A2626). The cluster can effectively be divided into three different environments: the cluster itself, a group environment in the periphery of the cluster (we call it the Swarm), and substructure in the cluster itself. We use these to study the dependence of the galaxy properties on environment. Aims. We have explored the relation between Ha ¯I deficiency, Ha ¯I morphology, and star formation deficiency for the galaxies in and around the A2626 galaxy cluster to investigate the environmental effects on these properties. Methods. To quantify the asymmetries of the outer Ha ¯I disc of a galaxy, we used 1) three visual classes based on the outermost reliable Ha ¯I contour (settled, disturbed, and unsettled Ha ¯I discs), 2) the offset between the Ha ¯I centre and the optical centre of a galaxy, and 3) the modified asymmetry parameter Amod as defined previously. Results. The Ha ¯I deficiency of a galaxy is strongly correlated with the projected distance from the centre of A2626. Furthermore, substructure galaxies tend to be more asymmetric than the isolated galaxies in A2626, probably because tidal interactions are more efficient within the substructures than outside the substructures. Moreover, asymmetric, offset, and smaller Ha ¯I discs are not necessarily the result of the cluster environment because they are also observed in substructures in A2626 and in the Swarm. This signifies that a pre-processing of the Ha ¯I discs of galaxies in groups or substructures plays an important role, together with the processing in the cluster environment. Finally, the star formation rates for the galaxies in all three environments are slightly lower than the typical star formation rate for normal galaxies, as manifested by their offset from the star formation main sequence. This implies effective gas removal mechanisms in all three environments.