An H α Survey of 8 Abell Clusters : the dependence of tidally - induced star formation on cluster density

We have undertaken a survey of Hα emission in a substantially complete sample of CGCG galaxies of types Sa and later within 1.5 Abell radii of the centres of 8 320 galaxies were surveyed, of which 116 were detected in emission (39% of spirals, 75% of peculiars). Here we present previously unpublished data for 243 galaxies in 7 clusters. Detected emission was classified as 'compact' or 'diffuse'. From an analysis of the full survey sample, we reconfirm our previous identification of compact and diffuse emission with circumnuclear starburst and disk emission respectively. The circumnu-clear emission is associated either with the presence of a bar, or with a disturbed galaxy morphology indicative of ongoing tidal interactions (whether galaxy–galaxy, galaxy–group, or galaxy–cluster). The frequency of such tidally-induced (circumnuclear) starburst emission in spirals increases from regions of lower to higher local galaxy surface density, and from clusters with lower to higher central galaxy space density. The percentages of spirals classed as disturbed, and of galaxies classified as peculiar show a similar trend. These results suggest that tidal interactions for spirals are more frequent in regions of higher local density and for clusters with higher central galaxy density. The prevalence of such tidal interactions in clusters is expected from recent theoretical modelling of clusters with a non-static potential undergoing collapse and infall. Furthermore, in accord with this picture, we suggest that peculiar galaxies are predominantly ongoing mergers. We conclude that tidal interactions are likely to be the main mechanism for the transformation of spirals to S0s in clusters. This mechanism operates more efficiently in higher density environments as is required by the morphological type–local surface density (T–Σ) relation for galaxies in clusters. For regions of comparable local density, the frequency of tidally-induced starburst emission is greater in clusters with higher central galaxy density. This implies that, for a given local density, morphological transformation of disk galaxies proceeds more rapidly in clusters of higher central galaxy density. This effect is considered to be due to subcluster merging and could account for the previously considered anomalous absence of a significant T–Σ relation for irregular clusters at intermediate redshift.