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

Abstract

We have undertaken a survey of H alpha emission in a substantially complete sample of CGCG galaxies of types Sa and later within 1.5 Abell radii of the centres of 8 low-redshift Abell clusters. Some 320 galaxies were surveyed, of which 116 were detected in emission (39% of spirals, 75% of peculiars). 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 circumnuclear emission is associated either with the presence of a bar, or with a disturbed galaxy morphology indicative of on-going tidal interactions. 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. We conclude that tidal interactions are likely to be the main mechanism for the transformation of spirals to S0s in clusters. Finally, 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 type - local surface density relation for irregular clusters at intermediate redshift.