Structural Evidence for Environment-driven Transformation of the Blue Galaxies in Local Abell Clusters: A85, A496, and A754

We present a detailed comparison of structural properties in the rest-frame V band of cluster and field galaxies, selected and analyzed in the same manner, to test the hypothesis that much of the current cluster galaxy membership resulted from the fairly rapid (1-2 Gyr) transformation of infalling, field spirals into red, cluster early types. Specifically, we have selected ~140 galaxies from three nearby Abell clusters (A85, A496, and A754) that have colors significantly bluer than the red sequence population and compared them to ~80 field galaxies with similar colors and luminosities from the Nearby Field Galaxy Survey. The comparison is based on the hypothesis that recent (1-4 Gyr) cluster arrivals were originally blue and star-forming, then stopped forming stars to dim and redden in a few gigayears. For the comparison we quantify galaxy internal structure and morphology from two-dimensional bulge/disk decompositions using GIM2D. We observe structural differences between blue galaxies in local (z < 0.06) clusters, compared to field environments. All cluster galaxies have spectroscopic membership. The majority of blue cluster members, presumably recent additions, are physically smaller and fainter than their equally colored field counterparts. At a matched size and luminosity, the newer cluster arrivals are quantifiably smoother in appearance, yet their total light is as disk dominated as in normal field spirals. Moreover, half of the blue cluster members appear to have blue cores or globally blue color profiles, in contrast with field spirals, which typically exhibit red inward color gradients. Blue cores suggest enhanced nuclear star formation, possibly a starburst, while uniformly blue profiles are consistent with an episode of fairly strong global star formation in the past few gigayears. Our previous work shows that the blue membership of local clusters is a recently infalling population that has yet to encounter the dense core. In a universe without environmentally dependent evolution outside of cluster cores, we would expect blue disk galaxies inhabiting field and cluster regions to have similar morphology, size, and color gradient distributions. Our findings show conclusively not only that the abundance of red and blue galaxies depends on environment, but also that fundamental structural and morphological galaxy properties do indeed reflect the environment in which the galaxy is found. Moreover, the data show that the transformation of accreted galaxies is not confined to the dense cluster core. The overall properties of bluer cluster members are best explained by environment-driven transformation of accreted field spirals, and our results suggest that the processes that govern color and morphological evolution occur separately.