Isophotometry of brightest elliptical galaxies in rich clusters

Abstract

This thesis presents ellipse-fitting isophotometry of 175 brightest elliptical galaxies in Abell clusters (BCEs). Galaxy structure is traced to an average major axis of 50 kpc, and some galaxies are traced beyond 100 kpc. The data consist not only of major- and minor-axis surface brightness profiles but also of isophote ellipticity, major axis position angle, and centroid position profiles. Almost all BCEs have some local structure that does not show in an azimuthally averaged brightness profile. However, local structure shows no correlation with global parameters such as galaxy luminosity or size. These results echo similar earlier findings for field ellipticals. The average ellipticity of a BCE is a strongly increasing function of physical radius as are most individual ellipticity profiles. This distinguishes BCEs as a class from other elliptical galaxies and explains why cD halos as a class are highly flattened. The deficit of large round isophotes is so marked that the isoluminosity surfaces of BCEs at large radii cannot be drawn from a population of randomly oriented oblate spheroids. This is the first sample of galaxies for which such an exclusion has been possible. The distinction of the isophotes from an oblate population becomes possible over the same range of radii (20-30 kpc) at which the isophotes become significantly aligned with the global position angle of the cluster. These results confirm and further quantify the long-suspected origin of BCEs in cluster material. The detection of nonconcentric light in 20 % of the BCEs in the sample suggests that some galaxy construction continues to the present day. Particular attention has been paid to the morphology of cD halos. The inner edges of these halos range from sharp to very gradual, are almost always associated with sudden ellipticity increases, and often also with isophote twists or nonconcentric light. This indicates that they have some dynamically significant identity, perhaps as material accreted from the cluster. An analytic method for separating the light distributions of overlapping binary galaxies has been derived and applied to images of 12 such systems. It is based solely on the assumption that each galaxy in the pair has point-reflection symmetry about its own center. Nonconcentric light is a probe of interactions between the members of the pair.