Authors: Bililign T. Dullo & Alister W. Graham
The inner and outer shapes and orientations of core-Sérsic galaxies may hold important clues to their formation and evolution. We have therefore measured the central and outer ellipticities and position angles for a sample of 24 core-Sérsic galaxies using archival HST images and data. By selecting galaxies with core-Séersic break radii Rb — a measure of the size of their partially depleted core — that are > 0.2 arcsec, we find that the ellipticities and position angles are quite robust against HST seeing. For the bulk of the galaxies, there is a good agreement between the ellipticities and position angles at the break radii and the average outer ellipticities and position angles determined over Re/2 < R < Re, where Re is the spheroids' effective half light radius. However there are some interesting differences. We find a median "inner" ellipticity at Rb of 0.13±0.01, rounder than the median ellipticity of the `outer' regions (0.20±0.01), which is thought to reflect the influence of the central supermassive black hole at small radii. In addition, for the first time we find a trend, albeit weak (2 σ significance), such that galaxies with larger (stellar deficit)-to-(supermassive black hole) mass ratios — thought to be a measure of the number of major dry merger events — tend to have rounder inner and outer isophotes, suggesting a connection between the galaxy shapes and their merger histories. We show that this finding is not simply reflecting the well known result that more luminous galaxies are rounder, but it is no doubt related.