We test the hypothesis that the apparent axis ratio of an elliptical galaxy is correlated with the age of its stellar population. We find that old ellipticals (with estimated stellar ages t > 7.5 Gyr) are rounder on average than younger ellipticals. The statistical significance of this shape difference is greatest at small radii; a Kolmogorov-Smirnov test comparing the axis ratios of the two populations at R = Re/16 yields a statistical significance greater than 99.96 percent. The relation between age and apparent axis ratio is linked to the core/powerlaw surface brightness profile dichotomy. Core ellipticals have older stellar populations, on average, than powerlaw ellipticals and are rounder in their inner regions. Our results are consistent with a scenario in which powerlaw ellipticals are formed in gas-rich mergers while core ellipticals form in dissipationless mergers, with cores formed and maintained by the influence of a binary black hole.
Categories: astro-ph