Structure and Formation of Elliptical and Spheroidal Galaxies

Abstract

New surface photometry of all known elliptical galaxies in the Virgo cluster is added to published data to derive composite profiles over large dynamic ranges. Sersic functions fit them remarkably well. Effective brightnesses and radii are derived via Sersic fits and by integrating the profiles nonparametrically. We strongly confirm two dichotomies: (1) Elliptical galaxies from cDs to M32 form a tight sequence in Fundamental Plane parameter space that is almost perpendicular to the sequence of spheroidal galaxies from NGC 205 to Draco. This is consistent with our understanding of their different formation processes: mergers for Es and conversion of late-type galaxies into spheroidals by environmental effects and by energy feedback from supernovae. (2) Ellipticals come in two varieties: e.g., our 10 brightest Es have cuspy cores; our 17 fainter Es do not have cores. We find a new distinct component in coreless Es. All have extra light at the center above the inward extrapolation of the outer Sersic profile. We suggest that extra light is made by starbursts in dissipational (wet) mergers, as in numerical simulations. Three other new aspects also point to an explanation of how the E-E dichotomy formed: extra light Es were made in wet mergers while core Es were made in dry mergers. We confirm that core Es do and extra light Es generally do not contain X-ray gas. This suggests why the E-E dichotomy arose. Only core Es and their progenitors are massive enough to retain hot gas that can make dry mergers dry and protect old star populations from late star formation.