An HST Survey of Cores of Early-Type Galaxies

Abstract

Photometry of the central parts of bulges and elliptical galaxies with the Hubble Space Telescope (HST) confirms and extends ground-based results. Most giant ellipticals have cuspy cores: at the “break radius” rb (formerly the core radius rc), the steep outer surface brightness profile turns down to a shallow inner power law I(r) ∝ r–γ, 0 ≤ γ ≲ 0.25. The corresponding slope of the deprojected profile is derived; the flattest cores allow box orbits to survive. Cores continue to satisfy fundamental plane parameter correlations like those found from the ground. In particular, HST confirms that the luminosity sequence of elliptical galaxies (from cDs to M 32) is physically unrelated to spheroidal galaxies like Fornax. The latter are closely related to late-type dwarfs. Low-luminosity ellipticals do not show cores: 0.5 ≲ γ ≲ 1.3. The most important new result is that global and core properties both show signs of a dichotomy between (i) low-luminosity ellipticals that rotate rapidly, that are nearly isotropic and oblate-spheroidal, that have disky-distorted isophotes, and that are coreless and (ii) giant ellipticals that are essentially nonrotating, anisotropic, and moderately triaxial, that are boxy-distorted, and that have cuspy cores.