Hubble Space Telescope observations of the centers of elliptical galaxies

Abstract

Hubble Space Telescope observations of the centers of elliptical galaxies reveal that there are two types of ellipticals, divided by luminosity and morphology. The first type, denoted Type I (or core galaxies), have luminosity profiles which can be fitted accurately with a double power law for radii smaller than ≈ 1 kpc, and are characterized by high luminosity, relatively low central surface brightness, and low ellipticity. Type II galaxies (power law galaxies) have power law brightness profiles that rise steeply into the very center, and are characterized by high central surface brightness, high ellipticity, and relatively low luminosity. The Type II galaxies often have a distinct, bright, thin (r<25 pc) nuclear disk. There are no elliptical galaxies with isothermal cores. Dust is the rule rather than the exception in Virgo ellipticals. Elliptical galaxies often have gaseous disks, dusty disks being the most common. Recent COSTAR plus FOS observations show that the small nuclear disks in two luminous radio ellipticals, M 87 and NGC 4261, are rotating around central masses greater than ≈109 M⊙. The respective mass to light ratios are M/L V ≈3000 and 5000, leading to the conclusion that these galaxies host massive black holes.