Hubble Space Telescope Spectroscopic Evidence
for a 1 × 10-M Black Hole in NGC 4594

Ralf Bender
Universitäts-Sternwarte, München, Germany

Edward A. Ajhar
Kitt Peak National Observatory

Alan Dressler
Carnegie Observatories, Pasadena, California

S. M. Faber
UCO/Lick Observatory, University of California, Santa Cruz

Karl Gebhardt
Department of Astronomy, University of Michigan

Carl Grillmair
Jet Propulsion Laboratory

Tod R. Lauer
Kitt Peak National Observatory

Douglas Richstone
Department of Astronomy, University of Michigan

Scott Tremaine
Canadian Institute for Theoretical Astrophysics
University of Toronto

The discovery by Kormendy of a 10 M massive dark object (MDO) in NGC 4594 is confirmed with higher-resolution spectroscopy from the Canada-France-Hawaii Telescope (CFHT) and the Hubble Space Telescope (HST ). CFHT measurements with the Subarcsecond Imaging Spectrograph improve the resolution from = 0."40 to 0."27 Gaussian dispersion radius of the PSF. The apparent central velocity dispersion rises from = 250 ± 7 km/s to 286 ± 7 km/s. As observed with the COSTAR-corrected HST, the Faint Object Spectrograph, and a 0."21 aperture, = 321 ± 7 km/s is still higher, and the central rotation curve is very steep. The published dynamical model with the highest black hole mass fits the new observations reasonably well when "observed" at HST resolution. The spatial resolution has now improved by a factor of ~ 5 since the discovery measurements, and the case for a black hole has strengthened correspondingly.

We confirm that NGC 4594 has a Seyfert spectrum; H is ~ 5200 km/s wide at zero intensity. However, gas velocities are lower than the circular velocities implied by the stars; they cannot be used to test the black hole case in NGC 4594. The gas may be in a ring, or it may be associated with patchy dust. HST images with the Second Wide Field and Planetary Camera show dust at some aperture positions.

NGC 4594 appears to have a bright point nucleus. However, the central absorption line strengths are low, consistent with dilution by enough nonthermal light to explain the "nucleus." There is no evidence for a distinct nuclear star cluster. NGC 4594 is similar to M87, which also has a nonthermal nuclear source, and not to M31 and NGC 3115, which have quiescent black holes and nuclear star clusters.

Subject headings: black hole physics - galaxies: individual (NGC 4594) - galaxies: kinematics and dynamics - galaxies: nuclei

The complete paper, including color figures, is available here.