Hubble Space Telescope Spectroscopic Evidence for a 2 10 9 M_sun Black Hole in NGC 3115

March 1996 • 1996ApJ...459L..57K

Authors • Kormendy, John • Bender, Ralf • Richstone, Douglas • Ajhar, E. A. • Dressler, Alan • Faber, S. M. • Gebhardt, Karl • Grillmair, Carl • Lauer, Tod R. • Tremaine, Scott

Abstract • The discovery by Kormendy & Richstone of an M• ~= 109 M⊙ massive dark object (MDO) in NGC 3115 is confirmed with higher resolution spectroscopy from the Canada-France-Hawaii Telescope (CFHT) and the Hubble Space Telescope (HST). Measurements with the CFHT and Subarcsecond Imaging Spectrograph improve the resolution from sigma * = 0."44 to sigma * = 0."244 ( sigma * = Gaussian dispersion radius of the point-spread function). The apparent central velocity dispersion rises from sigma = 295 +/- 9 km s-1 to sigma = 343 +/- 19 km s-1. The Faint Object Spectrograph and COSTAR-corrected HST provide a further improvement in resolution using a 0."21 aperture. Then, the measured sigma = 443 +/- 18 km s-1 is remarkably high, and the wings of the velocity profiles extend beyond 1200 km s-1 from the line centers. Similarly, the apparent rotation curve rises much more rapidly than is observed from the ground. Published dynamical models fit the new observations reasonably well when "observed" at the improved spatial resolution; V and sigma are at the high end of the predicted range near the center. Therefore, M• > 109 M⊙. The spatial resolution has now improved by a factor of ~5 since the discovery observations, and the case for a central MDO has strengthened correspondingly. With HST and the Second Wide Field and Planetary Camera, NGC 3115 also shows a bright nucleus. This is very prominent and distinct from the bulge when the superposed nuclear disk is subtracted. After bulge subtraction, the nucleus has sigma = 600 +/- 37 km s-1, the largest central dispersion seen in any galaxy. If the nucleus contained only old stars and not an MDO, its escape velocity would be ~352 km s-1, much smaller than the observed velocities of the stars. This is independent proof that an MDO is present. The new observations put more stringent constraints on the radius inside which the dark mass lies and strengthen the case that it is a 2 x 109 M⊙ black hole.

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