No Supermassive Black Hole in M 33 ? 1

We observed the nucleus of M33, the third-brightest galaxy in the Local Group, with the Space Telescope Imaging Spectrograph at a resolution at least a factor of 10 higher than previously obtained. Rather than the steep rise expected within the radius of gravitational influence of a supermassive black hole, the random stellar velocities showed a decrease within a parsec of the center of the galaxy. The implied upper limit on the mass of the central black hole is only 3000M ⊙ , about three orders of magnitude lower than the dynamically-inferred mass of any other supermassive black hole. Detecting black holes of only a few thousand solar masses is observationally challenging but is critical for establishing how supermassive black holes relate to their host galaxies and which mechanisms influence the formation and evolution of both. At a distance of 850 kpc from Earth, M33 is classified (1) as a late-type ScII-III spiral, consistent with its almost nonexistent bulge (2,3). The nucleus of M33 is very compact, reaching a stellar central mass density of several million solar masses per cubic parsec (4,5), larger than that of any globular cluster. While such high nuclear densities might be expected in the presence of a supermassive black hole (SMBH) (6), ground-based data show no evidence for a central rise in stellar velocities that would indicate the presence of a compact massive object in the nucleus (4). M33 was observed on 12 February 1999 with the Space Telescope Imaging Spectro-graph (STIS) on the Hubble Space Telescope (HST). Three sets of two long-slit spectra each, for a total exposure time of 7380 seconds, were obtained using the G750M grating centered on the CaII absorption triplet near 8561Å (1 ˚ A corresponds to 10 −10 meters), covering 19.6 km s −1 pixel −1. The pixel scale is 0 ′′ .05 with a spatial resolution of 0 ′′ .115 at 8561Å. While the two spectra in each set were obtained at the same position to facilitate removal of cosmic ray events, the nucleus was moved along the slit by 0 ′′ .216 between each consecutive set. This dithering procedure allows for optimal correction of residual