Announcing the discovery of three black holes in three normal galaxies, an international team of astronomers suggests that nearly all galaxies may harbor supermassive black holes which once powered quasars (extremely luminous nuclei of galaxies), but now are quiescent.
This conclusion is based on a census of 27 nearby galaxies carried out by NASA's Hubble Space Telescope and ground-based telescopes in Hawaii, which are being used to conduct a spectroscopic and photometric survey of galaxies to find black holes which have consumed the mass of millions of Sun-like stars.
The findings, being presented today at the 189th Meeting of the American Astronomical Society in Toronto, Canada, should provide insights into the origin and evolution of galaxies, as well as clarify the role of quasars in galaxy evolution.
The key results are:
Two of the black holes "weigh in" at 50 million and 100 million solar masses in the cores of galaxies NGC 3379 (also known as M105) and NGC 3377 respectively. These galaxies are in the "Leo Spur," a nearby group of galaxies about 32 million light- years away and roughly in the direction of the Virgo cluster.
Located 50 million light-years away in the Virgo cluster, NGC 4486B possesses a 500-million-solar-mass black hole. It is a small satellite of the very bright galaxy M87 in the Virgo cluster. M87 has an active nucleus and is known to have a black hole of about two billion solar masses.
Though several groups have previously found massive black holes dwelling in galaxies the size of our Milky Way or larger, these new results suggest smaller galaxies have lower-mass black holes, below Hubble's detection limit. The survey shows the black hole's mass is proportional to the host galaxy's mass.
It remains a challenging puzzle as to why black holes are so abundant, or why they should be proportional to a galaxy's mass. One idea, supported by previous Hubble observations, is that galaxies formed out of smaller "building blocks" consisting of star clusters. A massive "seed" black hole may have been present in each of these protogalaxies. The larger number of building blocks needed to merge and form very luminous galaxies naturally would have provided more seed black holes to coalesce into a single, massive black hole residing in a galaxy's nucleus.
An alternative model is that galaxies start at some early epoch with a modest black hole (not necessarily approaching the masses discussed here), but that the black hole consumes some fixed fraction of the total gas shed by the stars in the galaxy during their normal evolution. If that fraction is around one percent, the black holes easily could weigh as much as they do now, and naturally would track the current luminosity of the galaxy.
Critical ground-based observations to identify candidates were obtained for all three of these objects by John Kormendy with the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii. The NGC 4486b black hole detection also was based on CFHT spectra.
Hubble's high resolution then allowed the team to peer deep into the cores of the galaxies with extraordinary resolution unavailable from ground-based telescopes, and measure velocities of stars orbiting the black hole. A sharp rise in velocity means that a great deal of matter is locked away in the galaxy's core, creating a powerful gravitational field that accelerates nearby stars.
The team is confident their statistical search technique has allowed them to pinpoint all the black holes they expect to see, above a certain mass limit. "However, our result is complicated by the fact that the observational data for the galaxies are not of equal quality, and that the galaxies are at different distances," says Richstone.
One of the features of the February 1997 servicing mission to the Hubble will be the installation of the Space Telescope Imaging Spectrograph. This spectrograph will greatly increase the efficiency of projects, such as this black hole census, that require spectra of several nearby positions in a single object. This group will be continuing this census with the refurbished telescope.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
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