With 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 that once powered quasars -- extremely luminous nuclei of galaxies -- that are now quiescent.
This conclusion is based on a census of 27 nearby galaxies carried out by the Hubble Space Telescope and groundbased telescopes in Hawai'i, which are being used to conduct a spectroscopic and photometric survey of galaxies to find black holes that have consumed the mass of millions of Sun-like stars.
The findings, presented 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 the role of quasars in galaxy evolution.
As a result of the survey, the researchers now believe that supermassive black holes are so common that nearly every large galaxy harbors one. They also suggest that a black hole's mass is proportional to the mass of the host galaxy, so that, for example, a galaxy twice as massive as another would have a black hole that is also twice as massive. This discovery suggests that the growth of the black hole is linked to the formation of the galaxy in which it is located. The number and masses of the black holes found are consistent with what would have been required to power the quasars.
"We believe we are looking at 'fossil quasars' and that most galaxies at one time burned brightly as a quasar," says team leader Doug Richstone of the University of Michigan, Ann Arbor. These conclusions are consistent with previous HST observations showing quasars within a variety of galaxies, from isolated normal-looking galaxies to colliding pairs.
Two of the black holes in the cores of galaxies NGC 3379 (also known as M105) and NGC 3377 have masses of 50 million and 100 million Suns, respectively. These galaxies are in the "Leo Spur," a nearby group of galaxies about 32 million light years away, roughly in the direction of the Virgo cluster.
Located 50 million light years away in the Virgo cluster, NGC 4486B has 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 in galaxies the size of our Milky Way or larger, these new results suggest that smaller galaxies have lower-mass black holes, below Hubble's detection limit, where the black hole's mass is proportional to the host galaxy's mass.
The three galaxies (right) are believed to contain central, supermassive black holes. The galaxy NGC 4486B (lower left) shows a double nucleus (lower right).The images of NGC 3377 and NGC 4486B are 2.7 arcseconds on a side, and for NGC 3379 the size is 5.4 arcseconds; the lower right is a blow-up of the central 0.5 arcseconds of NGC 4486B.
Credit: Karl Gebhardt (University of Michigan) and Tod Lauer (NOAO)
It remains a challenging puzzle 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 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 observed in the Hubble census), and 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 be as massive as these observations indicate, and naturally would be proportional to the current luminosity of the galaxy.
Critical groundbased observations to identify galaxies that might contain black holes were obtained for all three of these objects by John Kormendy with the Canada-France-Hawai'i Telescope (CFHT) on Mauna Kea. The NGC 4486B black hole detection also was based on CFHT spectra.
Hubble then allowed the team to peer deep into the cores of the galaxies with resolution unavailable from groundbased 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 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 positions near one another in a single object. The group of researchers will continue the census with the refurbished telescope.