The observed distribution of black hole masses has a well-known gap. On the one hand we have ``black hole candidates'' in our own Galaxy with inferred masses between 3 and 10M, and on the other, there are supermassive black holes which lie in the centers of many, if not most, galaxies with mass in the range of 106 to 109M. The mass gap would be filled by ``middleweights'' of a thousand M, give or take a couple of orders of magnitude.
There are several reasonable ways to form middleweight black holes, including merging of solar-mass black holes in star clusters (Lee 1995), or even more directly from gravitational collapse of density fluctuations in the primordial ooze (Haiman, Thoul & Loeb 1996), although these mechanisms can be suppressed to some degree by supernova-driven ejection of matter. Yet until recently, there was virtually no evidence for the existence of middleweights. This might just be a selection effect: We find black hole candidates in X-ray binary systems because they are local and have a steady source of luminous matter flowing from a companion star. Supermassive holes, the central engines in active galactic nuclei, are observed because they are very bright, feeding off of the gas-rich environments in the centers of galaxies. In contrast, it is unclear what, if any, reservoirs of luminous matter might surround middleweights.
Within the past year, the observational situation changed dramatically. Two groups, Ptak & Griffiths (1999) and Colbert & Mushotzky (1999), reported data which suggest the presence of middleweight black holes in nearby galaxies. Ptak & Griffith used the ASCA satellite to measure the hard X-ray spectrum of the starburst galaxy M82. A compact X-ray source detected in the central region of the galaxy has a luminosity which is two orders of magnitude larger than the brightest black hole candidate, and two orders of magnitude dimmer than a typical active galactic nucleus. Assuming that the accretion flow onto the source is at the Eddington limit, the observed luminosity corresponds to a mass of 460M.
The Eddington limit does rule out solar-mass black holes, however, it is plausible that the X-ray source is just an unusually dim active galactic nucleus harboring a supermassive hole. Future high angular resolution observations of soft X-rays from an accretion disk around this source (by the Chandra X-ray Observatory, for example) might help distinguish a middleweight from an ``ordinary'' supermassive hole.
Colbert & Mushotzky used a combination of ROSAT X-ray imaging data and ASCA spectroscopy to examine X-ray sources in 21 nearby galaxies. They determined that a half dozen or so objects in the sample show X-ray emission from compact sources well away from the photometric center of their host galaxies. This adds some spice into the mix since some of these sources also have high luminosities and hence high Eddington masses. A suggestion that these sources are just unusually quiescent active galactic nuclei is significant, since it would be interesting to observe nuclear activity well outside the galactic nucleus.
Of the 21 objects in the sample, ASCA yielded hard X-ray spectra from three spiral galaxies and three ellipticals. The X-ray sources in the ellipticals are all coincident with the photometric centers (within positional errors). While their spectra show features similar to Galactic X-ray binaries, the luminosities are more than an order of magnitude higher, suggesting at least middleweight black holes. However, low-luminosity active galactic nuclei should not be ruled out. In two of the three ellipticals, Hubble Space Telescope observations of circumnuclear disks indicate the presence of supermassive black holes of more than 108M. We may not be able to say much, it seems, from the X-ray flux alone.
The spirals discussed by Colbert & Mushotzky perhaps make for better candidates as hosts of middleweight black holes. Two of them, NCG 1313 and NGC 5408, have an extranuclear X-ray source that is roughly a kiloparsec away from the photometric center of the galaxy. These objects have X-ray fluxes which are also about an order of magnitude greater than expected from known X-ray binaries containing a black hole candidate. The lack of any other evidence for behavior typical of active galactic nuclei, plus the dynamical awkwardness of placing a supermassive black hole in the suburbs of otherwise normal galaxies lends some credence to the idea that the sources are middleweights.
What could be wrong with the middleweight interpretation? The Eddington luminosities suggest mass limits of roughly 10M, hence the sources might only be pushing the boundary of what we now consider normal black hole candidates of stellar origin (e.g., Fryer 1999). Other possibilities include superluminous sources from which Eddington limits are not useful, and X-ray supernovae which can remain at constant brightness for periods of years. However, sub-Eddington accretion onto a middleweight is reasonable, and with luminosities at a percent or even a tenth of the Eddington limit, the mass of the black holes in NCG 1313 and NGC 5408 becomes interesting.
A middleweight black hole sounds at first like a gift from the heavens for the earth based gravity wave detectors. The frequency of peak sensitivity for these detectors corresponds to strong field processes for holes of a hundred or so M (Flanagan & Hughes 1998). But middleweights need more than mere existence to be the most fascinating objects in the gravitational night sky. The middleweights must also take part in some process that generates strong waves. The inspiral and plunge of a stellar mass compact object (hole or neutron star) would produce waves of the right frequency, but relatively low amplitude. What would be ideal would be the merger of two middleweights, perhaps as a step toward the formation of a supermassive hole. Whether this is a plausible astrophysical scenario, or a gift that the heavens won't deliver, depends on what sort of processes produce the middleweights, and what astrophysical neighborhoods they inhabit. These questions will be studied in the coming year, but near term answers are likely to be very speculative.
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