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Black holes in every galaxy?


(January 1997)


A Hubble Space telescope census reveals that black holes are common in galaxies, according to a January 13 release on the Internet. Three black holes have been identified in three normal galaxies, and the team responsible suggests that nearly all galaxies may harbour supermassive black holes which once powered quasars which are now no longer active.


They took a census of 27 nearby galaxies with NASA's Hubble Space telescope and the ground-based Canada-France-Hawaii telescope (CFHT) on Mauna Kea, 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 key results are that:


Supermassive black holes are so common that nearly every large galaxy has one.


A black hole's mass is proportional to the mass of the host galaxy, so 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.


Two of the black holes weigh 50 million and 100 million solar masses, and they lie in the cores of galaxies NGC 3379 (also known as M105) and NGC 3377 respectively. These galaxies are both in the "Leo Spur," a nearby group of galaxies about 32 million light-years away and roughly in the direction of the Virgo cluster. Some 50 million light-years away, also 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.


These new results suggest that smaller galaxies probably 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. Now cosmologists will need to work on explaining why the black holes are so common, and why they seem to be proportional to the masses of the home galaxies.


The Hubble telescope's high resolution allowed the team to measure the 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 February 1997 servicing mission to the Hubble telescope will involve installing 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.


And in yet another galaxy . . .


The nucleus of the spiral galaxy NGC 1068 has always been obscured from direct observation by gas and dust. But radio images now suggest that it conceals a black hole of 10 to 20 million solar masses, and that the gas around it is swirling into the hole so rapidly that the nucleus is radiating at close to the theoretical limit, according to a report in Nature in early January, by Mitchell C. Begelman and Joss Bland-Hawthorn. Black holes, it seems, are all the go .