Doing a census of the known quasars and other active galaxies at high redshifts indicates that the density of their remnants today is such that an average galaxy should contain a central black hole of about ten million solar masses. Simply by adding up the known quasars out to redshift 5 or so, and making some reasonable assumption about how long quasar episodes may last (about 100 million years), astronomers find that to accomodate their numbers, nearly every major galaxy contained a quasar or another type of active nucleus at some time in the past. Because quasars produce energy by accreting matter onto their central black holes, astronomers can work from the total amount of energy accreted to estimate how much mass was gobbled up by black holes. On average, it comes to about 10 million solar masses per galaxy. Some quasars may have no black hole; others may have a hundred million solar mass black hole; depending on their formation history. Some galaxies may undergo repeated episodes of quasarlike activity, depending on their feeding habits: A late, gas-rich merger could well wake up a dormant monster. This would account for the quasars seen at low redshifts. They would be rejuvenated, rather than young.
As for our own galaxy, arguments still range, but it is possible that a million solar-mass black hole lurks in its center. We may find out some day, if a nice gaseous morsel falls in it. If this happens, stock up on really good sunscreen lotion and consider moving to Alaska! Our very own quasar, which would be in the southern constellation of Sagittarius, would probably generate a lot of intense ultraviolet, X-ray, and gamma-ray radiation, in addition to being great (if brief) fun for astronomers. In our sky, such a quasar would appear as an intense, blue-white star in Sagittarius probably outshining Venus at its brightest.
From "Fires at Cosmic Dawn by S. George Djorgovski , Astronomy, September, 1995
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