Teach Astronomy – Black Holes in the Universe

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Statistical studies of quasars can tell us about the abundance of supermassive black holes in the universe. Surveys like the Sloan Digital Sky Survey have identified a hundred thousand quasars by their spectra and a million quasars by their colors. About one in a thousand galaxies harbors a quasar. So either one in a thousand galaxies is special in some way and has a unique history, or every galaxy spends a thousandth of its time in the quasar phase of activity. Arguing against the latter viewpoint is the fact that supermassive black holes, once created, cannot be destroyed, and it’s difficult to starve them entirely. So the likely explanation is that one in a thousand galaxies has a billion solar mass black hole. One in a hundred has a hundred million solar mass black hole. One in ten has a ten million solar mass black hole, and essentially every galaxy has a million solar mass black hole. This hypothesis is consistent with the evidence of a supermassive black hole in the Milky Way and of the abundance of black holes in nearby galaxies.