Early Natural Selection

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Science  17 Nov 2006:
Vol. 314, Issue 5802, pp. 1051
DOI: 10.1126/science.314.5802.1051b

The bright light emitted by quasars is powered by the infall of gas toward giant black holes in galactic cores. The first quasars are known to have had central black holes that comprised a billion solar masses within a region the size of a solar system. Because assembling such a massive black hole should take billions of years, astronomers have had difficulty explaining the presence of quasars in the early (billion-year-old) universe. Volonteri and Rees have modeled the growth of the first supermassive black holes, including the competing effects of gas accretion and the dynamics of black hole mergers in their calculations. Black holes may grow by accreting gas from their surroundings, but during the most efficient accretion periods, growth is slowed by increased radiation of energy. Mergers with small companion galaxies also contribute to growth but can be destructive as well. Coalescence may be prevented if merging black holes are expelled from the galaxy by recoil from asymmetric gravitational waves and multiple-body dynamics. The authors thus frame a Darwinian natural selection scenario for black hole growth in the very young universe. Rapid and efficient growth would proceed in the highest-density regions, where gas accretion was optimal and gentle mergers outcompeted recoil losses. — JB

Astrophys. J. 650, 669 (2006).

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