Rapid growth of seed black holes in the early universe by supra-exponential accretion

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Science  12 Sep 2014:
Vol. 345, Issue 6202, pp. 1330-1333
DOI: 10.1126/science.1251053

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Sheltered black hole seeds grow faster

Astrophysicists have struggled to find a way to grow supermassive black holes within the first billion years of the universe, in the first 6% of its current age. These objects power the extremely bright and distant quasars whose light from that era has finally reached us. To grow a black hole so massive so quickly seemed to violate the expected balance of both radiation pressure and angular momentum. Alexander and Natarajan report that the predicted accretion rate can be exceeded with the help of a protective gas cloud at the heart of a star cluster. There, the radiation from accretion is reverted back onto the black hole, enabling a phase of supraexponential growth for a brief 10 million years.

Science, this issue p. 1330


Mass accretion by black holes (BHs) is typically capped at the Eddington rate, when radiation's push balances gravity's pull. However, even exponential growth at the Eddington-limited e-folding time tE ~ few × 0.01 billion years is too slow to grow stellar-mass BH seeds into the supermassive luminous quasars that are observed when the universe is 1 billion years old. We propose a dynamical mechanism that can trigger supra-exponential accretion in the early universe, when a BH seed is bound in a star cluster fed by the ubiquitous dense cold gas flows. The high gas opacity traps the accretion radiation, while the low-mass BH's random motions suppress the formation of a slowly draining accretion disk. Supra-exponential growth can thus explain the puzzling emergence of supermassive BHs that power luminous quasars so soon after the Big Bang.

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