Journal Article Early cosmic merger of multiple black holes

Tagawa, H.  ,  Umemura, M.  ,  Gouda, N.  ,  Yano, T.  ,  Yamai, Y.

451 ( 2 )  , pp.2174 - 2184 , 2015-08 , Oxford University Press
We perform numerical simulations on the merger of multiple black holes (BHs) in primordial gas at early cosmic epochs. We consider two cases of BH mass: MBH = 30 and 104 M⊙. Attention is concentrated on the effect of the dynamical friction by gas in a host object. The simulations incorporate such general relativistic effects as the pericentre shift and gravitational wave emission. As a result, we find that multiple BHs are able to merge into one BH within 100 Myr in a wide range of BH density. The merger mechanism is revealed to be categorized into three types: gas-drag-driven merger (type A), interplay-driven merger (type B), and three-body-driven merger (type C). We find the relation between the merger mechanism and the ratio of the gas mass within the initial BH orbit (Mgas) to the total BH mass (∑MBH). Type A merger occurs if Mgas ≳ 105∑MBH, type B if Mgas ≲ 105∑MBH, and type C if Mgas ≪ 105∑MBH. Supposing the gas and BH density based on the recent numerical simulations on first stars, all the BH remnants from first stars are likely to merge into one BH in ∼ 107 yr through the type B or C mechanism. Also, we find that multiple massive BHs (MBH = 104 M⊙) distributed over several parsec can merge into one BH through the type B mechanism, if the gas density is higher than 5 × 106 cm−3. The present results imply that the BH merger may contribute significantly to the formation of supermassive BHs at high-redshift epochs.

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