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The route to massive black hole formation via merger-driven direct collapse: a review


Mayer, Lucio; Bonoli, Silvia (2019). The route to massive black hole formation via merger-driven direct collapse: a review. Reports on Progress in Physics, 82(1):016901.

Abstract

The direct collapse model for the formation of massive black holes has gained increased support as it provides a natural explanation for the appearance of bright quasars already less than a billion years from the Big Bang. In this paper we review a recent scenario for direct collapse that relies on multi-scale gas inflows initiated by the major merger of massive gas-rich galaxies at z  >  6, where gas has already achieved solar composition. Hydrodynamical simulations undertaken to explore our scenario show that supermassive, gravitationally bound compact gaseous disks weighing a billion solar masses, only a few pc in size, form in the nuclei of merger remnants in less than 105 yr. These could later produce a supermassive protostar or supermassive star at their center via various mechanisms. Moreover, we present a new analytical model, based on angular momentum transport in mass-loaded gravitoturbulent disks. This naturally predicts that a nuclear disk accreting at rates exceeding yr−1, as seen in the simulations, is stable against fragmentation irrespective of its metallicity. This is at variance with conventional direct collapse scenarios, which require the suppression of gas cooling in metal-free protogalaxies for gas collapse to take place. Such high accretion rates reflect the high free-fall velocities in massive halos appearing only at z  <  10, and occur naturally as a result of the efficient angular momentum loss provided by the merger dynamics. We discuss the implications of our scenario on the observed population of high-z quasars and on its evolution to lower redshifts using a semi-analytical galaxy formation model. Finally, we consider the intriguing possibility that the secondary gas inflows in the unstable disks might drive gas to collapse into a supermassive black hole directly via the General Relativistic radial instability. Such dark collapse route could generate gravitational wave emission detectable via the future Laser Interferometer Space Antenna (LISA).

Abstract

The direct collapse model for the formation of massive black holes has gained increased support as it provides a natural explanation for the appearance of bright quasars already less than a billion years from the Big Bang. In this paper we review a recent scenario for direct collapse that relies on multi-scale gas inflows initiated by the major merger of massive gas-rich galaxies at z  >  6, where gas has already achieved solar composition. Hydrodynamical simulations undertaken to explore our scenario show that supermassive, gravitationally bound compact gaseous disks weighing a billion solar masses, only a few pc in size, form in the nuclei of merger remnants in less than 105 yr. These could later produce a supermassive protostar or supermassive star at their center via various mechanisms. Moreover, we present a new analytical model, based on angular momentum transport in mass-loaded gravitoturbulent disks. This naturally predicts that a nuclear disk accreting at rates exceeding yr−1, as seen in the simulations, is stable against fragmentation irrespective of its metallicity. This is at variance with conventional direct collapse scenarios, which require the suppression of gas cooling in metal-free protogalaxies for gas collapse to take place. Such high accretion rates reflect the high free-fall velocities in massive halos appearing only at z  <  10, and occur naturally as a result of the efficient angular momentum loss provided by the merger dynamics. We discuss the implications of our scenario on the observed population of high-z quasars and on its evolution to lower redshifts using a semi-analytical galaxy formation model. Finally, we consider the intriguing possibility that the secondary gas inflows in the unstable disks might drive gas to collapse into a supermassive black hole directly via the General Relativistic radial instability. Such dark collapse route could generate gravitational wave emission detectable via the future Laser Interferometer Space Antenna (LISA).

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Additional indexing

Item Type:Journal Article, refereed, further contribution
Communities & Collections:07 Faculty of Science > Institute for Computational Science
Dewey Decimal Classification:530 Physics
Scopus Subject Areas:Physical Sciences > General Physics and Astronomy
Uncontrolled Keywords:General Physics and Astronomy
Language:English
Date:1 January 2019
Deposited On:15 Mar 2019 07:45
Last Modified:29 Jul 2020 09:57
Publisher:IOP Publishing
ISSN:0034-4885
OA Status:Closed
Publisher DOI:https://doi.org/10.1088/1361-6633/aad6a5

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