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

TL;DR

This review discusses a merger-driven scenario for massive black hole seed formation at high redshift, involving rapid gas inflows, stable supermassive disks, and potential direct collapse into black holes with observable gravitational waves.

Contribution

It introduces a new analytical model and hydrodynamical simulations supporting a merger-driven, metal-enriched gas inflow pathway for black hole seed formation, contrasting with traditional metal-free collapse models.

Findings

Supermassive, bound gaseous disks form within 10^5 years post-merger.

High accretion rates (>1000 M_sun/yr) are stable against fragmentation regardless of metallicity.

The scenario explains high-z quasar populations and predicts gravitational wave signals from direct collapse.

Abstract

In this paper we review a new scenario for the formation of massive black hole seeds that relies on multi-scale gas inflows initiated by the 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 gaseous disks, weighing a billion solar masses and of a few pc in size, form in the nuclei of merger remnants in less than $10^5$ 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 $1000 M_{\odot}$/yr, 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 at $z < 10$, and occur naturally as a result of the efficient angular momentum loss provided by mergers. 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 {\it dark collapse} route could generate gravitational wave emission detectable via the future Laser Interferometer Space Antenna (LISA). [Abridged]