DIRECT FORMATION OF SUPERMASSIVE BLACK HOLES IN METAL-ENRICHED GAS AT THE HEART OF HIGH-REDSHIFT GALAXY MERGERS
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abstract
We present novel 3D multi-scale SPH simulations of gas-rich galaxy mergers
between the most massive galaxies at $z \sim 8 - 10$, designed to scrutinize
the direct collapse formation scenario for massive black hole seeds proposed in
\citet{mayer+10}. The simulations achieve a resolution of 0.1 pc, and include
both metallicity-dependent optically-thin cooling and a model for thermal
balance at high optical depth. We consider different formulations of the SPH
hydrodynamical equations, including thermal and metal diffusion. When the two
merging galaxy cores collide, gas infall produces a compact, optically thick
nuclear disk with densities exceeding $10^{-10}$ g cm$^3$. The disk rapidly
accretes higher angular momentum gas from its surroundings reaching $\sim 5$ pc
and a mass of $\gtrsim 10^9$ $M_{\odot}$ in only a few $10^4$ yr. Outside
$\gtrsim 2$ pc it fragments into massive clumps. Instead, supersonic turbulence
prevents fragmentation in the inner parsec region, which remains warm ($\sim
3000-6000$ K) and develops strong non-axisymmetric modes that cause prominent
radial gas inflows ($> 10^4$ $M_{\odot}$ yr$^{-1}$), forming an ultra-dense
massive disky core. Angular momentum transport by non-axisymmetric modes should
continue below our spatial resolution limit, quickly turning the disky core
into a supermassive protostar which can collapse directly into a massive black
hole of mass $10^8-10^9$ $M_{\odot}$ via the relativistic radial instability.
Such a "cold direct collapse"' explains naturally the early emergence of high-z
QSOs. Its telltale signature would be a burst of gravitational waves in the
frequency range $10^{-4} - 10^{-1}$ Hz, possibly detectable by the planned
eLISA interferometer.
