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We present novel 3D multi-scale SPH simulations of gas-rich galaxy mergersbetween the most massive galaxies at $z \sim 8 - 10$, designed to scrutinizethe direct collapse formation scenario for massive black hole seeds proposed in\citet{mayer+10}. The simulations achieve a resolution of 0.1 pc, and includeboth metallicity-dependent optically-thin cooling and a model for thermalbalance at high optical depth. We consider different formulations of the SPHhydrodynamical equations, including thermal and metal diffusion. When the twomerging galaxy cores collide, gas infall produces a compact, optically thicknuclear disk with densities exceeding $10^{-10}$ g cm$^3$. The disk rapidlyaccretes higher angular momentum gas from its surroundings reaching $\sim 5$ pcand 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 turbulenceprevents fragmentation in the inner parsec region, which remains warm ($\sim3000-6000$ K) and develops strong non-axisymmetric modes that cause prominentradial gas inflows ($> 10^4$ $M_{\odot}$ yr$^{-1}$), forming an ultra-densemassive disky core. Angular momentum transport by non-axisymmetric modes shouldcontinue below our spatial resolution limit, quickly turning the disky coreinto a supermassive protostar which can collapse directly into a massive blackhole 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-zQSOs. Its telltale signature would be a burst of gravitational waves in thefrequency range $10^{-4} - 10^{-1}$ Hz, possibly detectable by the plannedeLISA interferometer.

Direct formation of supermassive black holes in metal-enriched gas at the heart of high-redshift galaxy mergers