Direct Formation of Massive Black Holes via Dynamical Collapse in Metal-enriched Merging Galaxies at z 10: Fully Cosmological Simulations

We present the results of the first fully cosmological hydrodynamical simulations studying the merger-driven model for massive black hole (BH) seed formation via direct collapse. Using the zoom-in technique as well as particle splitting, we achieve a final spatial resolution of 2 pc. We show that the major merger of two massive galaxies at redshift z similar to 8 results in the formation of a nuclear supermassive disk (SMD) of only 4 pc in radius, owing to a prodigious gas inflow sustained at 100-1000 M-circle dot yr(-1). The core of the merger remnant is metal-rich, well above solar abundance, and the SMD reaches a gaseous mass of 3 x 10(8)M(circle dot) in less than a million years after the merger, despite a concurrent prominent nuclear starburst. Dynamical heating as gas falls into the deepest part of the potential well, and heating and stirring by supernova blastwaves, generate a turbulent multiphase interstellar medium, with a gas velocity dispersion exceeding 100 km s(-1). As a result, only moderate fragmentation occurs in the inner 10-20 pc, despite the temperature falling below 1000 K. The SMD is Jeans-unstable as well as bar-unstable and will collapse further adiabatically, becoming warm and ionized. We show that the SMD, following inevitable contraction, will become general-relativistic-unstable and directly form a supermassive BH of mass in the range 10(6)-10(8)M(circle dot), essentially skipping the stage of BH seed formation. These results confirm that mergers between the most massive galaxies at z similar to 8-10 can naturally explain the rapid emergence of bright high-redshift quasars.