Direct formation of massive black holes via dynamical collapse in metal-enriched merging galaxies at $z \sim 10$: fully cosmological simulations

TL;DR

This study uses high-resolution cosmological simulations to demonstrate that galaxy mergers at high redshift can directly form supermassive black holes, bypassing seed stages, explaining early quasars.

Contribution

First fully cosmological hydrodynamical simulation showing direct collapse of massive black holes during galaxy mergers at z~8-10.

Findings

Major mergers produce nuclear supermassive disks within 1 million years.

Disks reach 3 x 10^8 solar masses with turbulent, multi-phase ISM.

Disks become unstable and collapse into supermassive black holes.

Abstract

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 $z \sim 8$ results in the formation of a nuclear supermassive disk (SMD) of only $4$ pc in radius, owing to a prodigious gas inflow sustained at $100$-$1000$ $M_{\odot}$ yr$^{-1}$. The core of the merger remnant is metal-rich, well above solar abundance, and the SMD reaches a gaseous mass of $3 \times 10^8$ $M_{\odot}$ 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 multi-phase 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 falls 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 $10^6$-$10^8$ $M_{\odot}$, essentially skipping the stage of BH seed formation. These results confirm that mergers between the most massive galaxies at $z \sim 8$-$10$ can naturally explain the rapid emergence of bright high-redshift quasars.