Direct Formation of Supermassive Black Holes via Multi-Scale Gas Inflows in Galaxy Mergers

Abstract: Observations of distant bright quasars suggest that billion solar mass supermassive black holes (SMBHs) were already in place less than a billion years after the Big Bang. Models in which light black hole seeds form by the collapse of primordial metal-free stars cannot explain their rapid appearance due to inefficient gas accretion. Alternatively, these black holes may form by direct collapse of gas at the center of protogalaxies. However, this requires metal-free gas that does not cool efficiently and thus is not turned into stars, in contrast with the rapid metal enrichment of protogalaxies. Here we use a numerical simulation to show that mergers between massive protogalaxies naturally produce the required central gas accumulation with no need to suppress star formation. Merger-driven gas inflows produce an unstable, massive nuclear gas disk. Within the disk a second gas inflow accumulates more than 100 million solar masses of gas in a sub-parsec scale cloud in one hundred thousand years. The cloud undergoes gravitational collapse, which eventually leads to the formation of a massive black hole. The black hole can grow to a billion solar masses in less than a billion years by accreting gas from the surrounding disk.

Direct Formation of Supermassive Black Holes via Multi-Scale Gas Inflows in Galaxy Mergers

This paper investigates a viable pathway for the rapid formation of supermassive black holes (SMBHs) through direct collapse in the context of galaxy mergers, addressing the challenge of explaining the presence of billion solar mass SMBHs less than a billion years after the Big Bang. Past models have predominantly relied on the growth of light black hole seeds formed from Population III stars. However, such models face challenges due to low accretion rates constrained by ionized gas environments and radiative feedback.

The researchers use three-dimensional numerical simulations to examine the potential of galaxy mergers to induce conditions favorable for direct SMBH formation. The simulation involves two massive protogalaxies representing high-density peaks collapsing at redshifts of (z \sim 7 - 8), consistent with the observational data on high-redshift quasars. Mergers induce significant gas inflow to the nuclear region of these galaxies, resulting in the formation of a dense, turbulent nuclear disk. This process avoids the need for primordial conditions, such as metal-free gas that otherwise could inhibit significant star formation, a limitation in other direct SMBH formation scenarios.

Key findings from the simulations include:

• Gas inflows driven by tidal torques from mergers can cascade down from kiloparsec scales to the inner few parsecs at inflow rates exceeding 10 M(_\odot)/yr. This rapid inflow accumulates sufficient mass for gravitational collapse within about 100,000 years post-merger.
• The resulting dense gas cloud in the nuclear disk avoids fragmentation and star formation, transitioning instead into a Jeans-unstable state that precedes collapse into a massive black hole.
• The initial formation of a massive black hole seed, coupled with subsequent Eddington-limited accretion, supports scenarios in which the seed can grow to reach a billion solar masses within appropriate cosmological timescales, compatible with observed z ∼ 6 quasars.

The simulations address critical barriers to the direct collapse model by framing galaxy mergers as natural mechanisms for inducing the necessary rapid gas inflows against conventional fragmentation and star formation processes. The study’s resolution permits an examination of stability conditions within the nuclear disk, revealing that high thermal and turbulent pressures maintain a stable disk configuration and prevent the fragmentation that might otherwise result from such inflows.

This research could have far-reaching implications for our understanding of early SMBH formation mechanisms. It suggests that galaxy mergers might provide an essential channel for early black hole growth, bypassing the inefficiencies associated with stellar-seeded models. As observational technology advances, instruments like JWST and ALMA may further probe these high-redshift configurations, potentially validating theoretical predictions regarding the mass and dynamics of early black holes against their host galaxies.

Despite offering a robust alternative framework for SMBH formation, challenges remain—particularly concerning the detailed thermodynamics and feedback processes within collapsing clouds, which could influence the final mass of the resultant black hole. Future work might explore the detailed evolution of the conditions post-collapse, incorporating additional effects like star formation feedback and the impact of AGNs, lending further accuracy and realism to the simulation outcomes.