Direct collapse of exceptionally heavy black holes in the merger-driven scenario

TL;DR

This paper proposes a merger-driven scenario for the rapid formation of supermassive black holes in the early universe, involving hydrostatic cores in supermassive discs collapsing into black holes within about half a million years, producing observable transients.

Contribution

It introduces a new pathway for supermassive black hole formation that does not depend on specific thermal or angular momentum assumptions, explaining early quasars.

Findings

Black hole masses range from 10^6 to 10^8 solar masses.

Collapse occurs within ~5×10^5 years of SMD formation.

Transient signals include electromagnetic, neutrino, and gravitational waves.

Abstract

We revisit the conditions present in supermassive discs (SMDs) formed by the merger of gas-rich, metal-enriched galaxies at red-shift $z\sim 10$. We find that SMDs naturally form hydrostatic cores which go through a rapidly accreting supermassive star phase, before directly collapsing into massive black holes via the general relativistic instability. The growth and collapse of the cores occurs within $\sim 5\times 10^5$ yr from the formation of the SMD, producing bright electromagnetic, neutrino and gravitational wave transients with a typical duration of a few minutes and, respectively, a typical flux and a typical strain amplitude at Earth of $\sim 10^{-8}$ erg s$^{-1}$ cm$^{-2}$ and $\sim4\times 10^{-21}$. We provide a simple fitting formula for the the resulting black hole masses, which range from a few $10^6$ M$_{\odot}$ to $10^8$ M$_{\odot}$ depending on the initial SMD configuration. Crucially, our analysis does not require any specific assumption on the thermal properties of the gas, nor on the angular momentum loss mechanisms within the SMD. Led by these findings, we argue that the merger-driven scenario provides a robust pathway for the rapid formation of supermassive black holes at $z > 6$. It provides an explanation for the origin of the brightest and oldest quasars without the need of a sustained growth phase from a much smaller seed. Its smoking gun signatures can be tested directly via multi-messenger observations.