Ultramassive black holes formed by triple quasar mergers at $z\sim 2$

TL;DR

This paper uses cosmological simulations to demonstrate that ultramassive black holes can form from successive galaxy mergers, often associated with triple quasar systems, and significantly influence their host galaxies at high redshift.

Contribution

It provides the first detailed simulation-based evidence that triple quasar mergers can lead to the formation of ultramassive black holes at z~2.

Findings

UMBH with MBH ~ 1e11 Msun formed at z~2

Triple quasar systems are linked to UMBH formation

Major galaxy mergers can produce multiple UMBHs at high redshift

Abstract

The origin of rare and elusive ultramassive black holes (UMBH, with MBH > 1e10 Msun) is an open question. Using the large volume cosmological hydrodynamic simulation ASTRID, we report on the formation of an extremely massive UMBH with MBH ~ 1e11 Msun at z~2. The UMBH is assembled as a result of two successive mergers of massive galaxies each with stellar mass M* > 3e11 Msun that also produces a bright, rare triple quasar system powered by three ~10^9 Msun black holes. The second merger of supermassive black holes (SMBHs) follows the first after 150 Myrs. The merger events lead to sustained Eddington accretion onto the central SMBH, forming an UMBH in the center of a massive compact stellar core with M* > 2e12 Msun. The strong feedback of the UMBH quenches the surrounding star formation to < 10 Msun/yr in the inner 50 kpc/h region. There are two more UMBHs with MBH > 5e10 Msun at z>2 in ASTRID which are also produced by major mergers of galaxies, and their progenitors can be observed as quasar triplets of lower luminosity. The rarely observed quasar multiples can be the cradle of UMBHs at high redshift, and likely end up in the center of the most massive clusters.