Wandering black holes

A ROMULUS simulation image showing an intermediate mass galaxie, its bright central region and its supermassive dark hole. The locations (and velocities of) "wandering supermassive" black holes are not confined to the nucleus. The 10kpc marker corresponds roughly to a distance approximately 31 thousand light-years. Simulations have examined the abundances and evolution of wandering supermassive dark holes. They were the largest source of mass in the early universe. Credit: Ricarte et al, 2021
A supermassive dark hole (SMBH), is thought to be a central feature of every massive galaxy. Its mass correlates with the mass in the host's inner regions. This is likely because the SMBH evolves and grows with the galaxy, through mergers and the inflow of material from intergalactic media. Material that reaches the galactic center and accretes onto SMBH creates an active galactic nucleus. Outflows from the AGN can disrupt star formation. Modern cosmological simulations can now consistently trace star formation and SMBH expansion in galaxies, from the beginning of the universe to the present. This confirms these ideas.

Naturally, some SMBHs are slightly off the center of an enlarged galaxy as a result of merger. It is not easy to combine multiple SMBHs. Sometimes, a binary SMBH may be formed first and then slowly merge into one. This process can produce detectable gravitational waves emission. However the merger can sometimes stall or be disruptedunderstanding why is one of the key puzzles in SMBH evolution. New cosmological simulations using the ROMULUS code show that some SMBHs, even after billions of years of evolutionary progress, do not join the nucleus and instead wander through the galaxy.

Angelo Ricarte, CfA Astronomer, led a group of colleagues to characterize wandering black holes. The ROMULUS simulations revealed that about ten per cent of black hole mass might be found in wanderers in the universe today (i.e., 13.7 billion years after big bang). These wanderers are more important and have most of the black hole mass at earlier times in the universe (two billion years after or younger). The scientists found that the majority of emission from the SMBH population is produced by wanderers in these early epochs. The astronomers also examine the observational signatures produced by the wandering SMBH populations in a related paper.

The research was published by Monthly Notices of the Royal Astronomical Society.

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More information: Angelo Ricarte and al, Origins, demographics, and wandering black holes. Monthly Notices of the Royal Astronomical Society (2021). Information for the Journal: Monthly Notices from the Royal Astronomical Society Angelo Ricarte and others, Origins & demographics of wandering Black Holes, (2021). DOI: 10.1093/mnras/stab866