Scientists Propose the Existence of Errant Black Holes

Supermassive black hole: not necessarily an inhabitant of the galactic center. Credit: ESO/L. Sidewalk

In general, one thinks about black holes inhabiting the center of galaxies, but a new study by American scientists throws another possibility for them: wandering through space. The study in this regard was published in the magazine Monthly Notices of the Royal Astronomical Society.

Every massive galaxy is believed to host a super massive black hole (SMBH) in its center. Its mass is correlated with the mass of its host’s internal regions (and some other properties as well), probably because the SMBH grows and evolves as the galaxy itself grows, through mergers with other galaxies and the fall of material from the intergalactic medium. When material travels to the galactic center and accumulates in the SMBH, it produces an active galactic core. Outflows or other feedback from the active galactic core act disruptively to quench star formation in the galaxy.

Modern cosmological simulations now self-consistently trace the formation of stars and the growth of SMBHs in galaxies from the beginning of the universe to the present day, confirming these ideas.

Romulus computer simulation image showing an intermediate-mass galaxy, its bright central region with its supermassive black hole, and the locations (and velocities) of “errant” supermassive black holes (those not confined to the core; the 10kpc marker corresponds to a distance of about 31 thousand light years). Simulations studied the evolution and abundance of wandering supermassive black holes; at the beginning of the universe, they contain most of the mass of black holes. Credit: Ricarte et al., 2021
journey through the galaxy

The fusion process naturally results in some SMBHs that are slightly displaced from the center of the enlarged galaxy. The path to a single combined SMBH is complex. Sometimes a binary SMBH is formed first, and then the two black holes gradually merge into one.

The emission of detectable gravitational waves ​​can be produced in this process. However, the fusion can sometimes stop or be interrupted. Understanding why this occurs is one of the main puzzles in the evolution of SMBH.

New cosmological simulations with Romulus code [simulações cosmológicas em grande escala com resolução equivalente às simulações de maior resolução desse tipo executadas até hoje] predict that, even after billions of years of evolution, some SMBHs did not join the nucleus but ended up wandering the galaxy.

Characterized objects

Angelo Ricarte, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA, USA), led a team of colleagues who characterized these wandering black holes. Using Romulus simulations, the team found that in today’s universe (ie, about 13.7 billion years after the big Bang), about 10% of the mass in black holes may be in errant black holes. In earlier times in the universe, 2 billion years after the Big Bang or younger, these wanderers seemed to be even more significant and contained most of the mass of black holes. In fact, scientists found that, in those early times, wanderers also produced most of the emissions from the population of SMBHs.

In a related article, astronomers explore the observational signatures of the wandering SMBH population.

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