New research led by astronomers from the Harvard & Smithsonian’s Center for Astrophysics shows that the current census of supermassive black holes is incomplete and that a substantial population of off-center wanderers likely exists.
A ‘wandering’ black hole in the lenticular galaxy GJ1417+52. The main panel has a wide-field, optical light image from the NASA/ESA Hubble Space Telescope. The black hole and its host galaxy are located within the box in the upper left. The inset on the left contains Hubble’s close-up view of GJ1417+52. Within this inset the circle shows a point-like source on the northern outskirts of the galaxy that may be associated with XJ1417+52. The inset on the right is an X-ray image of XJ1417+52 from NASA’s Chandra X-ray Observatory in purple, covering the same region as the Hubble close-up. Image credit: NASA / CXC / UNH / Lin et al. / STScI.
Supermassive black holes are thought to exist at the center of most large galaxies, including the center of our own Milky Way Galaxy.
Their masses correlate with properties of the inner regions of their host galaxies, probably because the black holes grow and evolve as galaxy themselves grow, through mergers with other galaxies and the infall of material from the intergalactic medium.
When material makes its way to the galactic center and accretes onto the supermassive black hole, it produces an active galactic nucleus.
Outflows or other feedback from the active galactic nucleus then act disruptively to quench star formation in the galaxy.
Modern cosmological simulations now self-consistently trace star formation and supermassive black hole growth in galaxies from the early Universe to the present day, confirming these ideas.
The merger process naturally results in some supermassive black holes that are slightly offset from the center of the enlarged galaxy.
The path to a single, combined supermassive black hole is complex. Sometimes a binary supermassive black hole is first formed which then gradually merges into one. Detectable gravitational wave emission can be produced in this process.
However, the merger can sometimes stall or be disrupted — understanding why is one of the key puzzles in the evolution of supermassive black holes.
According to the new research, even after a billions of years of evolution some supermassive black holes do not join the nucleus but end up instead wandering through the galaxy.
“We characterize the population of wandering black holes, defined as those physically offset from their halo centers, in the ROMULUS cosmological simulations,” said Dr. Angelo Ricarte from the Harvard & Smithsonian’s Center for Astrophysics and the Black Hole Initiative at Harvard University and colleagues.
“Unlike most other currently available cosmological simulations, black holes are seeded based on local gas properties and are permitted to evolve dynamically without being fixed at halo centers.”
“Tracking these black holes allows us to make robust predictions about the offset population.”
Using the ROMULUS cosmological simulations, the astronomers found that in today’s Universe about 10% of the mass in black holes might be in wanderers.
At earlier times in the Universe, two billion years after the Big Bang or younger, these wanderers appear to be even more significant and contain most of the mass in black holes.
Indeed, the researchers found that in these early epochs, the wanderers also produce most of the emission coming from the supermassive black hole population.
“Most wandering black holes, we find, remain close to the seed mass and originate from the centers of previously disrupted satellite galaxies,” they said.
“While most do not retain a resolved stellar counterpart, those that do are situated farther out at larger fractions of the virial radius.”
“Wanderers with higher luminosities are preferentially at lower radius, more massive, and either closer to their host’s mid-planes or associated with a stellar overdensity.”
“In future work, we will explore the observational signatures of the wandering population in greater detail.”
Their paper was published in the Monthly Notices of the Royal Astronomical Society.
Angelo Ricarte et al. 2021. Origins and demographics of wandering black holes. MNRAS 503 (4): 6098-6111; doi: 10.1093/mnras/stab866