Scientists from the Centre for Astrophysics characterized wandering black holes using the ROMULUS simulations and identified that in the early universe, these black holes contained most of the mass.
Each massive galaxy is presumed to host a supermassive black hole (SMBH) at its center. Its mass is related to the mass of the inner regions of its host (and also with certain other properties), presumably because the SMBH grows and evolves as the galaxy itself grows, via mergers with other galaxies and the infall of material from the intergalactic medium.
While material continues its path to the galactic center and accretes onto the SMBH, it creates an active galactic nucleus (AGN); outflows or other feedback from the AGN then act disruptively to quench star formation in the galaxy. At present, advanced cosmological simulations self-consistently track star formation and SMBH growth in galaxies from the early universe to the present time, substantiating these ideas.
The merger process normally results in certain SMBHs that are somewhat offset from the center of the enlarged galaxy. The path to a single, combined SMBH is complex. Occasionally, a binary SMBH is first developed, which then slowly merges into one.
Noticeable gravitational wave emission can be produced through this process. At times, the merger can stop or be disrupted — insight on why it happens is one of the major puzzles in SMBH evolution. New cosmological simulations with the ROMULUS code foretell that even after billions of years of evolution, certain SMBHs do not combine with the nucleus; instead, they end up wandering through the galaxy.
Centre for Astrophysics (CfA) astronomer Angelo Ricarte lead a group of co-workers characterizing such wandering black holes. By employing the ROMULUS simulations, the researchers observed that in today’s universe (that is, about 13.7 billion years after the big bang), around 10% of the mass in black holes may be in wanderers.
In the earlier universe, two billion years after the big bang or younger, these wanderers appear to be even more important and comprise most of the mass in black holes.
The researchers, indeed, observed that in these early epochs the wanderers also produced most of the emissions coming from the SMBH population. In similar research work, the astronomers investigated the observational signatures of the wandering SMBH population.
Ricarte, A., et al. (2021) Origins and demographics of wandering black holes. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/stab866.