Black holes in the newly formed massive galaxies began to grow rapidly, and in just a few hundred million years (recall that the age of the universe is approximately 13.8 billion years), they reached masses of approximately 50 billion, 65 billion, and 100 billion solar masses, after which their growth significantly slowed down.
“What we found are three ultramassive black holes that assembled their mass during the cosmic noon, the time 11 billion years ago when star formation, active galactic nuclei, and supermassive black holes in general reach their peak activity,” said Ni.
The simulation result agrees very well with observations, since the largest known black holes have masses of about 40 billion to 65 billion solar masses. Moreover, not only the masses of ultramassive black holes, but also the structures and luminosities of the galaxies hosting them, match observations almost perfectly, making the study even more reliable.
Another interesting feature of the simulation is that the masses of the ultramassive black holes turned out to be very close to the theoretical maximum, after which the black hole should nearly cease absorbing matter from the accretion disk surrounding it. This further confirms both the accuracy of the computer simulations and the correctness of our theoretical understanding of how black holes interact with matter.
Finding ultramassive black holes in the future
Only a few ultramassive black holes have been discovered to date, so further observations are needed to test the accuracy of this formation model.
Fortunately, there are many existing and planned telescopes, such as the James Webb Space Telescope (JWST), and gravitational-wave detectors, such as LIGO and VIRGO, that should help researchers detect more black holes and better understand their properties. (LIGO and VIRGO can currently only detect mergers of smaller stellar-mass black holes, as these detectors are not capable of detecting gravitational waves from mergers of supermassive or ultramassive black holes.)
“In addition, the future space-based Laser Interferometer Space Antenna (LISA) gravitational-wave observatory will give us a much better understanding the how these massive black holes merge and/or coalescence, along with the hierarchical structure, formation, and the galaxy mergers along the cosmic history,” said Ni. “This is an exciting time for astrophysicists, and it’s good that we can have simulation to allow theoretical predictions for those observations.”
Moreover, Ni’s research group is planning to use these observatories and the Astrid simulations to study not only ultramassive black holes, but also the properties of active galactic nuclei (AGN) — compact, ultrabright regions thought to be powered by supermassive black holes — and the galaxies that host them.
“They are a very important science target for JWST, determining the morphology of the active galactic nucleus host galaxies and how they are different compared to the broad population of the galaxy during cosmic noon,” said Ni.
Reference: Y. Ni, T. Di Matteo, N. Chen, R. Croft, and S. Bird, “Ultramassive Black Holes Formed by Triple Quasar Mergers at z ∼ 2,” The Astrophysical Journal Letters (2022), DOI: 10.3847/2041-8213/aca160.