Black holes are the most massive objects we know of in the universe. Not stellar mass black holes, not supermassive black holes (SMBHs,) but ultramassive black holes (UMBHs.) UMBHs sit at the center of galaxies like SMBHs, but they have more than five billion solar masses, an astonishing amount of mass. The largest black hole we know of is Phoenix A, a UMBH with up to 100 billion solar masses.
How can something grow so massively?
UMBHs are rare and elusive, and their origins are unclear. A team of astrophysicists working on the question used a simulation to uncover the formation of these massive objects. They traced UMBH’s origins back to the universe’s ‘cosmic dinner’ some 10 to 11 billion years ago.
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Their paper is “Ultramassive Black Holes Formed by Triple Quasar Mergers at z = 2,” and it’s published in The Astrophysical Journal Letters. The lead author is Yueying Ni, a postdoctoral fellow at the Center for Astrophysics/Harvard & Smithsonian.
“We found that a possible formation channel for ultramassive black holes is from the extreme merger of massive galaxies that is most likely to occur in the epoch of the ‘cosmic dinner,'” Ni said.
UMBHs are extremely rare. Creating them in scientific simulations requires a massive, complex simulation. This is where Astrid comes in. It is a large-scale cosmological hydrodynamic simulator running on the Frontera supercomputer at the University of Texas, Austin. Astrid’s large-scale simulations can track things like dark matter, temperature, metallicity and neutral hydrogen. Simulations like Astrid are ranked by the number of particles the simulations contain, and Astrid is at the top of that list.
“The science goal of Astrid is to study galaxy formation, the merging of supermassive black holes and re-ionization over cosmic history,” lead author Ni said in a press release. (Ni is a co-developer of Astrid.) A powerful tool like Astrid needs a powerful supercomputer. Fortunately, UT Austin has the most powerful academic supercomputer in the United States. “Frontera is the only system we performed Astrid from day one. It’s a pure Frontera-based simulation,” she explained.
Astronomers know that galaxies grow large through mergers, and it is likely that SMBHs grow more massive at the same time. But UMBHs are even more massive and much rarer. How are they formed?
The team’s work with Astrid provided answers.
“What we found are three ultramassive black holes that accumulated their mass during the cosmic dinner, the time 11 billion years ago when star formation, active galactic nuclei (AGN) and supermassive black holes, in general, reach their peak activity,” Ni said.
Cosmic noon is an important time period in the history of the universe. Astronomers believe that half of all stars were born during the period. It corresponds to redshift z=2 to z=3, or when the universe was about 2 to 3 billion years old. At that time, large amounts of gas from the intergalactic medium flowed into galaxies. Galaxies formed about half of their stellar mass during cosmic noon. So it’s no surprise that, as Ni says, they found three UMBHs gathering their mass during cosmic midday.
“In this epoch, we saw an extreme and relatively rapid merger of three massive galaxies,” Ni said. “Each of the galaxy masses is 10 times the mass of our own Milky Way, and a supermassive black hole sits at the center of each galaxy. Our findings raise the possibility that these quasar triplet systems are the progenitors of the rare ultramassive black holes after these triplets gravitationally interact and merge together.”
Quasar’s name is misleading. It means a quasi-stellar object, but the name dates from a time before astronomers knew what they were. Quasars are a subset of active galactic nuclei, but are extremely luminous. The luminosity comes from all the material falling into the SMBH at the center of a galaxy. The chances of triple quasar systems merging to form UMBHs are decreasing, according to the simulation.
Astrophysicists have determined a theoretical upper mass limit for black holes at around 50 billion solar masses, and the post-merger UMBH is approaching that limit. But the researchers caution that the Astrid simulation “…is not a prescription for a new upper limit on the mass of black holes.” That’s because simulations, even one as powerful as Astrid, cannot resolve the details of physical processes of black hole accretion below kiloparsec scales. After all, Astrid is a large-scale simulation.
However, if the simulation is correct, massive galaxy clusters in the local universe could host UMBHs of the same size as the one in the simulation. If they do, they likely also accumulated their mass via galaxy/BH mergers during cosmic dinner.
“We find that ultramassive black holes with extreme masses on them <50 milliarder solmasser> can form in the rare events that are several massive galaxy mergers that happen around z ~ 2, the epoch when both star formation and AGN reach their peak activity,” the authors conclude in their paper.
Only better observations can confirm these findings. JWST was built to probe the early universe and unravel some of its mysteries, and it’s already on its way. The team’s work with Astrid will help JWST, according to Ni. “We are pursuing a mock-up of observations for JWST data from the Astrid simulation,” Ni said.
Future telescopes will also help, especially NASA’s LISA space interferometer.
“In addition, the future space-based NASA Laser Interferometer Space Antenna (LISA) Gravitational Wave Observatory will give us a much better understanding of how these massive black holes merge and/or coalesce, along with the hierarchical structure, formation and galaxy mergers along cosmic history, ” Ni said. “This is an exciting time for astrophysicists, and it’s good that we can have simulation to allow theoretical predictions for these observations.”