Black Holes

Ultra-Massive Black Holes: How Does the Universe Produce Objects So Massive?

Black holes are the most massive objects that we know of in the Universe. Not stellar mass black holes, not supermassive black holes (SMBHs,) but ultra-massive black holes (UMBHs.) UMBHs sit in the center of galaxies like SMBHs, but they have more than five billion solar masses, an astonishingly large 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 massive?

UMBHs are rare and elusive, and their origins are unclear. A team of astrophysicists working on the question used a simulation to help uncover the formation of these massive objects. They traced UMBH’s origins back to the Universe’s ‘Cosmic Noon‘ around 10 to 11 billion years ago.

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 one possible formation channel for ultra-massive black holes is from the extreme merger of massive galaxies that are most likely to happen in the epoch of the ‘cosmic noon,'” said Ni.

UMBHs are extremely rare. Creating them in scientific simulations requires a massive, complex simulation. This is where Astrid comes in. It’s a large-scale cosmological hydrodynamical simulator that runs 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 their simulations contain, and Astrid is at the top of that list.

This figure shows some of Astrid’s output. The series of zoomed-in panels begins with a massive halo, then stars centred on an SMBH, then the morphology of individual simulated galaxies. Image Credit: Astrid/UT Austin/Ni et al. 2022.

“The science goal of Astrid is to study galaxy formation, the coalescence of supermassive black holes, and re-ionization over the cosmic history,” said lead author Ni in a press release. (Ni is a co-developer of Astrid.) A powerful tool like Astrid needs a powerful supercomputer. Luckily, UT Austin has the most powerful academic supercomputer in the USA. “Frontera is the only system that 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’s likely that SMBHs grow more massive at the same time. But UMBHs are even more massive and much rarer. How do they form?

The team’s work with Astrid delivered an answer.

“What we found are three ultra-massive black holes that assembled their mass during the cosmic noon, 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.

This figure from the research is an illustration of the quasar triplet system and its environment (host galaxies). BH1 is the most massive of the three quasars, and it sits in the center in the bottom row of images. Red and yellow lines show the trajectories of BH2 and BH3. Image Credit: Ni et al. 2023.

Cosmic Noon is an important time period in the history of the Universe. Astronomers think 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 quantities of gas flowed from the intergalactic medium 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 that assembled their mass during cosmic noon.

“In this epoch, we spotted an extreme and relatively fast 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 in the center of each galaxy. Our findings show the possibility that these quasar triplet systems are the progenitor of those rare ultra-massive blackholes after those triplets gravitationally interact and merge with each other.”

Quasars’ name is misleading. It means a quasi-stellar object, but the name stems 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 of the material falling into the SMBH at a galaxy’s center. Opportunities for triple quasar systems to merge and form UMBHs are dwindling, according to the simulation.

This figure from the research shows how the number of quasars (QSO=Quasi-Stellar Object) is dwindling over time. By the end of cosmic noon, there are almost no triple quasars, according to Astrid. The grey Shen 2020 line is from another study estimating the number of quasars in the Universe over time, and Astrid’s results agree with that research. Image Credit: Ni et al. 2023.

Astrophysicists have determined a theoretical upper mass limit for black holes at about 50 billion solar masses, and the post-merger UMBH approaches that limit. But the researchers caution that the Astrid simulation “… is not a prescription for a new upper limit for the black hole mass.” That’s because simulations, even one as powerful as Astrid, can’t resolve the details of physical processes of black hole accretion below kiloparsec scales. Astrid is a large-scale simulation, after all.

But if the simulation is correct, then massive galaxy clusters in the local Universe may host UMBHs the same size as the one in the simulation. If they do, then they likely also assembled their mass via galaxy/BH mergers during cosmic noon.

“We find that ultramassive black holes with extreme masses of <50 billion solar masses> can be formed in the rare events that are multiple massive galaxy mergers happening 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. The JWST was built to probe the early Universe and unravel some of its mysteries, and it’s already making headway. The team’s work with Astrid will help the JWST, according to Ni. “We’re 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 the how these massive black holes merge and/or coalescence, along with the hierarchical structure, formation, and the galaxy mergers along the 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 those observations.”

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Evan Gough

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