New Simulation Explains how SMBHs Grew so Quickly

One of the key scientific goals of next-generation observatories (like the JWST) has been to observe the first galaxies in the Cosmos – those that existed at Cosmic Dawn. This epoch is when the first stars, galaxies, and black holes in our Cosmos formed, about 50 million to 1 billion years post the Big Bang. By probing how these galaxies formed and evolved during the earliest cosmological epochs, astrophysicists will have a complete picture of how the Cosmos has changed with time.

The results of Webb‘s most distant observations have put forth a few surprises. Besides revealing that galaxies formed rapidly in the early Cosmos, astrophysicists also noted that these galaxies had massive supermassive black holes (SMBH) at their centers. This was particularly perplexing since, according to conventional models, these black holes and galaxies lacked enough time to form. In a new study, a team led by Penn State astronomers has developed a model that has the potential to explain how SMBHs grew so hastily in the early Universe.

The research team was led by W. Niel Brandt (the Eberly Family Chair Professor of Astronomy and Astrophysics at Penn State’s Eberly College of Science). Their study is described in two papers presented at the 244th meeting of the American Astronomical Society (AAS224), which was held from June 9th to June 13th in Madison, Wisconsin. Their first paper, “Mapping the Growth of Supermassive Black Holes as a Function of Galaxy Stellar Mass and Redshift,” appeared in The Astrophysical Journal on March 29th, while the second is still pending publication. the lead author of both paperswas Fan Zou (an Eberly College graduate student).

As they state in their papers, SMBHs grow through two chief channels: merging with the SMBHs of other galaxiesor by accreting cold gas from their host galaxy. When it comes to accretion, earlier research has shown that a black hole’s accretion rate (BHAR) is intensely linked to its galaxy’s stellar mass and the redshift of its general stellar population. “Supermassive black holes in galaxy centers have millions-to-billions of times the mass of the Sun,” expounded Zhou in a recent NASA press release. How do they turn into such monsters? This is a question that astrophysicists have been studying for decades, but it has been hard to track all the ways black holes can grow reliably.

For their study, the team depended on forefront X-ray sky survey data obtained by NASA’s Chandra X-ray Observatory, the Max Planck Institute for Extraterrestrial Physics’ eROSITA telescope, andthe ESA’s X-ray Multi-Mirror Mission-Newton (XMM-Newton). They measured the accretion-driven growth in a sample of 8000 AGNs (active galactic nuclei) located in 1.3 million galaxies. This was combined with IllustrisTNG (a suite of state-of-the-art cosmological simulations that model galaxy formation, evolution, and mergers from Cosmic Dawn to the present). This combined approach has offered the best modeling to date of SMBH growth over the past 12 billion years. Said Brandt:

“During the process of consuming gas from their hosting galaxies, black holes radiate strong X-rays, and this is the key to tracking their growth by accretion. We measured the accretion-driven growth using X-ray sky survey data accumulated over more than 20 years from three of the most powerful X-ray facilities ever launched into space.”

“In our hybrid approach, we combine the observed growth by accretion with the simulated growth through mergers to reproduce the growth history of supermassive black holes. With this new approach, we believe we have produced the most realistic picture of the growth of supermassive black holes up to the present day.”