How do black holes become supermassive?

Researchers have developed the most comprehensive model to date of the growth of supermassive black holes in galaxy centers using a combination of advanced X-ray observations and cutting-edge supercomputer simulations. 

Led by Penn State astronomers, this hybrid approach offers a complete picture of black hole growth over 12 billion years, from the Universe’s infancy at around 1.8 billion years old to the present day.

Mystery of supermassive black holes 

“Supermassive black holes in galaxy centers have millions-to-billions of times the mass of the Sun,” said lead author Fan Zou, a graduate student at Penn State. 

“How do they become such monsters? This is a question that astronomers have been studying for decades, but it has been difficult to track all the ways black holes can grow reliably.”

Mechanisms of black hole growth 

Supermassive black holes grow primarily through two mechanisms: consuming cold gas from their host galaxies (accretion) and merging with other supermassive black holes during galaxy collisions.

“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,” explained co-author W. Niel Brandt, a professor of astrophysics at Penn State. 

“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.”

Thousands of rapidly growing black holes 

The researchers utilized data from NASA’s Chandra X-ray Observatory, the European Space Agency’s X-ray Multi-Mirror Mission-Newton (XMM-Newton), and the Max Planck Institute for Extraterrestrial Physics’ eROSITA telescope. 

The team analyzed accretion-driven growth in a sample of 1.3 million galaxies containing over 8,000 rapidly growing black holes.

“All of the galaxies and black holes in our sample are very well characterized at multiple wavelengths, with superb measurements in the infrared, optical, ultraviolet, and X-ray bands,” Zou said. 

“This allows for robust conclusions, and the data show that, at all cosmic epochs, more massive galaxies grew their black holes by accretion faster. With the quality of the data, we were able to quantify this important phenomenon much better than in past works.”

Realistic picture of black hole growth

To track growth by mergers, the team used IllustrisTNG, a set of supercomputer simulations that model galaxy formation, evolution, and merging from shortly after the Big Bang to the present.

“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,” Brandt said. 

“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.”

Supermassive black holes in the early universe 

The researchers discovered that accretion predominantly drives black hole growth, with mergers contributing significantly, particularly in the last 5 billion years for the most massive black holes. 

Overall, supermassive black holes grew much more rapidly when the universe was younger, with the total number of supermassive black holes nearly settled by 7 billion years ago.

“With our approach, we can track how central black holes in the local universe most likely grew over cosmic time,” Zou explained. 

“As an example, we considered the growth of the supermassive black hole in the center of our Milky Way Galaxy, which has a mass of 4 million solar masses. Our results indicate that our Galaxy’s black hole most likely grew relatively late in cosmic time.”

Growth of supermassive black holes 

The growth of supermassive black holes is a fascinating and complex process that has intrigued astronomers and physicists for decades. 

Black holes, which reside at the centers of most galaxies, can reach masses ranging from millions to billions of times that of our Sun. 

Accretion disks 

The growth of black holes is primarily driven by the accretion of gas and dust from their surroundings. 

This material forms an accretion disk as it spirals inward, heating up and emitting vast amounts of energy in the form of electromagnetic radiation, particularly in the X-ray and ultraviolet wavelengths.

Violent mergers

Another significant factor in the growth of supermassive black holes is the merging of smaller black holes and other massive objects. 

When galaxies collide, their central black holes can eventually merge, forming an even more massive black hole. These mergers are violent events, releasing gravitational waves that ripple through the fabric of space-time.

Active galactic nuclei phases

Supermassive black holes also exhibit periods of intense activity known as active galactic nuclei (AGN) phases. 

During these phases, the black hole rapidly consumes surrounding material, resulting in powerful jets of particles and radiation being ejected at near-light speeds. The jets can influence the host galaxy by regulating star formation and redistributing gas and dust.

Mysterious origins

The initial formation of supermassive black holes remains a subject of ongoing research. One leading theory suggests that they could have formed from the direct collapse of massive primordial gas clouds in the early universe, bypassing the intermediate stages of star formation. 

Another theory proposes that they could have started as smaller, stellar-mass black holes formed from the deaths of the first generation of stars and then grew through subsequent accretion and mergers.

The Penn State study is detailed in two papers, one published in The Astrophysical Journal in April 2024, and another forthcoming paper. The research was also presented at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.

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