Black holes are astronomical objects with an incredibly strong gravitational pull, so strong that not even light can escape them. The most massive of these black holes, known as supermassive black holes, can weigh millions to billions times the mass of the Sun and are typically found in the centers of galaxies, including our own Milky Way.
The growth of supermassive black holes is a topic of interest among astrophysicists. To understand how these black holes became supermassive, researchers have looked back in time across the universe’s 13.8 billion-year history. They have constructed a model of the overall growth history of supermassive black holes spanning the past 12 billion years.
Supermassive black holes grow primarily in two ways: through accretion and mergers. Accretion occurs when black holes consume gas from their host galaxies. This process usually results in the emission of strong X-rays, a type of high-energy light. The X-rays are not emitted by the black hole itself but by the gas just outside it, which heats up and shines as it gets pulled towards the black hole.
Astronomers can capture X-rays from a large number of accreting supermassive black holes in the universe using data from powerful X-ray facilities like Chandra, XMM-Newton, and eROSITA. This data allows them to estimate the rate at which supermassive black holes grow by consuming gas. On average, a supermassive black hole can consume the mass of the Sun each year, although the exact rate depends on various factors.
Besides feeding on gas, supermassive black holes can also grow by merging with each other when galaxies collide. Supercomputer cosmological simulations predict the frequency of these events. These simulations show that galaxies and their supermassive black holes can undergo multiple mergers across cosmic history.
By tracking these two growth channels using X-rays and supercomputer simulations, researchers have constructed an overall growth history of black holes, mapping their growth across the universe over billions of years. This history reveals that supermassive black holes grew much faster billions of years ago when the universe was younger, as it contained more gas for black holes to consume. As the universe aged, the gas was gradually depleted, and supermassive black hole growth slowed. About 8 billion years ago, the number of supermassive black holes stabilized.
When there isn’t enough gas available for supermassive black holes to grow by accretion, the only way for them to get larger is through mergers. On average, the most massive black holes can accumulate mass from mergers at a rate up to the mass of the Sun every several decades.
This research has helped us understand how over 90% of the mass in black holes has accumulated over the past 12 billion years. However, more research is needed to explain the remaining few percentages of the mass in black holes, particularly in the very early universe. The astronomical community is making progress in this area, and more answers are expected soon.
