Supermassive black holes have masses of greater than one million suns – but their growth has slowed because the universe has aged

Black holes are remarkable astronomical objects with gravity so strong that nothing, not even light, can escape them. The most gigantic, called “supermassive” black holes, can weigh hundreds of thousands to billions of times the mass of the Sun.

These giants often live within the centers of galaxies. Our own galaxy, the Milky Way, accommodates a supermassive black hole also in his heart.

So how do these supermassive black holes develop into supermassive? To answer this query, our team by astrophysicists We looked back on the 13.8 billion yr history of the universe to trace how supermassive black holes have evolved from their beginnings to the current day.

We constructed a model of the Overall growth history of supermassive black holes over the past 12 billion years.

How do supermassive black holes grow?

Supermassive black holes grow in two principal ways. They can eat gas from their host galaxies in a process called accretionand so they may merge when two galaxies collide.

When supermassive black holes eat gas, they almost at all times emit powerful X-rays, a kind of high-energy light that’s invisible to the naked eye. You've probably heard of X-rays on the dentist, where they're sometimes used to look at your teeth. The X-rays utilized by astronomers generally have lower energies than medical X-rays.

So how can light, even invisible X-rays, escape from black holes? Strictly speaking, the sunshine comes not from the black holes themselves, but from the gas just outside them. When gas is pulled toward a black hole, it heats up and glows, producing light like X-rays. The more gas a supermassive black hole consumes, the more X-rays it is going to produce.

Thanks to data collected over greater than 20 years by three of essentially the most powerful X-ray systems ever sent into space – Chandra, XMM-Newton And ROSITE – Astronomers can capture X-rays from numerous accreting supermassive black holes within the universe.

These data allow our research team to estimate how briskly supermassive black holes grow by consuming gas. On average, a supermassive black hole can eat enough gas to grow about the mass of the sun yearly, although the precise value is dependent upon various aspects.

For example, The data shows that the expansion rate of a black hole, averaged over hundreds of thousands of years, is strongly related to the mass of all the celebrities in its host galaxy.

How often do supermassive black holes merge?

Supermassive black holes not only feed on gas, but may grow by merging as galaxies collide to form a single, more massive black hole.

Cosmological simulations on supercomputers can predict how often these events occur. These simulations aim to model how the universe grows and evolves over time. The countless galaxies flying through space are something just like the constructing blocks that make up the universe.

This Simulations show that Galaxies and the supermassive black holes they host can merge multiple times throughout cosmic history.

Our team tracked these two growth channels – gas consumption and mergers – using X-rays and supercomputer simulations, after which combined them to create this overall growth history that maps the expansion of black holes throughout the universe over billions of years.

Our growth story revealed that supermassive black holes grew much faster billions of years ago, when the universe was younger.

In the early days, the universe contained more gas that could possibly be consumed by supermassive black holes, and supermassive black holes continued to form. As the universe aged, the gas was steadily used up and the expansion of supermassive black holes slowed. About 8 billion years ago, the variety of supermassive black holes stabilized. Since then, it has not increased significantly.

If there isn’t enough gas for supermassive black holes to grow by accretion, they will only grow larger by merging. We haven’t seen very many cases of this in our growth history. On average, essentially the most massive black holes can accumulate mass through merging similar to the mass of the Sun every few a long time.

I'm looking forward to

This research has helped us understand how over 90% of the mass in black holes has gathered over the past 12 billion years.

However, we still need to review how they grew within the early Universe to clarify the remaining few percent of mass in black holes. The astronomical community is making progress in studying these early supermassive black holes and we hope to seek out more answers soon.