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This article was originally published by The Conversation The publication contributed the article to Space.com’s Expert Voices: Operational & Insights. Wait Zou He is a graduate student at Penn State University W. Neil Brandt is a professor of astronomy and astrophysics at Penn State.
Black holes are amazing astronomical objects whose gravity is so strong that nothing, not even light, can escape from them. The most gigantic ones, called “supermassive” black holes, can weigh millions to billions of times the mass of the sun.
These giants usually live in the center of galaxies. Our own galaxy, the Milky Way, also has a supermassive black hole at its heart.
So how do these supermassive black holes get so massive? To answer this question, our team of astrophysicists looked back in time across the 13.8 billion year history of the universe to find out how supermassive black holes grew from the earliest days to the present day.
We modeled the overall growth history of supermassive black holes over the past 12 billion years.
How do supermassive black holes grow?
Supermassive black holes grow mainly in two ways. They can eat gas from their host galaxies in a process called accretion, and they can also merge with each other when two galaxies collide.
When supermassive black holes consume gas, they almost always emit strong X-rays, a type of high-energy light invisible to the naked eye. You’ve probably heard of X-rays at the dentist, where they are sometimes used to examine your teeth. The X-rays used by astronomers are generally lower in energy than medical X-rays.
So how can any light, even invisible X-rays, escape from black holes? Strictly speaking, the light is not coming from the black holes themselves, but from the gas just outside them. When gas is drawn towards a black hole, it rises and shines to produce light, like X-rays. The more gas a supermassive black hole consumes, the more X-rays it will produce.
Thanks to the data accumulated over 20 years from three of the most powerful X-ray facilities ever launched in space – Chandra, XMM-Newton and eROSITA – astronomers can capture X-rays from a large number of supermassive black holes accrued in space. globe
This data allows our research team to estimate how fast supermassive black holes grow by consuming gas. On average, a supermassive black hole can consume enough gas to about the mass of the sun each year, and the exact value depends on various factors.
For example, the data show that a black hole’s growth rate, averaged over millions of years, is strongly tied to the mass of all the stars in its host galaxy.
How often do supermassive black holes merge?
In addition to feeding on gas, supermassive black holes can also grow by merging with each other to form one larger black hole when galaxies collide.
Supercomputer cosmological simulations can predict how often these events occur. These simulations aim to model how the universe grows and develops over time. The countless galaxies flying through space are like bricks, building up the universe.
These simulations show that galaxies and the supermassive black holes they host can undergo multiple mergers throughout cosmic history.
Our team traced these two growth paths – gas consumption and fusion – using X-rays and supercomputer simulations, and then combined them to build this overall growth history, which maps the growth of black holes on around the globe over billions of years.
Our growth history has shown that supermassive black holes grew much faster billions of years ago, when the universe was younger.
In the early days, there was more gas in the universe for supermassive black holes to consume, and supermassive black holes were emerging. As the universe aged, the gas was gradually depleted, and the growth of a supermassive black hole slowed. About 8 billion years ago, the number of supermassive black holes stabilized. It has not increased significantly since then.
When there is not enough gas available for supermassive black holes to grow through accretion, the only way to grow them is through mergers. We haven’t seen many instances of that in our growing history. On average, the most massive black holes can accumulate mass from mergers at a rate of up to the mass of the Sun every few years.
Looking forward
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This research has helped us understand how over 90% of the mass in black holes has accumulated over the past 12 billion years.
However, we still need to investigate how they grew in the very early universe to explain the percentages of mass left in black holes. The astronomical community is starting to make progress in investigating these supermassive black holes, and we hope to have more answers soon.
This article is republished from The Conversation under a Creative Commons license. Read the base.
