Black holes helped extinguish star formation in the early Universe

While some galaxies continue to make stars, others die out and lead a more passive existence. Why these galaxies stopped making stars at such a young age is unknown, not least because they are so distant and dim that they go undetected.

Hundreds of galaxies can be seen in this part of the sky called COSMOS. The farthest are seen as small, red dots, magnified along the edge of the image. Adding all these galaxies creates a unified signal that has led scientists to the cause of the galaxies’ deaths. Image credit: NAOJ.

A team of astronomers led by the University of Copenhagen found that black holes helped turn off star formation by combining the light from thousands of galaxies.

In our galaxy, the Milky Way, a new star is born about once a year.

Some galaxies make stars faster than others, and the most active galaxies in the early Universe produced hundreds or perhaps thousands of stars a year. Others, on the other hand, are pushed to the other extreme and completely stop generating new stars. Their population of stars gradually fades away, leaving only the small, reddish stars.

The explanation for this so-called extinction, particularly in the early universe, is unknown, although mankind knows it must have something to do with the depletion of star fuel – cold gas. However, it’s unclear if the gas is being blown out of the galaxy, heated to dangerously high temperatures, or something else.

Another concern is why galaxies go dormant: Intergalactic space in the early Universe was dense with gas, which should eventually move into galaxies and renew star formation.

Black holes light up by swallowing gas

One possibility is that a sleeping galaxy has a supermassive black hole at its center, consuming nearby matter while radiating excess energy. This form of “active galactic core” would be a less intense quasar with a lower brightness. Still, the energy released would be enough to heat the rest of the galaxy’s gas, preventing future star formation.

In X-ray and radio wavelengths, the galaxy should indicate a weak excess signal if this scenario is correct.

An international team of astronomers, led by postdoc Kei Ito of Japan’s SOKENDAI University, chose to verify the idea by searching a database of galaxies discovered in the “COSMOS field,” a region of the sky.

However, Ito and his team ran into an inherent problem with this strategy: Exploring early galaxies requires monitoring distant galaxies billions of years away because of the time it takes for light to reach us. However, since distant galaxies are small, any possible existing signal is not detected in an individual galaxy in the COSMOS database.

A stack of galaxies

To get around this problem, the researchers chose to “stack” the images of the galaxies, meaning they combined the light from all the galaxies and looked at the combined signal from all the galaxies at the same time.

Although we lose the information about the state of an individual galaxy, we can now see their »average« properties. And the result is clear: A typically extinct galaxy 10-12 billion years ago harbored an active, low-luminosity galactic core, which may have played a critical role in preventing rejuvenated star formation.

John Weaver, PhD student, Cosmic Dawn Center

The Niels Bohr Institute, the University of Copenhagen and DTU Space joined forces to establish the Cosmic Dawn Center.

The investigation involved several members of the Cosmic Dawn Center, including John Weaver. Most recently, he oversaw the collection, cataloging and analysis of the 1.7 million galaxies of the COSMOS field.

Now that we know the active galactic nuclei are there, we can target the galaxies individually. Future deep follow-up observations, for example with the new James Webb Space Telescope, will provide more evidence for our proposed scenario

John Weaver, PhD student, Cosmic Dawn Center

Magazine reference:

It’s OK., et al† (2022) COSMOS2020: ubiquitous AGN activity of massive silent galaxies at 0 z < 5 Revealed by X-Ray and Radio Stacking. The astrophysics


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