Black holes helped form extinction stars in the early universe

While some galaxies continue to create stars, others die out and live a more passive existence. Why these galaxies stopped creating stars at such a young age is unknown, especially since they are so distant and dim that they go unnoticed.

Hundreds of galaxies are visible in this region of the sky, called COSMOS. The farthest are seen as small red spots, magnified along the edge of the image. By “summing up” all these galaxies, a unified signal emerges, which has led scientists on the trail of the cause of galaxy death. Image credit: NAOJ.

However, a team of astronomers led by the University of Copenhagen found that black holes help disable star creation by combining 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 even thousands of stars a year. Others, on the other hand, are taken to the opposite extreme and stop generating new stars entirely. Their star population gradually fades, leaving only tiny reddish stars.

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

Another concern is why galaxies remain dormant: intergalactic space in the early Universe was dense with gas, which should eventually move towards galaxies, renewing star formation.

Black holes light up by swallowing gas

One possibility is that a dormant galaxy has a supermassive black hole at its center, which is consuming matter nearby while giving off excess energy. This form of “active galactic nucleus” would be a less intense quasar with a lower luminosity. Even so, the energy released would be enough to heat the rest of the galaxy’s gas, preventing future star formation.

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

An international team of astronomers, led by post-doctoral fellow Kei Ito of Japan’s SOKENDAI University, chose to verify the notion by combing through a database of galaxies discovered in the “COSMOS field”, a region from the sky.

However, Ito and his team ran into an inherent problem with this strategy: Examining early galaxies requires monitoring distant galaxies billions of years away because of the time it takes for light to reach us. However, because distant galaxies are small, any signals that may exist are not detected in any 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, which means 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 each individual galaxy, we can now see their “average” properties. And the result is clear: a typical galaxy that died out 10 to 12 billion years ago harbored a low-luminosity active galactic nucleus that may have played a crucial role in preventing rejuvenated star formation..

John Weaver, PhD Student, Cosmic Dawn Center

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

The search included several members of the Cosmic Dawn Center, including John Weaver. He recently oversaw the collection, cataloging and analysis of the 1.7 million galaxies in the COSMOS field.

Now that we know the active galactic nuclei are there, we can target the galaxies individually. Future deep-tracking 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

Journal reference:

Ito, K. et al. (2022) COSMOS2020: Pervasive AGN activity of massive galaxies at rest at 0 z < 5 Revealed by X-Ray and Radio Stacking. The Astrophysical Journal.


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