Supermassive black holes may have nixed star formation in ancient galaxies

Data from distant galaxies suggest that supermassive black holes may play a role in halting star formation, according to a recent study by an international team of astronomers.  

Supermassive black holes are the largest kind of black hole, which scientists theorize reside at the center of most galaxies. In some galaxies, specifically massive elliptical galaxies, stars form rapidly in their youth and suddenly cease. Although questions remain about what triggers and stops star formation, scientists suspect that supermassive black holes could bear some responsibility.

To test this hypothesis, the team of astronomers, led by postdoc Kei Ito at SOKENDAI (Graduate University for Advanced Studies) in Japan, surveyed galaxies in a region of space about 9.5 to 12.5 billion light-years away. The data was retrieved by a network of telescopes a part of the Cosmic Evolution Survey (COSMOS), a collaboration focused on better understanding the development of galaxies.

The scientists compared the activity of black holes in galaxies with stars forming and not forming. They did this by searching for x-rays and radio waves that black holes are known to emit.

X-ray and radio wave emissions were found to be stronger in galaxies without stars forming than those with stars forming during this period. Astronomers call these galaxies red and dead since they consist almost entirely of red stars reaching the end of their lives.

Hundreds of galaxies are seen in this region of the sky, called COSMOS. The most distant ones are seen as small, red specks, enlarged along the edge of the image. By “adding” all these galaxies a unified signal emerges, which has led scientists on the trail of the cause of the galaxies’ death (credit: NAOJ).

Because the galaxies are so far away, these signals can be difficult to detect. As a result, the astronomers stacked the images obtained from COSMOS on top of each other to produce a better signal-to-noise ratio.

“Although we lose the information about the state of any individual galaxy, we can now see their average properties,” says John Weaver, a Ph.D. student at a research center within the University of Copenhagen, in a statement. “And the result is clear: A typical quenched galaxy 10–12 billion years ago hosted a low-luminosity, active galactic nucleus which may have played a crucial role in preventing rejuvenated star formation.”

The remaining stars in red and dead galaxies are too weak to produce the emissions, but supermassive black holes could. The results of the study thus provide evidence for the hypothesis that supermassive black holes quenched more stars from forming in these ancient galaxies.

Looking ahead, the astronomers hope to corroborate their findings with additional observations.

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

The study reportedly marks the first time that these emissions have been detected from galaxies more than 10-billion light years from Earth. It is published in The Astrophysical Journal.

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