Black holes helped quench star formation in the early universe

Newswise — While some galaxies are forming stars at a steady rate, others are fading away and leading a more passive life. What caused these galaxies to stop forming stars at an early age is not well established, not least because they are so distant and faint that they escape observation. But by looking at the combined light from thousands of galaxies, a team of astronomers including the University of Copenhagen showed that black holes help turn off star formation.

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 “adding” all these galaxies, a unified signal emerges, which has led scientists on the trail of the cause of galaxy death (Credit: NAOJ).

About once a year, a new star is born in our galaxy, the Milky Way.

Some galaxies form stars faster, and in the early Universe the most vigorous galaxies formed hundreds or even thousands of stars per year. However, others are kind of pushed to the other extreme and stop forming new stars altogether. Slowly, their star population is depleting, leaving behind only the small reddish stars.

Particularly in the primordial Universe, the reason for this so-called quench is not well established, although we know it must be related to the exhaustion of the stars’ fuel – cold gas. But whether the gas is blown out of the galaxy, heated to too high temperatures, or something else is happening, is uncertain.

Another question is why the galaxies stay at rest: In the early Universe, intergalactic space was full of gas that would eventually gravitate toward the galaxies, reigniting star formation.

Black holes light up by swallowing gas

One possibility is that a galaxy at rest contains a supermassive black hole at its center, swallowing nearby matter and radiating excess energy. This type of “active galactic nucleus” would be a low-light version of the more energetic quasars. The energy emitted would nevertheless still be sufficient to heat the rest of the gas in the galaxy, preventing the formation of new stars.

If this scenario is true, the galaxy should show a small excess of X-ray and radio wave signal.

An international team of astronomers, led by postdoc Kei Ito at SOKENDAI University in Japan, decided to test the hypothesis by digging through a catalog of galaxies observed in a particular region of the sky, dubbed the “COSMOS field”.

However, Ito and his collaborators faced a problem inherent in this approach:

Due to the time it takes to get to us, explore early galaxies means to observe far galaxies billions of light-years away. But distant galaxies are faint, and so the signal, if there is, is undetectable in any individual galaxy in the COSMOS catalog.

A stack of galaxies

To overcome this hurdle, the team decided to “stack” the images of the galaxies, i.e. add the light from all the galaxies, looking 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 extinct galaxy 10 to 12 billion years ago harbored a low-luminosity active galactic nucleus that may have played a crucial role in preventing the formation of rejuvenated stars,explains John Weaver, PhD student at the Cosmic Dawn Center, a research center under the Niels Bohr Institute, the University of Copenhagen and DTU Space.

John Weaver is one of several researchers from the Cosmic Dawn Center to have participated in the study. He recently led the effort to collect, catalog and analyze 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.concludes John Weaver.

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