While some galaxies continue to form stars, others are disappearing and living more passively. What caused these galaxies to stop forming stars at such a young age is not known, not least because they are so distant and faint that they go unnoticed.
However, a team of astronomers led by the University of Copenhagen found that black holes helped turn off star formation by combining light from thousands of galaxies.
In our galaxy, the Milky Way, a new star is born about once a year.
Some galaxies form stars faster than others, and the most active galaxies in the early universe have produced hundreds or perhaps thousands of stars annually. Others, on the other hand, are pushed to the opposite extreme and stop generating new stars altogether. Their star populations gradually disappear, leaving only small, reddish stars.
The explanation for this so-called extinction, especially in the early universe, is unknown, although humanity knows that it must have something to do with the depletion of stellar fuel – cold gas. However, it is not clear whether the gas is being blown out of the galaxy, heated to dangerously high temperatures or anything else.
Another concern is why galaxies remain dormant: Intergalactic space in the early universe was dense with gas that should eventually move toward galaxies to restore star formation.
Black holes light up by swallowing gas
One possibility is that the sleeping galaxy has a supermassive black hole at its center, which consumes close matter while emitting excess energy. This form of “active galactic nucleus” would be a less intense quasar with lower luminosity. Even so, the energy released would be enough to heat the rest of the gas in the galaxy, preventing future star formation.
At 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 postdoctor Kei Item of SOKENDAI University in Japan, decided to verify this idea by studying a database of galaxies discovered in the “COSMOS field”, the sky region.
However, Ito and his team encountered an inherent problem with this strategy: exploring early galaxies requires observing distant galaxies billions of years away because of the time it takes light to reach us. However, because distant galaxies are small, any signal that may exist is not detected in any single galaxy in the COSMOS database.
A pile of galaxies
To solve this problem, the scientists decided to “compose” images of the galaxies, which means that they combined light from all the galaxies and looked at the combined signal from all the galaxies at the same time.
Even if we lose information about the state of each individual galaxy, we can now see their “average” properties. And the result is clear: A typical quenched galaxy 10-12 billion years ago hosted an active, low-luminosity galactic nucleus that may have played a key 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 establish the Cosmic Dawn Center.
The research involved several members of the Cosmic Dawn Center, including John Weaver. He has recently overseen the collection, cataloging and analysis of 1.7 million galaxies in the COSMOS field.
Now that we know there are active galactic nuclei, we can target 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 link:
It’s alright., et al. (2022) COSMOS2020: Ubiquitous AGN activity of massive calm galaxies at 0 of <5 X-Ray and Radio Stacking revealed. The Astrophysical Journal. doi.org/10.3847/1538-4357/ac5aaf.
Source: https://science.ku.dk/english
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