From The Niels Bohr Institute [Niels Bohr Institutet] (DK): “Black holes helped quenching star formation in the early Universe”

Niels Bohr Institute bloc

From The Niels Bohr Institute [Niels Bohr Institutet] (DK)


University of Copenhagen [Københavns Universitet] [UCPH] (DK)

2 June 2022

John Weaver
The Cosmic Dawn Center
Niels Bohr Institute
University of Copenhagen
Mobil: +45 35 33 64 09

Peter Laursen
Academic Research Staff
The Cosmic Dawn Center
Niels Bohr Institute
University of Copenhagen
Mobil: +45 30 26 59 69

Galaxies-While some galaxies form stars at a continuous rate, others die out and lead a more passive life. What made these galaxies stop forming stars at an early age is not well established, not the least because they are so distant and faint that they evade being observed. But looking at the combined light from thousands of galaxies, a team of astronomers including the University of Copenhagen showed that black holes helped turn off star formation.

Roughly 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 a year. However, others are somehow driven to the other extreme and completely stop forming new stars. Slowly their population of stars burn out, leaving only the small, reddish stars behind.

In particular in the early Universe, the reason for this so-called quenching is not well-established, although we know that it must have to do with the fuel for stars — cold gas — being depleted. But whether the gas is blown out of the galaxy, is heated to too high temperatures, or something else is going on, is uncertain.

Another question is why the galaxies stay quiescent: In the early Universe, the intergalactic space was full of gas which eventually should gravitate toward the galaxies, reviving star formation.

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).

Black holes light up by swallowing gas

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

If this scenario is true the galaxy should show a weak excess signal in X-ray and radio waves.

An international team of astronomers, led by postdoc Kei Ito at the 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 an inherent problem in this approach:

Because of the time it takes light time to reach us, exploring early galaxies means observing distant galaxies, billions of light years away. But distant galaxies are faint, and hence the signal, should it be there, is undetectable in any individual galaxy of the COSMOS catalog.

A stack of galaxies

To overcome this obstacle, the team decided to “stack” the images of the galaxies — that is, to add the light from all galaxies, looking at the combined signal from all galaxies at the same time.

“Although we lose the information about the state of any individual galaxy, we can now see their »average« properties. 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,” explains John Weaver, PhD student at the Cosmic Dawn Center, a research center under the Niels Bohr Institute, University of Copenhagen, and DTU Space.

John Weaver is one of several from the Cosmic Dawn Center who took part in the study. He recently led the effort of collecting, cataloging, and analyzing 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 follow-up observations — for instance with the new James Webb Space Telescope — will provide more evidence for our proposed scenario,” concludes John Weaver.

The study was published recently in The Astrophysical Journal.

See the full article here .


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Niels Bohr Institute Campus

The Niels Bohr Institutet (DK) is a research institute of the Københavns Universitet [UCPH] (DK). The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.

The Institute was founded in 1921, as the Institute for Theoretical Physics of the Københavns Universitet [UCPH] (DK), by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institutet (DK). Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.

During the 1920s, and 1930s, the Institute was the centre of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.

On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institutet (DK).

Københavns Universitet (UCPH) (DK) is the oldest university and research institution in Denmark. Founded in 1479 as a studium generale, it is the second oldest institution for higher education in Scandinavia after Uppsala University (1477). The university has 23,473 undergraduate students, 17,398 postgraduate students, 2,968 doctoral students and over 9,000 employees. The university has four campuses located in and around Copenhagen, with the headquarters located in central Copenhagen. Most courses are taught in Danish; however, many courses are also offered in English and a few in German. The university has several thousands of foreign students, about half of whom come from Nordic countries.

The university is a member of the International Alliance of Research Universities (IARU), along with University of Cambridge (UK), Yale University , The Australian National University (AU), and University of California-Berkeley , amongst others. The 2016 Academic Ranking of World Universities ranks the University of Copenhagen as the best university in Scandinavia and 30th in the world, the 2016-2017 Times Higher Education World University Rankings as 120th in the world, and the 2016-2017 QS World University Rankings as 68th in the world. The university has had 9 alumni become Nobel laureates and has produced one Turing Award recipient.