From The Niels Bohr Institute [Niels Bohr Institutet] (DK): “Radio and microwaves reveal true nature of dark galaxies in the early Universe”

Niels Bohr Institute bloc

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


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

11 July 2022

Galaxy Formation: Utilizing multiple radio telescopes across the world, a team of astronomers from the Cosmic Dawn Center, Copenhagen, have discovered several galaxies in the early Universe that, due to massive amounts of dust, were hidden from our sight. The observations allowed the team to measure the temperature and thickness of the dust, demonstrating that this type of galaxies contributed significantly to the total star formation when the Universe was only 1/10 of its current age.

Artist’s impression of a dust-enshrouded starburst (credit: ESO/M. Kornmesser). Artist’s impression of a dust-enshrouded starburst (Credit: M. Kornmesser/ESO).

Measuring the rate at which stars are born in galaxies across cosmic time is one of the fundamental ways astronomers describe the properties and the evolution of galaxies.

Various methods are used to estimate this so-called “star formation rate”, typically depending on the light that is emitted from either the stars, or from matter that is illuminated by the stars.

Cosmic dust

However, the stars that are formed tend, in turn, to create dust — particles made of heavy elements such as carbon, silicon, oxygen, and iron. The dust appears as thick clouds in the space between the stars, possibly hiding the stars completely from our eyes.

This makes it difficult to get a census of the star formation rate especially in young, “starburst” galaxies, where the dust has not yet had the time to disperse far from the compact sites of star formation.

As the dust is heated by the stars, it begins to glow in long-wavelength, infrared light which, although invisible to the human eye, may be detected by telescopes designed to observe these wavelengths.

But for the most compact, dust-enshrouded starbursts, we only see the surface of the clouds. These galaxies are invisible not only at the “humanly perceivable”, optical wavelengths, but also in the beginning of the infrared spectrum, utterly dark even to the Hubble Space Telescope.

Galaxies on the Radio

A team of astronomers — led by Shuowen Jin (靳硕文), Marie Curie postdoc fellow at the Cosmic Dawn Center, and including several other DAWNers — therefore decided to take a look at the early Universe at even longer wavelengths, using the radio/microwave antennae at two of the world’s largest radio observatories, the Atacama Large Millimeter Array (ALMA) in Chile, and the Northern Extended Millimeter Array (NOEMA) in France.

Six different views of the same galaxy (ID12646), seen less than a billion years after the Big Bang, at progressively longer wavelengths. The two first images show — or rather do not show — the galaxy in the near-infrared; the galaxy is completely invisible. Only when looking at the longer wavelengths is the galaxy revealed (credit: Shuowen Jin / Peter Laursen).

Together with observations of the same field on the sky acquired with other radio telescopes, Jin’s observations revealed a population of compact starburst galaxies, cloaked in extremely thick dust clouds.

Piercing through the clouds

The radio- and microwave observations allowed the astronomers to measure the star formation rate and the temperature of the dust.

“In these epochs, 1–2 billion years after the Big Bang, galaxies like these contributed significantly to the total star formation rate of the Universe, but pass unnoticed in optical and near-infrared observations,” says Shuowen Jin.

The study explains why these galaxies are so dark in optical and infrared: “Because the dust clouds are so thick and dense, optical and near-infrared light cannot travel through. Even the far-infrared light is partially absorbed,” Shuowen Jin explains.

The observations reveal not only dust, but also monoxide molecules (CO), mixed within the clouds. The light emitted by CO can help astronomers probe another important quantity of galaxies, namely the mass of all the gas in the galaxy. However, one of the key results of the work of Jin and his collaborators is that the standard way of inferring gas masses from CO emission is erroneous:

The observed light is emitted from the surfaces of the dusty clouds. Typical models do not consider that light is blocked inside the clouds, changing its wavelength before it escapes. Taking this effect into account has rather drastic implications:

“Our model accounts for the fact that even the infrared light does not escape directly from the center of the dust clouds. This shows us that previous estimates of gas masses have been overestimated by a factor of 2–3 in compact, dusty, star-forming galaxies,” Shuowen Jin explains.

The study has just been accepted for publication in Astronomy & Astrophysics.

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.