The true nature of dark galaxies in the early cosmos

July 11 () –

Using radio telescopes around the world, astronomers have discovered several galaxies in the early universe that, due to huge amounts of dust, They were hidden from our view.

The observations allowed the team at the Cosmic Dawn Center at the University of Copenhagen to measure the temperature and thickness of the dust, showing that this type of galaxy contributed significantly to total star formation when the universe was only 1/10 its current age.

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

Various methods are used to estimate this so-called star formation rate, which is generally it depends on the light emitted by the stars or by the matter illuminated by the stars.

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

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

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

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

A team of astronomers, led by Shuowen Jin, a Marie Curie Postdoctoral Fellow at the Cosmic Dawn Center, decided to take a look at the early universe at even longer wavelengths, using the radio/microwave antennas at two of the world’s largest radio observatories. world, the Atacama Large Millimeter Array (ALMA) in Chile and the Northern Extended Millimeter Array (NOEMA) in France.

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

Radio and microwave observations allowed astronomers to measure the rate of star formation and the temperature of the dust.

“In these epochs, 1 to 2 billion years after the Big Bang, galaxies like these contributed significantly to the total rate of star formation in the universe, but they go unnoticed in optical and near-infrared observations,” Says Shuowen Jin it’s a statement.

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 pass through them. Even far infrared light is partially absorbedShuowen Jin explains.

The observations reveal not only dust, but also monoxide (CO) molecules, mixed within the clouds. The light emitted by CO can help astronomers investigate another important number 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 wrong.

The observed light is emitted from the surface of the dust clouds. Typical models don’t consider light to be blocked within clouds, changing its wavelength before escaping. Accounting for this effect has rather drastic implications.

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

The study has just been accepted for publication in Astronomy & Astrophysics. A preprint is available at arXiv.org.