A team of astronomers from the Cosmic Dawn Center, Copenhagen, have discovered several galaxies in the early universe that were hidden from our sight, using multiple radio telescopes across the world. These galaxies from the early universe were hidden from our sight due to massive amounts of dust. With the help of these observations, the astronomers were able to measure the temperature and thickness of the dust, and demonstrate that this type of galaxies contributed significantly to the total star formation when the universe was only one-tenth of its current age.
A fundamental method of describing the properties and evolution of galaxies is measuring the rate at which stars are born in galaxies across cosmic time. Astronomers eliminate this so-called star formation rate using various methods. The star formation rate is dependent on the light that is emitted from either the stars, or from matter that is illuminated by the stars.
The study describing the findings has been accepted for publication in the journal Astronomy and Astrophysics.
Findings On Cosmic Dust
The stars formed tend to create dust. These are particles made of heavy elements such as carbon, silicon, oxygen, and iron. Appearing as thick clouds in the space between the stars, the dust hides the stars completely from our eyes.
Since the dust hides the stars completely from our eyes, it becomes difficult to get a census of the star formation especially in young, "starburst" galaxies. In these galaxies, the dust has not yet had the time to disperse far from the compact sites of star formation.
The dust begins to glow in long-wavelengths as it is heated by the stars. Although infrared light, which has long wavelengths, is invisible to the human eye, it may be detected by telescopes designed to observe these wavelengths.
Only the surface of the clouds can be seen for the most compact, dust-enshrouded starbursts. The starburst galaxies are invisible not only at the "humans perceivable", optical wavelengths, but also in the beginning of the infrared spectrum. This is utterly dark even to the Hubble Space Telescope.
Seeing Galaxies In Longer Wavelengths
Astronomers at the Cosmic Dawn Center decided to take a look at the early universe at even longer wavelengths. They used the radio/microwave antennae at two of the world's largest radio observatories. These are the Atacama Large Millimeter Array (ALMA) in Chile, and the Northern Extended Millimeter Array (NOEMA) in France.
Shuowen Jin, a researcher at the Cosmic Dawn Center, found a population of compact starburst galaxies, cloaked in extremely thick dust clouds, with the help of observations of the same field on the sky acquired with other radio telescopes.
Looking Through The Clouds
Astronomers were able to measure the star formation rate and the temperature of the dust with the help of the radio- and microwave observations.
In a statement released by Niels Bohr Institute, Shuowen Jin said that in these epochs, one to two 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.
The study has explained why these galaxies are so dark in optical and infrared. Shuowen Jin explained that because the dust clouds are so thick and dense, optical and near-infrared light cannot travel through.
Jin added that even the far-infrared light is partially absorbed.
Not only dust, but also monoxide molecules mixed within the clouds. The light emitted by the carbon monoxide molecules can help astronomers probe an important quantity of galaxies — the mass of all the gas in the galaxy.