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Astronomers find surprisingly mature galaxies in the early universe

Artist's illustration of a dusty, rotating distant galaxy, in the early universe. In this image, the red color represents gas, and blue/brown represents dust as seen in radio waves with ALMA. Many other galaxies are visible in the background, based on optical data from VLT and Subaru. Credit: B. Saxton NRAO/AUI/NSF, ESO, NASA/STScI; NAOJ/Subaru
by TR Pakistan

When the universe was only a tenth of its current age, its galaxies experienced a “growth spurt.”

It was this period that the scientists in the ALPINE project focused on when they used the ALMA telescope of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO) to carry out the first ever large survey of distant galaxies. To their surprise, these galaxies observed in the early stages of their life were far more mature than expected.

Their work is the subject of a series of articles published on 27 October, 2020 in the journal Astronomy & Astrophysics, signed among others by members of the CNRS and Aix-Marseille Université.

Galaxies began to form very early in the history of the universe. To study their infancy, it is therefore necessary to go back to the dawn of time, by observing very distant galaxies. The ALPINE project focused on a period between 1 and 1.5 billion years after the Big Bang, when the first galaxies experienced a phase of rapid growth.

Although such distant galaxies have already been observed, this is the first time that so many of them have been studied systematically. An international team of astronomers studied images of 118 massive galaxies which have already reached a mass similar to that of the Milky Way today.

These images were obtained with the Hubble (visible light) and Spitzer (near infrared) space telescopes, as well as spectra acquired using the ground-based VLT and Keck telescopes. It was supplemented by 70 hours of observation with ALMA at submillimetre wavelengths (between the infrared and radio waves).

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The Atacama Large Millimeter/submillimeter Array (ALMA) is a telescope designed for the study of radiation from the coldest objects in the universe, and consists of 66 antennas located in the Atacama desert in the Chilean Andes. It can quantify dust, a sign of maturity in galaxies, and cold gas, which provides information about their rate of growth and the number of stars they can form, as well as the motion of this gas, thus revealing the dynamics of galaxies.

“With ALMA we discovered a few distant galaxies for the first time. We call these Hubble-dark as they could not be detected even with the Hubble telescope,” said Lin Yan of Caltech.

And this turned up some surprising data.

For a start, the observed galaxies proved to be very rich not only in cold gas, which fuels star formation, but also in dust, which is thought to be a by-product of stars at the end of their lives. So despite their young age, these galaxies had apparently seen the formation and death of a first generation of stars.

The galaxies surveyed also exhibit an astonishing diversity of shapes: some are disordered, others already have a rotating disc that may end up as a spiral structure like the Milky Way, while yet others have been spotted in the process of merging.

Another surprising observation is that certain galaxies appear to be ejecting gas, forming mysterious haloes around them.

The survey thus raises a number of new questions about the early evolution of galaxies.

To learn more about distant galaxies, the astronomers want to point ALMA at individual galaxies for a longer time. “We want to see exactly where the dust is and how the gas moves around. We also want to compare the dusty galaxies to others at the same distance and figure out if there might be something special about their environments,” stated Paolo Cassata of the University of Padua in Italy, formerly at the Universidad de Valparaíso in Chile.

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