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When the Universe was only 10% of its current age, most early galaxies experienced a so-called “growth surge”. During this period, galaxies built up most of their stellar mass and other properties, such as dust, heavy element content, and spiral disk shapes. In a survey called ALPINE (the ALMA Large program to investigate C + at Early Times), astronomers used the Atacama Large Millimeter / submillimeter Array (ALMA) to observe 118 primordial galaxies that were experiencing a growth spurt.
An artist’s impression of a distant dusty and rotating galaxy in the early Universe: the red color represents gas and the blue / brown represents dust as seen in radio waves with ALMA; many other galaxies are visible in the background, based on optical data from ESO’s Very Large Telescope and NAOJ’s Subaru Telescope. Image credit: B. Saxton / NRAO / AUI / NSF / ESO / NASA / STScI / NAOJ / Subaru.
Galaxies are considered more “mature” than “primordial” when they contain a significant amount of dust and heavy elements, by-products of dying stars.
But the galaxies of the early Universe haven’t had much time to build stars yet, so astronomers don’t expect to see a lot of dust or heavy elements there too.
“To our surprise, many of them were much more mature than we expected,” said Dr. Andreas Faisst, astronomer at Caltech’s Infrared Processing and Analysis Center (IPAC).
“We didn’t expect to see so much dust and heavy elements in these distant galaxies.”
“From previous studies, we have learned that these young galaxies are low in dust,” said Dr. Daniel Schaerer, astronomer at the University of Geneva.
“However, we find that about 20% of the galaxies that assembled during this first epoch are already very dusty, and a significant fraction of the ultraviolet light from newborn stars is already hidden by this dust.”
Many of the galaxies were also considered relatively adult because they showed diversity in their structures, including early signs of spin-supported disks – which can later lead to galaxies with a spiral structure as seen in galaxies such as our Milky Modo.
“We see many galaxies colliding, but we also see a number of them rotating in an orderly fashion with no signs of collision,” said Dr. John Silverman, an astronomer at the Kavli Institute for the Physics and Mathematics of the Universe.
These are two of the early Universe galaxies that ALMA observed in radio waves; these galaxies are considered more “mature” than “primordial” because they contain large amounts of (yellow) dust; ALMA also revealed the gas (red), which is used to measure obscured star formation and movements in galaxies. Image Credit: B. Saxton / NRAO / AUI / NSF / ESO / NASA / STScI / NAOJ / ALPINE Team.
ALPINE is the first survey that allowed astronomers to study a significant number of galaxies in the early Universe and shows that they could evolve faster than expected.
“With ALMA we have discovered some distant galaxies for the first time,” said Dr. Lin Yan, Caltech astronomer.
“We call these galaxies ‘Hubble-dark’ because they haven’t even been detected with the NASA / ESA Hubble Space Telescope.”
“Such a large and complex investigation is only possible thanks to the collaboration between multiple institutes around the world,” said Dr. Matthieu Béthermin, astronomer of the Laboratoire d’Astrophysique de Marseille.
To learn more about distant galaxies, astronomers plan to point ALMA at individual galaxies for a longer time.
“We want to see exactly where the dust is and how the gas moves,” said Dr. Paolo Cassata, astronomer of the University of Padua.
“We also want to compare dusty galaxies with others at the same distance and see if there might be something special about their environments.”
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