“How Did They Get So Big So Fast?”- Primordial Galaxies in a Colossal Mashup Near the Beginning of Time


Galaxy Cluster SPT2349 i


An ancient  megamerger of 14 relic starbursting galaxies could become the most massive structure in our Universe. “The fact that this is happening so early in the history of the universe poses a formidable challenge to our present-day understanding of the way structures form in the universe,” said Scott Chapman, an astrophysicist at Dalhousie University in Halifax, Canada, about a massive galaxy clusters found in 2018 that date to times as early as three billion years after the Big Bang, containing stars that formed at even earlier epochs. Each of these galaxies is forming stars between 50 and 1,000 times more quickly than our own Milky Way.

“How It Got So Big So Fast is a Bit of a Mystery”

“How this assembly of galaxies got so big so fast is a bit of a mystery, it wasn’t built up gradually over billions of years, as astronomers might expect,” said Tim Miller, a doctoral candidate at Yale University and coauthor on the paper. “This discovery provides an incredible opportunity to study how galaxy clusters and their massive galaxies came together in these extreme environments.” Miller works with data from the Hubble Space Telescope and the Dragonfly Telephoto array to study galaxy sizes and the outskirts of galaxies at high and low redshift.

Peering deep into space—an astounding 90 percent of the way across the observable universe—astronomers have witnessed the beginnings of a gargantuan cosmic pileup, the impending collision of 14 young, starbursting galaxies. This ancient megamerger is destined to evolve into one of the most massive structures in the known universe: a cluster of galaxies, gravitationally bound by dark matter and swimming in a sea of hot, ionized gas.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), an international team of scientists has uncovered a startlingly dense concentration of 14 galaxies that are poised to merge, forming the core of what will eventually become a colossal galaxy cluster.

This tightly bound galactic smashup, known as a protocluster, is located approximately 12.4 billion light-years away, meaning its light started traveling to us when the universe was only 1.4 billion years old, or about a tenth of its present age. Its individual galaxies are forming stars as much as 1,000 times faster than our home galaxy and are crammed inside a region of space only about three times the size of the Milky Way. The resulting galaxy cluster will eventually rival some of the most massive clusters we see in the universe today.

How Did Gargantuan Galaxies Appear So Soon After the Big Bang?

“Having caught a massive galaxy cluster in the throes of formation is spectacular in and of itself,” said Chapman who specializes in observational cosmology and studies the origins of structure in the universe and the evolution of galaxies.

During the first few million years of cosmic history, normal matter and dark matter began to aggregate into larger and larger concentrations, eventually giving rise to galaxy clusters, the largest objects in the known universe. With masses comparable to a million billion suns, clusters may contain as many as a thousand galaxies, vast amounts of dark matter, gargantuan black holes, and X-ray emitting gas that reaches temperatures of over a million degrees.

Current theory and computer models suggest that protoclusters as massive as the one observed by ALMA, however, should have taken much longer to evolve.

Galaxy Cluster SPT2349

Zooming in to the galaxies discovered by ALMA that are evolving into a galaxy cluster. The outer field is from data taken by the Hershel Space Observatory. The middle image — a portion of a much-wider survey by NSF’s South Pole Telescope — uncovered the distant galactic source that was studied by ALMA to reveal the 14 galaxies. Credit: ALMA (ESO/NAOJ/NRAO), T. Miller & S. Chapman et al.; Herschel; South Pole Telescope; (NRAO/AUI/NSF) B. Saxton

First Observed as a Faint Smudge

This particular galactic protocluster, designated SPT2349-56, was first observed as a faint smudge of millimeter-wavelength light in 2010 with the National Science Foundation’s South Pole Telescope. Follow-up observations with the Atacama Pathfinder Experiment (APEX) telescope helped confirm that it was in fact an extremely distant galactic source and worthy of follow-up observations with ALMA. ALMA’s superior resolution and sensitivity allowed astronomers to distinguish no fewer than 14 individual objects in a shockingly small region of space, confirming the object was the archetypical example of a protocluster in a very early stage of development.

This cluster’s extreme distance and clearly defined components offer astronomers an unprecedented opportunity to study some of the first steps of cluster formation less than 1.5 billion years after the Big Bang. By using the ALMA data as the starting conditions for sophisticated computer simulations, the researchers were able to demonstrate how this current collection of galaxies will likely grow and evolve over billions of years.

Merging into a Single Gigantic Galaxy

“ALMA gave us, for the first time, a clear starting point to predict the evolution of a galaxy cluster. Over time, the 14 galaxies we observed will stop forming stars and will collide and coalesce into a single gigantic galaxy,” said Chapman.

Why Were the Earliest Galaxies In the Universe Brighter Than Those We See Today?

The Last Word –How the Cluster Got So Big, So Fast

When asked how this assembly of galaxies got so big so fast so early in the history of the Universe, Scott Chapman replied in an email to The Daily Galaxy:“There have been several subsequent studies by my group on this proton-cluster of galaxies. 

“These studies all corroborate the original 2018 hypothesis published in Nature that this structure indeed represents an immense, very massive cluster of galaxies that is assembling at an early time. The reason it ‘got so big so fast so early’ seems to simply be that it really is one of the most massive structures in the observable universe, and therefore started its collapse very early. Subsequent paper in 2020 showed the structure is much larger and more massive.

“In 2021, a study by astronomer Kaja Rotermund showed that there is an overdensity of optically selected galaxies as well, (not just dusty star forming galaxies) confirming that the massive collapsing structure showed that an immense amount of stellar mass had already assembled in the core. This finding  demonstrated that it started very early in the formation of the Universe, was rapidly building up stellar mass, and would become an unusually massive object today compared to anything else we see around us.

“A subsequent study of objects discovered in the South Pole Telescope data showed that SPT2349 is not unique, but that there are another eight comparable systems in the same survey, showing such clusters occur about 1/250 deg^2 on the the sky (or that there are about 160 of these in the observable universe). 

“A study in 2022 using Hubble Space Telescope observations showed that the cluster galaxies have evolved differently in this very dense environment of the infant Universe.”

In 2017, Chapman was part of the international group of scientists constructing the Cerro Chajnantor Atacama Telescope-prime (CCAT-p), located 5,600 meters above sea level at the summit of Cerro Chajnantor in Chile’s Atacama Desert This state-of-the-art, six-meter aperture telescope will help us see the sky in unprecedented ways. 

“We’re trying to understand the earliest history of galaxies like our own Milky Way, and the origins of the Universe,” said Chapman  “Many of the most interesting objects in the universe emit light in the far-infrared and submillimetre range, which is invisible to the naked eye.”

The image at the top of the page is an artist’s impression of the 14 galaxies detected by ALMA as they appear in the very early, very distant universe. These galaxies are in the process of merging and will eventually form the core of a massive galaxy cluster. (NRAO/AUI/NSF; S. Dagnello)

The Daily Galaxy Editorial Staff with Scott Chapman via National Radio Astronomy Observatory and Nature

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