Universe Today
We live on a tiny world, embedded in a galaxy that is, itself, part of a vast, interconnected lattice of galaxies, galaxy clusters, gas, dust, and dark matter called the Cosmic Web. This lattice traces its way through the entire Universe, making it the largest cosmic structure we've found. Astronomers have been probing this web with ever-more-powerful telescopes, looking for its seeds and roots in the early Universe.
Recently, a team analyzing data from a survey made using the Dark Energy Camera on the Victor M. Blanco telescope in Chile found the most distant-known piece of the web, a galaxy proto-supercluster called COSMOS-z3.1-A. They saw it as it looked about 12 billion years ago — a loose collection of galaxies that eventually coalesced into a gravitationally bound supercluster.
The structure of this infant proto-supercluster shows 10 dense groups of galaxies existing at a time when the Universe was only a couple of billion years old. They were in the process of forming clusters on their own before combining to form the larger proto-supercluster. The survey of these primordial structures gives astronomers a new look at how modern galaxy structures formed billions of years ago. The discovery team, led by Vandana Ramakrishnan, a Purdue graduate student, used the survey to go back through cosmic time on a hunt for answers to questions about early galaxy cluster environments. “With this project, we’re hoping to understand the growth of massive structures in the Universe and how they influence the evolution of galaxies within them,” said Ramakrishnan. “We also hope to get a better sense of how these protoclusters are connected to the larger cosmic web.”
The video below lets us zoom into the region that contains COSMOS-z3.1-A and its surroundings.
ODIN Peers Back Through the Eons
The data for the discovery came from the ODIN Survey (short for the One-hundred-deg2 DECam Imaging in Narrowbands). The survey was taken over 100 nights of observing throughout a three-year timespan. It captured some of the deepest images of the southern hemisphere sky ever taken, revealing very early eons of galaxy cluster formation. The team used the data to identify 150 very distant protoclusters. They formed sometime during the first three billion years of cosmic history. Seeing them that early gives scientists a chance to figure out the origins of not just the galaxies and their clusters, but the role they played in the formation of the cosmic web we see today.
The Dark Energy Camera that performed the ODIN survey is attached to the Victor M. Blanco Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. CREDIT: NOIRLab.
The connection to the modern Universe of superclusters is one that astronomers have been tracing for a while now. According to astronomer Eric Gawiser at Rutgers University, determining the link is important for understanding details of the overall cosmic evolution. “When we look at galaxy clusters in the nearby Universe, we are seeing the finished product,” he said. “This distant structure takes us back to a much earlier stage when the individual pieces were still coming together. It allows us to study how the Universe built structures on its largest scales.”
The data from the ODIN survey also shows that COSMOS-z3.1-A is a rare proto-supercluster, with a mass about 5,000 times that of the Milky Way. That's quite a large structure to be found at such an early time in cosmic time. “COSMOS-z3.1-A represents the most extreme, most overdense regions of the Universe,” said Ramakrishnan. “We think there should be fewer than one such object for every 10,000 galaxy clusters!”
Supporting a Hierarchical Evolutionary Process
Gawiser pointed out that the discovery of these rare early protoclusters supports the idea that the Universe's structures developed hierarchically — that is, from the bottom up. It's a complex process, but here's a simplified look at how cosmologists suggest the cosmos evolved. The earliest Universe was a soup of protons that eventually combined to create molecules of hydrogen. The hydrogen was shepherded by the action of dark matter into regions of greater density. Those clumped together and began to form the first stars and galaxies. The galaxies were gravitationally attracted to form clusters, and then on to superclusters, which became the backbone of the cosmic web.
The team's 3D maps of these clusters made from the ODIN data show that the ancient structures are largely clumpy and irregularly shaped. Not only that, but they appear to be pegged at the intersections of multiple cosmic web filaments. The science team suggests that this clumpy substructure is strong evidence that the Universe was a bottom-up construction job. The clumps will collapse together as the protoclusters evolve. Eventually, as cosmic time progressed, they gave rise to clusters similar to what we see in the modern, local Universe. These are much rounder in shape.
Galaxy clusters are the Universe’s largest individual structures (containing many galaxies held together by gravity). Each cluster and supercluster can have hundreds of thousands of galaxies stretched across millions of light-years of space. For example, the Virgo Supercluster (which contains the Virgo Cluster and our home cluster, the Local Group), spans nearly 150 million light-years. The Virgo Supercluster itself is part of a larger fragment of the cosmic web called the Laniakea Supercluster. There are about 10 million superclusters known in the observable Universe.
Galaxy clusters can contain thousands of galaxies and span millions of light-years. They also contain much of the dark matter in the Universe, which we can detect because of its gravitational effect on the matter we can see. Future surveys done by the Vera C. Rubin Observatory should contribute more discoveries like COSMOS-z3.1-A and its siblings in the early Universe. The result should be a much better understanding of galaxy supercluster evolution from the earliest epochs of the cosmos across the eons of cosmic time.
For More Information
Extremely Massive Galaxy Proto-Supercluster Smashes Distance Records
Rutgers Astronomers Help Identify 10 developing Galaxy Clusters from About 12 Billion Years Ago
ODIN: Characterizing the Three-dimensional Structure of Two Protocluster Complexes at z=3.1
