Scientists have discovered the galaxy protoclusters COSMOS-z3.1-A and COSMOS-z3.1-C in the depths of space, billions of light-years away from us. At present, these colossal structures resemble ordinary collections of galaxies surrounded by large amounts of gas and dust, but in the future they are expected to grow into superclusters containing tens of thousands of stellar systems similar to the Milky Way.
Colossal Structures of the Universe and Their Precursors
An international team of astronomers has discovered the most distant precursor of a galaxy supercluster ever found. The discovery confirms existing theories about the evolution of galaxy clusters and reveals how they are connected to the larger cosmic web. This is reported by noirlab.edu.
Galaxy clusters are the most massive gravitationally bound structures in the Universe. These colossal groupings, containing hundreds to thousands of galaxies, extend across millions of light-years. They are held together by a large concentration of dark matter, which serves as a building block for the larger structure of the Universe.
The clusters that scientists observe relatively close to us in space and time are mature structures that evolved from so-called “protoclusters.” These protoclusters are enormous, loosely bound concentrations of galaxies that are in the process of merging but have not yet formed stable clusters. By studying very distant protoclusters that formed when the Universe was relatively young, scientists can understand how modern galaxy clusters grew and evolved over time.
For this purpose, an international team led by Vandana Ramakrishnan, who was a graduate student at Purdue University at the time of the study, looked billions of years into the past to find these ancient precursors of galaxy clusters.
“With this project, we hope to understand how massive structures in the Universe grow and how they influence the evolution of the galaxies within them,” says Ramakrishnan. “We also hope to gain a better understanding of how these protoclusters are connected to the broader cosmic web.”
Creating a Three-Dimensional Map
In their study, the authors used data from the One-hundred-deg2 DECam Imaging in Narrowbands (ODIN) survey. This survey is being conducted with the Dark Energy Camera (DECam), built by the U.S. Department of Energy (DOE), installed on the National Science Foundation’s 4-meter Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, which is part of NSF NOIRLab. Thanks to its wide field of view and 570-megapixel resolution, DECam has spent more than 100 nights over the past three years taking deep images of a vast region of the Southern Hemisphere sky.
In total, the team identified 150 distant protoclusters that formed when the Universe was approximately 1–3 billion years old. They focused on two clusters that showed an impressive overdensity of galaxies and were named COSMOS-z3.1-A and COSMOS-z3.1-C. The ODIN survey provided two-dimensional coordinates for the positions of these structures in the sky. However, to gain a true understanding of the distribution of galaxies within them and how they fit into the large-scale cosmic web, a three-dimensional perspective was needed.
To create a 3D map of these structures, Ramakrishnan was joined by two other graduate students — Byonga Moon (KASI) and Nicole Firestone (Rutgers University) — for follow-up observations using a set of instruments called spectrographs. Unlike two-dimensional imaging of the sky, spectrographs use the properties of light to measure the distance to an object, allowing scientists to determine its position in three-dimensional space.
The team obtained additional spectra using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini South telescope in Chile — one of the components of the International Gemini Observatory, which is partly funded by the National Science Foundation (NSF) and operated by NSF NOIRLab — as well as using the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope located on Maunakea in Hawaii.
This study is one of the first in which such detailed 3D maps of several distant protoclusters have been created. These maps allowed the team to predict what kinds of clusters COSMOS-z3.1-A and COSMOS-z3.1-C will eventually evolve into. They found that both will become more massive than the largest known galaxy cluster in our local Universe — the Coma Cluster.
Clumpy Protoclusters at the Dawn of Our Universe
Moreover, scientists established that COSMOS-z3.1-A is something even rarer than a protocluster, which is already a relatively rare phenomenon. It is a protosupercluster, or the precursor of a “cluster of clusters” of galaxies. This object, observed when the Universe was only 2.1 billion years old, is the earliest and most distant protosupercluster ever discovered, and it has an impressive mass approximately 5,000 times that of the Milky Way.
Based on their detailed 3D maps, the team concluded that these ancient protoclusters have a highly clumpy and irregular appearance and are located at the intersections of numerous filaments of the cosmic web. For the first time, scientists have directly observed such features in the distant Universe, and these observations are consistent with their expectations for how matter is distributed in space.
Current models suggest that structure formation proceeds according to a “bottom-up” principle: smaller structures form first and then merge to create larger ones. The clumpy substructures visible in the 3D maps are likely evidence of this bottom-up growth. As the protoclusters evolve, these clumps will merge to form clusters similar to those seen in the local Universe, which have a much rounder shape.
New Discoveries with New Instruments
Thanks to its wide survey area and observational depth, ODIN is well positioned to discover more such massive cosmic structures in the distant Universe. “The 3D reconstruction methodology presented in this work will become an important component of these efforts, allowing us to clearly distinguish the cores and outskirts of protoclusters, as well as the cosmic filaments that feed them,” Ramakrishnan notes.
The team is eagerly anticipating new discoveries in the coming decade, when the NSF–DOE Vera C. Rubin Observatory conducts its groundbreaking Legacy Survey of Space and Time (LSST). By imaging the entire Southern Hemisphere sky every few nights, Rubin Observatory will create a rich dataset that will complement ODIN’s deep images in overlapping regions of the sky. Together, these data will provide a profound view of both the nearby and distant Universe, allowing scientists to study the evolution of galaxy clusters across cosmic time.
