The Most Massive Objects in the Modern Universe –Primordial Relics of the Cosmic Microwave Background

Merging Galaxies of the Early Universe


“In Big Bang cosmology, the cosmic microwave background (CMB) is in some sense a map of fluctuations thought to be linked to variations in the matter density of the primordial expansion of the universe,” explains Harvard astronomer Matthew Ashby in an email to The Daily Galaxy about the formation of massive galaxy clusters from the cosmic web of filaments, nodes, and voids, with the nodes being clusters of galaxies. “Their amplitude is very small. But, because they trace fluctuations in the primordial density of matter, they will grow over cosmic time through the action of gravity.  And for that reason, massive coherent structures such as galaxy cluster SPT2349-56, the most massive objects in the modern Universe, are of great interest, being among the very first to form (that is, collapse) out of those ancient fluctuations.  

Galaxy clusters and protoclusters are relics of the primordial density fluctuations

“Because galaxy clusters and protoclusters are in a sense relatively nearby relics of the primordial density fluctuations,” Ashby continued, “many astronomers are working hard to characterize the masses, distributions, sizes, and distances to those objects.  Doing so has the potential to shed new light on how the universe grew during its very first moments.  An advantage of the SPT for this science is its ability to reliably detect so many high-redshift clusters, of which SPT2349-56 is among the very most interesting.”

Cosmic Web Structure of the Universe

The structure of the universe, reports the  Harvard CfA, is often described as being a cosmic web of filaments, nodes, and voids, with the nodes being clusters of galaxies, the largest gravitationally bound objects known. These nodes are thought to have been seeded by small-amplitude density fluctuations like those observed in the cosmic microwave background (CMB) which grew until they collapsed into the structures seen today. While the CMB is well understood, and the details of present-day galaxy clusters are well-described, the intermediate phases of evolution lack sufficient observations to constrain the models. Traditional galaxy cluster searches assume these objects have had enough time to equilibrate so that the intergalactic gas has heated up enough to be detected in X-ray emission. To detect the more distant galaxies and protoclusters that are too faint to detect in the X-ray, astronomers use their bright infrared or submillimeter emission instead.

Before the “Bang” –The Primordial Universe 

Supercluster SPT2349−56

The supercluster SPT2349−56, discovered in the submillimeter band by the South Pole Telescope, is so distant that its light has been traveling for over twelve billion years. It hosts over thirty submillimeter-bright galaxies and dozens of other luminous and/or spectroscopically confirmed star-forming galaxies. It is one of the most active star forming complexes known, producing over ten thousand stars per year. One of its bright sources appears to be the merger of over twenty galaxies. The stellar mass of the system, however, was not known, making it impossible for example to know whether the huge burst of stars was the result of an extraordinary efficiency or simply arose because the system was so extremely large.

Spectral energy distribution (SED) analyses

Harvard CfA astronomer Ashby was a member of a team that has now completed very deep observations at optical and infrared wavelengths to obtain the stellar masses through spectral energy distribution (SED) analyses. They used the Gemini and Hubble Space Telescopes to obtain optical/near infrared flux measurements and Spitzer’s IRAC camera for the infrared flux. In order to model the SEDs, the many point sources detected need to be matched to one another at all wavelengths. This is a complex undertaking, and the scientists describe the processes for doing so while also addressing the serious blending that can occur due to inadequate spatial resolution in the infrared.

Galaxy Clusters -“Dark Skeletons of the Cosmos”

According to their results published in Monthly Notices of the Royal Astronomical Society, the astronomers find that the stellar mass in this primordial cluster as compared with its star formation rate is close to the value measured in nearby (“normal”) galaxies, a conclusion that suggests that the star formation processes at work are similar to those in the local universe. The cluster does, however, show a deficit of molecular gas, suggesting that the activity is nearing the end of this tumultuous phase as the gaseous raw material for stars is being dissipated.

More information: Ryley Hill et al, Rapid build-up of the stellar content in the protocluster core SPT2349−56 at z = 4.3, Monthly Notices of the Royal Astronomical Society (2021). DOI: 10.1093/mnras/stab3539

Matthew Ashby and Harvard Center for Astrophysics

Image credit: The artist’s impression of SPT2349-56 at the top of the page shows a group of interacting and merging galaxies in the early Universe. Such mergers have been spotted using the ALMA and APEX telescopes and represent the formation of galaxies clusters, the most massive objects in the modern Universe. Astronomers thought that these events occurred around three billion years after the Big Bang, so they were surprised when the new observations revealed them happening when the Universe was only half that age.

Credit: ESO/M. Kornmesser

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