Astronomers have reconstructed detailed three-dimensional maps of two enormous protoclusters seen when the universe was only about 2.1 billion years old, revealing multiple dense clumps connected to the surrounding cosmic web.One structure, COSMOS-z3.1-A, contains 10 distinct density peaks and is so large that researchers classify it as a proto-supercluster, making it the earliest and most distant structure of its kind yet identified.The observations support the \u201cbottom-up\u201d picture of cosmic structure formation, in which smaller concentrations of matter grow and merge along filaments to produce the massive, more rounded galaxy clusters seen in the modern universe.<\/p>\n
A sprawling network of young galaxies seen more than 11 billion years in the past may represent the earliest known ancestor of one of the universe\u2019s largest structures<\/a>.<\/p>\n The enormous formation, called COSMOS-z3.1-A, existed when the universe was only about 2.1 billion years old. Rather than resembling a mature galaxy cluster, it contains numerous dense clumps woven together by filaments of galaxies and matter.<\/p>\n Astronomers now classify it as a proto-supercluster, an exceptionally early predecessor of a future collection of galaxy clusters. Its estimated mass is roughly 5,000 times that of the Milky Way.<\/p>\n The discovery comes from an international team led by Vandana Ramakrishnan, then a graduate student at Purdue University<\/a>. Published in The Astrophysical Journal<\/a>, the work combines observations from the ODIN survey with extensive spectroscopy to reconstruct two distant structures in three dimensions.<\/p>\n Spectroscopically confirmed sources for COSMOS-z3.1-A (top) and COSMOS-z3.1-C (bottom), overlaid on the 2D LAE surface density map. (CREDIT: Vandana Ramakrishnan et al, The Astrophysical Journal) <\/p>\n \u201cWith 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,\u201d Ramakrishnan said. \u201cWe also hope to get a better sense of how these protoclusters are connected to the larger cosmic web.\u201d<\/p>\n Finding the seeds of today\u2019s largest structures<\/p>\n Galaxy clusters are among the most massive gravitationally bound structures in the universe. Hundreds or even thousands of galaxies can occupy a cluster containing enormous quantities of dark matter and hot gas.<\/p>\n The clusters astronomers see nearby are comparatively mature. Their distant ancestors looked very different.<\/p>\n Protoclusters consist of galaxies and dark matter<\/a> that have not yet collapsed into one stable, gravitationally bound structure. At high redshift, they can stretch across enormous regions and contain several separate concentrations that will eventually evolve into massive clusters.<\/p>\n The ODIN survey, short for One-hundred-deg\u00b2 DECam Imaging in Narrowbands, is designed to find these early structures. It uses the 570-megapixel Dark Energy Camera on the V\u00edctor M. Blanco 4-meter Telescope in Chile to locate galaxies emitting strongly at the hydrogen Lyman-alpha wavelength.<\/p>\n Earlier ODIN observations identified about 150 protocluster candidates at redshifts of 2.4 and 3.1. The researchers concentrated on two particularly striking systems at redshift 3.1, COSMOS-z3.1-A and COSMOS-z3.1-C.<\/p>\n<\/p>\n Turning a flat picture into a 3D cosmic map<\/p>\n Images can show where galaxies appear across the sky, but that provides only two dimensions. Galaxies that seem close together could actually sit millions of light-years apart along our line of sight.<\/p>\n The team therefore added spectroscopic measurements capable of determining much more precise galaxy redshifts and distances.<\/p>\n Most of the spectroscopy came from the Dark Energy Spectroscopic Instrument<\/a>, or DESI, which can collect spectra from as many as roughly 5,000 objects in a single pointing. Additional measurements came from the DEIMOS instrument on the Keck II telescope in Hawaii and the Gemini Multi-Object Spectrograph on Gemini South in Chile.<\/p>\n For COSMOS-z3.1-A, dedicated Keck observations confirmed 111 of 161 targeted Lyman-alpha-emitting galaxies and seven of eight targeted Lyman-alpha blobs. Gemini observations confirmed another 27 of 38 targeted emitters around COSMOS-z3.1-C.<\/p>\n The researchers combined those measurements with the larger photometric sample using a probabilistic reconstruction technique. They divided space into three-dimensional cells and repeatedly assigned probable distances to galaxies lacking spectroscopy, producing 500 realizations before averaging them into a final map.<\/p>\n Tests against simulated galaxy populations showed that this method reproduced the underlying matter distribution more accurately than spectroscopy alone.<\/p>\n<\/p>\n Ten enormous clumps form a proto-supercluster<\/p>\n The resulting view transformed COSMOS-z3.1-A from a projected galaxy overdensity into a complex three-dimensional structure.<\/p>\n It contains about 400 Lyman-alpha-emitting galaxies and 17 Lyman-alpha blobs, with spectroscopy confirming 249 of the emitters and 13 blobs. The reconstruction identified 10 distinct high-density peaks.<\/p>\n Those peaks are not isolated. Long structures extend between and away from them, consistent with cosmic filaments<\/a> feeding matter into the developing system.<\/p>\n Each major peak is expected to evolve into a cluster-sized dark matter halo. Taken together, the researchers conclude that COSMOS-z3.1-A is the progenitor of an exceptionally massive system, with the combined descendant mass of its principal components estimated at roughly twice that of the Coma Cluster.<\/p>\n That makes it more than an ordinary protocluster.<\/p>\n The team identifies COSMOS-z3.1-A as a proto-supercluster comparable to Hyperion, a previously discovered seven-component structure seen at redshift 2.45. COSMOS-z3.1-A appears at redshift 3.1, meaning astronomers are seeing it at an earlier point in cosmic history.<\/p>\n<\/p>\n The two systems occupy similarly enormous comoving volumes of roughly 100,000 cubic megaparsecs, although their orientation relative to Earth differs substantially.<\/p>\n The cosmic web appears to be feeding the clusters<\/p>\n COSMOS-z3.1-C also proved highly structured. Its three-dimensional map revealed four major density peaks connected through lower-density regions.<\/p>\n Both systems show elongated shapes, extended tails and multiple separate concentrations. These features resemble the filamentary structures predicted by cosmological simulations.<\/p>\n That architecture supports the hierarchical, or bottom-up, theory of structure formation. In this picture, small concentrations of matter form first. Gravity gradually draws them together while additional gas and dark matter<\/a> flow in through the cosmic web.<\/p>\n Over billions of years, separate clumps merge and collapse into the smoother, more centrally concentrated galaxy clusters seen closer to Earth today.<\/p>\n The observations also provided an intriguing clue about Lyman-alpha blobs, enormous clouds of glowing hydrogen associated with intense activity in the early universe. Their surface density around the two structures was about four times the average, yet none of the confirmed blobs occupied the densest cores.<\/p>\n Left: 3D galaxy distribution with green and blue lines marking two selected lines of sight. Right: corresponding probability distributions used to assign redshifts to nearby LAEs, with the N501 transmission curve shown as a gray dashed line. (CREDIT: Vandana Ramakrishnan et al, The Astrophysical Journal) <\/p>\n Instead, their median distance from the nearest density peak was about 17 comoving megaparsecs. That places them mainly around cluster outskirts or filaments, strengthening evidence that their formation is connected to the broader cosmic environment.<\/p>\n An extremely rare glimpse of cosmic construction<\/p>\n COSMOS-z3.1-A appears extraordinarily unusual even by protocluster<\/a> standards.<\/p>\n The researchers estimate that structures like it have a number density no higher than about five per 100 million cubic comoving megaparsecs. Comparable objects were absent from the cosmological simulation volume the team used for testing.<\/p>\n Orientation creates another challenge. The three-dimensional maps show that the same structure can appear compact, elongated or broken into separate objects depending on the observer\u2019s viewing angle.<\/p>\n That means some enormous structures could be difficult to recognize using images alone. Combining wide-field imaging with spectroscopy may therefore be essential for finding the most extreme structures in the early universe.<\/p>\n