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The study Unveiling the largest structures in the nearby Universe: Discovery of the Quipu superstructure, published in Astronomy & Astrophysics, introduces the Quipu superstructure, the largest cosmic structure discovered to date. This research, led by Hans Böhringer and collaborators, provides a comprehensive assessment of large-scale cosmic structures using X-ray galaxy clusters from the CLASSIX survey.
Introduction to the Quipu Superstructure
The paper highlights how local large-scale structures influence cosmological measurements, such as the Hubble constant and the cosmic microwave background (CMB). Using X-ray galaxy clusters from the CLASSIX survey, the researchers mapped structures at distances between 130 and 250 Mpc. Among five significant superstructures, Quipu stands out with a length exceeding 400 Mpc and an estimated mass of approximately 2×10¹⁷ solar masses. The structure contains 68 galaxy clusters, constituting a major part of the observed universe within the target redshift range of 0.03 to 0.06.
Mapping the Universe with X-ray Galaxy Clusters
The CLASSIX cluster sample, derived from the ROSAT All-Sky Survey, is pivotal to this study. With nearly 86% sky coverage, this survey identifies galaxy clusters through their X-ray emissions, which correlate closely with cluster mass. The survey is over 90% complete outside the Galactic band, providing a robust dataset for large-scale structure mapping.
The researchers employed the friends-of-friends algorithm to identify superstructures, adjusting the linking length to cluster density. Five prominent superstructures were discovered, with Quipu being the most extensive and massive.
Characteristics of the Quipu Superstructure
Quipu extends 428 Mpc and contains 68 galaxy clusters with a combined mass of approximately 2.4×10¹⁷ solar masses. It lies partially in the Zone of Avoidance, making its full extent uncertain. The structure resembles a long filament with several side extensions, akin to the Incan quipu used for recording information.
The study also explores potential connections between Quipu and the Vela supercluster, which lies just beyond the Zone of Avoidance. The researchers note that with a slightly increased linking length, Quipu and Vela could merge into a single superstructure, indicating the vastness and complexity of cosmic structures.
Comparison with Other Superstructures
The study identifies four other major superstructures:
- Shapley Supercluster: Compact and dense, known for its significant gravitational pull on the Local Group.
- Serpens-Corona Borealis: Extensive but less dense than Shapley.
- Hercules Supercluster: Composed of many low-mass systems.
- Sculptor-Pegasus Supercluster: An elongated filamentary structure.
Among these, Shapley is the most massive, while Quipu is the largest in spatial extent.
Tracing Matter Distribution with Galaxy Surveys
The researchers compared the distribution of CLASSIX galaxy clusters with the 2MASS galaxy redshift survey, which maps galaxies based on their near-infrared brightness. The comparison revealed a strong correlation, with all five superstructures identifiable in the galaxy distribution. This alignment underscores the reliability of using galaxy clusters as tracers of large-scale cosmic structures.
The study also examined galaxy density around superstructures, finding that galaxy density remains elevated up to 45 Mpc from the nearest cluster, significantly farther than for clusters outside superstructures. This suggests that superstructures exert a strong influence on their surrounding environments.
Insights from Cosmological Simulations
The study compared its observations with cosmological simulations from the Millennium simulation, which models the formation of cosmic structures under the ΛCDM model. The simulations produced a similar number of superstructures, confirming that structures like Quipu are consistent with the prevailing cosmological model. The matter density distribution around superstructures in simulations also closely matched observations, validating the methodology used to identify these structures.
Implications for Cosmology
Impact on the Local Group’s Motion and the Hubble Constant
The study highlights how large-scale structures like Shapley and Quipu influence local cosmic flows. The gravitational pull of these superstructures contributes to the peculiar velocity of the Local Group relative to the CMB. This motion affects local measurements of the Hubble constant, which could lead to a better understanding of the Hubble tension.
Integrated Sachs-Wolfe Effect
The researchers investigated the potential imprint of superstructures on the CMB through the Integrated Sachs-Wolfe (ISW) effect. Large structures can cause slight temperature fluctuations in the CMB by altering the gravitational potential through which CMB photons travel. Using Planck satellite data, they searched for the ISW signal from the superstructures but only detected a signal with low statistical significance. However, the magnitude of the detected signal matched theoretical predictions, indicating the potential for more precise future detections with larger datasets or advanced detection techniques.
Summary
The study Unveiling the largest structures in the nearby Universe: Discovery of the Quipu superstructure presents the discovery of Quipu, the largest cosmic structure found to date. This structure provides critical insights into the large-scale architecture of the universe and its impact on cosmological measurements. The comparison with simulations reinforces the validity of current cosmological models, while the search for the ISW effect highlights the potential for future discoveries in CMB research. These findings underscore the importance of large-scale structure studies in understanding the universe’s evolution and properties.
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