Abstract
CLUSTERS of galaxies are not distributed randomly in space, but are themselves clustered, reflecting inhomogeneities in the early Universe1. The degree of clustering–usually expressed as a correlation length, which measures the characteristic scale for clustering–can therefore be used to determine the size of the initial density fluctuations that gave rise to the clusters1–3. Optical studies of galaxy clusters4–7 have indicated a correlation length that con-flicts with the predictions of some theories of large-scale structure formation1–3, leading to the suggestion that these optical samples are biased in that foreground or background galaxies not phys-ically associated with a cluster are counted as part of it8–10. Here we report a measurement of the correlation length for a sample of clusters that were selected based on their X-ray emission, which is free from the bias that is inherent to optical studies. We find a correlation length of 13–15 h-l Mpc, where h is the Hubble con-stant in units of 100 km s-1 Mpc-1. There is no evidence for clusters being significantly elongated along the line of sight, contrary to previous suggestions11,12.
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Romer, A., Collins, C., Böhringer, H. et al. The large-scale distribution of X-ray clusters of galaxies. Nature 372, 75–77 (1994). https://doi.org/10.1038/372075a0
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DOI: https://doi.org/10.1038/372075a0
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