Cosmic acceleration and galaxy cluster growth

Article By:

Forman, William R. Smithsonian Astrophysical Observatory, Cambridge, Massachusetts.

Vikhlinin, Alexey Smithsonian Astrophysical Observatory, Cambridge, Massachusetts.

Last reviewed:2010

DOI:https://doi.org/10.1036/1097-8542.YB100067

Galaxy clusters are the largest and most massive gravitationally bound objects in the universe. Even though they are made up of thousands of galaxies, each with tens of billions of stars, the bulk of the visible mass in clusters (that is, mass in atoms and nuclei) resides in a hot (10⁷–10⁸ K) gas that radiates predominantly in the 1–10-keV (equivalent to photons of wavelength 1.24–0.124 nm) x-ray band (Fig. 1). Clusters are unmistakable lighthouses in the x-ray universe—they are x-ray luminous (10³⁶– 10³⁸ J/s) and large, with typical sizes of about 1 megaparsec (3 × 10²² m = 3 million light-years). They are detectable to large distances (redshift z ∼ 1, when the universe was less than half its present age) and appear as extended sources (they are resolvable) at all distances with the current generation of x-ray telescopes. While the cluster gas is hot, it also has very low density, typically 0.1–1 atom/liter, and yet even this low density is sufficient to illuminate the intra-cluster medium with thermal bremsstrahlung radiation, which is emitted by electrons that are accelerated in collisions with ions. In addition, extensive studies show that the x-ray properties are tightly coupled to the total cluster mass. Therefore, clusters are excellent sources for studying the evolution of the universe.

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