Journal Article

Constraints on Ω<sub>0</sub> and cluster evolution using the <i>ROSAT</i> log <i>N</i>-log <i>S</i> relation

B. Mathiesen and A. E. Evrard

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 295, issue 4, pages 769-780
Published in print April 1998 | ISSN: 0035-8711
Published online April 1998 | e-ISSN: 1365-2966 | DOI:
Constraints on Ω0 and cluster evolution using the ROSAT log N-log S relation

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We examine the likelihoods of different cosmological models and cluster evolutionary histories by comparing semi-analytical predictions of X-ray cluster number counts with observational data from the ROSAT satellite. We model cluster abundance as a function of mass and redshift using a Press-Schechter distribution, and assume that the temperature T(M, z) and bolometric luminosity LX(M, z) scale as power laws in mass and epoch, in order to construct expected counts as a function of X-ray flux. The LXM scaling is fixed using the local luminosity function, while the degree of evolution in the X-ray luminosity with redshift LX ∝ (1 + z)s is left open, with s an interesting free parameter which we investigate. We examine open and flat cosmologies with initial, scale-free fluctuation spectra having indices n = 0, −1 and −2. An independent constraint arising from the slope of the luminosity-temperature relation strongly favours the n = −2 spectrum.

The expected counts demonstrate a strong dependence on Ω0 and s, with lesser dependence on λ0 and n. Comparison with the observed counts reveals a ‘ridge’ of acceptable models in the Ω0s plane, roughly following the relation s ∼ 6Ω0 and spanning low-density models with a small degree of evolution to Ω = 1 models with strong evolution. Models with moderate evolution are revealed to have a strong lower limit of Ω0 ≳ 0.3, and low-evolution models imply that Ω0 < 1 at a very high confidence level. We suggest observational tests for breaking the degeneracy along this ridge, and discuss implications for evolutionary histories of the intracluster medium.

Keywords: galaxies: clusters: general; cosmology: observations; cosmology: theory; dark matter; X-rays: general

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Subjects: Astronomy and Astrophysics

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