Measuring the mean and scatter of the X-ray luminosity - Optical richness relation for maxBCG galaxy clusters

E. S. Rykoff, T. A. McKay, M. R. Becker, A. Evrard, D. E. Johnston, B. P. Koester, E. Rozo, E. S. Sheldon, R. H. Wechsler

Research output: Contribution to journalArticlepeer-review

65 Scopus citations


Determining the scaling relations between galaxy cluster observables requires large samples of uniformly observed clusters. We measure the mean X-ray luminosity-optical richness (L̄X-N̄200) relation for an approximately volume-limited sample of more than 17,000 optically selected clusters from the maxBCG catalog spanning the redshift range 0.1 < z < 0.3. By stacking the X-ray emission from many clusters using ROSAT All-Sky Survey data, we are able to measure mean X-ray luminosities to ∼10% (including systematic errors) for clusters in nine independent optical richness bins. In addition, we are able to crudely measure individual X-ray emission from ∼800 of the richest clusters. Assuming a lognormal form for the scatter in the LX-N200 relation, we measure σln L = 0.86 ± 0.03 at fixed N200. This scatter is large enough to significantly bias the mean stacked relation. The corrected median relation can be parameterized by L̃X = e α(N̄200/40)β × 10 42 h-2 ergs s-1, where α = 3.57 ± 0.08 and β = 1.82 ± 0.05. We find that X-ray - selected clusters are significantly brighter than optically selected clusters at a given optical richness. This selection bias explains the apparently X-ray - underluminous nature of optically selected cluster catalogs.

Original languageEnglish (US)
Pages (from-to)1106-1124
Number of pages19
JournalAstrophysical Journal
Issue number2
StatePublished - Mar 10 2008


  • Galaxies: clusters: general
  • X-rays: galaxies: clusters

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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