SIMBAD references

Determining the scaling relations between galaxy cluster observables requires large samples of uniformly observed clusters. We measure the mean X-ray luminosity-optical richness (L{d1}_X-N{d1}₂₀₀) 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 L_X-N₂₀₀ relation, we measure σ_lnL=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 L{d5}_X=e^α(N{d1}₂₀₀/40)^βx10⁴²/h2/ergs/s, 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.