Antibias in Clusters: The Dependence of the Mass-to-Light Ratio on Cluster Temperature

We show that the observed mass-to-light ratio of galaxy clusters increases with cluster temperature as expected from cosmological simulations. Contrary to previous observational suggestions, we find a mild but robust increase of M/L from poor (T = 1 - 2 keV) to rich (T = 12 keV) clusters; over this range, the mean M/LV increases by a factor of about 2. The best fit relation satisfies M/LV = (170 +- 30) T (keV) ^ (0. 3 +- 0. 1) h at z=0, with a large scatter. This trend confirms predictions from cosmological simulations which show that the richest clusters are antibiased, with a higher ratio of mass per unit light than average. The antibias increases with cluster temperature. The effect is caused by the relatively older age of the high-density clusters, where light has declined more significantly than average since their earlier formation time. Combining the current observations with simulations, we find a global value of M/LV = 240 +- 50 h, and a corresponding mass density of the universe of Omegaₘ = 0. 17 +- 0. 05.