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

Abstract

We show that the observed mass-to-light (M/L) 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 from poor ( -2 keV) to rich ( keV) clusters; over this range, the mean M/L T∼ 1 T ∼ 12 increases by a factor of about 2. The best-fit relation satisfies at , witha large scatter. This trend confirms predictions from cosmological simulations that 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.

Astrophysics in 2002

“…Numbers for the density of matter (in all forms) found from clusters of galaxies are often somewhat smaller than those from more global considerations, like cosmic shear and the CMB, and this may (or may not) constitute a problem (Bahcall & Comerford 2002).…”

SARCS strong-lensing galaxy groups

“…The presence of a higher fraction of galaxies with evolved stellar populations in higher mass objects has also been invoked to explain the increase in the mass-to-light ratio with mass (e.g. Bahcall & Comerford 2002). However, this assumption has been ruled out by Popesso et al (2007) and their study of the fundamental plane of the cluster ellipticals, showing that they have a constant mass-to-light ratio that cannot explain the variations in the total mass-to-light ratio of clusters.…”

The Cluster Mass Function from Early Sloan Digital Sky Survey Data: Cosmological Implications

“…For the observed SDSS Schechter luminosity function parameters of the HMF clusters (within 0.6 h À1 Mpc), ¼ À1:08 AE 0:01 and M Ã r ¼ À21:1 AE 0:02 (h ¼ 1; S. Hansen et al 2003, in preparation; Goto et al 2002), we adopt a correction factor of 1:42 AE 0:08 for the added contribution of faint galaxies to the total HMF cluster luminosity. The cluster mean luminosity is then converted to cluster mass, M (<0.6 h À1 Mpc physical), using the mean observed cluster M=L r ratio for each richness threshold (Bahcall & Comerford 2002). The observed best-fit M=L is used (on the basis of the means of 20 clusters and 33 groups): M=L v;tot ðz ¼ 0Þ ¼ 142 AE 32þ ð23 AE 5ÞT keV h (Bahcall & Comerford 2002).…”

“…The cluster mean luminosity is then converted to cluster mass, M (<0.6 h À1 Mpc physical), using the mean observed cluster M=L r ratio for each richness threshold (Bahcall & Comerford 2002). The observed best-fit M=L is used (on the basis of the means of 20 clusters and 33 groups): M=L v;tot ðz ¼ 0Þ ¼ 142 AE 32þ ð23 AE 5ÞT keV h (Bahcall & Comerford 2002). The mild increase of M=L with temperature, T keV (seen both in observations and in simulations; e.g., Bahcall et al 2000), is accounted for at each richness threshold using the observed correlation between richness and velocity dispersion (see below) and the observed mean relation between velocity dispersion and temparature ( v ¼ 332 T 0:6 keV km s À1 ; Lubin & Bahcall 1993).…”