The Mass‐to‐Light Function: Antibias and \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \…

We use large-scale cosmological simulations to estimate the mass-to-lightratio of galaxy systems as a function of scale, and compare the results withobservations of galaxies, groups, clusters, and superclusters of galaxies. Wefind remarkably good agreement between observations and simulations.Specifically, we find that the simulated mass-to-light ratio increases withscale on small scales and flattens to a constant value on large scales, assuggested by observations. We find that while mass typically follows light onlarge scales, high overdensity regions --- such as rich clusters andsuperclusters of galaxies --- exhibit higher M/L_B values than average, whilelow density regions exhibit lower M/L_B values; high density regions are thusantibiased in M/L_B, with mass more strongly concentrated than blue light. Thisis true despite the fact that the galaxy mass density is unbiased or positivelybiased relative to the total mass density in these regions. The M/L_B antibiasis likely due to the relatively old age of the high density regions, wherelight has declined significantly since their early formation time, especiallyin the blue band which traces recent star formation. Comparing the simulatedresults with observations, we place a powerful constraint on the mass densityof the universe; using, for the first time, the entire observed mass-to-lightfunction, from galaxies to superclusters, we find Omega =0.16+/-0.05.Comment: 26 pages, 4 figs, minor changes, to be published in ApJ, Vol.541 (Oct.1, 2000