Tracing mass and light in the Universe: where is the dark matter?

Abstract

How is mass distributed in the Universe? How does it compare with the\ndistribution of light and stars? We address these questions by examining the\ndistribution of mass, determined from weak lensing observations, and starlight,\naround $>10^5$ SDSS MaxBCG groups and clusters as a function of environment and\nscale, from deep inside clusters to large cosmic scales of $22 h^{-1}$ Mpc. The\nobserved cumulative mass-to-light profile, $M/L (< r)$, rises on small scales,\nreflecting the increasing $M/L$ of the central bright galaxy of the cluster,\nthen flattens to a nearly constant ratio on scales above $\\sim 300 h^{-1}$ kpc,\nwhere light follows mass on all scales and in all environments. A trend of\nslightly decreasing $M/L (r)$ with scale is shown to be consistent with the\nvarying stellar population following the morphology-density relation. This\nsuggests that stars trace mass remarkably well even though they represent only\na few percent of the total mass. We determine the stellar mass fraction and\nfind it to be nearly constant on all scales above $\\sim 300 h^{-1}$ kpc, with\n$M_{*}/M_{tot} \\simeq 1.0\\pm0.4\\%$. We further suggest that most of the dark\nmatter in the Universe is located in the large halos of individual galaxies\n($\\sim 300$ kpc for $L^{*}$ galaxies); we show that the entire $M/L (r)$\nprofile -- from groups and clusters to large-scale structure -- can be\naccounted for by the aggregate masses of the individual galaxies (whose halos\nmay be stripped off but still remain in the clusters), plus gas. We use the\nobserved mass-to-light ratio on large scales to determine the mass density of\nthe Universe: $\\Omega_{m} = 0.24 \\pm 0.02 \\times b_{M/L}^{2} = 0.26 \\pm 0.02.$\n