Where is the Dark Matter? A Study of the Mass to Light Ratio Across Large Scales

Abstract

This thesis explores a fundamental question about dark matter: where is it distributed in the Universe? Is it found primarily with the halos of individual galaxies which are then assembled into groups, clusters, and the large-scale structure of the Universe or does a substantial diffuse component exist beyond these halos?

We use the mass-to-light ratio ($M/L$) as a diagnostic across cosmic scales. We begin by calibrating galaxy-scale $M/L$ ratios for $L^*$ spiral and elliptical galaxies using multiple independent approaches including dynamical mass estimates for the Milky Way and Andromeda compiled from the literature, weak gravitational lensing measurements from the Dark Energy Spectroscopic Instrument (DESI) and the Dark Energy Survey (DES) extending into the dwarf galaxy regime, and stellar-to-halo mass constraints from the Cosmic Evolution Survey (COSMOS) survey. We also calibrate a luminosity-dependent $M/L$ relation, which is then integrated over the Schechter galaxy luminosity function to predict the total $M/L$ expected from galaxy halos alone.

The predicted galaxy halo $M/L$ is compared to observational measurements of groups and clusters to assess whether galaxy halos alone can account for the dark matter we observe at larger scales. Our results are consistent with the dark matter content on all cosmic scales, with no compelling evidence found for the presence of dark matter beyond what is accounted for by the assembly of galaxy halos. We explore the sensitivity of our predictions to the spread in dynamical mass estimates for anchor galaxies like the Milky Way and Andromeda, the choice of faint-end slope $\alpha$ in the luminosity function, the assumed galaxy morphology-density relation, the treatment of $M/L$ at the bright end, and more.