Dark matter and the assembly history of massive galaxies and clusters

In Part I of this thesis we study the distribution of dark matter and baryons in a sample of seven massive, relaxed galaxy clusters by combining multiple observational tools. Our aim is to make comprehensive mass profile measurements and compare these to the form of the universal density profile derived in numerical cold dark matter (CDM) simulations. By joining weak and strong gravitational lensing observations with resolved stellar kinematic data within the central brightest cluster galaxy (BCG), we constrain the density profile over the wide dynamic range of 3-3000 kpc in radius for the first time. We first compare lensing- and X-ray-derived mass measures to constrain the line-of-sight geometry of the clusters in our sample. We then show that the logarithmic slope of the total density profile -- comprising both stars and dark matter -- agrees closely with numerical simulations containing only dark matter down to radii of ∼ 7 kpc, despite the significant contribution of stellar material on such small scales. Our unique stellar kinematic data allow us to constrain two-component models of the stellar and dark matter distributions in the cluster cores. We find a mean logarithmic slope for the dark matter density of beta = 0.50 +/- 0.10 (random) +0.14, -0.13 (systematic) at small radii, where rho DM ∼ r-beta. This is significantly shallower than a canonical CDM cusp having beta = 1. Alternatively, a cored dark matter profile with log rcore / kpc = 1.14 +/- 0.13 (random) +0.14,-0.22 (systematic) provides an equally good description. The mean mass-to-light ratio of the stars in the BCGs, derived from lensing and dynamics, is found to be consistent with estimates from stellar population synthesis modeling provided that a Salpeter initial mass function (IMF), or one with a similarly high mass-to-light ratio, is adopted. We find some evidence for a correlation between the inner dark matter profile and the size or luminosity of the BCG, which suggests a connection between the inner dark matter distribution and the assembly history of stars in the central galaxy. We discuss physical processes that might account for the small-scale dark matter distribution that we observe.;In Part II we turn to the assembly of stars in massive, quiescent galaxies. Many observations now indicate that the extended stellar envelopes seen in these systems today have grown over the last 11 Gyr. We present a two-pronged program aimed at understanding the remarkable growth observed in the size of their stellar distributions over this interval. First, we conducted deep spectroscopic observations of a sample of 17 spheroidal galaxies at z = 1.05--1.60 to derive their internal velocity dispersions from stellar absorption lines. These dynamical data provide a robust verification of their compactness and masses, which were previously inferred photometrically. Second, in order to investigate the likely role of galaxy mergers in contributing to the observed size growth, we searched for satellites around a sample of massive, quiescent galaxies at z = 0.4--2. Using HST/WFC3 imaging from the CANDELS survey, we are able to probe faint companions with stellar masses down to 10% of that of the host galaxy. By coupling measurements of the number and stellar mass content of such companions with results from published merger simulations, we estimate the rate of size growth attributable to major and minor galaxy mergers. We compare this to the rate of size growth measured in the same large, homogeneous sample of quiescent galaxies, based on deep, high-resolution imaging in the rest-frame optical. We find that observed impending mergers might account for the size growth seen at z ≲ 1, provided that a relatively short merger timescale is valid. At progressively higher redshifts, however, the estimated merger rate is outstripped by the rate of size growth. Either the merger physics is not currently well understood, or additional processes must contribute significantly to early size growth of quiescent galaxies.;In the final chapter we summarize our results and describe future prospects for elucidating the small-scale distribution of dark matter at intermediate mass scales, as well as the physical drivers of early spheroid evolution.