The Morphology and Dynamics of Galaxies from LCDM Models

Abstract

The formation and evolution of galaxies is studied using two alternative modelling techniques within the context of the LCDM cosmogony. In particular, we consider the capacity of the models to reproduce the fundamental components and internal structure of galaxies. We begin by outlining the theoretical basis for the study, describing the standard paradigm for structure formation and the details of the modelling techniques we employ. The extent of our current understanding of the processes responsible for morphological transformation is explored and we discuss what the structure of the Milky Way and its satellites can reveal about the wider pictures of galaxy formation and cosmology. Using data from two semi-analytical models, we investigate the origin of the disks and spheroids that represent the coarse-grain detail of galactic structure. Major galaxy mergers, which have long been regarded as the main mechanism by which elliptical galaxies and spiral bulges are generated, are found to play only a minor role in spheroid formation for all but the most massive systems. The models make similar predictions in many respects but disagree on the importance of gravitationally unstable disks in forming spheroids, serving to illustrate the uncertainty that remains in modelling morphology even at this basic level. We go on to introduce a smoothed particle hydrodynamics code which includes models for radiative gas heating and cooling, the structure of the interstellar medium, star formation and evolution, chemical enrichment and feedback from supernovae. We present a set of simulations that focus on a single Milky Way-mass galaxy and its local environment and summarise its bulk properties and formation history. We also show results from a code comparison project, which aims to quantify the effects of different physical mechanisms and modelling approaches on the formation of a galaxy disk. Our simulations also resolve the formation of a population of satellite galaxies which have a luminosity function similar to those found around the Milky Way. Through comparison with a dissipationless version of the same simulation, we determine that the baryonic component in each satellite has little effect on the structure of its dark matter halo. We also find a statistically significant discrepancy between the central mass densities of the simulated and observed satellites. Finally, we consider the formation of the haloes of hot gas and stars which surround the main galaxy disk. We find qualitative agreement with several observed properties of the Milky Way's stellar halo and with recent theoretical studies of the two components in the literature