Studying various properties of stellar populations within a galaxy provides rich insight into its formation, and current surveys enable us to do so with the goal of reconstructing their formation histories. However, it is ambiguous as to when the main progenitor of a galaxy like the Milky Way (MW) formed, and what its constituent mergers were like. In the first half of this work, I use the FIRE-2 cosmological simulations to study these low-mass galaxies at early times, and their later mergers. I quantify the formation times of MW/M31-mass galaxies based on different definitions of formation, as well as the mass function of its constituent building blocks. I find that the MW-mass galaxies in Local Group (LG)-like pairs form earlier, highlighting the role that environment may play in galaxy formation. I also find a similar ubiquitous feature of metal-poor stars in the disks of the MW-mass hosts on preferentially prograde orbits, which are also found in the MW. These stars largely originate ex-situ, from a gas-rich merger of mass similar to the Magellanic Clouds, that sometimes shapes the orientation of the stellar disk.

We call the gravitationally bound low-mass galaxies that survive the hierarchical formation process satellite galaxies. The satellite galaxies in the LG are the only low-mass galaxies that we can extensively study through fully resolved stellar populations and their full phase-space coordinates. In the second half of this work, I study the infall and orbit histories of satellite galaxies around the MW-mass hosts in the simulations. I examine trends in their present-day orbital dynamics, including total velocity, specific angular momentum, and total energy, as well as their orbital histories versus present-day distance, stellar mass, and the lookback time of infall. Surprisingly, the majority of satellites that experience multiple orbits have larger recent pericenters than their minimum, contradictory to the assumption that satellite orbits always shrink. Finally, I compare the satellite galaxy orbits in a static, axisymmetric MW-mass potential to the simulations to quantify the extent to which this common orbit modeling technique works. I find that orbital energy and specific angular momentum are not always conserved, and orbit properties that occurred recently, such as pericenters, are better recovered than events further in the past, such as infall time.

Understanding galaxy formation and evolution requires characterizing elemental abundance distributions in galaxies. Chemical tagging is a useful tool to understand the evolutionary history of the Milky Way (MW) because it takes advantage of the fact that stellar abundances at present-day are identical to the abundances with which stars formed. Thus, stellar elemental abundances provide an observable that can, in principle, be used to determine the birth location of a star.

Elemental abundance observations of gas and stars in the MW and nearby galaxies show that the median elemental abundance typically decreases with increasing radius with little scatter about the mean at each radius. These observations are robust for nearby galaxies, however they are less certain at high redshift because of the observational difficulties associated with obtaining spatially resolved spectra of high redshift galaxies. As a result, many galactic elemental evolution models rely on assumptions that observed properties of the MW and nearby external galaxies, crucially the lack of azimuthal scatter in abundances, are a time-independent property. Alternatively, some researchers use physical models of galaxy evolution to which they fit a multitude of free parameters such that their model recreates the present-day observed properties of the MW and external galaxies. However, these models typically rely on overly-simplified assumptions of physical processes and include multiple free functions unconstrained by physical models.

Cosmological zoom-in hydrodynamic simulations can help to precisely characterize the spatial distribution of stellar elemental abundances in MW-mass galaxies across cosmic time. Our results challenge the status quo of galactic elemental evolution models. We find that the minimal azimuthal abundance variations in MW-mass galaxies are not an intrinsic property, rather galaxies evolve from an epoch of extreme azimuthal abundance variation to their present-day state. Additionally, radial abundance gradients in galaxies were nearly non-existent at sufficiently high redshifts, despite being relatively strong at present-day. These results suggest a higher degree of difficulty in chemically tagging stars than previously assumed. Thus, chemical tagging techniques may only be able to loosely constrain birth locations of older stellar populations. To help address this, we characterize the scale of radial redistribution of stars in simulated MW-mass galaxies as a function of both stellar age and stellar location. Accounting for stellar radial redistribution as a function of present-day radial location can help break degeneracies of birth location for older stellar populations. Our results on stellar radial redistribution suggest that inferring a time-dependent radial abundance gradient of the MW from present-day observations is non-trivial; at present-day, old stellar populations are often at very different radii than the radii at which they formed. Therefore, more emphasis must be placed on models derived from cosmological simulations.

Satellite dwarf galaxies are singularly useful for studies of galaxy formation because they are highly susceptible to environmental and stellar feedback effects, we can observe them in unparalleled detail in the Local Group (LG), and we can model them at high resolution in simulations. Of particular interest in galaxy formation are the Milky Way's (MW) satellites, which appear to be nearly uniformly quenched inside the MW's virial radius, likely because of both internal stellar feedback and environmental effects of the gaseous host halo. Many observational campaigns have been devoted to measuring LG satellite star formation histories and high precision proper motions from resolved stellar populations, critical to determining orbital effects on formation. However, LG surveys will revolutionize galaxy formation, only if we can provide sufficiently rigorous theoretical models to interpret them.

The spatial distribution, kinematics, and star formation histories of satellite dwarf galaxies in the LG present a unique opportunity to test galaxy formation and numerical resolution in cosmological hydrodynamic simulations. We find excellent agreement between LG observations and the inner radial distributions of ‘classical’ dwarf satellites around MW/M31-mass host galaxies from the FIRE simulations. These results also suggest potentially undiscovered classical dwarf-mass satellites in the outer halos of the MW and M31. Furthermore, we investigate the prevalence, longevity, and potential causes of so-called ‘satellite planes’ in the LG. Although satellites in the simulations are typically distributed almost isotropically around their hosts, we find that hosts with an LMC-like satellite are more likely to have MW-like satellite planes, and that M31's satellite distribution is much more common. Preliminary results on the gas content and star formation histories of simulated satellites indicate agreement with the high quenched fraction of satellites in the LG. However, the simulations and LG observations appear to be in tension with the small fraction of quenched satellites around MW analogs in the nearby universe. Our results suggest that environmental effects of the host halo may be the dominant mechanisms for quenching satellite dwarf galaxies.

Mergers between distinct objects are a natural part of hierarchical structure formation. Mergers are also one of the most critical elements in the evolution of both galaxies and halos. I use high-resolution, cosmological volume simulations to explore galaxy and halo evolution and merging activity in a cosmological context, including environmental dependence, merger rates and dynamics, and how these processes in halos connect with those of galaxies.

I first explore halo merging and evolution, focusing on its interplay with large-scale environment. While halo spatial clustering has been thought to depend only on mass, I ex- amine how spatial clustering depends on secondary parameters such as halo formation time, concentration, and recent merger history, a phenomenon known as "assembly bias". Next, I examine the extent to which close spatial pairs of objects can be used to predict mergers, finding limited utility to the pair-merger method arising from a competition between merger efficiency and completeness. I also explore the dependence of merging on environmental density, discovering that merging is less efficient in overdense environments. I then investigate how a massive galaxy/halo population at high redshift connects to a massive population of the same number density today, finding that scatter in mass growth and mergers between massive objects preclude a direct population mapping either forward or backward in time.

In the latter part of this work, I explore the dynamics and mergers of galaxies in groups and clusters. I first examine the orbital distributions of satellite halos/galaxies at the time of infall onto a more massive host halo, finding that satellite orbits become more radial and penetrate deeper at higher host halo mass and higher redshift. I then track the evolution of galaxies in groups directly, examining the merger rates of galaxies over time and finding that galaxy mergers do not simply trace halo mergers. I also examine the small-scale environments of galaxy mergers, discovering that recently merged galaxies exhibit enhanced small-scale spatial clustering for a short time after a merger. Finally, by using abundance matching to assign stellar mass to subhalos, I explore the importance of merging vs. disruption processes for satellite galaxy evolution. I rigorously test the connection of galaxies to subhalos by comparing simulations against observed galaxy spatial clustering, satellite fractions, and cluster satellite luminosity functions, finding agreement in all cases.