I present an investigation of the impact of various bulge fraction and the inclusion of gas on stellar bar evolution through use of N-body and hydrodynamic isolated galaxy simulations, each with live dark matter halos. My work looks at the orbital resonance distribution to quantify the rate of the resonance evolution processes and to inform our understanding of observations of morphological structures in disk galaxies within the local universe, or even up to cosmic noon. With this work we are better equipped to interpret bar and bulge structures from cosmic noon until today.
In Chapter 1, we present evidence for resonance crossing population process to form the BPX pseudobulge and describe the conditions that stabilize this process. We use a suite of isolated N-body disk galaxy models with live dark matter halo, stellar disk, and classical bulge components ranging from 0% to 16% of the stellar disk mass. We use frequency analysis in the live, time-evolving potential to characterize the bar-resonance populations of the stellar orbits. In agreement with observations and prior work, we find that more massive bulges facilitate formation of longer, stronger bars and that the inclusion of a bulge stabilizes the bar against sudden onset of instability and vertical asymmetry events. Our orbital characterization shows that the BPX pseudobulge is primarily supported by the brezel orbital family and that BPX-feature growth is driven through resonance passage of horizontal 2:1 stars crossing the vertical 2:1 resonance with a short time of peak populations showing Omega_z=Omega_r. Our work shows that this process proceeds steadily in the presence of a stabilizing bulge and is most efficient during radial overlap of the vertical and horizontal resonances in the plane of the disk.
Chapter 2 presents evidence that classical bulges may not be long lived in disk galaxies with a strong stellar bar. We demonstrate that 50% of each included classical bulge exhibits characteristic bar-orbits within 3-7 Gyr of the time each model (using the same disk galaxy models of Chapter 1 is measured to have a significant strong stellar bar. Our results build on prior simulation work that showed significant exchange of angular momentum between the stellar bar with an accompanying classical bulge component at the bar resonances. We quantify the efficiency of this process of secular attrition by measuring the fraction of bar-orbits in the initial bulge population across 10 Gyr of evolution in the context of kinematically derived pseudo-observations and within the context of a sample of over 200 MaNGA IFU disk galaxy observations. Our work shows that significant classical bulges may not be long lived and that after at least 3 Gyr of secular evolution, observed classical bulges are at most 75% of their initial component by mass.
The central region of our Milky Way galaxy has a bulky bulge extending above (and below) the galactic plane. Study of the kinematic properties and metallicities of the stars within this region aim to uncover the history of this structure. Recent observations of the SDSS-V Milky Way Mapper stellar survey that show the low metalicity population of this bulge have velocity dispersions on the order of 120 km/s, three times higher than the dispersion measured in the extended regions of the Galactic disk, providing possible new evidence for a central classical stellar bulge in our galaxy. In this work, we challenge this explanation with N-body and hydrodynamical isolated galaxy models with various bulge fractions and on-going star formation and show that this central high velocity dispersion population can be reproduced without any initial classical bulge component. We show with our hydrodynamical models the stellar populations that form later after the bar is evolved remain largely constrained to the disk despite achieving high velocity dispersion, as is seen in these observations. We use our frequency analysis techniques and resonance classifications to quantify the timescale of the high-velocity dispersion signal that accompanies rapid growth of BPX pseudobulges and the role of the bar stars in this process.