Building Theoretical Frameworks to Interpret the Dynamics of Low-mass and High-mass Galaxy Pairs Across Cosmic Time

Abstract

Galaxy interactions and mergers help drive galaxy evolution by triggering starbursts, AGN activity, and morphological changes. Understanding the frequency and dynamics of interacting galaxy pairs across cosmic time is thus necessary to quantify their importance for galaxy evolution and for testing predictions of hierarchical assembly in a $\Lambda$CDM paradigm. At low redshift, the Local Group (LG) is a great testbed for galaxy dynamics studies as it hosts several galaxy pairs, including major pairs (mass ratio $>1:4$, e.g., Milky Way (MW)–M31) and minor pairs ($1:4 > \mbox{mass ratio} > 1:10$, e.g., MW–LMC, M31–M33, and LMC–SMC). High-precision observational data have facilitated tight constraints on measurements of the 6D phase-space information (distance, line of sight velocity, and proper motions) for several LG pairs, which in turn has permitted detailed studies of their orbital histories and dynamics. However, it is still unclear how to place LG pairs in a cosmological context in terms of their mass and dynamics and how to connect the evolution of LG pairs across redshift with expectations from $\Lambda$CDM. In this dissertation, I study the dynamics of galaxy pairs to develop a deeper understanding of the LG and how observations and simulations can be integrated to study these types of systems at redshifts $z=0-4$.First, I study the impact of the disequilibrium state of the MW on dynamical mass measurements of the Local Group, and use the first measurements of the MW disk travel velocity to show that perturbations to the MW lead to inflated mass estimations in frameworks that do not account for perturbations. With updated mass measurements of the LG, I then use the Illustris TNG100 simulation to study the evolution of LG pairs in a cosmological context in terms of their mass and dynamics as a function of redshift. I start by studying the redshift evolution of pair fractions of isolated low-mass ($10^8<M_*<5\times 10^9\Msun$) and high-mass ($5\times10^9<M_*< 10^{11}\Msun$) pairs between $z=0-4$, and find that the frequency of low-mass major and minor pairs is considerably lower than high-mass major and minor pairs at low redshift. I also show that MW--LMC-mass analogs are most common at $z=0$, but that MW--LMC-mass analogs that also match the observed kinematic properties of the real MW--LMC system (i.e. have small separations and high velocities) are 2-3 times more common at redshift $z\sim2$. Additionally, LMC--SMC-mass pairs are 3 times more common at $z>2$ than at $z=0$. Further, I develop a framework for pair selection criteria that permits self-consistent studies of pair fractions across mass scales and cosmic time. I show that traditional physical-separation selection criteria yield biased measurements of the pair fraction evolution of galaxy pairs. I then extend this framework by calculating the orbits of these low-mass and high-mass pairs out to $z=6$, and show that low-mass and high-mass major pairs have identical merger timescales only when pairs are selected via separation criteria that vary with mass and redshift. These frameworks will help facilitate interpretations of the dynamics of low-mass and high-mass galaxy pairs, both in the Local Group and across cosmic time, in the coming era of more precise, deeper, and wider observational programs permitted by \textit{Gaia}, \textit{JWST}, \textit{Roman Space Telescope}, and the Rubin Observatory.