UC Santa Cruz

The majority of stars in the universe reside in the stellar disks of galaxies. These disks are fragile but are known to undergo dynamical heating events that increase their scale height and velocity dispersion. Particularly, mergers are a critical part of a galaxy's mass assembly. Thus, understanding how disks are affected by and how they can survive being heated is an important part of understanding how our universe evolves. In this thesis, I study the present day kinematics of disk stars in two nearby spiral galaxies, Andromeda and Triangulum, to comment on their potential dynamical heating history. Studying nearby galaxies allows us to resolve individual stars while making measurements across an entire disk, which gives us a detailed global view of a galaxy's dynamics.

First, I describe a way of calculating asymmetric drift that does not rely on models of a galaxy's potential. I show that asymmetric drift, especially as a function of stellar age, is a tracer of dynamical heating. When compared to simulated galaxy analogs, observed asymmetric drift measurements can be used to constrain a galaxy's merger history. Using this technique, I find that the Andromeda galaxy likely had a major merger in the past 4 Gigayears, which helps shed light on the ongoing debate about Andromeda's recent merger history.

I then go on to describe the largest stellar spectroscopic survey in the Triangulum galaxy, which is Andromeda's largest satellite galaxy. I discuss initial results from this survey, including a lack of a trend between asymmetric drift/velocity dispersion and stellar age, which is unexpected because of what we see in other nearby galaxies and in our own. I show there are many questions still left to be answered about this galaxy and discuss how answering them will help us understand the pasts and futures of Triangulum and Andromeda.