I am broadly interested in the connection between cosmology, galaxy formation, and the elemental composition of stars. This has led me to “near field cosmology”, which uses observations of nearby objects to understand the evolution of the universe. I’m especially interested in studying the swarm of little dwarf galaxy satellites that orbit our Milky Way galaxy. These dwarf galaxies contain clues about the first stars and galaxies, the origin of the elements, the history of the Milky Way galaxy, and the nature of dark matter.

Ultra-faint Dwarf Galaxies

My research focuses on observing and interpreting the chemical content of stars in the faintest known galaxies (creatively termed “ultra-faint” dwarf galaxies). I think that ultra-faint dwarf galaxies are the most fascinating type of object in the universe. They have only a few thousand stars (fewer stars than many individual star clusters!), and yet they sit in dark matter halos and show evidence for extended star formation. But not too extended: all their stars formed in just the first 1-2 billion years of the universe’s history. As a result, each ultra-faint dwarf galaxy is a repository of ancient stars with a common formation history, and there’s dozens of these little galaxies surrounding our Milky Way.

I like to think of each ultra-faint dwarf galaxy as a small experiment: the universe took lots of little galaxies, let them form stars for a short time, then turned them off and left them lying around for us to study. We’re still in the early stages of this process, only having looked at 15 of the dozens of ultra-faint dwarf galaxies. So far, I have studied five of these galaxies: Reticulum II, Tucana II, Bootes II, and Grus I and Triangulum II. Stay tuned for many more coming soon!

Origin of the heaviest elements

The heaviest elements in the periodic table cannot be created through nuclear fusion. Instead, they are synthesized through neutron-capture processes, a “slow” and a “rapid” process. The origin of the rapid process (or “r-process”) has been a long-standing astrophysical mystery, but recent evidence has slowly coalesced around neutron star mergers as the likely dominant source.

The dwarf galaxy Reticulum II is an extremely unique galaxy: nearly every star in it is highly enhanced with r-process elements! This galaxy appears to preserve the signature of a neutron star merger in the early universe. Our paper in Nature describes this discovery (arXiv version). A more detailed companion paper can be found in ApJ. Since this galaxy’s stars preserve a pure r-process signature, I have also used detailed abundances to understand the nature of neutron star merger ejecta (paper).

We’ve long known that similar r-process stars can be found in the Milky Way’s stellar halo (e.g. here, here, here, and more). I hypothesized here that such stars might exclusively originate from ultra-faint dwarf galaxies. Graduate student Kaley Brauer and I have since constructed models showing that only half of such stars originate just from the ultra-faint dwarf galaxies, with the rest likely explainable by larger disrupted satellite galaxies (paper).

The First Stars and Galaxies

The first few generations of star formation are a unique time in the history of the universe. In particular, the very first stars in our universe have a fundamentally different character than stars that form today. The differences may be reflected in the nucleosynthetic yields of elements created when these stars go supernovae, which can be indirectly observed in chemical abundances of old (“metal-poor”) stars.

I have investigated how well these signatures can be preserved in typical early star forming environments (paper, some related code). In short, they’re preserved only by the oldest stars, as the signatures tend to get wiped out after even a single additional generation of star formation. Even the oldest stars are complicated, tracing the combined signatures from several stars and requiring knowledge of the star formation environment to extract quantitative conclusions.

The very most iron-poor stars are very likely to be true second-generation stars. There are only ~5 of these stars known, and I’ve helped chase down and study the origin of two of them (Star 1, Star 2).

I have also looked at the critical metallicity for the transition from the top-heavy Population III IMF to today’s bottom-heavy IMF. We proposed an observational criterion to assess the role of dust thermal cooling in creating the first low-mass stars. (paper, code)

As part of the Caterpillar project, I have continued investigating how to use metal-poor stars to understand chemical abundances (paper) and tracing them to the present day (paper 1; paper 2).

Milky Way Substructure

One of the biggest challenges when studying the local universe is that we only get one local universe, but our cosmological model can only predict statistical distributions of physical properties. As a result, we must run a lot of simulations to disentangle observations specific to our corner of the universe from general facts about cosmology.

I’m a core member of the Caterpillar project, a large suite of cosmological zoom-in simulations of Milky Way mass galaxies. These will help us understand the formation history of our Milky Way and potentially constrain our models of dark matter. I’ve played a large role in running and postprocessing these simulations, which has taken millions of CPU-hours. One of my main contributions was an adaptation of the halo finder ROCKSTAR that implements iterative unbinding (link here). (Caterpillar flagship paper)