I am a near-field cosmologist. I study nearby stars and galaxies to understand the first stars and galaxies, the origin of the elements, the history of our galaxy, and the nature of dark matter.

The First Stars and Galaxies

The first stars and galaxies formed in the first billion years of our universe (at redshifts z > 6). This early epoch is one of the last unexplored frontiers in the history of our universe.

Relics of the first stars

I study the chemical composition of metal-poor stars to learn about the first generation of stars. The very first stars in our universe form in a universe without heavy elements. As a result, they were probably unusually massive, which should be reflected in the nucleosynthetic yields of elements created when these stars explode. The ashes of these stars are preserved in old (“metal-poor”) stars.

The very most iron-poor stars are very likely to be true second-generation stars. There are only ~7 of these stars known, and I’ve helped chase down and study the origin of two of them (Star 1, Star 2).

On the theoretical side, I have studied whether unique abundance signatures of the first stars can be preserved in typical early star forming environments (paper, some related code). I have looked at the critical metallicity for the transition from the top-heavy Population III IMF to today’s bottom-heavy IMF. (paper, code) As part of the Caterpillar project, I have investigated how to use metal-poor stars to understand chemical abundances (paper) and tracing them to the present day (paper 1; paper 2).

Relics of the first galaxies

Much of my research focuses on observing and interpreting the chemical content of stars in surviving relics of the first galaxies. These are very faint 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). I continue to pursue these questions with the R-Process Alliance.

I have also used the Milky Way’s r-process stars to set baseline expectations for upcoming gravitational wave followup of neutron star mergers (paper).

Milky Way Assembly and 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. I led the post-processing effort for Caterpillar, and one of my main contributions was an adaptation of the halo finder ROCKSTAR that implements iterative unbinding (link here). (Caterpillar flagship paper)

I also study stars in stellar streams with the S5 Survey.