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. This early epoch is one of the last unexplored frontiers in the history of our universe.

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, the so-called “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.

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 ~20 of the hundreds of ultra-faint dwarf galaxies expected around our Milky Way. So far, I have studied seven of these galaxies: Reticulum II, Tucana II, Bootes II, Grus I and Triangulum II, and Carina II and III.

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. 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).

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 that such stars might exclusively originate from ultra-faint dwarf galaxies. Graduate student Kaley Brauer and I have since constructed models showing that about 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.

We have now directly detected r-process elements made in a neutron star merger coincident with a gravitational wave detection. This is a new and exciting way to understand the r-process. Using the Milky Way’s r-process stars, I have made predictions for optical followup of neutron star mergers detected through gravitational waves (paper).

Milky Way Assembly and Substructure

One of the biggest challenges of near-field cosmology is that we only get one nearby universe, but our cosmological model only predicts 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 am also interested in other denizens of our Milky Way’s halo. I have recently become particularly interested in stellar streams. These are galaxies and star clusters that are in the process of being tidally disrupted by the Milky Way. I lead the high-resolution abundance analysis for the S5 Survey (paper). More recently, I co-chair the Milky Way Halo Working group as part of SDSS-V.