I am broadly interested in the connection between cosmology, dark matter, and galaxy formation. This has led me to “near field” cosmology, which uses observations of nearby objects to understand the evolution of the universe. In particular, our Milky Way is surrounded by a swarm of dwarf galaxy satellites. These 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.

Chemical Abundances of Stars in 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 have studied three such galaxies: Reticulum II, Tucana II, and Bootes II.

The most interesting of these three galaxies is Reticulum II, which I examined with Anna Frebel, Ani Chiti, and Josh Simon. To our surprise, the stars in this galaxy are chock-full of r-process elements! Along with the other ultra-faint dwarf galaxies, this implies that a rare and prolific event created the r-process elements in this galaxy. This galaxy may help unravel a 60-year-old question about the origin of r-process elements. Our paper in Nature describes this discovery (arxiv:1512.01558). A more detailed companion paper can be found in ApJ (arxiv:1607.07447).

Early Universe Star and Galaxy Formation

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. Anna Frebel, Volker Bromm, and I have investigated how well these signatures can be preserved in typical early star forming environments (arxiv:1508.06137, 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.

Anna, Volker, and 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. (arxiv:1307.2239, code)

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. With postdocs Brendan Griffen and Facundo Gomez, and graduate student Greg Dooley, 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). (arxiv:1509.01255)