Theoretical Astrophysicist · Carnegie Observatories
I am a Staff Scientist at the Carnegie Observatories. My research is focused on understanding the nature of dark matter and the process of galaxy formation — combining analytic models, numerical simulations, and large astronomical surveys.
Research focus
Three threads tie my research together: building a coherent theoretical model of galaxy formation; constraining the microphysics of dark matter; and designing the synthetic universes that next-generation surveys need to interpret their data.
Galaxy formation
An open-source semi-analytic model of galaxy formation used to interpret observations across cosmic time.
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Dark matter
Using halo substructure, gravitational lensing, and dwarf galaxies to test warm, self-interacting, and other non-CDM models.
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Surveys
Building galaxy mocks for the Roman GRS Project Infrastructure Team and the NASA Open Universe initiative.
Read more →Recent work
These cards are rebuilt automatically from my NASA ADS library on a weekly schedule. Summaries and figures are generated from the paper itself.
For galaxies picked by their mass in stars, the extra clustering that depends on more than the mass of their dark matter halos tracks most closely how gas cools and how stars blow gas back out, regardless of how common the galaxies are. For galaxies picked by how fast they form stars, the most closely linked process shifts from star formation itself to gas cooling as the sample includes more common galaxies.
The pattern of distortions in 28 quadruply imaged quasars observed by JWST shows that dark matter still clumps on scales below about 10^7 solar masses, limiting the distance its particles stream before structure forms to under 6 to 7 kpc and requiring a thermal relic particle heavier than about 7 keV. Assuming dark matter is cold instead, the mass held in small clumps around elliptical lens galaxies is measured directly, confirming a central prediction of the standard picture.
Ultralight dark matter produced with structure on small scales behaves less like the "fuzzy" dark matter usually assumed and more like warm dark matter, with an additional wave-interference effect that adds small-scale structure back in. Matching this against the observed population of Milky Way satellite galaxies sets a new lower limit on the particle mass that depends on the scale at which the field's structure peaks.
Every dark matter halo forms with a dense spike at its center, and if dark matter is "warm" enough to erase small-scale structure those spikes are heavy enough to speed up the stars in the smallest galaxies — yet the stars in Tucana V and Triangulum II move far too slowly to match. Taken together, the three faintest Milky Way satellites rule out dark matter particles lighter than 5.8 keV at 95% confidence, a limit that better velocity measurements or the discovery of more such galaxies could sharpen considerably.
Infrared images of the warm dust around 31 multiply imaged quasars, taken with JWST, double the sensitivity to a cutoff in the number of low-mass dark matter clumps relative to earlier work, and show no sign of one down to about 10^7.8 solar masses. That implies dark matter particles produced thermally in the early universe must be heavier than roughly 6 keV.
Open source
Most of my modeling work happens inside Galacticus, an open-source semi-analytic model of galaxy formation that I wrote and continue to develop. It's used by groups around the world to study dark matter, galaxy evolution, and forecast observations for upcoming surveys. See the full software stack →