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.
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.
Even if dark matter is perfectly cold, 28 strong gravitational lenses require that halos keep forming down to at least 10^8.3 solar masses, a limit comparable to or stronger than the one from the Milky Way's satellite galaxies and one that does not depend on those halos containing any stars. A sample of 200 such lenses, a small fraction of what Rubin, Euclid, and Roman are expected to find, would push the limit more than a factor of ten lower.
Adding the detailed shapes of lensed arcs to the brightnesses of multiply imaged quasars in 28 lens systems tightens limits on how far dark matter particles travel before clumping, ruling out any smoothing of structure above about 10^7.4 solar masses and requiring a thermally produced dark matter particle heavier than about 7 keV. The mass found in small clumps around the lens galaxies matches semi-analytic predictions, but sits mildly above recent N-body simulations.
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.
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 →