Theoretical Astrophysicist · Carnegie Observatories

Andrew Benson

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.

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Research focus

What I work on

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

The Galacticus model

An open-source semi-analytic model of galaxy formation used to interpret observations across cosmic time.

Dark matter

Constraining dark matter microphysics

Using halo substructure, gravitational lensing, and dwarf galaxies to test warm, self-interacting, and other non-CDM models.

Surveys

Synthetic skies for next-gen surveys

Building galaxy mocks for the Roman GRS Project Infrastructure Team and the NASA Open Universe initiative.

Recent work

Selected recent papers

These cards are rebuilt automatically from my NASA ADS library on a weekly schedule. Summaries and figures are generated from the paper itself.

Figure from JWST lensed quasar dark matter survey. IV. Stringent warm dark matter constraints from the joint reconstruction of extended lensed arcs and quasar flux ratios

Physical Review D 2026

JWST lensed quasar dark matter survey. IV. Stringent warm dark matter constraints from the joint reconstruction of extended lensed arcs and quasar flux ratios

Gilman, D., A. Benson, et al.

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.

ADS · arXiv · DOI

Physical Review D 2026

JWST lensed quasar dark matter survey. III. Dark matter sensitive flux ratios and warm dark matter constraint from the full sample

Keeley, R. E., A. Benson, et al.

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.

ADS · arXiv · DOI

The Astrophysical Journal 2026

Testing Warm Dark Matter with Kinematics of the Smallest Galaxies

Delos, M. Sten, A. Benson, et al.

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.

ADS · arXiv · DOI

Physical Review Letters 2026

Measurement of the Minimum Cold Dark Matter Halo Mass with Strong Gravitational Lensing

Nierenberg, A. M., A. Benson, et al.

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.

ADS · arXiv · DOI

Physical Review Letters 2026

Free-Streaming Length of Dark Matter from JWST Observations of 28 Strong Gravitational Lenses

Gilman, D., A. Benson, et al.

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.

ADS · arXiv · DOI

The Astrophysical Journal Letters 2026

Warm, Not Fuzzy: Generalized Ultralight Dark Matter Limits from Milky Way Satellites

Nadler, Ethan O., A. Benson, et al.

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.

ADS · arXiv · DOI

See all recent papers →

Open source

Galacticus

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 →