Research

I study a wide range of topics related to the formation and evolution of the earliest small-scale structures in the Universe, using a variety of theoretical techniques including high-resolution hydrodynamical cosmological simulations, semi-analytic modeling, and analytic methods. Systems of interest to me and my collaborators include the first dark matter minihalos that hosted Pop III stars, early dwarf galaxies, high-redshift star clusters, and high-redshift nuclear star clusters.

Small-scale structures sit at the intersection of dark matter and baryonic physics, offering a unique astrophysical probe of key physical processes. Thanks to new and upcoming observational capabilities (including Webb, Rubin, and other facilities), we have the opportunity to study these effects at ever higher redshift.

A full list of my publications can be found on my publications page.

This work has spanned several different areas of astrophysics, including cosmology, structure formation, galaxy formation, (first) star formation and turbulence, probes of dark matter, and astrophysical dynamics. Below, I provide an overview of the various research areas that my work has explored, and linked to the publications that are relevant to each topic.

Research Topics

01
Star Clusters at Cosmic Dawn
I study the formation and properties of the very first star clusters that formed at high redshift in the ΛCDM framework. These compact, massive, and metal-poor objects are relevant to understanding the progenitors of present-day globular clusters and nuclear star clusters, and are increasingly accessible through JWST observations. My work investigates their structural properties (stellar densities, sizes, morphologies), their equilibrium states, and the environments in which they form. I also explore dynamical processes in high redshift systems, particularly stellar collisions, which have a variety of interesting physical implications at early times. My recent work explores how these clusters compare to modern observational candidates such as compact high-redshift lensed clusters and Little Red Dots.
Download and use our cluster catalog
02
The Supersonic Streaming Velocity & Properties of Early Baryonic Perturbations
Prior to the formation of the first galaxies, baryonic matter and dark matter were moving supersonically relative to one another, a velocity field imprinted in ΛCDM cosmology at the time of recombination. This "streaming velocity" introduces a phase shift between baryon and dark matter overdensities, causing baryons to form collapsed objects (Supersonically Induced Gas Objects, or SIGOs) outside the virial radii of nearby dark matter halos. An interesting consequence of this effect is the production of gas-dominated, dark-matter-poor objects are candidate progenitors of globular clusters. Along with the Supersonic Project Collaboration, I study the formation, morphology, rotation, and evolution of SIGOs and their dark-matter-rich relatives at slightly higher mass (DM GHOSts) through high-resolution hydrodynamical simulations. These simulations can also reveal the physical imprints of the stream velocity on early cosmological structures, especially primordial gas.
SIGOs & the Supersonic Project DM GHOSts & Structure Formation
03
JWST Predictions & the Early UV Luminosity Function
The streaming velocity has a dramatic effect on early star formation: in high-streaming regions, gas is initially advected away from dark matter halos, suppressing star formation at the smallest scales but triggering a subsequent starburst when it returns. This may produce a population of temporarily over-luminous dwarf galaxies at cosmic dawn that could be detectable with JWST. My work connects cosmological simulations of streaming to predictions for the UV luminosity function at high redshift, offering a test of the cold dark matter model with current JWST data.
Public description of Williams et al. 2024
04
Small-Scale Structure Problems & Dwarf Galaxies
The ΛCDM model is highly successful on large scales, but tensions persist at small scales: observed dwarf galaxies exhibit anomalous rotation curves, shallow dark matter density profiles, and scatter in the stellar-to-halo mass relation that are difficult to reproduce in standard simulations. The streaming velocity offers a physical mechanism that could naturally produce some of these anomalies, by creating a population of dark-matter-poor objects and altering the formation histories and structural properties of the faintest galaxies. I investigate connections between these high-redshift formation pathways and the present-day observed population of dwarf satellites.
Problems in Structure Formation
05
Testing Fundamental Physics with Gravitational Waves
Prior to joining the Supersonic Project, I worked with the LIGO-Virgo collaboration on tests of general relativity using gravitational wave observations. In particular, I contributed to measurements of the propagation speed of gravitational waves using data from the first and second observing runs of Advanced LIGO and Advanced Virgo. Constraints on the gravitational wave speed provide stringent tests of Lorentz invariance and alternative theories of gravity.

The Supersonic Project