Research

The abundance of galaxies in the first billion years after the Big Bang has important implications for the physics of galaxy formation and evolution as well as the process of hydrogen reionization. It is expected that galaxies form in the potential wells of dark matter halos with their star formation rates linked to halo accretion rates. However, compared to the present-day Universe, the physical processes that regulate star formation may be extremely different at very early times. Additionally, the abundance of early galaxies impacts the production of ionizing photons that can ionize the hydrogen in the intergalactic medium, thus affecting the rate at which cosmic reionization progresses.

I have used data from the JWST Advanced Deep Extragalactic Survey, a large JWST imaging and spectroscopy program targeting legacy extragalactic fields, to quantify the abundance of galaxies at z ~ 9 - 16 and examine implications for early galaxy evolution and reionization. Read more:

• Whitler et al. (2025)

Understanding the cosmic star formation history is a key component of understanding both galaxy formation and evolution in the early Universe and how these objects drove the process of cosmic reionization. While we cannot directly observe the full history of a galaxy's formation and evolution, the properties it has at the time we observe it provides indirect insights into its past, providing us with independent constraints on galaxies at very early times (which are very complementary to direct constraints on galaxies being directly observed at z > 12).

I have investigated the star formation histories (SFHs) of galaxies in the first billion years of cosmic time over a range of redshifts and luminosities in a series of papers. Read more:

• Methods development and the SFHs of very bright galaxies at z~7 with ground-based observations: Whitler et al. (2023b)

• SFHs of moderately bright galaxies at z~8.5-11 with early JWST observations: Whitler et al. (2023a)

Lyα emission is highly susceptible to attenuation by neutral hydrogen, so can be used to infer information about the quantity and distribution of neutral hydrogen in the intergalactic medium (IGM) during reionization. When compared to the expected intrinsic Lyα emission that escapes a galaxy, the amount of Lyα that transmits through the reionizing IGM provides insights into the volume-averaged fraction of neutral hydrogen in the IGM at a given time. The details of the line shape and the spatial distribution of galaxies that emit Lyα provides insights into the topology of the reionization process. Additionally, it is hypothesized that a large overdensity of galaxies is responsible for producing the ionizing photons that can create the large ionized bubbles that contribute to determining the observed  Lyα properties.

I have searched for an overdensity of galaxies near two of the earliest known Lyα emitting galaxies at a redshift of z=8.7 and discussed the implications for the existence of a very large ionized bubble at very early times. Read more:

• Whitler et al. (2024)

As an undergraduate, I also worked at the Center for Astrophysics | Harvard & Smithsonian with Dr. Charlotte Mason to improve a physical model of Lyα transmission through the IGM, to be used to infer the timeline of cosmic reionization from observations of Lyα emission from galaxies. Specifically, I implemented scatter in the relation between galaxy UV luminosity and dark matter halo mass and inferred the volumetric neutral fraction at z~7 with the improved model. Read more:

• Whitler et al. (2020)