My research interests are broadly focused on galaxy formation and evolution with a specific aim towards understanding how very early galaxies contributed to the phenomenon of hydrogen reionization. 


Currently, my work is focused on observations of the brightest galaxies present in the reionization era using telescopes from the optical through far-infrared (Subaru, MMT, VISTA, Spitzer, and ALMA). Because very early galaxies are so distant and therefore generally faint, this extremely luminous subset offers a unique opportunity to study the properties of reionization-era galaxies in great detail. In particular, we can use this luminous population to understand the ionizing efficiency of very early galaxies and thereby help inform how galaxies helped drive reionization. 


In addition to using data from existing telescopes, I have also applied theoretical models to predict what we can learn about early galaxies and reionization from future observatories such as the James Webb Space Telescope (JWST) and the Giant Magellan Telescope (GMT). My full publication list can be found with ADS.

Characterizing Ionized Bubbles in the Reionization Era



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Thanks to a variety of observational efforts over the past decade, we have begun to understand that reionization mainly took place at z>6. However, very little remains known about the sizes of ionized bubbles formed during reionization as well as the galaxy overdensities which drove their growth. Fortunately, we can use spectroscopy to begin identifying and studying these bubbles. In a nutshell, Lyman-alpha is far easier to detect from early galaxies situated within ionized bubbles. Because of this effect, regions of the early Universe where galaxies show relatively strong Lyman-alpha emission are likely volumes where a large ionized bubble has formed. Once we identify these bubbles, we can begin characterizing their size as well as the surrounding galaxy overdensities to better understand how reionization happened.

In Endsley & Stark (2021d), we presented ultra-deep MMT/Binospec observations of several galaxies surrounding a tentative ionized bubble we previously identified in Endsley et al. (2021b). We detected Lyman-alpha emission at z=6.70-6.88 in 9 of 10 observed galaxies, revealing the the large-scale volume spanned by these sources is highly overdense with >3x the average number density of UV-bright galaxies. We also verified that Lyman-alpha emission is unusually strong from galaxies situated in this volume, suggesting the presence of a large (R~3 physical Mpc) ionized bubble. These results are consistent with the expectation that the largest ionized bubbles formed around strong galaxy overdensities due to the excess ionized photons produced within these regions. Upcoming facilities (e.g., Roman and the GMT) will enable us to characterize many more and even larger ionized bubbles in the early Universe, thereby delivering detailed insight into how reionization happened.

The Ionizing Efficiency of Reionization-Era Galaxies


One of the key questions for understanding how galaxies contributed to reionization is how efficiently they produced ionizing photons (i.e. light with enough energy to ionize hydrogen) and what fraction of that light was able to escape from early galaxies. Recent studies have shown that the rest-optical [OIII]+Hβ emission lines are valuable probes of the ionizing efficiency (both production and escape) of early galaxies. In Endsley et al. (2021a), we used a set of highly luminous galaxies to infer the [OIII]+Hβ EW distribution at z~7, finding that a significant fraction of z~7 galaxies are likely to be highly efficient ionizing agents, far more than at even z~2.

Lyman-Alpha in the Reionization Era

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Lyman-alpha (an emission line caused by hydrogen recombination) can act as a valuable probe of reionization, constraining how quickly reionization occurred and where it first took place. Since 2019, I have been leading an MMT/Binospec program aimed at characterizing Lyman-alpha emission in very luminous z~7 galaxies. The first results from this program were presented in Endsley et al. (2020) where we describe a Lyman-alpha detection from a very luminous z=6.85 galaxy with extremely strong [OIII]+Hβ emission (~4000 Angstrom EW). I am also actively involved in a large ALMA program (REBELS) targeting [CII] and dust emission in a large sample of extremely luminous z>6.5 galaxies, several of which possess Lyman-alpha constraints from our Binospec program. With these datasets, I am investigating the properties of Lyman-alpha emission in massive z~7 galaxies to gain general insight on reionization. Stay tuned for more information!

Clustering Measurement Predictions at z=4-10 with JWST

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Within a small fraction of a second after the Big Bang, a process referred to as inflation created large-scale ripples across the Universe, resulting in regions that were either overdense or underdense. Because galaxies represent locations of the Universe where a large amount of mass has congregated together, they preferentially reside in overdense regions and are therefore clustered together in space. By measuring how clustered these galaxies are, we can infer the total amount of mass they hold (i.e. the halo mass) and compare that to the amount of mass we see locked into stars (i.e. the stellar mass) to calculate their integrated star formation efficiency. In this work, we used an empirical model to predict that planned Cycle 1 surveys with JWST (e.g. JADES and CEERS) would be able to measure the clustering of galaxies at z=4-10 with >5σ significance, and thereby enable the inference of halo masses with <0.3 dex precision at these early epochs (about <10% of the current age of the Universe). This suggests that early JWST observations will deliver the first precise picture of how the stellar-halo mass relation evolves in the reionization era.