Research

How did we get here?

My research seeks to answer this question in the broadest sense - how did our universe go from an infinite sea of hydrogen and helium to the fireworks display of stars and galaxies that light up our telescopes today? How did the first stars and galaxies form? How did those nascent galaxies grow into the massive elliptical and grand spiral galaxies we see today? And how did stars drive the formation of the heavier elements that make up our home planet, and us?

I try to answer these questions using images and spectra of galaxies and their components, including HII regions (gas clouds), star clusters, and individual stars. In order to see these small features of distant galaxies, we rely on a boost from the universe itself in the form of massive galaxy clusters. The mass of these "cities" of galaxies bends the fabric of space around them, which bends the path of light passing through from background objects. The process is similar to light bending when it passes through a glass lens, so we call this phenomenon "gravitational lensing". Gravitationally lensed galaxies appear distorted, but they are magnified to allow us to see star clusters and individual stars in distant galaxies.

Chemical Evolution Within Galaxies

Gravitational lensing allows us to look inside distant galaxies. In the case of the Sunburst Arc (JWST imaging shown at right), the boost from lensing allowed us to explore the connections between massive stars and the abundances of elements in the surrounding gas. In two related papers, we measured the abundances of elements including oxygen, nitrogen, sulfur, argon, neon, iron, and helium (Welch et al. 2025), and connected the abundance pattern, particularly the measured excess of nitrogen relative to oxygen, to the presence of massive Wolf-Rayet stars with strong stellar winds (Rivera-Thorsen et al. 2024).

The Light of Earendel

Earendel is the most distant star that has yet been observed. It has a redshift of z = 6.2, meaning that we are seeing the light it emitted a mere 900 million years after the Big Bang. The star is extremely magnified by gravitational lensing, so the image we see appears thousands of times brighter than it otherwise would. This high magnification allows us to distinguish the light of the star from the rest of the host galaxy. Typically we can only see full galaxies at this distance, with the light of millions of stars blending together.

Images of Earendel taken by JWST (left) have confirmed that this is a stellar object. The images suggest that Earendel is either a single, evolved B-type giant star, or perhaps a binary consisting of one relatively hot (30,000 K) and one cooler (10,000 K) star. (JWST Earendel Paper; HST Earendel Paper)

Star Clusters at Cosmic Dawn

Combining the spatial resolution of the Hubble Space Telescope with gravitational lensing enables us to see the substructures that make up distant galaxies. Using images taken as part of the RELICS program, we modeled the morphology of three lensed arcs at Cosmic Dawn (z > 6). We found that each galaxy is made up of smaller clumps, with some as small as 1 parsec in radius, making them similar to the compact star clusters seen in nearby galaxies. (Figure at right showing HST images and morphological models from Welch et al. 2023).

Relevant Links:

Publications: First author publications; All coauthor publications

Code: https://github.com/bwelch94

Other Cool Stuff:

Cosmic Spring - A collection of awesome JWST/lensing information

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