Jones, Olivia

Ferguson, Annette

Snodgrass, Colin

Mann, Bob

Nally, Conor

2026-03-31T13:08:42Z

2026-03-31

Infrared observations of stellar populations trace the evolutionary cycle of dust and baryonic matter in their host galaxy. Understanding this cycle is key to building a picture of galaxy formation and evolution, star formation, and chemical enrichment in the Universe. Local Group galaxies offer a unique opportunity to characterise the chemical evolution under conditions analogous to those in higher redshift systems. The previous generation of great infrared observatories, Herschel and Spitzer, were capable of mapping galaxies in our immediate neighbourhood to ∼1 Mpc. This space includes galaxies such as the Large and Small Magellanic Clouds (50 and 60 kpc respectively), which, with their low metallicities (0.5 and 0.2 𝑍⊙) and active star-formation, serve as ideal test- beds for theories on how dust forms, evolves, and is destroyed on both small and large scales. The launch of JWST vastly expands this observation window to an order of magnitude greater distances, and encompasses a vast array of parameter spaces to search where, for the first time, we can conduct high-resolution imaging and spectroscopy. This provides the depth and spatial scales previously only obtainable in our Galaxy. With unprecedented depth and high-resolution data provided by JWST, come new challenges in data processing. The second chapter of this thesis outlines the development of Starbug2, a PSF photometric suite tuned to detect and recover photometry from deeply embedded objects in complex and crowded fields. This open source software has been applied to a number of local galaxies, enabling the first resolved stellar population studies from JWST to be conducted. One such galaxy, and the focus of the remainder of this thesis, is NGC 6822. This is a nearby (490 kpc) irregular dwarf. Its tidal isolation, low metallicity (𝑍∼0.3 𝑍⊙), and ongoing star formation make it similar to galaxies during the epoch of peak star formation. NGC 6822 underwent a rapid increase in star formation approximately 3 Gyrs ago, and it remains unclear what triggered the event. In chapter three, using Starbug2, I produce a deep stellar catalogue of the central bar of NGC 6822, the photometry reaching ∼7 magnitudes deeper than previous 𝐽𝐻𝐾𝑠 surveys of the area. This deep NIRCam catalogue reveals stellar populations ∼3 magnitudes below the Red Clump for the first time. With MIRI, I detect sources ∼2 magnitudes below the Tip of the Red Giant Branch. This near- to mid-infrared combined catalogue contains ∼900, 000 point sources, allowing a comprehensive survey of the central bar, revealing features from various populations that have never been seen in NGC 6822 in the infrared. Asymptotic Giant Branch (AGB) stars evolve from low- to intermediate mass (0.8-8 M⊙) progenitors in their final stages of nuclear burning. Due to their high infrared luminosity, stellar winds, and mass loss, they are some of the most influential stars in their systems. Dust-driven wind in the final stages of an AGB stars evolution will, at its peak, inject 10−4M⊙yr−1 produced gas and dust into the interstellar medium (ISM) to form the natal material for the next generation of star formation. However, their dust production mechanisms and how this affects the subsequent rounds of evolution are not well understood, especially in low- metallicity environments. The large number of AGB stars in NGC 6822 offers an excellent vantage point from which to study these processes. In the fourth chapter, I produce an H-R diagram of the NGC 6822 bar, derived from blackbody modelling of the photospheres of stars to a photometric depth of the Red Clump. I identify the AGB stars within my JWST NGC 6822 catalogue through a series of CMD colour-cuts, measure dust production through SEDs fitting with the grams grid of AGB models and determine their chemical subtypes. The models enable the robust separation of oxygen- and carbon- rich AGB types, which is crucial for disentangling the effects of age, mass, and metallicity within the population. I determine the quantity of dust produced in the central stellar bar of NGC 6822 to be 5.6 × 10−7 M⊙yr−1, with ∼60% of this being produced by oxygen-rich AGB stars. This contradicts the recent paradigm that carbon stars are the primary source of dust production in metal- poor galaxies. The separated AGB species allows me to analyse the carbon-star luminosity functions, as well as map the spatial distribution of metallicity across the field and develop observational proxies for measuring dust production rates.

https://era.ed.ac.uk/handle/1842/44547

https://doi.org/10.7488/era/7064

en

Nally C., 2023, StarbugII: JWST PSF photometry for crowded fields, Astrophysics Source Code Library, record ascl:2309.012 (ascl:2309.012)

Nally C., et al., 2024, MNRAS, 531, 183

JWST (James Webb Space Telescope)

NGC 6822

AGB (Asymptotic Giant Branch) stars

Star formation

Dust production

Resolved stellar populations of NGC 6822 with JWST

Thesis

Doctoral

PhD Doctor of Philosophy