Thesis Supervisor: Annalisa De Cia

Abstract

The chemical evolution of galaxies is driven by the interplay between stars and gas. Core-collapse supernovae quickly enrich galaxies with α-elements, while Type Ia supernovae contribute iron over longer timescales, making [α/Fe] a sensitive tracer of star formation history and stellar populations. Gas inflows, star formation, and metal exchange—modulated by dust depletion— shape the chemical makeup of the interstellar (ISM) and circumgalactic medium (CGM). Until recently, studies assumed a homogeneous ISM composition, but new methodologies (e.g., De Cia et al.) now allow measurement of dust depletion from relative metal abundances, enabling more precise characterisation of ISM/CGM chemistry. These advances have revealed metallicity variations in the Milky Way, α-element enhancements in the Magellanic Clouds and distant galaxies, and extreme elemental signatures at z ~ 6 possibly linked to very massive stars. Recent observations in the early Universe have uncovered unexpectedly high abundances of elements such as nitrogen (from JWST spectroscopy) and aluminium (from VLT/X-Shooter) in some galaxies at z ≳ 6. Similar abundance patterns are also seen in globular cluster stars. These anomalies may be tied to the nucleosynthetic output of very or super-massive stars, potentially hundreds of solar masses, which are short-lived but may have been common in the early Universe. Very massive stars may be ubiquitous even in the local Universe, for example as observed in the Tarantula Nebula (R136). The study of relative metal abundances in the gas offers rare observational windows into the chemical fingerprints of very and super-massive stars, as well as the first stellar generations. Ongoing programmes such as HST’s STIS-ISM Large Programme (GO 17703, PI De Cia) and the ULLYSES project provide high- and mid-resolution UV spectroscopy to probe metallicity gradients, dust depletion, and elemental anomalies across different environments. This PhD project will focus on gas-phase abundances in local and distant galaxies, with emphasis on signatures from very massive stars—particularly overabundances of N, Al, and Na in low-metallicity regimes. The first goal targets the poorly explored Milky Way outskirts (9–14.5 kpc), using 36 STIS-ISM spectra to measure metallicity gradients and search for peculiar abundances. The second goal examines the LMC, mapping metallicity and dust depletion in star-forming regions, especially around R136. The third goal investigates GRB host galaxies at z ≳ 6 to search for chemical fingerprints of very massive or Population III stars. This work will contribute significantly to understanding ISM chemical diversity, the role of massive stars in the chemical enrichment of the Universe, and the broader evolution of galaxies across cosmic time. It will also help pave the way for the era of the Extremely Large Telescope and the Habitable Worlds Observatory. 

Return to list of thesis topics