Resolved Stellar Populations and Evolution of Star-Forming Metal-Poor Dwarf Galaxies

by Giacomo Bortolini (Stockholm University)

Wednesday 10 Jun 2026, 13:30 → 14:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

In this seminar, I will present the research I have carried out during my PhD at Stockholm University on the resolved stellar populations and star formation histories of metal-poor, star-forming dwarf galaxies.

Dwarf galaxies (M < 10⁹ M☉) are not only the most common type of galaxy across cosmic time, but are also thought to be the fundamental building blocks of more massive galaxies through hierarchical merging. Although they are not perfect analogues of their high-redshift counterparts, dwarf galaxies within the Local Volume (D < 20 Mpc) provide unique laboratories for studying star formation and chemical evolution in low-metallicity environments at high spatial resolution. Thanks to the exquisite spatial resolution of the Hubble Space Telescope (HST) in the ultraviolet and optical, and the James Webb Space Telescope (JWST) in the near-infrared, we can now resolve individual stars in these systems across multiple wavelengths using point-spread-function fitting photometry. By constructing color–magnitude diagrams (CMDs) and comparing the observed stellar populations with predictions from state-of-the-art stellar evolution models, we can estimate stellar ages and masses, and reconstruct how the star-formation rate has evolved over time. In this sense, stellar populations act as fossil records of galaxy evolution, much as archaeological artifacts reveal the history of human civilizations.

I will present recent HST and JWST deep imaging of one of the most metal-poor (Z ~ 3% Z☉) star-forming dwarf galaxy known in the nearby Universe: I Zw 18, a prime laboratory to study stellar evolution and feedback of young massive stars at extremely low metallicity. I will also present new NIRCam JWST imaging of the interacting dwarf galaxy pair NGC 4485/4490 and the irregular dwarf galaxy NGC 4449, obtained as part of the JWST cycle 1 program FEAST (Feedback in Emerging extrAgalactic Star clusTers, PI: A. Adamo). I will show how we can probe the stellar populations of these galaxies at an unprecedented level of detail given their distance from us (d > 4 Mpc), and how to reconstruct their star-formation histories across cosmic time throgh CMD modelling.