PhD ceremony Barbara Siljeg

On Tuesday 26 January 2026, Barbara Siljeg defends her thesis called
Looking through the neutral hydrogen glasses: the hidden population and clustering of gas-rich dwarf galaxies

Summary of her thesis:
Dwarf galaxies, defined as having at least ten times less stellar mass than our Milky Way, are the most abundant galaxy population in the Universe. Nevertheless, their observed properties often deviate from theoretical expectations, making them one of the least understood classes of galaxies. In her thesis, Barbara Siljeg explores the diversity and clustering of dwarf galaxies using observations of an emission line of neutral hydrogen (HI) present in their interstellar gas. Unlike traditional optical surveys that rely on starlight, HI can be detected even in optically faint and diffuse galaxies, allowing dwarf galaxies to be identified regardless of their stellar distributions.

Siljeg shows that HI-detected dwarf galaxies are systematically larger for their stellar mass than those found in optical surveys and that ultra-diffuse galaxies—large but extremely faint systems—naturally emerge as part of the normal dwarf galaxy population, indicating that optical observations miss a significant fraction of the dwarf galaxy population. Siljeg also demonstrates that interactions between dwarf galaxies are more common than previously indicated by optical studies and can significantly influence their star formation activity. In addition, she reveals a faint stellar counterpart to a previously identified HI source thought to be a candidate dark galaxy, showing that some apparently starless systems may instead be distant or extremely faint dwarfs. Overall, Siljeg highlights how observing galaxies through their gas provides a more complete picture of the low-mass Universe and helps refine models of galaxy evolution on the smallest mass scales.

Link to Barbara's thesis

If available, you can follow the PhD ceremony via this livestream