Code of Conduct
Anna Frebel (MIT)
The first stars and first galaxies formed a few hundred million years after the Big Bang. Their emergence transformed the universe: the first heavy elements changed the gas physics and high energy photons reionized their surroundings. Hence, understanding this early era is at the frontier of modern astrophysics and cosmology. It can be well probed with ancient ultra-faint dwarf galaxies that orbit the Milky Way today. Two ultra-faint dwarf galaxies have been particularly interesting in this regard. Reticulum II is the first known "r-process galaxy". A prolific nucleosynthesis event must have gone off in this system very early on so that subsequent stars formed from gas enriched in large amounts of the very heaviest elements. This has helped solving a 60-year-old puzzle about the astrophysical site of the rapid (r-) process. Tucana II is a spatially extended metal-poor galaxy with member stars found at several half-light radii. Other systems also contain stars in their outskirts, suggesting that such extended "halos" may not be uncommon among the tiniest dwarf galaxies, possibly being a signature of the very first mergers between galaxies at the earliest times.
