Thesis Defense: Dwarf galaxies galore: stellar populations of disrupted Milky Way satellites

 

Abstract: Unveiling the driving mechanisms behind galaxy growth is one of the main goals of Astrophysics, from the tiniest ultra-faint dwarfs in the Local Group to the most luminous blue galaxies beyond the epoch of reionization. In this thesis, we combine observations of Galactic halo stars from Gaia space mission and large-scale spectroscopic surveys to study the stellar populations of tidally disrupted dwarf galaxies that merged with the Milky Way during its process of hierarchical assembly. Overall, we cover a stellar mass range of 10^7 < M*/Msun < 10^9 within a redshift interval of 0.5 < z < 2.0. This thesis compiles papers either published (Limberg 2021, 2022, 2023, 2024) or submitted during the period of this PhD program (2020--2024). First, we explored the stellar populations and globular clusters from the bright dwarf galaxy responsible for the Last Major Merger experienced by the Milky Way, the Gaia-Sausage/Enceladus (GSE). We demonstrate that GSE is the best available candidate, out of known accreted satellites, to be the original host galaxy of Omega Centauri, the most massive Galactic globular cluster, also known to be the stripped nuclear star cluster of a fully disrupted galaxy. Also, we show that the recently discovered intermediate- mass black hole inside Omega Centauri/GSE is consistent with the stellar mass-- central black hole mass relation extrapolated from massive galaxies. Then, we analyzed the abundance patterns of alpha elements for stars in another accreted satellite, the Helmi streams. Our most important finding is that this destroyed dwarf galaxy experienced a revived episode of star formation before being fully quenched by tidal disruption during its merger. The mechanism that causes this extra star- formation burst remains unclear and should be explored with hydrodynamical simulations of satellites around Milky Way-mass hosts. We also studied the chemical evolution in the dwarf-galaxy stellar stream named Waking/LMS-1. Our main result is that this disrupted satellite is the first example of a galaxy that experienced chemical enrichment in heavy elements produced by the rapid neutron-capture (r-)process predominantly by neutron star mergers. Lastly, we investigated the origin of distinct chemo-dynamical populations in the tidal tails of Sagittarius dSph galaxy, the most massive stellar stream in the Milky Way's halo. We found that stars in the more metal-poor and dynamically hotter component of the stream originated in the outskirts of Sagittarius dSph and were stripped first. On the contrary, the more metal-rich and dynamically colder component only forms after the second pericentric passage and its member stars were originally more centrally concentrated. The conclusion is that Sagittarius dSph must have been capable of developing a metallicity gradient prior to infall already at redshift z ~ 0.5 (>6 Gyr ago). Collectively, our results represent significant advances in dwarf- galaxy archaeology. Not only this thesis provides fresh insights into aspects of galaxy evolution at small scales (occupation by central black holes and chemical evolution with neutron star mergers), but also offers new paths for dwarf galaxy research; for example, what is the driving mechanism for star-formation bursts in both surviving and disrupted dwarf satellites? or how can dwarf galaxies form early metallicity gradients at high redshift? and more. The future is certainly bright for bright dwarf galaxies.

 

Palavras-chave: dwarf galaxies, stellar populations, chemical abundance.