Unravelling the evolution of the Galactic stellar disk and bulge

Abstract

The formation and evolution of spiral galaxies is a research topic central to modern Astronomy. In this context, the Milky Way offers a unique opportunity for astronomers to study a spiral galaxy in detail and thus informs many aspects of galaxy formation theory.

The observational signatures of Galactic stellar components provide clues to its assembly history. This thesis is focused on two main components of the Galaxy: the stellar disk and bulge. In particular, we examine the chemical properties of these components and their implications for Galactic evolution. The data in this thesis were obtained with HERMES, a new high-resolution optical spectrograph on the Anglo Australian Telescope. The disk sample consists of over 3000 giant-branch stars, extending up to 4 kpc in height from the Galactic plane. The thin disk (low-α population) exhibits a steep negative vertical metallicity gradient, a signature observed in galaxy evolution models where radial migration plays an important role. The thick disk (high-α population) has a weaker vertical metallicity gradient, which could have arisen from a settling phase of the primordial disk. The [α/Fe] ratios of the thin and thick disk populations are distinct and nearly constant with height. This indicates the two populations were formed in very different conditions, and although the high-α population likely experienced a settling phase, its formation timescale was fast still, in the order of a few Gyrs.

To investigate the chemistry of the Galactic bulge and its connection to the disk, we obtained abundance ratios of 18 elements for more than 800 red giants. The [α/Fe] abundance ratios show vertical variations that are consistent with the distribution of bulge metallicity components: at high latitudes [α/Fe] is enhanced as the metal-poor component dominates; closer to the plane, the metal-rich components contribute lower [α/Fe]. However, at fixed metallicity, all elements show uniform abundance ratios with latitude. We observe normal [Na/Fe] ratios that do not vary as a function of latitude at fixed metallicity, indicating that the bulge does not contain strongly helium-enhanced populations as observed in globular clusters.

By comparing our results with that of the GALAH survey, we conclude that there are similarities between the bulge and disk in terms of their chemistry. However, the more metal-poor bulge population ([Fe/H] ≲ -0.8) shows enhanced abundance ratios compared to the disk for some light, alpha, and iron-peak elements that are associated with core-collapse supernovae (SNeII). This population may have experienced a different evolution to bulge stars of disk origin. Moreover, the [La/Eu] abundance ratios suggest higher r-process contribution in the bulge, which indicates that overall the bulge experienced a higher star formation rate than the disk. Keywords: Galaxy, stellar populations, stellar abundances, disk, bulge, galaxy formation, galaxy evolution Show more