From the bulge region to the outer disk: Tracing the α-enhancement level in the Galactic thin and thick disks

Abstract: One of the biggest mysteries astronomers wish to unveil is the formation and evolution of galaxies. Properties of our own galaxy, the Milky Way, can provide great insight into these. The star formation rate (SFR) and whether it varies with distance from the galactic centre (GC), is one of these properties. Studies indicate that the SFR is higher in the bulge region than in the local thick disk of our galaxy. One would expect it to continue to decrease outwards since the gas density is lower further away from the GC. This thesis aims to trace the SFR "knee", which is an indication of SFR when analysing the α-element abundances of a star. In this study, we will investigate if the knee's position changes for stars residing in different regions of the Milky Way. To trace the position of the knee, generalised Gaussian distributions were constructed for the α-element abundances in different [Fe/H] intervals. The peaks of these could then be used to pinpoint where the star-count was largest in each individual metallicity interval. The knee appears most prominent in the [Mg/Fe] abundance trend, which was therefore chosen as the focus of the analysis. Data from APOGEE DR16 will be used, which also contains Bailer-Jones distance estimates from Gaia DR2. Stars believed to have originated from mergers were excluded. These are not of interest since the study only wishes to trace the α-abundance trends of the Milky Way itself. Furthermore, to avoid any selection bias, only giants were included, since dwarfs are intrinsically fainter and difficult to observe at greater distances. The results found that the knee in the abundance trends of [Mg/Fe] with [Fe/H] seemed shifted to higher metallicities towards the GC, which implies a varying SFR throughout the Milky Way. However, this shift is only found if we look at the shift of the slope itself. Previous studies have also found that the bulge region has elevated [α/Fe] abundances at higher [Fe/H], compared to the local thick disk. An estimated shift of [Fe/H]≈+1.0 was found, in accordance with the results of other studies. The results also found that the IMF appeared larger in the bulge region than in the local thick disk. In conclusion, the study found that the bulge region could have had a higher SFR and IMF than the local thick disk. These are indicators that they are separate components, and that the bulge region is not solely a superposition of thick disk, thin disk, halo, and bar components. Instead, it could be a separate component with its own stellar population.