The quest for a classical bulge - a study of kinematics, abundances, and ages using microlensed dwarf stars and Gaia

Abstract

Context: We present an kinematical analysis of 90 F and G type dwarf, turn-off, and
subgiant stars in the Galactic bulge. These stars were observed with high-resolution spectrographs during gravitational microlensing events This allowed for their ages, abundances, and radial velocities to be ascertained while they were significantly brighter. Plotting these stars as [C/O] vs [O/H], it is revealed that there appears to be a distinct sub-group of stars that is chemically different. Aims: The aims of the thesis was then to investigate whether these chemically distinct stars also exhibit distinct kinematics compared to the entire sample of stars. By undertaking a kinematical analysis on these Sun-like dwarf stars it will shed some light on the tentative nature of the formation and evolution of the Milky Way bulge. Method: Through the use of finding charts we will cross-match our sample with stellar catalogues such as Gaia Data Release 3, and VVV InfraRed Astrometric Catalogue in order to extract relevant kinematical parameters; we then use these parameters in tandem with Galpy to make orbital simulations of our sample of stars. Results: Orbital integrations were successfully completed for 63 out of 90 stars, resulting in four parameters
that we could use for the kinematical analysis: apocentre, pericentre, eccentricity, and
Zmax of the orbits. By plotting [C/O] vs [O/H] as a function of these parameters we found there is no kinematical distinctness between this chemically distinct sub-group of stars and the rest of the stars. Plotting eccentricity vs apocentre and Zmax showed a large degree of similarity to plots of the same parameters but with stars in the disk, which would indicate that the bulge and disk shared similar evolutions and history. Plotting the galactocentric radial velocity vs galactic longitude/latitude revealed characteristic trends of cylindrical rotation, indicative of a buckling instability scenario for the bulge. Conclusions: We conclude that the sub-group of chemically distinct stars are not kinematically unique or indicative of a remnant group of stars from a early merger event. The kinematics point more towards a secular origin of the Milky Way bulge rather than a classical bulge resulting from mergers early on in the universe.

Popular Abstract

The Milky Way, a spiral galaxy, possesses a complex structure and evolutionary his-
tory that remains partially elusive. The formation of the bulge has undergone significant changes in explanation over the years, making it a crucial point of debate in Galactic archaeology. By examining the chemical abundance and kinematic properties of stars, we aim to uncover the puzzle pieces that reveal the evolution and formation of our galaxy. This approach enables us to peer into the past and fill in gaps in our understanding.

Our aim was to investigate whether a chemically distinct sub-group of stars found in
the bulge region of the Milky Way exhibit distinct kinematics. By cross-matching our
sample with Gaia Data Release 3, and VVV InfraRed Astrometric Catalogue, we were
able to extract relevant kinematical parameters and use them in tandem with Galpy to
make orbital simulations of our sample of stars.

We found that there was no kinematical distinctness between this chemically distinct sub-
group of stars and the rest of the stars. We plotted many different parameters, and found a large degree of similarity to plots of the same parameters but with stars in the disk, which would indicate that the bulge and disk shared similar evolutions and history. For example, plotting the galactocentric radial velocity vs galactic longitude/latitude revealed characteristic trends of cylindrical rotation, indicative of a buckling instability scenario for the bulge.

We concluded that the sub-group of chemically distinct stars are not kinematically unique or indicative of a remnant group of stars from an early merger event. The kinematics point more towards a secular origin of the Milky Way bulge rather than a classical bulge resulting from mergers early on in the universe.

In summary, our study provides insight into the tentative nature of the formation and
evolution of the Milky Way bulge. We have shown that the chemically distinct sub-group
of stars is not kinematically unique, providing evidence for the secular origin of the Milky Way bulge. Our findings contribute to our understanding of the formation and evolution of the Milky Way and provide a basis for future research.

• author: Kvernes Macpherson, Calum
• supervisor: Thomas Bensby
• organization: Astrophysics; Department of Physics
• course: ASTM32 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Bulge, Milky Way, Kinematics, Galpy, Dwarf Stars
• report number: 2023-EXA209
• language: English
• id: 9126001
• date added to LUP: 2023-06-16 15:39:09
• date last changed: 2023-08-30 15:08:44