Our current, leading theory describing the history of the Universe is the ΛCDM model. The model predicts that larger structures are formed through hierarchical clustering, and, thus, merger events are vital to the formation and evolution of larger galaxies such as the Milky Way. The remnants of such accretion events will be scattered throughout the Galaxy, building the Galactic halo that we can observe today.

In this thesis we use blue straggler stars (BSS), red giant branch (RGB) stars and blue horizontal branch (BHB) stars to determine whether globular clusters or dwarf galaxies are the dominant building blocks of the local Milky Way halo. Since the BSSs are expected to form through mass transfer in binary systems or stellar collisions, their number differs depending on the mass and type of the astronomical object in which they are born, making them ideal for our purposes.
With photometric data from the Gaia Early Third Data Release we determine the BSS/BHB and BSS/RGB ratios in the local halo, probing distances out to 1.83 kpc. We find BSS/BHB ratios in the range [2.97,6.09] and BSS/RGB ratios in the range [0.36,0.63]. The former range is consistent with the ratios found in dwarf galaxies, thereby favouring these as the primary building blocks of the halo. We also find that the relatively high BSS/BHB ratios are at odds with the ratios found in the faintest dwarf galaxies, suggesting that the halo was built by the accretion of one or few larger dwarf galaxies. The BSS/RGB ratios found in the halo does not allow for such a distinction between less-massive GCs and dwarf galaxies; however, it does support the conclusions drawn from the BSS/BHB ratio.

Zoom link: https://aarhusuniversity.zoom.us/j/61126214976