In this thesis we use state-of-the-art observations and simulations to provide the best constraints to date on the HVS ejection rate and constraints on the stellar content and dynamics at the GC, including the progenitor binary populations and their star formation history. The challenge in identifying HVSs is often the observational volume of the sample that is used, which is a common thread through most of the chapters in this thesis. We first attempt to identify new HVSs by increasing the number of stars with radial velocity solutions using the 220 million low resolution spectra provided by Gaia DR3, before switching to a method that looks for the fastest (>800 km/s) HVSs in all ~1.5 billion sources in Gaia. In Chapter 4 we combine these measurements with observations of the GC to perform a robust analysis linking the S-stars with the stars captured by the Hills mechanism and in the final chapter we use the newly released DESI DR1 MWS to further increase our observational footprint and search instead for HVSs ejected over billions of years that have remained bound to the Galaxy. We briefly summarise each chapter below.
In Chapter 2 we start out by attempting to measure radial velocities from the aforementioned Gaia BP/RP spectra in Gaia DR3. Our goal is to increase the number of sources with 3D velocities, since that makes it more straightforward to identify HVS candidates. Using model grids of stellar spectra, we fit every available BP/RP spectrum and marginalise over all stellar parameters other than radial velocity. We find that BP/RP spectra in Gaia can be used to measure radial velocities, albeit at much lower precision compared to, for instance, Gaia RVS. We perform calibration of the radial velocities in colour-magnitude-extinction space and publish a catalogue of BP/RP measured radial velocities. Our extended catalogue, containing some 125M radial velocity measurements is to our knowledge the largest radial velocity catalogue to date. Despite the size of our catalogue, we find that identifying HVSs from our catalogue is non-trivial, due to the large nominal uncertainties and a fraction of sources with unreliable measurements.
In Chapter 3 we follow a different approach to identify HVS candidates. Instead of only looking at stars with 3D velocity measurements, we use a novel method to select stars that appear to be travel on radial trajectories from the GC, which can be applied to all ~1.5 billion stars with astrometric solutions in Gaia. We create a catalogue of 600 HVS candidates, based on their proper motions, sky position, and colour and perform follow-up observations for about 200 of those using ground-based spectroscopic instruments. Because we use both northern and southern hemisphere telescopes, we can cover our HVS candidates across the sky. We use the non-detection of new HVSs among our 200 observed HVS candidates in combination with sophisticated simulations to significantly improve constraints on the ejection rate and mass function of HVSs and provide predictions for the undiscovered population of HVSs in Gaia.
Chapter 4 in a way is the odd one out in this thesis, since we do not attempt to discover new HVSs. Instead we combine observations of the GC with our own observational catalogue described in Chapter 3 and compare to simulations to investigate if the Hills mechanism alone can explain the observed properties of the S-star cluster. We use realistic star formation histories for stars near the GC and simulate binary disruptions. We find that no single progenitor binary population can explain all the observed properties of both the S-star cluster and HVSs simultaneously. Instead, we find that at least two progenitor populations are required: an old population and a young population. Assuming that the young population is the CWD near Sgr A* and the old population undergoes Hills mechanism disruptions at a constant rate, we find that the recent star formation episode at the GC which formed the CWD has boosted the Hills mechanism disruption rate by about an order of magnitude compared to the background rate over the past ~10 Myr.
Finally in Chapter 5 we again aim to identify new HVSs by going to an even larger potential discovery space: stars ejected from the GC that have remained bound to the Galaxy. Since HVS ejections will have been happening for billions of years and a significant fraction of these is expected to remain bound to the Galaxy, a population of stars ejected from the GC should have accumulated in the stellar halo. Using the recently released DESI DR1 MWS survey data, we aim to statistically identify an overdensity of high metallicity stars on low angular momentum orbits, characteristic of stars ejected from the GC. Although we obtain a null-detection for a GC ejected population of stars, we use this null-detection to put upper limits on the ejection rate of stars from the GC over the past ~5 Gyr, which is ejection model independent.
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