Supermassive black holes — located in galactic nuclei— and dark matter —dominant in galactic halos— are vital ingredients in the cosmological process of galaxy formation, including that of our own Galaxy. However, probing our Galactic Centre (GC) and halo has proven observationally challenging. Hypervelocity stars (HVSs) are unique in that they deliver key information on both. Following interactions with black holes in the vicinity of Sagittarius A* (Sgr A*), HVSs are ejected on fast trajectories through the halo. They thus bear testimony not only to the black hole and stellar populations within the hard-to-access innermost parsec, but also to the Galactic mass distribution, imprinted on their orbits. This information is essential to understand the assemble history of our GC and our Galaxy, respectively.

So far, the utilisation of HVSs has been limited by the paucity and quality of data. The ESA Gaia mission and new spectroscopic surveys are about to dramatically change this. Their upcoming data releases contain a few hundred HVSs with unprecedented astrometric measurements, but identifying them requires careful analysis of the basic data.

My ERC project “VEGA-P” unprecedentedly combines three key facets: i) identifying and characterising HVSs; ii) modelling HVS data in a full statistical framework and theoretically interpreting our results for the GC; and iii) performing a joint analysis with complementary Galactic halo probes, never observed so abundantly before Gaia.

Currently, the team work is focused on Gaia data mining, which has resulted in the tightest and most robust determination of the HVSs’ ejection rate, and on developing a self-consistent, holistic modelling framework. This framework is based on a realistic Milky Way potential, where stellar/compact object binaries are deflected from as far as the central molecular zone (~200 pc) onto radial orbits that result into their tidal separation or their tidal disruption by our massive black hole, Srg A. The result of the separation is that one star/stellar compact object is deposited very close to Srg A, while the other is ejected at high speed into the halo as a HVS. This framework will allow us to extract GC content, spatial distribution and dynamics of stellar binaries, stars and compact objects. These will be used to discriminate between assembly scenarios for the GC, and to uniquely calibrate the rate estimates for related galactic nuclear phenomena observed in other galaxies, including gravitational wave sources (e.g., Extreme Mass Ratio Inspirals), Tidal Disruption Events, and Quasi-Periodic Eruptions.