Searching for new Hypervelocity Stars with Gaia DR3 and VLT/FORS2 Spectroscopy

Abstract

Hypervelocity stars are unique objects moving through the Milky Way at speeds exceeding the local escape velocity, providing valuable insights into the Galactic gravitational potential and the properties of its central supermassive black hole. The advent of Gaia DR3 offers an unprecedented astrometric precision, enabling the discovery of new hypervelocity stars and facilitating their characterization. This study seeks to identify and characterize hypervelocity star candidates using Gaia DR3 data, focusing on stars lacking radial velocity measurements. Our goal was to estimate the total velocities of these stars and establish their origin within the Galactic framework, if possible. We applied strict selection criteria to Gaia DR3 data, focusing on sources with low parallax uncertainties and high astrometric fidelity. The distributions of the total velocities in the Galactic rest frame were derived and used to identify candidates. Spectroscopic follow-up with VLT/FORS2 provided radial velocity measurements for a subset of these candidates. We evaluated the probabilities of stars exceeding local escape velocities under different Galactic potential models and traced their past orbits to identify possible origins. From Gaia DR3, we identified 149 hypervelocity star candidates with Pesc > 50% of exceeding local escape velocities. Our follow-up spectroscopy for 23 of those sources confirms that the selected targets are traveling at high velocities, with many appearing to escape the Galaxy, depending on the Galactic potential adopted. Our analysis suggests that nearly one-third of the stars may have an extra-Galactic origin. These findings highlight the need for more precise astrometric and spectroscopic data to conclusively determine the origins of hypervelocity stars and improve models of the Galactic potential.