The so-called Hypervelocity stars (HVSs; with velocities even greater than 1000 km/s) were first predicted from the theoretical arguments of Hills (1988), and attributed to be the result of tidal interaction between a close stellar binary system and a supermassive black hole (SMBH) in the Galactic center, commonly referred to as the “Hills mechanism”. HVSs or High-Velocity stars (HiVels) can also be ejected by extending the Hills mechanism with alternative assumptions: (1) a SMBH binary or the pair of a SMBH and an intermediate-mass black hole (IMBH) in the Galactic center or (2) individual IMBHs (or SMBHs) in the Galactic dwarf galaxies or globular clusters, or even massive BH binaries in such Galactic subsystems.

 

Using data from current large-scale spectroscopic surveys (e.g., LAMOST) and the Gaia mission, we present an extensive sample of over 1000 HVSs/HiVels spanning all spectral types. Through backward orbital integrations, combined with chemical fingerprints and age similarities, we uncover clear evidence of HVSs/HiVels ejected from both dwarf galaxies and globular clusters. For the first time, we report the discovery of a high-velocity star J07 ejected from globular cluster M15 approximately 21 million years ago, providing strong evidence for the presence of an IMBH constrained to within a few AU of the central region of M15. With the increasing power of ongoing Gaia and large-scale spectroscopic surveys, we expect to discover more such HVSs/HiVels ejected from the Galactic center, dwarf galaxies, or globular clusters, which should shed light on the understanding of the evolutionary path from stellar-mass BHs to SMBHs.