Constraints on the Galactic Centre environment from Gaia hyper-velocity stars

Following a dynamical encounter with Sgr A∗, binaries in the Galactic Centre (GC) can be tidally separated and one member star ejected as a hyper-velocity star (HVS) with a velocity beyond the escape speed of the Milky Way. As GC-born objects located in more observationally accessible regions of the sky, HVSs offer insight into the stellar population in the inner parsecs of the Milky Way. We perform a suite of simulations ejecting stars from the GC, exploring how detectable HVS populations depend on assumptions concerning the GC stellar population, focusing on HVSs that would appear in current and/or future data releases from the Gaia space mission with precise astrometry and measured radial velocities. We show that predictions are sensitive to two parameters in particular: the shape of the stellar initial mass function (IMF) in the GC and the ejection rate of HVSs. The absence of confident HVS candidates in Gaia Data Release 2 excludes scenarios in which the HVS ejection rate is ≳ 3 × 10-2 yr-1. Stricter constraints will be placed on these parameters when more HVS candidates are unearthed in future Gaia data releases - assuming recent determinations of the GC IMF shape, one confident HVS at minimum is expected in Gaia DR3 and DR4 as long as the HVS ejection rate is greater than ∼ 10-3 and ∼ 10-5 yr-1, respectively.