Constraints on the Galactic Centre environment from Gaia hypervelocity stars

TL;DR

This paper uses Gaia data and simulations to constrain the environment of the Galactic Centre by analyzing hyper-velocity stars, revealing how their observed absence limits the ejection rate and informs about the stellar population in the inner Milky Way.

Contribution

It provides new constraints on the HVS ejection rate and the stellar initial mass function in the Galactic Centre based on Gaia data and simulation comparisons.

Findings

Absence of confident HVS candidates in Gaia DR2 limits ejection rate to below 3×10^{-2} yr^{-1}.

Future Gaia data releases will tighten constraints on the HVS ejection rate and IMF shape.

Predictions suggest at least one HVS should be detected in Gaia DR3 and DR4 if ejection rates are above certain thresholds.

Abstract

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 which would appear in current and/or future data releases from the \textit{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 \textit{Gaia} Data Release 2 excludes scenarios in which the HVS ejection rate is $\gtrsim3\times10^{-2} \, \mathrm{yr^{-1}}$. Stricter constraints will be placed on these parameters when more HVS candidates are unearthed in future \textit{Gaia} data releases -- assuming recent determinations of the GC IMF shape, one confident HVS \textit{at minimum} is expected in \textit{Gaia} DR3 and DR4 as long as the HVS ejection rate is greater than $\sim 10^{-3} \, \mathrm{yr^{-1}}$ and $\sim10^{-5} \, \mathrm{yr^{-1}}$, respectively.