# Lessons from the curious case of the `fastest' star in Gaia DR2

## TL;DR

This paper investigates a high-velocity star in Gaia DR2, finds the reported extreme velocity was due to measurement error caused by star misalignment, and proposes quality cuts to improve Gaia data reliability for Galactic studies.

## Contribution

The study identifies the cause of an erroneous extreme velocity measurement in Gaia DR2 and introduces data quality criteria to enhance the accuracy of Gaia RVS star selections.

## Key findings

- The Gaia DR2 velocity was affected by a star misalignment, not an actual high velocity.
- Misalignment can cause spurious velocity shifts up to 620 km/s.
- Applying quality cuts reduces false high-velocity star identifications.

## Abstract

Gaia DR2 5932173855446728064 was recently proposed to be unbound from the Milky Way based on the $-614.3\pm2.5\;\mathrm{km}\;\mathrm{s}^{-1}$ median radial velocity given in Gaia DR2. We obtained eight epochs of spectroscopic follow-up and find a very different median radial velocity of $-56.5 \pm 5.3\;\mathrm{km}\;\mathrm{s}^{-1}$. If this difference were to be explained by binarity, then the unseen companion would be an intermediate-mass black hole; we therefore argue that the Gaia DR2 radial velocity must be in error. We find it likely that the spectra obtained by Gaia were dominated by the light from a star $4.3\;\mathrm{arcsec}$ away, and that, due to the slitless, time delay integration nature of Gaia spectroscopy, this angular offset corresponded to a spurious $620\;\mathrm{km}\;\mathrm{s}^{-1}$ shift in the calcium triplet of the second star. We argue that such unanticipated alignments between stars may account for 105 of the 202 stars with radial velocities faster than $500\;\mathrm{km}\;\mathrm{s}^{-1}$ in Gaia DR2 and propose a quality cut to exclude stars that are susceptible. We propose further cuts to remove stars where the colour photometry is suspect and stars where the radial velocity measurement is based on fewer than four transits, and thus produce an unprecedentedly clean selection of Gaia RVS stars for use in studies of Galactic dynamics.