Flying far and fast: the distribution of distant hypervelocity star candidates from Gaia DR2 data

Abstract

Context. Hypervelocity stars move fast enough to leave the gravitational field of their home galaxies and venture into intergalactic space. The most extreme examples known have estimated speeds in excess of 1000 km/s. These can be easily induced at the centres of galaxies via close encounters between binary stars and supermassive black holes; however, a number of other mechanisms operating elsewhere can produce them as well. Aims. Recent studies suggest that hypervelocity stars are ubiquitous in the local Universe. In the Milky Way, the known hypervelocity stars are anisotropically distributed, but it is unclear why. Here, we used Gaia Data Release 2 (DR2) data to perform a systematic exploration aimed at confirming or refuting these findings. Methods. We assume that the farther the candidate hypervelocity stars are, the more likely they are to be unbound from the Galaxy. We used the statistical analysis of both the spatial distribution and kinematics of these objects to achieve our goals. Results. Focussing on nominal Galactocentric distances greater than 30 kpc, which are the most distant candidates, we isolated a sample with speeds in excess of 500 km/s that exhibits a certain degree of anisotropy but remains compatible with possible systematic effects. We find that the effect of the Eddington-Trumpler-Weaver bias is important in our case: over 80% of our sources are probably located further away than implied by their parallaxes; therefore, most of our velocity estimates are lower limits. If this bias is as strong as suggested here, the contamination by disc stars may not affect our overall conclusions. Conclusions. The subsample with the lowest uncertainties shows stronger, but obviously systematic, anisotropies and includes a number of candidates of possible extragalactic origin and young age with speeds of up to 2000 km/s.

17 pages, 12 figures, 7 tables, 1 appendix. Published in Astronomy and Astrophysics. Structured abstract abridged

References in corpus (16)

• The Gaia mission
• An eclipsing binary distance to the Large Magellanic Cloud accurate to 2 per cent
• Ejection of Supermassive Black Holes from Galaxy Cores
• Constraining the thermally-pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud
• Distances and parallax bias in Gaia DR2
• Estimating distances from parallaxes. II. Performance of Bayesian distance estimators on a Gaia-like catalogue
• Activity indicators and stellar parameters of the Kepler targets. An application of the ROTFIT pipeline to LAMOST-Kepler stellar spectra
• The local high velocity tail and the Galactic escape speed
• Maximum speed of hypervelocity stars ejected from binaries
• The Anisotropic Spatial Distribution of Hypervelocity Stars
• Lessons from the curious case of the `fastest' star in Gaia DR2
• Comparison of Gaia DR2 Parallaxes of Stars with VLBI Astrometry
• Discovery of two new hypervelocity stars from the LAMOST spectroscopic surveys
• The central dynamics of M3, M13, and M92: Stringent limits on the masses of intermediate-mass black holes
• Eclipsing binary stars in the Large Magellanic Cloud. Results from the EROS-2, OGLE and VMC surveys
• The Lutz-Kelker Paradox

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