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Hypervelocity Globular: A beacon of merging clusters Oleg Gnedin (University of Michigan) with Alexey Vikhlinin (CfA) While measuring radial velocities of globular clusters around M87 in Virgo Cluster, Caldwell et al. (2014) found an outlier… A triple interaction with a binary black hole (slingshot) can lead to a very high ejection velocity. M87 A source is offset by 2300 km/s from M87. How to achieve it? A known analog: Hypervelocity Stars • Discovered in 2005 by Warren Brown • More than 20 are now known • Move with radial velocity +500 to +800 km/s, escaping the Galaxy Problem: star clusters are not as dense as stars Tidal disruption of a single star at r < 0.00001 pc for Mbh = 4e6 Msun as in Milky Way Tidal disruption of a star binary at r < 0.001 pc Tidal disruption of a star cluster at r < 10 pc for Mbh = 6e9 Msun as in M87 To survive tidal disruption, HVGC would have to have > 10 times higher density than any known stellar system Velocity kick ≈ (2 v dv)1/2 v ≈ (2GMbh/r)1/2 dv ≈ 500 – 1500 km/s for BH mass ratio 1:10 to 1:3 To achieve the observed offset of 2300 km/s, HVGC needs to have come within d = 2-6 pc of M87 BH To survive tidal field of BH at distance d, HVGC needs to have minimum average density 4e9 Msun/pc3 d(pc)-3 (Misgeld & Hilker 2011) Another solution: Structure of Virgo Cluster Group centered on M86 is merging head-on with the main cluster HCGC (Forman et al. 1979, Bohringer et al. 1994, Randall et al. 2008, Urban et al. 2011) M86 group contains its own system of globular clusters, one of which could be HVGC Globular clusters are found between M86 and M87 Expected number of GCs from M86 group as bright as HVGC within the survey area of Caldwell et al. ≈ 3 (Lee et al. 2010) Probability of observing the HVGC velocity Radial velocity is a combination of: • M86 mean velocity • group velocity dispersion • gravitational potential gain Depending on the merger mass ratio and M86 distance, the probability of HVGC velocity being as extreme as observed is 5% to 35% Not extreme! Additional acceleration due to double scattering by cluster halo and then group halo (Samsing 2015) Orbits and origin of Hypervelocity stars Astrometric measurement of proper motions with HST over 3 to 6 years. Origin consistent with ejection from the Galactic center, but uncertainty is very large. Measurement will be improved with Gaia and JWST. Brown, Anderson, OG et al. 2015 arXiv:1502.05069 Summary • A hypervelocity globular cluster in the Virgo cluster: radial velocity = -1000 km/s • Cannot survive tidal forces of supermassive black hole in M87: requires different acceleration mechanism • Likely belongs to the globular cluster system of M86 group, which is merging head-on with the Virgo cluster: significant probability of reaching the observed velocity • Extreme negative velocity outliers are signposts of cluster mergers: could be detected to a distance of > 300 Mpc