Download Hypervelocity Globular: A beacon of merging clusters Oleg Gnedin with Alexey Vikhlinin

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