Merging Galaxy Clusters 2119.4 Merging Galaxy Clusters
In isolated galaxies and galaxy clusters all matter components contributing to the
common gravitational potential are more or less centrally-symmetrically coincident.
This makes it difficult to discern DM from the baryonic components, and dependent
on parametrization, as we have discussed in the section on Dynamics. In merging
galaxy clusters however, the separate distributions of galaxies, intracluster gas and
DM may become spatially segregated permitting separate observations. The visually
observable galaxies behave as collisionless particles, the baryonic intracluster plasma
is fluid-like, it experiences ram pressure and emits X-rays, but noninteracting DM
does not feel that pressure, it only makes itself felt by its contribution to the common
gravitational potential.
Major cluster mergers are the most energetic events in the Universe since the Big
Bang. Shock fronts in the intracluster gas are the key observational tools in the study
of these systems. When a subcluster traverses a larger cluster it cannot be treated as a
solid body with constant mass moving at constant velocity. During its passage through
the gravitational potential of the main cluster it is shrinking over time, stripped of gas
envelope and decelerating. Depending on the ratio of the cluster masses, the gas forms
a bow shock in front of the main cluster, and this can even be reversed at the time when
the potentials coincide.
In several examples of galaxy cluster mergers the presence of DM could be inferred
from the separation of the gravitational potential from the position of the radiating
plasma.
The Bullet Cluster 1E0657-558. The exceptionally hot and X-ray luminous galaxy
cluster 1E0657-558, theBullet clusterat redshift푧= 0 .296, was discovered by Tucker
et al. in 1995 [13] inChandraX-ray data. Its structure as a merger of a 2.3× 1014 푀⊙
subcluster with a main 2.8× 1014 푀⊙cluster was demonstrated as the first clear exam-
ple of a bow shock in a heated intracluster plasma. With the advent of high-resolution
lensing, a technique could be developed combining multiple strongly-lensedHubble
Space Telescopemulti-color images of identified galaxies with weakly lensed and ellip-
tically distorted background sources. The reconstructed gravitational potential does
not trace the X-ray plasma distribution which is the dominant baryonic mass com-
ponent, but rather approximately traces the distribution of bright cluster member
galaxies.
In early pictures, often reproduced, the center of the total mass was offset from
the center of the baryonic mass peaks, proving that the majority of the matter in the
system is unseen. In front of the bullet cluster which has traversed the larger one
about 100 Myr ago with a relative velocity of 4500km s−^1 , a bow shock is evident in
the X-rays. The main cluster peak and the distinct subcluster mass concentration are
both clearly offset from the location of the X-ray gas. We do not reproduce that well
known figure here because recent analysis of this system [14] do not confirm these
results, rather they find that dark matter forms three distinct clumps.