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[1007.3902]
Bullet Clusters in the MareNostrum Universe

Authors:  J.E. ForeroRomero, S. Gottloeber, G. Yepes 
Abstract:  We estimate the expected distribution of displacements between the dark
matter and gas cores in simulated clusters. We use the MareNostrum Universe,
one of the largest non radiative, SPH Lambda CDM cosmological simulations. We
find that projected 2D displacements between dark matter and gas, equal or
larger than the observed in the Bullet Cluster, are expected in 1% to 2% of the
clusters with masses larger than 10^{14} Msun. The 2D displacement
distribution is roughly the same between redshifts 0<z<0.5 when multiplied by a
factor of (1+z)^{1/2}. We conclude that the separations between dark matter
and gas as observed in the bullet cluster can be easily found in a Lambda CDM
universe. Furthermore we find that the displacement distribution is not very
sensitive to the normalization of the power spectrum. Upcoming surveys could
extend the measurements of these displacements between dark matter and gas into
large samples of hundreds of clusters, providing a potential test for Lambda
CDM. 

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Molly Swanson
Joined: 22 Jun 2009 Posts: 9 Affiliation: University College London

Posted: July 30 2010 


This paper is an interesting counterpoint to Lee and Komatsu 1003.0939 regarding the likelihood of an object like the Bullet Cluster existing in a LCDM universe.
Lee and Komatsu make the claim that the bullet is moving too fast: based on Nbody simulations, they find that the estimated initial relative velocity of 3000 km/s is very rare  none of the bulletlike clusters in the Nbody simulations (with a volume comparable to the Hubble volume) have such high velocities. They estimate the probability of finding a system with 3000 km/s relative velocity is 3.3x10^{−11} to 3.6x10^{−9.}
Here ForeroRomero et al take a different approach: they use hydrodynamical simulations with gas and use the projected separation between the gas and dark matter as their main indicator rather than the relative velocity. In contrast to Lee and Komatsu, they find that 1−2% of the simulated clusters show a DMgas displacement as large as the Bullet Cluster, from which they conclude that bulletclusterlike objects can easily be found in an LCDM universe.
They are measuring somewhat different things, so they are not necessarily inconsistent with each other, despite coming to the opposite conclusions. ForeroRomero et al are working with a smaller simulation volume so they need to extrapolate to higher masses, but the claim of Lee and Komatsu depends on the estimate of 3000 km/s for the initial velocity, which is not directly measurable. The 3000 km/s number comes from the best fit model to noncosmological hydrodynamic simulations of 0711.0967  if the structure of the Bullet Cluster can be explained with a lower infall velocity, the disagreement with LCDM would no longer be valid.
So, which of these is more convincing? Do we need to worry about the Bullet Cluster disproving LCDM? 

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