[astro-ph/0409652] Cluster Alignments and Ellipticities in LCDM Cosmology

Authors:  Philip F. Hopkins, Neta Bahcall, Paul Bode (Princeton University)
Abstract:  The ellipticities and alignments of clusters of galaxies, and their evolution with redshift, are examined in the context of a Lambda-dominated cold dark matter cosmology. We use a large-scale, high-resolution N-body simulation to model the matter distribution in a light cone containing ~10^6 clusters out to redshifts of z=3. Cluster ellipticities are determined as a function of mass, radius, and redshift, both in 3D and in projection. We find strong cluster ellipticities: the mean ellipticity increases with redshift from 0.3 at z=0 to 0.5 at z=3, for both 3D and 2D ellipticities; the evolution is well-fit by e=0.33+0.05z. The ellipticities increase with cluster mass and with cluster radius; the main cluster body is more elliptical than the cluster cores, but the increase of ellipticities with redshift is preserved. Using the fitted cluster ellipsoids, we determine the alignment of clusters as a function of their separation. We find strong alignment of clusters for separations <100 Mpc/h; the alignment increases with decreasing separation and with increasing redshift. The evolution of clusters from highly aligned and elongated systems at early times to lower alignment and elongation at present reflects the hierarchical and filamentary nature of structure formation. These measures of cluster ellipticity and alignment will provide a new test of the current cosmological model when compared with upcoming cluster surveys.
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Sarah Bridle
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[astro-ph/0409652] Cluster Alignments and Ellipticities in L

Post by Sarah Bridle » September 30 2004

The idea of using cluster ellipticities and the correlations in their ellipticities to constrain cosmology seems really nice to me. This paper quantifies these effects and their evolution using high resolution simulations.

This reminds me of studies of galaxy intrinsic alignments where there is a big question about how the apparent galaxy ellipticity, as measured by the light emitted, traces the ellipticity of the mass distribution, which is closer to what simulations predicts. For example astro-ph/0201095 (van den Bosch et al) find that there is a misalignment of about 30 per cent.

I was wondering, to what extent do we expect the observed cluster ellipticities as traced eg. by the galaxy number density to match the dark matter ellipticity?

Anze Slosar
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Re: [astro-ph/0409652] Cluster Alignments and Ellipticities

Post by Anze Slosar » September 30 2004

Sarah Bridle wrote:I was wondering, to what extent do we expect the observed cluster ellipticities as traced eg. by the galaxy number density to match the dark matter ellipticity?
:) Well there must correlation, but is probably not one-to-one... So you probably need to introduce a bias parameter and then might want to use simulations to see how this bias parameter depends on halo mass...

[arxiv]astro-ph/9906160[/arxiv] claims that DM profile is less round than baryons, although they talk about IGM in clusters rather than distibution of galaxies, but something similar must apply to galaxies..

Henk Hoekstra
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Post by Henk Hoekstra » October 06 2004

It is not clear to me whether this is a good probe of the cosmology, but it is definitely a good consistency check. It is a relatively old test, and it is something I have been thinking of doing using RCS. Using RCS2 (1000 sq deg.) would give an even better constraint. So far, I haven't done this though... The problem is indeed to link the simulations (DM) to the galaxies, but comparison of the galaxy distribution and weak lensing mass maps typically gives a good correlation on >Mpc scales, so that might not be too bad after all.

The situation is different compared to intrinsic galaxy alignments, though. In the case of clusters the alignments arise from the inflow of material, whereas in the case of galaxies, typically the alignment of angular momentum vectors is involved. Also, dynamical interactions tend to destroy these initial correlations, something which is less relevant for clusters as giant mergers are not that common.

Pengjie Zhang
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Post by Pengjie Zhang » November 01 2004

I think that the cluster allignment and correlation will eventually be able to constrain cosmology, but more work needs to be done first. This paper is based on N-body simulaiton and thus neglects gas physics, which one intuitively would think to has trivial effect. But surprisingly some simulations (e.g. Kazantzidis, et al. astro-ph/0405189) found that gas cooling makes cluster much more spherical. These simulations may suffer the overcooling problem and thus overestimate the effect of gas cooling. But they at least suggest that gas physics may not be negligible.

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