Shape Of Galaxy Clusters Research Articles

Aspherical galaxy clusters: Effects on cluster masses and gas mass fractions

We present an investigation of the effects of asphericity on the estimates of total mass and gas mass fraction in galaxy clusters from X-ray observations. We model the aspherical shape of galaxy clusters by a triaxial model and compare the true total mass and the true total gas mass fraction with the corresponding quantities obtained with the assumption of spherical symmetry. In the triaxial model we allow the extent along the line of sight to vary in order to describe elongated and compressed cluster shapes. Using a sample of 10 ROSAT clusters and a recent CHANDRA observation we find the following results. For prolate or oblate shapes the difference between triaxial and spherical model both in the mass and in the gas mass fraction are negligible (less than 3%). For more aspherical shapes the total mass is underestimated (overestimated) in the centre, if the cluster is compressed (elongated). The gas mass fraction is overestimated for compressed clusters and slightly underestimated for elongated clusters. Comparing X-ray masses with gravitational lensing estimates, we find that elongations along the line of sight can resolve discrepancies of masses determined by the two different methods of up to ∼30%. The combination of Sunyaev- Zel'dovich and X-ray observations is useful to measure the elongation of the cluster along the line of sight. As an application, we estimate the elongation of the cluster CL0016+16 with two different approaches, Sunyaev-Zel'dovich measurements and comparison of weak lensing and X-ray masses, and find reasonable agreement.

Erratum - The Shapes of Galaxy Clusters

The paper 'The shapes of galaxy clusters' was published in Mon. Not. R. Astron Soc. 273,30-46(1995). Our description of the work of PBF in Section 2.2 was incomplete, as their analysis was based on about 400 clusters instead of ∼100. Also, our description in Section 2.2, as well as in Fig. 3 (bottom panel), may suggest that their e-estimates are based only at the overdensity level at which the clusters are defined. However, they have also analysed the cluster shape at lower overdensity levels, and therefore their e-estimates are in general based on more than the 6-8 cells mentioned in Section 4.3.

The shapes of galaxy clusters

We have reanalysed a data set of 99 low-redshift (z < 0.1) Abell clusters studied previously by Rhee, van Haarlem & Katgert, and determined their shapes.For this, three different measures are used: two of which were originally used by Rhee et al., and one of which was used by Plionis, Barrow & Frenk in their investigation of clusters in the Lick catalogue. We use Monte Carlo simulations of clusters to investigate the errors in the methods. For low ellipticity, all methods overestimate the cluster elongation, whereas the opposite is true for a highly flattened system. Also background galaxies and shot noise have a rather large influence on the measured quantities. The corrected distribution of cluster ellipticities shows a peak at ε ∼ 0.4 and extends to ε ∼ 0.8 , consistent with results of some previous studies. However, the present study uses more than twice as many clusters as the earlier studies, and is self-consistent. That is, with the corrected distribution over projected cluster shapes we can reconstruct the observed distribution over projected cluster shapes and the observed relation between the number of galaxies in a cluster and its ellipticity. To achieve this, we have to assume that there is an anti-correlation between the true (projected) ellipticity of a cluster and its number of galaxies. It is not necessary to assume that the ellipticity of a cluster increases when one only includes the brighter galaxies (as suggested by Binney). Using a redshift-independent richness criterion of Vink & Katgert, it is shown that the richer clusters are intrinsically more nearly spherical than the poorer ones. Furthermore, the corrected distribution of cluster shapes is found to be more consistent with a population that consists of purely prolate clusters than with a purely oblate population. We compare the corrected true distribution of (projected) ellipticities with predictions from N-body simulations. For this, we use a catalogue of 75 N-body simulated clusters which assume a CDM spectrum with Ω = 1.0 . The simulations include a recipe for galaxy formation and merging. The model clusters are expected to be a representative sample of all real clusters. They show a good resemblance with the data in both radial profile and number of galaxies. 'Observation' of these simulated clusters in exactly the same way as the real clusters produces an ellipticity distribution that extends to much higher epsilon and that has too few nearly spherical clusters. Preliminary results of simulations of the formation of clusters in an Ω = 0.2 universe suggest that, on average, clusters are more nearly spherical in this case, as is expected on theoretical grounds. This shows that the elongations of clusters can provide a useful constraint on the value of Ω.

Projected and intrinsic shapes of galaxy clusters

Identifications of a large number of galaxy clusters in the Lick map, using an algorithm based on an overdensity criterion, are presented. The resulting catalogues contain ∼ 6000 clusters (with |b| ≥ 40°) out of which 753 are Abell clusters. Ellipticities and major axis orientations for a suitable subset of this sample, including 397 Abell clusters, are determined. It is found that the distribution of projected axial ratios is approximately Gaussian. Methods are investigated to invert the distribution of apparent axial ratios in order to obtain the distribution of intrinsic axial ratios. The apparent shapes of clusters in the sample are found inconsistent with those expected from a population of oblate spheroids seen in projection