The Shapes of BCGs and normal Ellipticals in Nearby Clusters

TL;DR

This study compares the shapes of Brightest Cluster Galaxies and normal ellipticals, revealing that cD BCGs are strongly prolate and likely reflect the shape of their dark matter halos, contrasting with previous findings.

Contribution

It provides new insights into the triaxial shapes of BCGs and ellipticals, especially highlighting the prolateness of cD BCGs and their relation to dark matter halo shapes.

Findings

BCGs and ellipticals are both triaxial, with BCGs, especially cDs, being more prolate.

Normal ellipticals share similar shape distributions with non-cD BCGs.

The shape differences are influenced by sample selection criteria and cluster properties.

Abstract

We compare the apparent axial ratio distributions of Brightest Cluster Galaxies (BCGs) and normal ellipticals (Es) in our sample of 75 galaxy clusters from the WINGS survey. Most BCGs in our clusters (69%) are classified as cD galaxies. The sample of cDs has been completed by 14 additional cDs (non-BCGs) we found in our clusters. We find that: (i) Es have triaxial shape, the triaxiality sharing almost evenly the intrinsic axial ratios parameter space, with a weak preference for prolateness; (ii) the BCGs have triaxial shape as well. However, their tendence towards prolateness is much stronger than in the case of Es. Such a strong prolateness appears entirely due to the sizeable (dominant) component of cDs inside the WINGS sample of BCGs. In fact, while the 'normal'(non-cD) BCGs do not differ from Es, as far as the shape distribution is concerned, the axial ratio distribution of BCG_cD galaxies is found to support quite prolate shapes; (iii) our result turns out to be strongly at variance with the only similar, previous analysis by Ryden et al.(1993)(RLP93), where BCGs and Es were found to share the same axial ratio distribution; (iv) our data suggest that the above discrepancy is mainly caused by the different criteria that RLP93 and ourselves use to select the cluster samples, coupled with a preference of cDs to reside in powerful X-ray emitting clusters; (v) the GIF2 N-body results suggest that the prolateness of the BCGs (in particular the cDs) could reflect the shape of the associated dark matter halos.