Shapes of clusters and groups of galaxies: comparison of model predictions with observations

PAZ, D. J.; MERCHÁN, M.; PADILLA, N.; LAMBAS, D. G.

Santiago de Chile

Congreso; “Groups and Clusters in the nearby Universe”, European Southern Observatory; 2005

European Southern Observatory

We have performed several analyses of shapes of gravitationally bound systems in numerical simulations, mockgroup catalogues, and observational group catalogues derived from the 2dFGRS and the SDSS-DR3 galaxysurveys. The three-dimensional shapes of dark matter haloes on simulations are well described by triaxialellipsoids that tend to be more prolate as the halo mass increases. This can also be detected by means of otherstatistical parameters; both the asphericity c/a, and the projected semi-axis ratios b/a show a clear decrementwith halo mass. We demonstrated that low number statistics tend to bias the measured shapes toward oblateshapes. Performing a Monte-Carlo analysis, we show that the smaller semi-axes are more affected by noise, andconsequently, prolate shapes, characterized by two small semi-axes, are more distorted. The properties of thedistribution of shapes in the mock catalogue closely resemble those in the observations. In particular thetendency of groups with low number of members to show elongated shapes is entirely consistent in both realand mock catalogues. We demonstrate that the observed trend where richer groups are rounder is an artifact oflow number statistics. Finally, by considering a fixed number of members per group, we were able to detect astatistically reliable trend of 2PIGG group shapes with mass. Our findings indicate that more massive groupstend to show more elongated shapes, in excellent agreement with results from numerical simulations.