The Shapes of Galaxy Groups - Footballs or Frisbees ?

We derive probability density functions for the projected axial ratios of the real and mock 2PIGG galaxy groups, and use this data to investigate the intrinsic three dimensional shape of the dark matter ellipsoids that they trace. As well as analysing the raw data for groups of varying multiplicities, a convolution corrected form of the data is also considered which weights the probability density function according to the results of multiple Monte-Carlo realizations of discrete samples from the input spatial distributions. The important effect observed is that the best fit distribution for all the raw data is a prolate ellipsoid with a Gaussian distribution of axial ratios with β̄ = 0.36 and σ = 0.14, whilst for the convolved data the best fit solution is that of an oblate ellipsoid β̄ = 0.22 and σ = 0.1. Previously only prolate distributions were thought compatible with the data, this being interprated as evidence of filamentary collapse at nodes. We also find that even after allowing for the sampling effects, the corrected data is better fit using separate multiplicity bins, which display a trend towards more spherical halos in higher multiplicity groups. Finally, we find that all results in the real data are in good agreement with the mock data from ΛCDM simulations, KS tests showing that all comparative data have been drawn from the same distributions within the 1σ confidence limits. Subject headings: galaxies: clusters: general; galaxies: halos