Galaxy groups in the 2dF Galaxy Redshift Survey:: large-scale structure with groups

We use the 2dF Galaxy Group Catalogue constructed by Merchan & Zandivarez to study the large-scale structure of the Universe traced by galaxy groups. We concentrate on the computation of the power spectrum and the two-point correlation function. The resulting group power spectrum shows a similar shape to the galaxy power spectrum obtained from the 2dF Galaxy Redshift Survey by Percival et al., but with a higher amplitude quantified by a relative bias in redshift space of b(s)(k) similar to 1.5 on the range of scales analysed in this work, 0.025 < k/h Mpc(-1) < 0.45. The group two-point correlation function for the total sample is well described by a power law with correlation length s(0) = 8.9 +/- 0.3 h(-1) Mpc and slope gamma = 1.6 +/- 0.1 on scales of s < 20 h(-1) Mpc. In order to study the dependence of the clustering properties on group mass, we split the catalogue into four subsamples defined by different ranges of group virial masses. Our results are consistent with a 40 per cent increase of the correlation length s0 when the minimum mass of the sample increases from M-vir > 5 x 10(12) h(-1) M-. to M-vir > 1 x 10(14) h(-1) M-.. These computations allow a fair estimate of the relation described by the correlation length s(0) and the mean intergroup separation d(c) for galaxy systems of low mass. Our results show that an empirical scaling law s(0) = 4.7d(c)(0.32) c provides a very good fit to the results from this work, as well as to previous results obtained for groups and clusters of galaxies. The same law describes the predictions for dark matter haloes in N-body simulations of Lambda cold dark matter (LambdaCDM) models. We also extend our study to the redshift space distortions of galaxy groups, where we find that the anisotropies in the clustering pattern of the 2dF Galaxy Group Catalogue are consistent with gravitational instability, with a flattening of the redshift-space correlation function contours in the direction of the line of sight. The group pairwise velocities found from this analysis for a sample of groups with masses M-vir > 5 x 10(12) h(-1) M-. are consistent with [w(2)](1/2) = (280(-110)(+50)) km s(-1), in agreement with LambdaCDM cosmological simulations. The bias factor for the 2dF groups of moderate masses M-vir > 2 x 10(13) h(-1) M-. is consistent with the values predicted by the combination of a CDM model and the ellipsoidal collapse model for the formation of structures.