Dominant galaxies in 2dF groups

We investigate whether the spectral-type of a locally dominant (most luminous) galaxy can be used to select sets of galaxies that are physically associated (groups). We assume that passive dominants trace a group-like potential, and SF-dominants a field-like environment. The group sample includes 988 groups selected in the 2dFGRS applying a maximum magnitude difference criterion. We find that the average number of passive galaxies associated to a dominant is larger when the dominant is passive, a result supporting our assumption that galaxy associations around a passive dominant are reliable groups. Finally we show that, to reduce the contamination by unbound galaxy associations (SF-dominant), a >3 passive-members criterion is more efficient than a >6 all-members criterion. 1. Dominant Group Galaxy selection criteria Each 2dF galaxy with >4 neighbours (projected separation less than 1hMpc and line of sight velocity difference less than 1000km s), that is more luminous (by >0.2 mag) than all of its neighbours, is a dominant group galaxy and identifies a group. Given the bright and faint apparent magnitude limits of the 2dF, a maximum magnitude difference criterion has been applied providing, for each dominant, complete identification of all neighbours from ∼equally-luminous to ∼2÷2.5 magnitudes fainter. Dominants are selected in the range bj∈[17 ÷ 17.5], neighbours in range bj∈[17 ÷ 19.5]. The criteria for selection are 0.03 6 zdom 6 0.12 and −22 6 (M − 5logh)dom 6 −19. The sample includes 988 groups (7281 galaxies): 639 with a passive (Type 1 in Madgwick et al. 2002) dominant galaxy (P-dG), and 349 with a SF (Type>1) dominant (SF-dG). P-dGs are typically richer than SF-dGs. 2. Passive and Star-Forming dominants If we assume that P-dGs trace a group-like potential, and SF-dGs a field-like environment (galaxy associations not embedded within a common massive halo), differences in the spectral mix of P-dGs and SF-dGs members should reflect the role of a group environment on the spectral evolution of galaxies. Figure 1 shows the abundance of PdGs, and SF-dGs (top panels) as a function of absolute magnitude of the dominant. Non-luminous dominants are equally likely to be passive or SF, but the former are much more common than the latter among luminous dominants. That SF galaxies represent only a small fraction among luminous dominant galaxies is consistent with X-ray studies that show diffuse hot X-ray halo typically occur in groups hosting a dominant luminous elliptical. It remains unclear whether groups with a faint passive dominant exist at all, or whether these are typically embedded within larger systems (Kelm & Focardi 2004). The bottom panels show the distributions of non-dominants that are passive and starforming, as a function of the luminosity of the dominant, in P-dGs (left) and SF-dGs (right). P-dGs with a luminous dominant exhibit more passive than SF non-dominant