The halo distribution of 2dF galaxies

We use the clustering results obtained by Madgwick et al. (2003) for a sample of 96,791 2dF galaxies with redshift 0.01 < z < 0.15 to study the distribution of late-type and early-type galaxies within dark matter haloes of different mass. Galaxies of different classes present different spatial concentrations: while star-forming galaxies seem to prefer steep ρ(r) ∝ r −3 profiles, early-type galaxies can only be described by shallower (NFW-like) distributions. We also find evidence for morphological segregation, as latetype galaxies appear to be distributed within haloes of mass scales corresponding to groups and clusters up to about two virial radii, while passive objects show a preference to reside closer to the halo centre. If we assume a broken power-law of the form 〈Ngal(m) 〉 = (m/m0) α1 for mcut ≤ m < m0 and 〈Ngal(m) 〉 = (m/m0) α2 at higher masses to describe the dependence of the average number of galaxies within haloes on the halo mass, fits to the data show that star-forming galaxies start appearing in haloes of masses mcut ≃ 10 10.8 m⊙, smaller than what is obtained for early-type galaxies (mcut ≃ 10 11.6 m⊙). In the high-mass regime m ≥ m0, 〈Ngal 〉 increases with halo mass more slowly (α2 ≃ 0.6) in the case of late-type galaxies than for passive objects which present α2 ≃ 0.9. The above results imply that, while on small-to-intermediate mass scales galaxies within haloes mainly belong to the late-type population, the situation is reversed in the high-mass regime m ∼> 10 15 m⊙, where passive galaxies dominate the counts. We stress that – at variance with previous statements – there is no degeneracy in the determination of the best functional forms for ρ(r) and 〈Ngal〉, as they affect the behaviour of the galaxy-galaxy correlation function on different scales.