Satellite number density profiles of primary galaxies in the 2dFGRS

Abstract

We analyse the projected radial distribution of satellites around bright primary galaxies in the 2dFGRS. We have considered several primary-satellite subsamples to search for dependences of the satellite number density profile, ρ(r_p), on properties of satellites and primaries. We find significant differences of the behaviour of ρ(r_p) depending on primary characteristics. In star-forming primaries, the satellite number density profile is consistent with power laws within projected distance 20<r_p<500 kpc. On the other hand, passively star forming primaries show flat profiles for 20<r_p<70 kpc, well fitted by generalized King models with a large core radius parameter (r_c ~ 68 kpc). In the external regions of the haloes (r_p>100 kpc), the density profiles of all primaries is well described by power laws, although we notice that for red, early spectral type primaries, the outer slope obtained is steeper than that corresponding to blue, late spectral type ones. We have tested our results by control samples of galaxies identical to the samples of satellites in apparent magnitude and projected distance to the primary, but with a large relative velocity. This sample of unphysical primary-galaxy pairs shows a flat radial density beyond r_p=20 kpc indicating that our results are not biased toward a decrease of the true number of objects due to catalogue selection effects. Our results can be understood in terms of dynamical friction and tidal stripping on satellites in the primary haloes. These processes can effectively transfer energy to the dark matter, flattening the central steep profiles of the satellite distribution in evolved systems.