Accurate fitting formula for the two-point correlation function of the dark matter halos

An accurate fitting formula is reported for the two-point correlation function of the dark matter halos in hierarchical clustering models, It is valid for the the linearly clustering regime, and its accuracy is about 10% in the correlation amplitude for the halos with mass M greater than 1/100 -- 1/1000 of the characteristic non-linear mass M_c. The result is found on the basis of a careful analysis for a large set of scale-free simulations with 17 million particles. The fitting formula has a weak explicit dependence on the index n of the initial power spectrum, but can be equally well applied to the cold dark matter (CDM) cosmological models if the effective index n_{eff} of the CDM power spectrum at the scale of the halo mass replaces the index n. The formula agrees with the analytical formula of Mo & White (MW96) for massive halos with M>M_c, but the MW96 formula significantly underpredicts the correlation for the less massive halos. The difference between the fitting and the analytical formulae amounts to a factor => 2 in the correlation amplitude for M=0.01 M_c. One of the most interesting applications of this fitting formula would be the clustering of galaxies since the majority of halos hosting galaxies satisfies M<< M_c.