DETERMINE THE GALAXY BIAS FACTORS ON LARGE SCALES USING THE BISPECTRUM METHOD

We study whether the bias factors of galaxies can be unbiasedly recovered from their power spectra and bispectra. We use a set of numerical N-body simulations and construct large mock galaxy catalogs based upon the semianalytical model of Croton et al. We measure the reduced bispectra for galaxies of different luminosity, and determine the linear and first nonlinear bias factors from their bispectra. We find that on large scales down to that of the wavenumber k = 0.1 h Mpc(-1), the bias factors b(1) and b(2) are nearly constant, and b(1) obtained with the bispectrum method agrees very well with the expected value. The nonlinear bias factor b(2) is negative, except for the most luminous galaxies with M(r) < -23 which have a positive b(2). The behavior of b(2) of galaxies is consistent with the b(2) mass dependence of their host halos. We show that it is essential to have an accurate estimation of the dark matter bispectrum in order to have an unbiased measurement of b(1) and b(2). We also test the analytical approach of incorporating halo occupation distribution to model the galaxy power spectrum and bispectrum. The halo model predictions do not fit the simulation results well on the precision requirement of current cosmological studies.