Modeling Galaxy-mass Correlations in Dissipationless Simulations

We use high-resolution, dissipationless simulations of the concordance flat ΛCDM model to make predictions for the galaxy-mass correlations and compare them with the recent Sloan Digital Sky Survey (SDSS) weak-lensing measurements of Sheldon et al. The simulations resolve both isolated galaxy-sized host halos and satellite halos (subhalos) within them. We use a simple scheme based on matching the circular velocity function of halos to the galaxy luminosity function and on using the observed density-color correlation of the SDSS galaxies to assign luminosities and colors to the halos. This allows us to closely match the selection criteria used to define observational samples. The simulations reproduce the observed galaxy-mass correlation function and the observed dependence of its shape and amplitude on luminosity and color, if a reasonable amount of scatter between galaxy luminosity and circular velocity is assumed. We find that the luminosity dependence of the correlation function is primarily determined by the changing relative contribution of central and satellite galaxies at different luminosities. The color dependence of the galaxy-mass correlations reflects the difference in the typical environments of blue and red galaxies. We compare the cross-biases, bx ≡ b/r, measured in simulations and observations and find a good agreement at all probed scales. We show that the galaxy-mass correlation coefficient r is close to unity on scales greater than ~1 h-1 Mpc. This indicates that the cross-bias measured in weak-lensing observations should measure the actual bias b of galaxy clustering on these scales. In agreement with previous studies, we find that the aperture mass-to-light ratio is independent of galaxy color in the range of luminosities probed by observational samples. The aperture mass scales approximately linearly with luminosity at Lr > 1010 h-2 L☉, while at lower luminosities the scaling is shallower: MΔΣ ∝ L. We show that most of the luminous galaxies (Mr < -21) are near the centers of their halos and that their galaxy-mass correlation function at r ≲ 100 h-1 kpc can therefore be interpreted as the average dark matter density profile of these galaxies. Finally, we find that for galaxies in a given narrow luminosity range, there is a broad and possibly non-Gaussian distribution of halo virial masses. Therefore, the average relation between mass and luminosity derived from the weak-lensing analyses should be interpreted with caution.