Galaxy-Galaxy Weak-lensing Measurements from SDSS. II. Host Halo Properties of Galaxy Groups

In this second paper in a series studying galaxy-galaxy lensing signals using Sloan Digital Sky Survey Data Release 7 (SDSS DR7), we present our measurement and modeling of the lensing signals around groups of galaxies. We divide the groups into four halo mass bins and measure the signals around four different halo-center tracers: brightest central galaxies (BCGs), luminosity-weighted centers, number-weighted centers, and X-ray peak positions. For groups cross-identified in both X-ray and SDSS DR7, we further split the groups into low and high X-ray emission subsamples, both of which are assigned to two halo-center tracers, BCGs and X-ray peak positions. The galaxy-galaxy lensing signals show that BCGs, among the four candidates, are the best halo-center tracers. We model the lensing signals using a combination of four contributions: the off-center NFW host halo profile, subhalo contribution, stellar contribution, and projected two-halo term. We sample the posterior of five parameters, i.e., the halo mass, concentration, off-centering distance, subhalo mass, and fraction of subhalos, via a Monte Carlo Markov Chain (MCMC) package using the galaxy-galaxy lensing signals. After taking into account the sampling effects (e.g., Eddington bias), we found that the best-fit halo masses obtained from lensing signals are quite consistent with those obtained in the group catalog based on an abundance matching method, except in the lowest mass bin.