2021MNRAS.506.3385S

We present a measurement of average histograms of line-of-sight velocities over pairs of galaxies and galaxy clusters. Since the histogram can be measured at different galaxy-cluster separations, this observable is commonly referred to as the stacked phase-space density. We formulate the stacked phase-space density based on a halo-model approach so that the model can be applied to real samples of galaxies and clusters. We examine our model by using an actual sample of massive clusters with known weak-lensing masses and spectroscopic observations of galaxies around the clusters. A likelihood analysis with our model enables us to infer the spherical-symmetric velocity dispersion of observed galaxies in massive clusters. We find the velocity dispersion of galaxies surrounding clusters with their lensing masses of 1.1×10¹⁵ h^–1 M_☉ to be 1180⁺⁸³_–70 kms^–1 at the 68 per cent confidence level. Our constraint confirms that the relation between the galaxy velocity dispersion and the host cluster mass in our sample is consistent with the prediction in dark-matter-only N-body simulations under General Relativity. Assuming that the Poisson equation in clusters can be altered by an effective gravitational constant of G_eff, our measurement of the velocity dispersion can place a tight constraint of 0.88 < G_eff/G_N < 1.29 (68 per cent) at length-scales of a few Mpc about 2.5 Giga years ago, where G_N is the Newton's constant.