Clustering of luminous red galaxies - II. Small-scale redshift-space distortions

This is the second paper of a series where we study the clustering of luminous red galaxies (LRG) in the recent spectroscopic Sloan Digital Sky Survey (SDSS) data release, DR6, which has 75 000 LRG covering over 1 Gpc(3) h(-3) for 0.15 < z < 0.47. Here, we focus on modelling redshift-space distortions in xi(sigma, pi), the two-point correlation in separate line-of-sight and perpendicular directions, at small scales and in the line-of-sight. We show that a simple Kaiser model for the anisotropic two-point correlation function in redshift space, convolved with a distribution of random peculiar velocities with an exponential form, can describe well the correlation of LRG on all scales. We show that to describe with accuracy the so-called 'fingers-of-God' (FOG) elongations in the radial direction, it is necessary to model the scale dependence of both bias b and the pairwise rms peculiar velocity sigma(12) with the distance. We show how both quantities can be inferred from the xi(sigma, pi) data. From r similar or equal to 10 Mpc h(-1) to r similar or equal to 1 Mpc h(-1), both the bias and sigma(12) are shown to increase by a factor of 2: from b = 2 to 4 and from sigma(12) = 400 to 800 km s(-1). The latter is in good agreement, within a 5 per cent accuracy in the recovered velocities, with direct velocity measurements in dark matter simulations with Omega(m) = 0.25 and sigma(8) = 0.85.