Velocity bias from the small-scale clustering of SDSS-III BOSS galaxies

Abstract

We present the measurements and modelling of the projected and redshift-space clustering of CMASS galaxies in the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey Data Release 11. For a volume-limited luminous red galaxy sample in the redshift range of 0.48 < z < 0.55, we perform halo occupation distribution modelling of the smalland intermediate-scale (0.1-60 h −1 Mpc) projected and redshift-space two-point correlation functions, with an accurate model built on high resolution N -body simulations. To interpret the measured redshift-space distortions, the distribution of galaxy velocities must differ from that of the dark matter inside haloes of ∼ 10 13 -10 14 h −1 M ⊙ , i.e. the data require the existence of galaxy velocity bias. Most notably, central galaxies on average are not at rest with respect to the core of their host haloes, but rather move around it with a 1D velocity dispersion of 0.22 +0.03−0.04 times that of the dark matter, implying a spatial offset from the centre at the level of 1 per cent of the halo virial radius. The luminous satellite galaxies move more slowly than the dark matter, with velocities 0.86−0.03 times those of the dark matter, which suggests that the velocity and spatial distributions of these satellites cannot both be unbiased. The constraints mainly arise from the Fingers-of-God effect at nonlinear scales and the smoothing to the Kaiser effect in the translinear regime; the robustness of the results is demonstrated by a variety of tests. We discuss the implications of the existence of galaxy velocity bias for investigations of galaxy formation and cosmology.

Properties and Origin of Galaxy Velocity Bias in the Illustris Simulation

“…Since we assume that the central galaxy and the host halo are located at the same spatial position (potential minimum), a central velocity bias naturally leads to a small offset between the position of the central galaxy and the halo potential minimum, as discussed in detail in Guo et al (2015a).…”

“…When trying to compare the velocity bias measurements in the hydrodynamical simulation to the result obtained from HOD modeling of observed SDSS galaxy samples (Guo et al 2015a,c), we need to take into account the impact of baryon that is ignored in the HOD models based on the DMO simulations. We show in Figure 11 the 3D velocity dispersions of the dark matter particles in halos of different masses for the DMO (solid line) and hydrodynamical simulations (dotted line).…”

“…For example, the halo velocity in the ROCKSTAR halo finder (Behroozi et al 2013) is defined as the average velocity of the dark matter particles within the innermost 10% of the halo radius, which would be less affected by the disturbance in the outer part of the halo during merger events. Guo et al (2015a) define the halo velocity as the bulk velocity of the inner 25% of the particles (as also used in Li et al 2012). To investigate the effect of the different halo velocity definitions, we compare the effect on the galaxy velocity bias between our fiducial model and three other halo velocity definitions, the average core velocities of the innermost 25% and 10% of the dark matter particles, and also those within the central galaxy radius (twice the stellar half-mass radius).…”

Galaxy power spectrum in redshift space: Combining perturbation theory with the halo model

“…The halo model has been adopted to the galaxy clustering in redshift space [9,45,46,[55][56][57][58][59]. The halo power spectrum and correlation function were directly measured from N -body simulations in [59,60] and [9], respectively, to fully take into account the nonlinearities of halo clustering in the halo model. However, for analytical approaches linear PT has been used to describe RSD of halos, i.e., the linear Kaiser model [1], to combine with the halo model in previous studies [e.g., 46].…”

Fingers-of-God effect of infalling satellite galaxies

“…The velocity distribution of satellites has been studied by conducting various numerical simulations (e.g., Ghigna et al 2000;Diemand et al 2004;Faltenbacher et al 2005;Wu et al 2013). A non-zero velocity of central galaxies relative to the host halo has been found in several studies (e.g., van den Bosch et al 2005;Guo et al 2015).…”