The impact of environment on the dynamical structure of satellite systems

We examine the effects of environment on the dynamical structure of satellite systems based on the Millennium--II Simulation. Satellite halos are defined as sub--halos within the virial radius of a host halo. The satellite sample is restricted to those sub--halos which showed a maximum circular velocity above 30 km/s at the time of accretion. Host halo masses range from 10^11 to 10^14 Msol/h. We compute the satellites' average accretion redshift, z_acc, velocity dispersion, sigma, and velocity anisotropy parameter, beta, utilising stacked satellite samples of equal mass hosts at similar background densities. The main results are: (1) On average satellites within hosts in high density environments are accreted earlier (Delta z~ 0.1$) compared to their counterparts at low densities. For host masses above 5 times10^13 Msol/h this trend weakens and may reverse for higher host masses; (2) The velocity dispersion of satellites in low density environments follows that of the host, i.e. no velocity bias is observed for host halos at low densities independent of host mass. However, for low mass hosts in high density environments the velocity dispersion of the satellites can be up to ~30% larger than that of the host halo, i.e. the satellites are dynamically hotter than their host halos. (3) The anisotropy parameter depends on host mass and environment. Satellites of massive hosts show more radially biased velocity distributions. Moreover in low density environments satellites have more radially biased velocities (Delta beta > 0.1) compared to their counterparts in high density environments. We believe that our approach allows to predict a similar behaviour for observed satellite galaxy systems.Comment: 7 pages, 4 figures, accepted for publication in MNRA

Entropy of gas and dark matter in galaxy clusters

On the basis of a large scale 'adiabatic', namely non-radiative and non-dissipative, cosmological smooth particle hydrodynamic simulation we compare the entropy profiles of the gas and the dark matter (DM) in galaxy clusters. The quantity K_g = T_g \rho_g^{-2/3} provides a measure for the entropy of the intra-cluster gas. By analogy with the thermodynamic variables of the gas the velocity dispersion of the DM is associated with a formal temperature and thereby K_DM = \sigma_DM^2 \rho_DM^{-2/3} is defined. This DM entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}. In accord with other studies the DM phase space density follows a power law behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}. The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4) R_s, where R_s is the NFW scale radius. The outer profile follows the DM behaviour, K_g \propto r^{1.21}, in close agreement with X-ray observations. Upon scaling the DM and gas densities by their mean cosmological values we find that outside the entropy core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18 prevails. By extending the definition of the gas temperature to include also the bulk kinetic energy the ratio of the DM and gas extended entropy is found to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to that of the DM implies that observations of the intra-cluster gas can provide an almost direct probe of the DM.Comment: 7 pages, 8 figures, accepted for publication in MNRAS, web page of the The Marenostrum Numerical Cosmology Project : http://astro.ft.uam.es/~marenostrum

Detection of the large scale alignment of massive galaxies at z~0.6

We report on the detection of the alignment between galaxies and large-scale structure at z~0.6 based on the CMASS galaxy sample from the Baryon Oscillation Spectroscopy Survey data release 9. We use two statistics to quantify the alignment signal: 1) the alignment two-point correlation function which probes the dependence of galaxy clustering at a given separation in redshift space on the projected angle (theta_p) between the orientation of galaxies and the line connecting to other galaxies, and 2) the cos(2theta)-statistic which estimates the average of cos(2theta_p) for all correlated pairs at given separation. We find significant alignment signal out to about 70 Mpc/h in both statistics. Applications of the same statistics to dark matter halos of mass above 10^12 M_sun/h in a large cosmological simulation show similar scale-dependent alignment signals to the observation, but with higher amplitudes at all scales probed. We show that this discrepancy may be partially explained by a misalignment angle between central galaxies and their host halos, though detailed modeling is needed in order to better understand the link between the orientations of galaxies and host halos. In addition, we find systematic trends of the alignment statistics with the stellar mass of the CMASS galaxies, in the sense that more massive galaxies are more strongly aligned with the large-scale structure.Comment: 6 pages, 3 figures, accepted for publication in ApJ Letter

Velocity distributions in clusters of galaxies

We employ a high-resolution dissipationless N-body simulation of a galaxy cluster to investigate the impact of subhalo selection on the resulting velocity distributions. Applying a lower limit on the present bound mass of subhalos leads to high subhalo velocity dispersions compared to the diffuse dark matter (positive velocity bias) and to a considerable deviation from a Gaussian velocity distribution (kurtosis -0.6). However, if subhalos are required to exceed a minimal mass before accretion onto the host, the velocity bias becomes negligible and the velocity distribution is close to Gaussian (kurtosis -0.15). Recently it has been shown that the latter criterion results in subhalo samples that agree well with the observed number-density profiles of galaxies in clusters. Therefore we argue that the velocity distributions of galaxies in clusters are essentially un-biased. The comparison of the galaxy velocity distribution and the sound speed, derived from scaling relations of X-ray observations, results in an average Mach number of 1.24. Altogether 65% of the galaxies move supersonically and 8% have Mach numbers larger than 2 with respect to the intra cluster gas.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS(Letters

The concentration-velocity dispersion relation in galaxy groups

Based on results from cold dark matter N-body simulations we develop a dynamical model for the evolution of subhaloes within host haloes of galaxy groups. Only subhaloes more massive than 5 times 10^8 M_{sol} at the time of accretion are examined because they are massive enough to possibly host luminous galaxies. As they orbit within a growing host potential the subhaloes are subject to tidal stripping and dynamical friction. We consider groups of equal mass (M_{vir} = 3.9 times 10^{13} M_{sol}) at redshift z=0 but with different concentrations associated with different formation times. We investigate the variation of subhaloe (or satellite galaxy) velocity dispersion with host concentration and/or formation time. In agreement with the Jeans equation the velocity dispersion of subhaloes increases with the host concentration. Between concentrations ~5 and ~20 the subhaloe velocity dispersions increase by ~25 per cent. By applying a simplified tidal disruption criterion, i.e. rejection of all subhaloes with a tidal truncation radius below 3 kpc at z=0, the central velocity dispersion of 'surviving' subhaloes increases substantially for all concentrations. The enhanced central velocity dispersion among surviving subhaloes is caused by a lack of slow tangential motions. Additionally, we present a fitting formula for the velocity anisotropy parameter \beta(r) which does not depend on concentration if the group-centric distances are scaled by r_s, the characteristic radius of the NFW-profile.Comment: 12 pages, 11 figures, published in MNRA

The velocity--shape alignment of clusters and the kinetic Sunyaev--Zeldovich effect

We use the Millennium simulation to probe the correlation between cluster velocities and their shapes and the consequences for measurements of the kinetic Sunyaev-Zeldovich (kSZ) effect. Halos are generally prolate ellipsoids with orientations that are correlated with those of nearby halos. We measure the mean streaming velocities of halos along the lines that separate them, demonstrating that the peculiar velocities and the long axes of halos tend to be somewhat aligned, especially for the most massive halos. Since the kSZ effect is proportional to the line-of-sight velocity and the optical depth of the cluster, the alignment results in a strong enhancement of the kSZ signature in clusters moving along the line of sight. This effect has not been taken into account in many analyses of kSZ signatures.Comment: 5 pages, 5 figures, 1 table; accepted for publication in MNRA