Peculiar Velocities Of Galaxy Clusters Research Articles

The pairwise and cross-pairwise y-type polarised kinetic Sunyaev Zeldovich effect from transverse velocity of galaxy clusters

We develop a new theoretical framework for studying the pairwise and cross-pairwise polarised kinetic Sunyaev Zeldovich (pkSZ) effect arising from the transverse peculiar velocity of galaxy clusters. The pkSZ effect is second order in peculiar velocities and has a spectrum that can be decomposed into y-type and blackbody components, whereas the unpolarised linear kSZ effect has only the blackbody component. Thus, the detectability of the pkSZ effect depends only on the sensitivity and the number of frequency channels of the survey and not on the other primary and secondary CMB anisotropies. We consider pairing of clusters with other clusters as well as cross-pairing of clusters with galaxies from spectroscopic galaxy surveys. The pairwise pkSZ signal is a function of intra-pair spatial separation. We develop and compare estimators of the pairwise pkSZ effect and study the detectability of the pairwise signal with cluster catalogs consisting of a few hundred thousand clusters expected from surveys such as eROSITA and CMB-S4. We find that cross-pairing clusters with galaxies from a large overlapping spectroscopic survey having a few billion galaxies will enable us to detect the pairwise pkSZ effect with CMB-S4. The pairwise pkSZ effect will thus open up a new window into the large-scale structure of the Universe in the coming decades.

Peculiar velocity estimation from kinetic SZ effect using deep neural networks

ABSTRACT The Sunyaev–Zel’dolvich (SZ) effect is expected to be instrumental in measuring velocities of distant clusters in near future telescope surveys. We simplify the calculation of peculiar velocities of galaxy clusters using deep learning frameworks trained on numerical simulations to avoid the independent estimation of the optical depth. Images of distorted photon backgrounds are generated for idealized observations using one of the largest cosmological hydrodynamical simulations, the Magneticum simulations. The model is tested to determine its ability of estimating peculiar velocities from future kinetic SZ observations under different noise conditions. The deep learning algorithm displays robustness in estimating peculiar velocities from kinetic SZ effect by an improvement in accuracy of about 17 per cent compared to the analytical approach.

Effect of electron nonextensivity on oblique propagation of arbitrary ion acoustic waves in a magnetized plasma

The combined effects of the obliqueness and nonextensive electrons are incorporated in the study of ion acoustic (IA) waves in a magnetized plasma. The propagation properties of two possible modes (in the linear regime) are investigated. It is found that the electron nonextensivity decreases the phase velocities of both two modes. Also obliqueness leads to increase of separation between two modes. The nonlinear evolution of IA solitary waves is governed by an energy-like equation. The influence of electron nonextensivity, obliqueness and electron population on the existence domain of solitary waves and the soliton characteristics are examined. It is shown that the existence domain of the IA soliton and its profile is significantly depended on the deviation of electrons from thermodynamic equilibrium and obliqueness. Interestingly, the present model supports compressive as well as rarefactive IA solitary waves. Our finding should elucidate the nonlinear electrostatic structures that propagate in astrophysical and cosmological plasma scenarios where nonextensive and magnetized plasma can exist; like instellar plasma stellar polytropes, solar neutrino problem, peculiar velocities of galaxy clusters, dark-matter halos, protoneutron stars, hadronic matter, quark-gluon plasma, and magnetosphere, etc.

MEASURING THE DARK FLOW WITH PUBLIC X-RAY CLUSTER DATA

We present new results on the "dark flow" from a measurement of the dipole in the distribution of peculiar velocities of galaxy clusters, applying the methodology proposed and developed by us earlier. Our latest measurement is conducted using new, low-noise 7-yr WMAP data as well as an all-sky sample of X-ray selected galaxy clusters compiled exclusively from published catalogs. Our analysis of the CMB signature of the kinematic Sunyaev-Zeldovich (SZ) effect finds a statistically significant dipole at the location of galaxy clusters. The residual dipole outside the cluster regions is small, rendering our overall measurement 3-4 sigma significant. The amplitude of the dipole correlates with cluster properties, being larger for the most X-ray luminous clusters, as required if the signal is produced by the SZ effect. Since it is measured at zero monopole, the dipole can not be due to the thermal SZ effect. Our results are consistent with those obtained earlier by us from 5-yr WMAP data and using a proprietary cluster catalog. In addition, they are robust to quadrupole removal, demonstrating that quadrupole leakage contributes negligibly to the signal. The lower noise of the 7-yr WMAP also allows us, for the first time, to obtain tentative empirical confirmation of our earlier conjecture that the adopted filtering flips the sign of the KSZ effect for realistic clusters and thus of the deduced direction of the flow. The latter is consistent with our earlier measurement in both the amplitude and direction. Assuming the filtering indeed flips the sign of the KSZ effect from the clusters, the direction agrees well also with the results of independent work using galaxies as tracers at lower distances. We make all maps and cluster templates derived by us from public data available to the scientific community to allow independent tests of our method and findings.

Correlation Properties of the Kinematic Sunyaev‐Zel’dovich Effect and Implications for Dark Energy

In the context of a cosmological study of bulk flows in the universe, we present a detailed study of the statistical properties of the kinematic Sunyaev-Zel'dovich (kSZ) effect. We first compute analytically the correlation function and the power spectrum of the projected peculiar velocities of galaxy clusters. By taking into account the spatial clustering properties of these sources, we perform a line-of-sight computation of the all-sky kSZ power spectrum and find that at large angular scales (l < 10), the local bulk flow should leave a visible signature above the Poisson-like fluctuations dominant at smaller scales, while the coupling of density and velocity fluctuations should give a much smaller contribution. We conduct an analysis of the prospects of future high-resolution CMB experiments (such as ACT and SPT) for detecting the kSZ signal and extracting cosmological information and dark energy constraints from it. We present two complementary methods, one suitable for deep and narrow surveys such as ACT and one suitable for wide and shallow surveys such as SPT. Both methods can constraint the equation of state of dark energy (w) to about 5%-10% when applied to forthcoming and future surveys and can probe w in complementary redshift ranges, which could shed some light on its time evolution. This is mainly due to the high sensitivity of the peculiar velocity field to the onset of the late acceleration of the universe. We stress that this determination of w does not rely on the knowledge of cluster masses, although it relies on cluster redshifts and makes minimal assumptions about cluster physics.

Measuring cluster peculiar velocities with the Sunyaev-Zel'dovich effect: scaling relations and systematics

The fluctuations in the cosmic microwave background (CMB) intensity due to the Sunyaev-Zel'dovich (SZ) effect are the sum of a thermal and a kinetic contribution. Separating the two components to measure the peculiar velocity of galaxy clusters requires radio and microwave observations at three or more frequencies, and knowledge of the temperature T e of the intracluster medium (ICM) weighted by the electron number density. To quantify the systematics of this procedure, we extract a sample of 117 massive clusters at redshift z = 0 from an N-body hydrodynamical simulation, with 2 x 480 3 particles, of a cosmological volume 192 h -1 Mpc on a side of a flat cold dark matter model with Ω 0 = 0.3 and Ω Λ = 0.7. Our simulation includes radiative cooling, star formation and the effect of feedback and galactic winds from supernovae. We find that: (i) our simulated clusters reproduce the observed scaling relations between X-ray and SZ properties; (ii) bulk flows internal to the ICM affect the velocity estimate by less than 200 km s -1 in 93 per cent of the cases; (iii) using the X-ray emission weighted temperature, as an estimate of T e , can overestimate the peculiar velocity by 20-50 per cent, if the microwave observations do not spatially resolve the cluster. For spatially resolved clusters, the assumptions on the spatial distribution of the ICM, required to separate the two SZ components, still produce a velocity overestimate of 10-20 per cent, even with an unbiased measure of T e . Thanks to the large size of our cluster samples, these results set a robust lower limit of ∼200 km s -1 to the systematic errors that will affect upcoming measures of cluster peculiar velocities with the SZ effect.

Modelling peculiar velocities of dark matter haloes

We present a simple model that accurately describes various statistical properties of peculiar velocities of dark matter haloes. We pay particular attention to the following two effects. First, the evolution of the halo peculiar velocity depends on the local matter density, instead of the global density. Secondly, dark matter haloes are biased tracers of the underlying mass distribution, thus haloes tend to be located preferentially at high density regions. For the former, we develop an empirical model calibrated with N-body simulations while, for the latter, we use a conventional halo bias model based on the extended Press–Schechter model combined with an empirical lognormal probability distribution function of the mass density distribution. We find that, compared with linear theory, the present model significantly improves the accuracy of predictions of statistical properties of the halo peculiar velocity field including the velocity dispersion, the probability distribution function, and the pairwise velocity dispersion at large separations. Thus, our model predictions may be useful in analysing future observations of the peculiar velocities of galaxy clusters.

The rms peculiar velocity of galaxy clusters for different cluster masses and radii

We investigate the rms peculiar velocity of galaxy clusters in the Lambda cold dark matter ($\Lambda$CDM) and tau cold dark matter ($\tau$CDM) cosmological models using N-body simulations. Cluster velocities for different cluster masses and radii are examined. To identify clusters in the simulations we use two methods: the standard friends-of-friends (FOF) method and the method, where the clusters are defined as the maxima of the density field smoothed on the scale $R\sim 1h^{-1}$ Mpc (DENSMAX). If we use the DENSMAX method, the size of the selected clusters is similar for all clusters. We find that the rms velocity of clusters defined with the DENSMAX method is almost independent of the cluster density and similar to the linear theory expectations. The rms velocity of FOF clusters decreases with the cluster mass and radius. In the $\Lambda$CDM model, the rms peculiar velocity of massive clusters with an intercluster separation $d_{cl}=50h^{-1}$ Mpc is $\approx$15% smaller than the rms velocity of the clusters with a separation $d_{cl}=10h^{-1}$Mpc.

How accurately can the SZ effect measure peculiar cluster velocities and bulk flows?

The Sunyaev-Zel'dovich eect is a powerful tool for cosmology that can be used to measure the radial peculiar velocities of galaxy clusters, and thus to test, and constrain theories of structure formation and evolution. This requires, in principle, an accurate measurement of the eect, a good separation between the Sunyaev- Zel'dovich components, and a good understanding of the sources contributing to the signal and their eect on the measured velocity. In this study, we evaluate the error in the individual radial peculiar velocities determined with Sunyaev-Zel'dovich measurements. We estimate, for three cosmological models, the errors induced by the major contributing signals (primary Cosmic Microwave Background anisotropies, Sunyaev-Zel'dovich eect due to the background cluster population, residuals from component separation and instrumental noise). We generalise our results to estimate the error in the bulk velocity on large scales. In this context, we investigate the limitation due to the Sunyaev-Zel'dovich source (or spatial) confusion in a Planck-like instrumental conguration. Finally, we propose a strategy based on the future all-sky Sunyaev-Zel'dovich survey, that will be provided by the Planck mission, to measure accurately the bulk velocities on large scales up to redshift 1, or more.

Power Spectrum of Velocity Fluctuations in the Universe

We investigate the power spectrum of velocity fluctuations in the universe starting from four different measures of velocity: the power spectrum of velocity fluctuations from peculiar velocities of galaxies, the rms peculiar velocity of galaxy clusters, the power spectrum of velocity fluctuations from the power spectrum of density fluctuations in the galaxy distribution, and the bulk velocity from peculiar velocities of galaxies. There are various way of interpreting the observational data.

Reconstructing Positions and Peculiar Velocities of Galaxy Clusters within 25,000 Kilometers per Second: The Bulk Velocity

Using a dynamical three-dimensional reconstruction procedure, we estimate the peculiar velocities of R ≥ 0 Abell/ACO galaxy clusters from their measured redshift within 25,000 km s-1. The reconstruction algorithm relies on the linear gravitational instability hypothesis, assumes linear biasing, and requires an input value of the cluster β-parameter (βc ≡ Ω0.60/bc), which we estimated in Branchini & Plionis to be βc 0.21. The resulting cluster velocity field is dominated by a large-scale streaming motion along the Perseus-Pisces/Great Attractor baseline directed toward the Shapley concentration, in qualitative agreement with the galaxy velocity field on smaller scales. Fitting the predicted cluster peculiar velocities to a dipole term, in the Local Group frame and within a distance of ~18,000 km s-1, we recover extremely well both the Local Group velocity and direction, in disagreement with the Lauer & Postman observation. However, we find a ~6% probability that their observed velocity field could be a realization of our corresponding one, if the latter is convolved with their large distance-dependent errors. Our predicted cluster bulk velocity amplitude agrees well with that deduced by the POTENT and the da Costa et al. analyses of observed galaxy motions at ~5000-6000 km s-1; it decreases thereafter, while, at the Lauer & Postman limiting depth (~15,000 km s-1), its amplitude is ~150 km s-1, in comfortable agreement with most cosmological models.

Peculiar velocities of galaxy clusters: a comparison of mixed dark matter and low-density cold dark matter

Peculiar velocities of galaxy clusters: a comparison of mixed dark matter and low-density cold dark matter

The fundamental plane of galaxy clusters

Velocity dispersion $\sigma$, radius $R$ and luminosity $L$ of elliptical galaxies are known to be related, leaving only two degrees of freedom and defining the so-called ``fundamental plane". In this {\em Letter} we present observational evidence that rich galaxy clusters exhibit a similar behaviour. Assuming a relation $L \propto R^{\alpha}\sigma^{2 \beta}$, the best-fit values of $\alpha$ and $\beta$ are very close to those defined by galaxies. The dispersion of this relation is lower than 10 percent, i.e. significantly smaller than the dispersion observed in the $L-\sigma$ and $L-R$ relations. We briefly suggest some possible implications on the spread of formation times of objects and on peculiar velocities of galaxy clusters.