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Register a new userPeculiar Velocities of Clusters in CDM Models
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S. Borgani
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Recently, peculiar velocity measurements became available for a new sample of galaxy clusters. From an accurately calibrated Tully-Fisher relation for spiral galaxies, we compute the rms cluster peculiar velocity and compare it to the linear theory predictions of COBE-normalized low-density and open CDM models (LambdaCDM and OCDM, respectively). Confidence levels for model rejection are estimated using a Monte Carlo procedure to generate for each model a large ensemble of artificial data sets. Following Zaroubi et al. (1997), we express our results in terms of constraints on the (Omega_0,n_pr,h) parameter space. Such constraints turn into sigma_8 Omega_0^{0.6}=0.50^{+0.25}_{-0.14} at the 90% c.l., thus in agreement with results from cluster abundance. We show that our constraints are also consistent with those implied by the shape of the galaxy power spectrum within a rather wide range for the values of the model parameters. Finally, we point out that our findings disagree at about the 3sigma level with respect to those by Zaroubi et al. (1997), based on the Mark III catalogue, which tend to prefer larger Omega_0 values within the CDM class of models.
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Type Ia Supernovae (SNe Ia) are widely used to measure the expansion of the Universe. To perform such measurements the luminosity and cosmological redshift ($z$) of the SNe Ia have to be determined. The uncertainty on $z$ includes an unknown peculiar velocity, which can be very large for SNe Ia in the virialized cores of massive clusters. We determine which SNe Ia exploded in galaxy clusters. We then study how the correction for peculiar velocities of host galaxies inside the clusters improves the Hubble residuals. Using 145 SNe Ia from the Nearby Supernova Factory we found 11 candidates for membership in clusters. To estimate the redshift of a cluster we applied the bi-weight technique. Then, we use the galaxy cluster redshift instead of the host galaxy redshift to construct the Hubble diagram. For SNe Ia inside galaxy clusters the dispersion around the Hubble diagram when peculiar velocities are taken into account is smaller in comparison with a case without peculiar velocity correction, with a $wRMS=0.130pm0.038$ mag instead of $wRMS=0.137pm0.036$ mag. The significance of this improvement is 3.58 $sigma$. If we remove the very nearby Virgo cluster member SN2006X ($z<0.01$) from the analysis, the significance decreases to 1.34 $sigma$. The peculiar velocity correction is found to be highest for the SNe Ia hosted by blue spiral galaxies, with high local specific star formation rate and smaller stellar mass, seemingly counter to what might be expected given the heavy concentration of old, massive elliptical galaxies in clusters. As expected, the Hubble residuals of SNe Ia associated with massive galaxy clusters improve when the cluster redshift is taken as the cosmological redshift of the SN. This fact has to be taken into account in future cosmological analyses in order to achieve higher accuracy for cosmological redshift measurements. Here we provide an approach to do so.
We propose two new methods for measuring tangential peculiar velocities of rich clusters of galaxies. Our first method is based on weak gravitational lensing and takes advantage of the differing images of background galaxies caused by moving and stationary gravitational potentials. Our second method is based on measuring relative frequency shifts between multiple images of a single strongly lensed background galaxy. We illustrate this method using the example of galaxy cluster CL 0024+1654.
We investigate peculiar velocities predicted for clusters in Lambda cold dark matter ($Lambda$CDM) models assuming that the initial density fluctuation field is Gaussian. To study the non-linear regime, we use N-body simulations. We investigate the rms velocity and the probability distribution function of cluster peculiar velocities for different cluster masses. To identify clusters in the simulation we use two methods: the standard friends-of-friends (FOF) method and the method, where the clusters are defined as maxima of a smoothed density field (DMAX). The density field is smoothed with a top-hat window, using the smoothing radii $R_s=1.5h^{-1}$ Mpc and $R_s=1.0h^{-1}$ Mpc. The peculiar velocity of the DMAX clusters is defined to be the mean peculiar velocity of matter within a sphere of the radius $R_s$. We find that the rms velocity of the FOF clusters decreases as the cluster mass increases. The rms velocity of the DMAX clusters is almost independent of the cluster mass and is well approximated by the linear rms peculiar velocity smoothed at the radius $R=R_s$. The velocity distribution function of the DMAX clusters is similar to a Gaussian.
This paper presents detailed analysis of large-scale peculiar motions derived from a sample of ~ 700 X-ray clusters and cosmic microwave background (CMB) data obtained with WMAP. We use the kinematic Sunyaev-Zeldovich (KSZ) effect combining it into a cumulative statistic which preserves the bulk motion component with the noise integrated down. Such statistic is the dipole of CMB temperature fluctuations evaluated over the pixels of the cluster catalog (Kashlinsky & Atrio-Barandela 2000). To remove the cosmological CMB fluctuations the maps are Wiener-filtered in each of the eight WMAP channels (Q, V, W) which have negligible foreground component. Our findings are as follows: The thermal SZ (TSZ) component of the clusters is described well by the Navarro-Frenk-White profile expected if the hot gas traces the dark matter in the cluster potential wells. Such gas has X-ray temperature decreasing rapidly towards the cluster outskirts, which we demonstrate results in the decrease of the TSZ component as the aperture is increased to encompass the cluster outskirts. We then detect a statistically significant dipole in the CMB pixels at cluster positions. Arising exclusively at the cluster pixels this dipole cannot originate from the foreground or instrument noise emissions and must be produced by the CMB photons which interacted with the hot intracluster gas via the SZ effect. The dipole remains as the monopole component, due to the TSZ effect, vanishes within the small statistical noise out to the maximal aperture where we still detect the TSZ component. We demonstrate with simulations that the mask and cross-talk effects are small for our catalog and contribute negligibly to the measurements. The measured dipole thus arises from the KSZ effect produced by the coherent large scale bulk flow motion.
We present results of a statistical analysis of the SFI catalog of peculiar velocities, a recently completed survey of spiral field galaxies with I-band Tully-Fisher distances (Haynes et al. 1999). The velocity field statistic utilized is the velocity correlation function, $psi_1(r)$ (Gorski et al. 1989). The analysis is performed in redshift space, so as to circumvent potential ambiguities connected with inhomogeneous Malmquist bias corrections. The results from the SFI sample are compared with linear-theory predictions. We generate a large set of mock samples, extracted from N-body simulations, which are used to assess the reliability of our analysis and to estimate the associated uncertainties. Defining $eta_8=sigma_8Omega_0^{0.6}$, we find that the measured $psi_1(r)$ implies a degenerate constraint in the $eta_8-Gamma$ plane, with $eta_8=(0.3 +/- 0.1) (Gamma/0.2)^{0.5}$, at the $2sigma$ level, for the inverse Tully-Fisher (ITF) calibration presented in this paper. Model constraints are quite sensitive to the ITF calibration. The other ITF calibrations by Giovanelli et al. (1997) and da Costa et al. (1998) both give, for $Gamma=0.2$, $eta_8simeq 0.6$ as the best-fitting value.
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