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Register a new userMerging Galaxy Clusters: Offset Between the Sunyaev-Zeldovich Effect and X-ray Peaks
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Galaxy clusters, the most massive collapsed structures, have been routinely used to determine cosmological parameters. When using clusters for cosmology, the crucial assumption is that they are relaxed. However, subarcminute resolution Sunyaev-Zeldovich (SZ) effect images compared with high resolution X-ray images of some clusters show significant offsets between the two peaks. We have carried out self-consistent N-body/hydrodynamical simulations of merging galaxy clusters using FLASH to study these offsets quantitatively. We have found that significant displacements result between the SZ and X-ray peaks for large relative velocities for all masses used in our simulations as long as the impact parameters were about 100-250 kpc. Our results suggest that the SZ peak coincides with the peak in the pressure times the line-of-sight characteristic length and not the pressure maximum (as it would for clusters in equilibrium). The peak in the X-ray emission, as expected, coincides with the density maximum of the main cluster. As a consequence, the morphology of the SZ signal and therefore the offset between the SZ and X-ray peaks change with viewing angle. As an application, we compare the morphologies of our simulated images to observed SZ and X-ray images and mass surface densities derived from weak lensing observations of the merging galaxy cluster CL0152-1357. We find that a large relative velocity of 4800 km/s is necessary to explain these observations. We conclude that an analysis of the morphologies of multi-frequency observations of merging clusters can be used to put meaningful constraints on the initial parameters of the progenitors.
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Observations reveal that the peaks of the X-ray map and the Sunyaev-Zeldovich (SZ) effect map of some galaxy clusters are offset from each other. In this paper, we perform a set of hydrodynamical simulations of mergers of two galaxy clusters to investigate the spatial offset between the maxima of the X-ray and the SZ surface brightness of the merging clusters. We find that significantly large SZ-X-ray offsets (>100kpc) can be produced during the major mergers of galaxy clusters. The significantly large offsets are mainly caused by a `jump effect occurred between the primary and secondary pericentric passages of the two merging clusters, during which the X-ray peak may jump to the densest gas region located near the center of the small cluster, but the SZ peak remains near the center of the large one. Our simulations show that merging systems with higher masses and larger initial relative velocities may result in larger offset sizes and longer offset time durations; and only nearly head-on mergers are likely to produce significantly large offsets. We further investigate the statistical distribution of the SZ-X-ray offset sizes and find that (1) the number distribution of the offset sizes is bimodal with one peak located at low offsets ~0 and the other at large offsets ~350-450kpc/h, but the objects with intermediate offsets are scarce; and (2) the probabilities of the clusters in the mass range higher than 2x10^{14}Msun/h that have offsets larger than 20, 50, 200, 300, and 500kpc/h are 34.0%, 11.1%, 8.0%, 6.5%, and 2.0% respectively at z=0.7. The probability is sensitive to the underlying pairwise velocity distribution and the merger rate of clusters. Future observations on the offsets for a large number of clusters may put strong constraints on the cosmic velocity fields on the cluster scale and the cluster merger rate. (Abridged)
We present scaling relations between the integrated Sunyaev-Zeldovich Effect (SZE) signal, $Y_{rm SZ}$, its X-ray analogue, $Y_{rm X}equiv M_{rm gas}T_{rm X}$, and total mass, $M_{rm tot}$, for the 45 galaxy clusters in the Bolocam X-ray-SZ (BOXSZ) sample. All parameters are integrated within $r_{2500}$. $Y_{2500}$ values are measured using SZE data collected with Bolocam, operating at 140 GHz at the Caltech Submillimeter Observatory (CSO). The temperature, $T_{rm X}$, and mass, $M_{rm gas,2500}$, of the intracluster medium are determined using X-ray data collected with Chandra, and $M_{rm tot}$ is derived from $M_{rm gas}$ assuming a constant gas mass fraction. Our analysis accounts for several potential sources of bias, including: selection effects, contamination from radio point sources, and the loss of SZE signal due to noise filtering and beam-smoothing effects. We measure the $Y_{2500}$--$Y_{rm X}$ scaling to have a power-law index of $0.84pm0.07$, and a fractional intrinsic scatter in $Y_{2500}$ of $(21pm7)%$ at fixed $Y_{rm X}$, both of which are consistent with previous analyses. We also measure the scaling between $Y_{2500}$ and $M_{2500}$, finding a power-law index of $1.06pm0.12$ and a fractional intrinsic scatter in $Y_{2500}$ at fixed mass of $(25pm9)%$. While recent SZE scaling relations using X-ray mass proxies have found power-law indices consistent with the self-similar prediction of 5/3, our measurement stands apart by differing from the self-similar prediction by approximately 5$sigma$. Given the good agreement between the measured $Y_{2500}$--$Y_{rm X}$ scalings, much of this discrepancy appears to be caused by differences in the calibration of the X-ray mass proxies adopted for each particular analysis.
Cosmography provides a direct method to map the expansion history of the Universe in a model-independent way. Recently, different kinds of observations have been used in cosmographic analyses, such as SNe Ia and gamma ray bursts measurements, weak and strong lensing, cosmic microwave background anisotropies, etc. In this work we examine the prospects for constraining cosmographic parameters from current and future measurements of galaxy clusters distances based on their Sunyaev-Zeldovich effect (SZE) and X-ray observations. By assuming the current observational error distribution, we perform Monte Carlo simulations based on a well-behaved parameterization for the deceleration parameter to generate samples with different characteristics and study the improvement on the determination of the cosmographic parameters from upcoming data. The influence of galaxy clusters (GC) morphologies on the $H_0- q_0$ plane is also investigated.
We used optical imaging and spectroscopic data to derive substructure estimates for local Universe ($z < 0.11$) galaxy clusters from two different samples. The first was selected through the Sunyaev-Zeldovich (SZ) effect by the Planck satellite and the second is an X-ray selected sample. In agreement to X-ray substructure estimates we found that the SZ systems have a larger fraction of substructure than the X-ray clusters. We have also found evidence that the higher mass regime of the SZ clusters, compared to the X-ray sample, explains the larger fraction of disturbed objects in the Planck data. Although we detect a redshift evolution in the substructure fraction, it is not sufficient to explain the different results between the higher-z SZ sample and the X-ray one. We have also verified a good agreement ($sim$60$%$) between the optical and X-ray substructure estimates. However, the best level of agreement is given by the substructure classification given by measures based on the brightest cluster galaxy (BCG), either the BCG$-$X-ray centroid offset, or the magnitude gap between the first and second BCGs. We advocate the use of those two parameters as the most reliable and cheap way to assess cluster dynamical state. We recommend an offset cut of $sim$0.01$times$R$_{500}$ to separate relaxed and disturbed clusters. Regarding the magnitude gap the separation can be done at $Delta m_{12} = 1.0$. The central galaxy paradigm (CGP) may not be valid for $sim$20$%$ of relaxed massive clusters. This fraction increases to $sim$60$%$ for disturbed systems.
This is the second of two papers investigating the spherical averaging of ellipsoidal galaxy clusters in the context of X-ray and Sunyaev-Zeldovich (SZ) observations. In the present study we quantify the orientation-average bias and scatter in observables that result from spherically averaging clusters described by ellipsoidal generalizations of the NFW profile or a nearly scale-free logarithmic potential. Although the mean biases are small and mostly <1%, the flattest cluster models generally have a significant mean bias; i.e., averaging over all orientations does not always eliminate projection biases. Substantial biases can result from different viewing orientations, where the integrated Compton-y parameter (Y_SZ) and the concentration have the largest scatter (as large as sigma ~10% for Y_SZ), and the emission-weighted temperature (T_X) has the smallest (sigma < ~0.5%). The very small scatter for T_X leads to Y_X and M_gas having virtually the same orientation biases. Substantial scatter is expected for individual clusters (up to sigma ~8%) in the correlation between Y_SZ and Y_X in comparison to the small mean bias (sigma < ~1%) applicable to a random sample of clusters of sufficient size. For ellipsoidal NFW models we show that the orientation bias for the total cluster mass attains a minimum near the radius r_2500 so that the spherically averaged mass computed at this radius is always within ~0.5% of the true value for any orientation. Finally, to facilitate the accounting for orientation bias in X-ray and SZ cluster studies, we provide cubic polynomial approximations to the mean orientation bias and 1-sigma scatter for each cluster observable as a function of axial ratio for the ellipsoidal NFW models.
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