Offsets Between the X-Ray and the Sunyaev-Zel'dovich-Effect Peaks in Merging Galaxy Clusters and Their Cosmological Implications

Abstract

Observations reveal that the peaks of the X-ray map and the Sunyaev-Zel'dovich (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 (> 100 kpc) can be produced during the major mergers of galaxy clusters (with mass > 1 × 10 14 M ⊙ ). 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 ∼350-450h −1 kpc, but the objects with intermediate offsets are scarce; and (2) the probabilities of the clusters in the mass range higher than 2 × 10 14 h −1 M ⊙ that have offsets larger than 20, 50, 200, 300, and 500h −1 kpc 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. We suggest that the SZ-X-ray offsets provide a probe to the cosmic velocity fields on the cluster scale and the cluster merger rate, and future observations on the SZ-X-ray offsets for a large number of clusters may put strong constraints on them.Our simulation results suggest that the SZ-X-ray offset in the Bullet Cluster, together with the mass ratio of the two merging clusters, requires a relative velocity larger than 3000 km s −1 at an initial separation 5 Mpc. The cosmic velocity distribution at the high-velocity end is expected to be crucial in determining whether there exists an incompatibility between the existence of the Bullet Cluster and the prediction of a ΛCDM model.

Sunyaev-Zel’dovich detection of the galaxy cluster Cl J1449+0856 at z = 1.99: The pressure profile in uv space

“…That detection is consistent across different observations (Mantz et al 2018) and thus rather unlikely to be a product of noise. Offsets of this amplitude between either the BCG or X-ray peak and the SZ centroid are not unexpected, especially in clusters that are in a unrelaxed state (e.g., Zhang et al 2014) as we would expect Cl1449 to be given its relative youth, and are commonly observed in high-redshift clusters (e.g., Brodwin et al 2016;Strazzullo et al 2019). We perform a Monte Carlo simulation to estimate the significance of this offset (see Appendix B), subtracting the combined astrometric uncertainty of ACA/ALMA and Chandra in quadrature.…”

Close-up view of an ongoing merger between the NGC 4839 group and the Coma cluster – a post-merger scenario

“…Each body is further modelled as a spherical object consisting of a dark matter (DM) halo and gas atmosphere. A detailed description of the simulation method has been given in Zhang et al (2014Zhang et al ( , 2015. Here, we only provide a brief summary.…”

“…The merger plane coincides with the x − y plane. The DM halos and gas halos follow the Navarro-Frenk-White (NFW) profile (Navarro et al 1997) and the Burkert profile (Burkert 1995) within the virial radius for the density profiles (see equations (1)-(4) in Zhang et al 2014). We fix the masses of halos as described below and set the concentration parameter of the main cluster c = 4 according to the weak-lensing measurement (Okabe et al 2014).…”

Encounters of merger and accretion shocks in galaxy clusters and their effects on intracluster medium

“…We have explored the cluster merger process for a wide range of merger parameters in our previous works (see e.g. Zhang et al 2014Zhang et al , 2016) 5 , which provided some intuitions on this question. For example, Zhang et al (2019a) and Lyskova et al (2019) presented analysis of two merging systems with ξ = 10 and zero and large impact parameters respectively.…”