Search for: All records

Galaxy cluster mergers are excellent laboratories for studying a wide variety of different physical phenomena. An example of such a cluster system is the distant SPT-CLJ2228-5828 merger located atz ≈ 0.77. Previous analyses via the thermal Sunyaev-Zeldovich effect and weak lensing (WL) data suggested that the system was potentially a dissociative cluster post-merger, similar to the Bullet cluster. In this work, we perform an X-ray and optical follow-up analysis of this rare system. We used new deepXMM-Newtondata to study the hot gas in X-rays in great detail, spectroscopicGeminidata to precisely determine the redshift of the two mass concentrations, and newHubbleSpace Telescope data to improve the total mass estimates of the two components. We find that SPT-CLJ2228-5828 constitutes a pre-merging double cluster system instead of a post-merger as previously thought. The merging process of the two clusters has started, with their gas on the outskirts colliding with a ∼22° −27° on the plane of the sky. Both clusters have a similar radius ofR₅₀₀ ∼ 700 kpc, with the two X-ray emission peaks separated by ≈1 Mpc (2.1′). We fully characterized the surface brightness, gas density, temperature, pressure, and entropy profiles of the two merging clusters for their undisturbed non-interacting side. The two systems have very similar X-ray properties, with a moderate cluster mass ofM_tot ∼ (2.1 − 2.4)×10¹⁴ M_⊙according to X-ray mass proxies. Both clusters show good agreement with known X-ray scaling relations when their merging side is ignored. The WL mass estimate of the western cluster agrees well with the X-ray-based mass, whereas the eastern cluster is surprisingly only marginally detected from its WL signal. A gas bridge with ≈333 kpc length connecting the two merging halos is detected at a 5.8σlevel. The baryon overdensity of the excess gas (not associated with the cluster gas) isδ_b ∼ (75 − 320) across the length of the bridge, and its gas mass isM_gas ∼ 1.4 × 10¹² M_⊙. The gas density and temperature jumps at ∼10⁻³cm⁻³and ∼5.5 keV, respectively, are also found across the gas bridge, revealing the existence of a weak shock front with a Mach number ℳ ∼ 1.1. The gas pressure and entropy also increase at the position of the shock front. We estimate the age of the shock front to be ≲100 Myr and its kinetic energy ∼2.4 × 10⁴⁴erg s⁻¹. SPT-CLJ2228-5828 is the first such high-zpre-merger with a gas bridge and a shock front, consisting of similarly sized clusters, to be studied in X-rays. 

We use galaxy cluster abundance measurements from the South Pole Telescope enhanced by multicomponent matched filter confirmation and complemented with mass information obtained using weak-lensing data from Dark Energy Survey Year 3 (DES Y3) and targeted Hubble Space Telescope observations for probing deviations from the cold dark matter paradigm. Concretely, we consider a class of dark sector models featuring interactions between dark matter (DM) and a dark radiation (DR) component within the framework of the effective theory of structure formation (ETHOS). We focus on scenarios that lead to power suppression over a wide range of scales, and thus can be tested with data sensitive to large scales, as realized, for example, for DM–DR interactions following from an unbroken non-Abelian $SU\left(N\right)$gauge theory (interaction rate with power-law index $n=0$within the ETHOS parametrization). Cluster abundance measurements are mostly sensitive to the amount of DR interacting with DM, parametrized by the ratio of DR temperature to the cosmic microwave background (CMB) temperature, ${\xi }_{\mathrm{DR}}=T_{\mathrm{DR}}/T_{\mathrm{CMB}}$. We find an upper limit ${\xi }_{\mathrm{DR}}<17\%$at 95% credibility. When the cluster data are combined with Planck 2018 CMB data along with baryon acoustic oscillation (BAO) measurements we find ${\xi }_{\mathrm{DR}}<10\%$, corresponding to a limit on the abundance of interacting DR that is around 3 times tighter than that from CMB + BAO data alone. We also discuss the complementarity of weak lensing informed cluster abundance studies with probes sensitive to smaller scales, explore the impact on our analysis of massive neutrinos, and comment on a slight preference for the presence of a nonzero interacting DR abundance, which enables a physical solution to the $S_8$tension. Published by the American Physical Society2025