Deep Chandra observations of merging galaxy cluster ZwCl 2341+0000

Abstract

Context. Knowledge of X-ray shock and radio relic connection in merging galaxy clusters has been greatly extended in terms of both observation and theory over the last decade. ZwCl 2341+0000 is a double-relic merging galaxy cluster; previous studies have shown that half of the southern relic is associated with an X-ray surface brightness discontinuity, while the other half not. The discontinuity was believed to be a shock front. Therefore, it is a mysterious case of an only partial shock-relic connection. Aims. By using the 206.5 ks deep Chandra observations, we aim to investigate the nature of the southern surface brightness discontinuity. Meanwhile, we aim to explore new morphological and thermodynamical features. Methods. We perform both imaging and spectroscopic analyses to investigate the morphological and thermodynamical properties of the cluster. In addition to the X-ray data, we utilize the GMRT 325 MHz image and JVLA 1.5 GHz and 3.0 GHz images to compute radio spectral index maps. Results. Surface brightness profile fitting and the temperature profile suggest that the previously reported southern surface brightness discontinuity is better described as a sharp change in slope or as a kink. This kink is likely contributed by the disrupted core of the southern subcluster. The radio spectral index maps show spectral flattening at the south-eastern edge of the southern relic, suggesting that the location of the shock front is 640 kpc away from the kink, where the X-ray emission is too faint to detect a surface brightness discontinuity. We update the radio shock Mach number to be ℳradio, S = 2.2 ± 0.1 and ℳradio, N = 2.4 ± 0.4 for the southern and northern radio relics based on the injection spectral indices. We also put a 3σ lower limit on the X-ray Mach number of the southern shock to be ℳX-ray, S > 1.6. Meanwhile, the deep observations reveal that the northern subcluster is in a perfect cone shape, with a ∼400 kpc linear cold front on each side. This type of conic subcluster has been predicted by simulations but is observed here for the first time. It represents a transition stage between a blunt-body cold front and a slingshot cold front. Strikingly, we found a 400 kpc long gas trail attached to the apex of the cone, which could be due to the gas stripping. In addition, an over-pressured hot region is found in the south-western flank of the cluster.

Using the polarization properties of double radio relics to probe the turbulent compression scenario

“…The southern relic shows few patches of polarized emission with < p >= 13 ± 2 %. Projection effects are likely to play an important role in the depolarization of these relics, as suggested by the disturbed X-ray morphology of the cluster (Zhang et al 2021), merger components along the line-of-sight (Golovich et al 2019b) and high σ RM value (i.e., 35 rad m −2 in the source restframe).…”

“…Several optical studies of this system (Boschin et al 2013;Benson et al 2017;Golovich et al 2019b) found that it is composed of at least three sub-clusters: two of them are aligned along the NW-SE elongation of the X-ray emission and their merger is possibly responsible for the radio relics formation, while the third one in the north-east is likely to be involved in a secondary merger along the line-of-sight. Zhang et al (2021) performed a detailed analysis of a deep 206.5 ks Chandra observation of ZwCl2341. They discovered the presence of numerous substructures within this cluster and confirmed its complex dynamical state.…”

“…The northern relic lies instead at the apex of a conic X-ray structure delimited by cold fronts on both sides. Zhang et al (2021) also presented resolved spectral index maps between 325 MHz GMRT and 1.5 GHz JVLA observations (the same that are used in this work). Both relics show a spectral steeping towards the center of the cluster although the trend is not very clear, also due to the patchy shape of the two relics.…”

“…However, given the disturbed structure of this cluster, the non-detection of surface brightness edges from deep Chandra observations (Zhang et al 2021) and the possible presence of a secondary merger along the line-of-sight (Golovich et al 2019b), it is very likely that these relics are not seen edge-on and that projection effects play a role in determining their depolarization fraction. This is also supported by the RM dispersion of the southern relic which is the highest observed in our sample.…”

Using the polarization properties of double radio relics to probe the turbulent compression scenario

“…Several optical studies of this system (Boschin et al 2013;Benson et al 2017;Golovich et al 2019b) found that it is composed of at least three subclusters: two of them are aligned along the northwest-southeast elongation of the Xray emission and their merger is possibly responsible for the radio relics' formation, while the third one in the northeast is likely to be involved in a secondary merger along the line of sight. Zhang et al (2021) performed a detailed analysis of a deep 206.5 ks Chandra observation of ZwCl2341. They discovered the presence of numerous substructures within this cluster and confirmed its complex dynamical state.…”