The pre-merging system of galaxy clusters Abell 3391-Abell 3395 located at a mean redshift of 0.053 has been observed at 1 GHz in an ASKAP/EMU Early Science observation as well as in X-rays with eROSITA. The projected separation of the X-ray peaks of
the two clusters is $sim$50$$ or $sim$ 3.1 Mpc. Here we present an inventory of interesting radio sources in this field around this cluster merger. While the eROSITA observations provide clear indications of a bridge of thermal gas between the clusters, neither ASKAP nor MWA observations show any diffuse radio emission coinciding with the X-ray bridge. We derive an upper limit on the radio emissivity in the bridge region of $langle J rangle_{1,{rm GHz}}< 1.2 times 10^{-44} {rm W}, {rm Hz}^{-1} {rm m}^{-3}$. A non-detection of diffuse radio emission in the X-ray bridge between these two clusters has implications for particle-acceleration mechanisms in cosmological large-scale structure. We also report extended or otherwise noteworthy radio sources in the 30 deg$^2$ field around Abell 3391-Abell 3395. We identified 20 Giant Radio Galaxies, plus 7 candidates, with linear projected sizes greater than 1 Mpc. The sky density of field radio galaxies with largest linear sizes of $>0.7$ Mpc is $approx 1.7$ deg$^{-2}$, three times higher than previously reported. We find no evidence for a cosmological evolution of the population of Giant Radio Galaxies. Moreover, we find seven candidates for cluster radio relics and radio halos.
We used dedicated SRG/eROSITA X-ray, ASKAP/EMU radio, and DECam optical observations of a 15 sq.deg region around the interacting galaxy cluster system A3391/95 to study the warm-hot gas in cluster outskirts and filaments, the surrounding large-scale
structure and its formation process. We relate the observations to expectations from cosmological hydrodynamic simulations from the Magneticum suite. We trace the irregular morphology of warm-hot gas of the main clusters from their centers out to well beyond their characteristic radii, $r_{200}$. Between the two main cluster systems, we observe an emission bridge; thanks to eROSITAs unique soft response and large field of view, we discover tantalizing hints for warm gas. Several matter clumps physically surrounding the system are detected. For the Northern Clump, we provide evidence that it is falling towards A3391 from the hot gas morphology and radio lobe structure of its central AGN. Many of the extended sources in the field detected by eROSITA are known clusters or new clusters in the background, including a known SZ cluster at redshift z=1. We discover an emission filament north of the virial radius, $r_{100}$, of A3391 connecting to the Northern Clump and extending south of A3395 towards another galaxy cluster. The total projected length of this continuous warm-hot emission filament is 15 Mpc, running almost 4 degrees across the entire eROSITA observation. The DECam galaxy density map shows galaxy overdensities in the same regions. The new datasets provide impressive confirmation of the theoretically expected structure formation processes on the individual system level, including the surrounding warm-hot intergalactic medium distribution compared to the Magneticum simulation. Our spatially resolved findings show that baryons indeed reside in large-scale warm-hot gas filaments with a clumpy structure.
Using Chandra data for a sample of 26 galaxy groups, we constrained the central cooling times (CCTs) of the ICM and classified the groups as strong cool-core (SCC), weak cool-core (WCC) and non-cool-core (NCC) based on their CCTs. The total radio lum
inosity of the brightest cluster galaxy (BCG) was obtained using radio catalog data and literature, which was compared to the CCT to understand the link between gas cooling and radio output. We determined K-band luminosities of the BCG with 2MASS data, and used it to constrain the masses of the SMBH, which were then compared to the radio output. We also tested for correlations between the BCG luminosity and the overall X-ray luminosity and mass of the group. The observed cool-core/non-cool-core fractions for groups are comparable to those of clusters. However, notable differences are seen. For clusters, all SCCs have a central temperature drop, but for groups, this is not the case as some SCCs have centrally rising temperature profiles. While for the cluster sample, all SCC clusters have a central radio source as opposed to only 45% of the NCCs, for the group sample, all NCC groups have a central radio source as opposed to 77% of the SCC groups. For clusters, there are indications of an anticorrelation trend between radio luminosity and CCT which is absent for the groups. Indications of a trend of radio luminosity with black hole mass observed in SCC clusters is absent for groups. The strong correlation observed between the BCG luminosity and the cluster X-ray luminosity/cluster mass weakens significantly for groups. We conclude that there are important differences between clusters and groups within the ICM cooling/AGN feedback paradigm.
We present a statistical study of the occurrence and effects of the cooling cores in the clusters of galaxies in a flux-limited sample, HIFLUGCS, based on ROSAT and ASCA observations. About 49% of the clusters in this sample have a significant, class
ically-calculated cooling-flow, mass-deposition rate. The upper envelope of the derived mass-deposition rate is roughly proportional to the cluster mass, and the fraction of cooling core clusters is found to decrease with it. The cooling core clusters are found to have smaller core radii than non-cooling core clusters, while some non-cooling core clusters have high $beta$ values (> 0.8). In the relation of the X-ray luminosity vs. the temperature and the mass, the cooling core clusters show a significantly higher normalization. A systematic correlation analysis, also involving relations of the gas mass and the total infrared luminosity, indicates that this bias is shown to be mostly due to an enhanced X-ray luminosity for cooling core clusters, while the other parameters, like temperature, mass, and gas mass may be less affected by the occurrence of a cooling core. These results may be explained by at least some of the non-cooling core clusters being in dynamically young states compared with cooling core clusters, and they may turn into cooling core clusters in a later evolutionary stage.
Results of a systematic study of substructure in X-ray surface brightness distributions of a combined sample of 470 REFLEX+BCS clusters of galaxies are presented. The fully automized morphology analysis is based on data of the 3rd processing of the R
OSAT All-Sky survey (RASS-3). After correction for several systematic effects, $52pm 7$ percent of the REFLEX+BCS clusters are found to be substructured in metric apertures of 1 Mpc radius ($H_0= 50 {rm km} {rm s}^{-1} {rm Mpc}^{-1}$). Future simulations will show statistically which mass spectrum of major and minor mergers contributes to this number. Another important result is the discovery of a substructure-density relation, analogous to the morphology-density relation for galaxies. Here, clusters with asymmetric or multi-modal X-ray surface brightness distributions are located preferentially in regions with higher cluster number densities. The substructure analyses techniques are used to compare the X-ray morphology of 53 clusters with radio halos and relics, and 22 cooling flow clusters with the REFLEX+BCS reference sample. After careful equalization of the different `sensitivities of the subsamples to substructure detection it is found that the halo and relic sample tends to show more often multi-modal and elongated X-ray surface brightness distributions compared to the REFLEX+BCS reference sample. The cooling flow clusters show more often circular symmetric and unimodal distributions compared to the REFLEX+BCS and the halo/relic reference samples. Both findings further support the idea that radio halos and relics are triggered by merger events, and that pre-existing cooling flows might be disrupted by recent major mergers.
