Non-cool Core Research Articles

XLSSC 122 caught in the act of growing up

Context. How protoclusters evolved from sparse galaxy overdensities to mature galaxy clusters is still not well understood. In this context, detecting and characterizing the hot intracluster medium (ICM) at high redshifts (z ∼ 2) is key to understanding how the continuous accretion from the filamentary large-scale structure and the mergers along it impact the first phases of cluster formation. Aims. We study the dynamical state and morphology of the z = 1.98 galaxy cluster XLSSC 122 with high-resolution observations (≈5″) of the ICM through the Sunyaev–Zeldovich (SZ) effect. XLSSC 122 is the highest redshift optically confirmed galaxy cluster found in an unbiased, widefield survey. Methods. Via Bayesian forward modeling, we mapped the ICM on scales from the virial radius down to the core of the cluster. To constrain such a broad range of spatial scales, we employed a new technique that jointly forward-models parametric descriptions of the pressure distribution to interferometric ACA and ALMA observations and multiband imaging data from ACT. Results. We detect the SZ effect with 11σ significance in the ALMA+ACA observations and find a flattened inner pressure profile that is consistent with a noncool core classification with a significance of ≥3σ. In contrast to the previous works, we find better agreement between the SZ effect signal and the X-ray emission as well as the cluster member distribution. Further, XLSSC 122 exhibits an excess of SZ flux in the south of the cluster where no X-ray emission is detected. By reconstructing the interferometric observations and modeling in the uv-plane, we obtain a tentative detection of an infalling group or filamentary-like structure in the southeast that is believed to boost and heat up the ICM while the density of the gas is still low. In addition to characterizing the dynamical state of the cluster, we provide an improved SZ mass estimate M500,c = 1.66−0.20+0.23 × 1014 M⊙. Conclusions. Altogether, the observations indicate that we see XLSSC 122 in a dynamic phase of cluster formation while a large reservoir of gas is already thermalized.

The heart of galaxy clusters: Demographics and physical properties of cool-core and non-cool-core halos in the TNG-Cluster simulation

We analyzed the physical properties of the gaseous intracluster medium (ICM) at the center of massive galaxy clusters with TNG-Cluster, a new cosmological magnetohydrodynamical simulation. Our sample contains 352 simulated clusters spanning a halo mass range of 1014 < M500c/M⊙ < 2 × 1015 at z = 0. We focused on the proposed classification of clusters into cool-core (CC) and non-cool-core (NCC) populations, the z = 0 distribution of cluster central ICM properties, and the redshift evolution of the CC cluster population. We analyzed the resolved structure and radial profiles of entropy, temperature, electron number density, and pressure. To distinguish between CC and NCC clusters, we considered several criteria: central cooling time, central entropy, central density, X-ray concentration parameter, and density profile slope. According to TNG-Cluster and with no a priori cluster selection, the distributions of these properties are unimodal, whereby CCs and NCCs represent the two extremes. Across the entire TNG-Cluster sample at z = 0 and based on the central cooling time, the strong CC fraction is fSCC = 24%, compared to fWCC = 60% and fNCC = 16% for weak and NCCs, respectively. However, the fraction of CCs depends strongly on both halo mass and redshift, although the magnitude and even direction of the trends vary with definition. The abundant statistics of simulated high-mass clusters in TNG-Cluster enabled us to match observational samples and make a comparison with data. The CC fractions from z = 0 to z = 2 are in broad agreement with observations, as are the radial profiles of thermodynamical quantities, globally as well as when divided as CC versus NCC halos. TNG-Cluster can therefore be used as a laboratory to study the evolution and transformations of cluster cores due to mergers, AGN feedback, and other physical processes.

AMUSE-Antlia. I. Nuclear X-Ray Properties of Early-type Galaxies in a Dynamically Young Galaxy Cluster

To understand the formation and growth of supermassive black holes (SMBHs) and their coevolution with host galaxies, it is essential to know the impact of environment on the activity of active galactic nuclei (AGNs). We present new Chandra X-ray observations of nuclear emission from member galaxies in the Antlia cluster, the nearest non-cool core and the nearest merging galaxy cluster, residing at D = 35.2 Mpc. Its inner region, centered on two dominant galaxies NGC 3268 and NGC 3258, has been mapped with three deep Chandra ACIS-I pointings. Nuclear X-ray sources are detected in 7/84 (8.3%) early-type galaxies (ETG) and 2/8 (25%) late-type galaxies with a median detection limit of 8 × 1038 erg s−1. All nuclear X-ray sources but one have a corresponding radio continuum source detected by MeerKAT at the L band. Nuclear X-ray sources detected in early-type galaxies are considered the genuine X-ray counterpart of low-luminosity AGN. When restricted to a detection limit of and a stellar mass of , six of 11 ETGs are found to contain an X-ray AGN in Antlia, exceeding the AGN occupation fraction of 7/39 (18.0%) and 2/12 (16.7%) in the more relaxed, cool core clusters, Virgo and Fornax, respectively, and rivaling that of the AMUSE-Field ETG of 27/49 (55.1%). Furthermore, more than half of the X-ray AGN in Antlia is hosted by its younger subcluster, centered on NGC 3258. We believe that this is because SMBH activity is enhanced in a dynamically young cluster compared to relatively relaxed clusters.

Redshift Evolution of the Feedback–Cooling Equilibrium in the Core of 48 SPT Galaxy Clusters: A Joint Chandra–SPT–ATCA Analysis

We analyze the cooling and feedback properties of 48 galaxy clusters at redshifts 0.4 < z < 1.3 selected from the South Pole Telescope (SPT) catalogs to evolve like the progenitors of massive and well-studied systems at z ∼ 0. We estimate the radio power at the brightest cluster galaxy (BCG) location of each cluster from an analysis of Australia Telescope Compact Array data. Assuming that the scaling relation between the radio power and active galactic nucleus (AGN) cavity power P cav observed at low redshift does not evolve with redshift, we use these measurements in order to estimate the expected AGN cavity power in the core of each system. We estimate the X-ray luminosity within the cooling radius L cool of each cluster from a joint analysis of the available Chandra X-ray and SPT Sunyaev–Zel’dovich (SZ) data. This allows us to characterize the redshift evolution of the P cav/L cool ratio. When combined with low-redshift results, these constraints enable investigations of the properties of the feedback–cooling cycle across 9 Gyr of cluster growth. We model the redshift evolution of this ratio measured for cool-core clusters by a log-normal distribution - and constrain the slope of the mean evolution to β = −0.05 ± 0.47. This analysis improves the constraints on the slope of this relation by a factor of two. We find no evidence of redshift evolution of the feedback–cooling equilibrium in these clusters, which suggests that the onset of radio-mode feedback took place at an early stage of cluster formation. High values of P cav/L cool are found at the BCG location of noncool-core clusters, which might suggest that the timescales of the AGN feedback cycle and the cool core–noncool core transition are different. This work demonstrates that the joint analysis of radio, SZ, and X-ray data solidifies the investigation of AGN feedback at high redshifts.

A test of linearity of the ratio of dark matter to baryonic matter in galaxy clusters

We search for a linearity in the ratio of dark matter to baryonic matter as a function of radius for galaxy clusters, motivated by a recent result by Lovas (arXiv:2206.11431), who has discovered such a linearity for a diverse suite of galaxies in the SPARC sample. For our analysis, we used a sample of 54 non-cool core clusters from the HIFLUGCS sample. We do not find any evidence for a linear trend in the aforementioned ratio as a function of radius for individual clusters. We then repeat this analysis for the stacked sample, which also does not show this linearity. Therefore, the linear scaling found by Lovas is not a universal property of dark matter haloes at all scales.

The evolving cluster cores: Putting together the pieces of the puzzle

Context. In this work we address the issue of whether the division of clusters in cool cores (CCs) and non-cool cores (NCCs) is due to a primordial difference or to how clusters evolve across cosmic time. Aims. Our first goal is to establish if spectra from the central regions of a subclass of NCCs known as cool core remnants (CCRs) are consistent with having a small but significant amount of short cooling time gas, thereby allowing a transformation to CC systems on a timescale of a giga year. Our second goal is to determine if low ionization Fe lines emitted from this residual cool gas will be detectable by the calorimeters that will fly on board XRISM and Athena. Methods. We performed a spectral analysis of CCR systems with a multi temperature model and, assuming the different components to be in pressure equilibrium with one another, derived entropy and cooling time distributions for the X-ray emitting gas. Results. We find that in most of our systems, the spectral model allows for a fraction of low entropy, short cooling time gas with a mass that is comparable to the one in CC systems. Moreover, simulations show that future spectrometers on board XRISM and Athena will have the power to directly resolve emission lines from the low temperature gas, thereby providing incontrovertible evidence for its presence. Conclusions. Within the scenario that we have explored, the constant fraction of CCs measured across cosmic time emerges from a dynamical equilibrium where CCs transformed in NCCs through mergers are balanced by NCCs that revert to CCs. Furthermore, CCs and NCCs should not be viewed as distinct sub classes, but as “states” between which clusters can move.

The eROSITA Final Equatorial-Depth Survey (eFEDS)

Context.Morphological parameters are the estimators for the dynamical state of clusters of galaxies. Surveys performed at different wavelengths through their selection effects may be biased toward, different populations of clusters. For example, X-ray surveys are biased to detecting cool-core clusters as opposed to Sunyaev-Zel’dovich (SZ) surveys being more biased toward non-cool-core systems. Understanding the underlying population of clusters of galaxies in surveys is of the utmost importance when these samples are to be used in astrophysical and cosmological studies.Aims.We present an in-depth analysis of the X-ray morphological parameters of the galaxy clusters and groups detected in the eROSITA Final Equatorial-Depth Survey (eFEDS). The eFEDS, completed during the performance verification phase of the Spectrum-Roentgen-Gamma(SRG)/eROSITA telescope, is designed to provide the first eROSITA X-ray selected sample of galaxy clusters and groups.Methods.We studied the eROSITA X-ray imaging data for a sample of 325 clusters and groups that were significantly detected in the eFEDS field. We characterized their dynamical properties by measuring a number of dynamical estimators: concentration, central density, cuspiness, centroid shift, ellipticity, power ratios, photon asymmetry, and the Gini coefficient. The galaxy clusters and groups detected in eFEDS cover a luminosity range of more than three orders of magnitude and a wide redshift range out to 1.2. They provide an ideal sample on which the redshift and luminosity evolution of the morphological parameters can be studied and the underlying dynamical state of the sample can be characterized. Based on these measurements, we constructed a new dynamical indicator, the relaxation score, for all the clusters in the sample.Results.We find no evidence for a bimodality in the distribution of the morphological parameters of our clusters. We instead observe a smooth transition from the cool core to non-cool core and from relaxed to disturbed states, with a preference for skewed distributions or log-normal distributions. A significant evolution in redshift and luminosity is also observed in the morphological parameters we examined after taking the selection effects into account.Conclusions.We determine that in contrast do ROSAT-based cluster samples, our eFEDS-selected cluster sample is not biased toward cool-core clusters, but contains a similar fraction of cool-cores as SZ surveys.

A deep learning view of the census of galaxy clusters in IllustrisTNG

ABSTRACT The origin of the diverse population of galaxy clusters remains an unexplained aspect of large-scale structure formation and cluster evolution. We present a novel method of using X-ray images to identify cool core (CC), weak cool core (WCC), and non-cool core (NCC) clusters of galaxies that are defined by their central cooling times. We employ a convolutional neural network, ResNet-18, which is commonly used for image analysis, to classify clusters. We produce mock Chandra X-ray observations for a sample of 318 massive clusters drawn from the IllustrisTNG simulations. The network is trained and tested with low-resolution mock Chandra images covering a central 1 Mpc square for the clusters in our sample. Without any spectral information, the deep learning algorithm is able to identify CC, WCC, and NCC clusters, achieving balanced accuracies (BAcc) of 92 per cent, 81 per cent, and 83 per cent, respectively. The performance is superior to classification by conventional methods using central gas densities, with an average ${\rm BAcc}=81{{\ \rm per\ cent}}$, or surface brightness concentrations, giving ${\rm BAcc}=73{{\ \rm per\ cent}}$. We use class activation mapping to localize discriminative regions for the classification decision. From this analysis, we observe that the network has utilized regions from cluster centres out to r ≈ 300 kpc and r ≈ 500 kpc to identify CC and NCC clusters, respectively. It may have recognized features in the intracluster medium that are associated with AGN feedback and disruptive major mergers.

On the Assembly Bias of Cool Core Clusters Traced by Hα Nebulae

Abstract Do cool-core (CC) and non-cool-core (NCC) clusters live in different environments? We make novel use of Hα emission lines in the central galaxies of redMaPPer clusters as proxies to construct large (1000's) samples of CC and NCC clusters and measure their relative assembly bias using both clustering and weak lensing. We increase the statistical significance of the bias measurements from clustering by cross-correlating the clusters with an external galaxy redshift catalog from the Sloan Digital Sky Survey III, the LOWZ sample. Our cross-correlations can constrain assembly bias up to a statistical uncertainty of 6%. Given our Hα criteria for CC and NCC, we find no significant differences in their clustering amplitude. Interpreting this difference as the absence of halo assembly bias, our results rule out the possibility of having different large-scale (tens of Mpc) environments as the source of diversity observed in cluster cores. Combined with recent observations of the overall mild evolution of CC and NCC properties, such as central density and CC fraction, this would suggest that either the cooling properties of the cluster core are determined early on solely by the local (&lt;200 kpc) gas properties at formation or that local merging leads to stochastic CC relaxation and disruption in a periodic way, preserving the average population properties over time. Studying the small-scale clustering in clusters at high redshift would help shed light on the exact scenario.

Exploring the multiphase medium in MKW 08: from the central active galaxy up to cluster scales

Context. The study of the brightest cluster galaxy (BCG) coronae embedded in noncool core (NCC) galaxy clusters is crucial to understand the BCG’s role in galaxy cluster evolution as well as the activation of the self-regulated cooling and heating mechanism in the central regions of galaxy clusters. Aims. We explore the X-ray properties of the intracluster medium (ICM) of the NCC galaxy cluster MKW 08 and the BCG corona, along with their interface region. With recent and deep archival Chandra observations, we study the BCG corona in detail, and with archival XMM-Newton observations, we investigate the implications of the central active galactic nuclei (AGN) on the BCG. Methods. We carry out imaging and spectral analyses of MKW 08 with archival XMM-Newton and Chandra X-ray observations. Results. Our spectral analysis suggests the presence of a central AGN by a power-law with a photon index of Γ ≃ 1.8 at the core of its BCG. Although the ICM does not exhibit a cluster scale cool core, the BCG manifests itself as a mini cool core characterized by a cooling time as short as 64 Myr at r = 3 kpc centered at the galaxy. The isothermality of the BCG corona seems to favor mechanical feedback from the central AGN as the major source of gas heating. The gas pressure profile of this mini cool core suggests that the BCG coronal gas reaches pressure equilibrium with the hotter and less dense ICM inside an interface of nearly constant pressure, delimited by radii 4 ≤ r ≤ 10 kpc at the galactic center. As revealed by the presence of a metal enriched tail (Z ≃ 0.5–0.9 Z⊙) extending up to 40 kpc, the BCG corona seems to be experiencing ram-pressure stripping by the surrounding ICM and/or interacting with a nearby galaxy, IC 1042.

The Most Massive galaxy Clusters (M2C) across cosmic time: link between radial total mass distribution and dynamical state

We study the dynamical state and the integrated total mass profiles of 75 massive (M500 &gt; 5 × 1014 M⊙) Sunyaev–Zeldovich(SZ)-selected clusters at 0.08 &lt; z &lt; 1.1. The sample is built from the Planck catalogue, with the addition of four SPT clusters at z &gt; 0.9. Using XMM-Newton imaging observations, we characterise the dynamical state with the centroid shift ⟨w⟩, the concentration CSB, and their combination, M, which simultaneously probes the core and the large-scale gas morphology. Using spatially resolved spectroscopy and assuming hydrostatic equilibrium, we derive the total integrated mass profiles. The mass profile shape is quantified by the sparsity, that is the ratio of M500 to M2500, the masses at density contrasts of 500 and 2500, respectively. We study the correlations between the various parameters and their dependence on redshift. We confirm that SZ-selected samples, thought to most accurately reflect the underlying cluster population, are dominated by disturbed and non-cool core objects at all redshifts. There is no significant evolution or mass dependence of either the cool core fraction or the centroid shift parameter. The M parameter evolves slightly with z, having a correlation coefficient of ρ = −0.2 ± 0.1 and a null hypothesis p-value of 0.01. In the high-mass regime considered here, the sparsity evolves minimally with redshift, increasing by 10% between z &lt; 0.2 and z &gt; 0.55, an effect that is significant at less than 2σ. In contrast, the dependence of the sparsity on dynamical state is much stronger, increasing by a factor of ∼60% from the one third most relaxed to the one third most disturbed objects, an effect that is significant at more than 3σ. This is the first observational evidence that the shape of the integrated total mass profile in massive clusters is principally governed by the dynamical state and is only mildly dependent on redshift. We discuss the consequences for the comparison between observations and theoretical predictions.

Expanding the Sample: The Relationship between the Black Hole Mass of BCGs and the Total Mass of Galaxy Clusters

Abstract Supermassive black holes (BHs) residing in brightest cluster galaxies (BCGs) are overly massive when considering the local relationships between the BH mass and stellar bulge mass or velocity dispersion. Because of the location of these BHs within the cluster, large-scale cluster processes may aid the growth of BHs in BCGs. In this work, we study a sample of 71 galaxy clusters to explore the relationship between the BH mass, stellar bulge mass of the BCG, and the total gravitating mass of the host clusters. Because of difficulties in obtaining dynamically measured BH masses in distant galaxies, we use the Fundamental Plane relationship of BHs to infer their masses. We utilize X-ray observations taken by Chandra to measure the temperature of the intracluster medium, which is a proxy for the total mass of the cluster. We analyze the M BH–kT and M BH–M bulge relationships and establish the best-fitting power laws: = −0.35+2.08 and = −1.09+1.92 . Both relations are comparable with that established earlier for a sample of brightest group/cluster galaxies with dynamically measured BH masses. Although both the M BH–kT and the M BH–M bulge relationships exhibit large intrinsic scatter, on the basis of Monte Carlo simulations, we conclude that dominant fraction of the scatter originates from the Fundamental Plane relationship. We split the sample into cool core and noncool core resembling clusters but do not find statistically significant differences in the M BH–kT relation. We speculate that the overly massive BHs in BCGs may be due to frequent mergers and cool gas inflows onto the cluster center.

Chandra Observations of the AS0295 Cluster

Abstract We present deep (205 ks) Chandra observations of the AS0295 binary merging cluster (z = 0.30). In the X-ray image, the secondary component is clearly visible as a surface brightness peak, while the primary cluster has a flatter distribution. We found cool gas (∼6 keV) associated with the secondary, while the central temperature of the primary does not deviate significantly from the mean temperature of the cluster of ∼9.5 keV. In the vicinity of the primary’s core we found the hottest region in the cluster accompanied by a surface brightness discontinuity. We propose that this region corresponds to a shock, for which we estimate a Mach number of . We found other merger signatures such as a plume of cool gas emerging from the primary cluster and a cold front and a possible bow shock (Mach number of ) leading the secondary cluster. Based on the observed properties in comparison to binary merger simulations from the literature we propose for AS0295 a low-mass ratio, off-axis merging scenario, with the secondary close to the first apocenter. Comparison of our results with strong-lensing observations of AS0295 from Cibirka et al. shows an offset between the total mass peak and the bulk of the gas distribution in the primary cluster. The properties of the merger and the existence of the offset between mass peak and gas make AS0295 a promising candidate for the study of mergers involving non-cool core clusters and the nature of dark matter.

The Megaparsec-scale Gas-sloshing Spiral in the Remnant Cool Core Cluster Abell 1763

Abstract We present a multiwavelength study of the massive galaxy cluster Abell 1763 at redshift z = 0.231. Image analysis of a 19.6 ks Chandra archival observation reveals a cluster-wide spiral of enhanced surface brightness in the intracluster medium (ICM). While such spirals are understood to form in clusters with sloshing strong cool cores (SCCs), the gas comprising the spiral’s apex is of intermediate entropy (∼110 keV cm2) and cooling time (∼6.8 Gyr), indicating core disruption is occurring throughout the spiral formation process. Two subclusters dominated by the second- and third-ranked galaxies in the system lie along a line parallel to the elongation axis of the primary cluster’s ICM. Both subsystems appear to have fallen in along a previously discovered intercluster filament and are each considered candidates as the perturber responsible for initiating disruptive core sloshing. Dynamical analysis indicates infall is occurring with a relative radial velocity of ∼1800 km s−1. The brightest cluster galaxy of Abell 1763 possesses a high line-of-sight peculiar velocity (v pec ∼ 650 km s−1) and hosts a powerful (P 1.4 ∼ 1026 W Hz−1) bent double-lobed radio source, likely shaped by the relative bulk ICM flow induced in the merger. The cluster merger model of SCC destruction invokes low impact parameter infall as the condition required for core transformation. In contrast to this, the high angular momentum event occurring in Abell 1763 suggests that off-axis mergers play a greater role in establishing the non-cool core cluster population than previously assumed.

Thermodynamic profiles of galaxy clusters from a joint X-ray/SZ analysis

We jointly analyze Bolocam Sunyaev-Zeldovich (SZ) effect and Chandra X-ray data for a set of 45 clusters to derive gas density and temperature profiles without using spectroscopic information. The sample spans the mass and redshift range $3 \times 10^{14} M_{\odot} \le M_{500} \le 25 \times 10^{14} M_{\odot}$ and $0.15\le z \le 0.89$. We define cool-core (CC) and non-cool core (NCC) subsamples based on the central X-ray luminosity, and 17/45 clusters are classified as CC. In general, the profiles derived from our analysis are found to be in good agreement with previous analyses, and profile constraints beyond $r_{500}$ are obtained for 34/45 clusters. In approximately 30% of the CC clusters our analysis shows a central temperature drop with a statistical significance of $>3\sigma$; this modest detection fraction is due mainly to a combination of coarse angular resolution and modest S/N in the SZ data. Most clusters are consistent with an isothermal profile at the largest radii near $r_{500}$, although 9/45 show a significant temperature decrease with increasing radius. The sample mean density profile is in good agreement with previous studies, and shows a minimum intrinsic scatter of approximately 10% near $0.5 \times r_{500}$. The sample mean temperature profile is consistent with isothermal, and has an intrinsic scatter of approximately 50% independent of radius. This scatter is significantly higher compared to earlier X-ray-only studies, which find intrinsic scatters near 10%, likely due to a combination of unaccounted for non-idealities in the SZ noise, projection effects, and sample selection.

A census of cool-core galaxy clusters in IllustrisTNG

The thermodynamic structure of hot gas in galaxy clusters is sensitive to astrophysical processes and typically difficult to model with galaxy formation simulations. We explore the fraction of cool-core (CC) clusters in a large sample of $370$ clusters from IllustrisTNG, examining six common CC definitions. IllustrisTNG produces continuous CC criteria distributions, the extremes of which are classified as CC and non-cool-core (NCC), and the criteria are increasingly correlated for more massive clusters. At $z=0$, the CC fractions for $2$ criteria are in reasonable agreement with the observed fractions but the other $4$ CC fractions are lower than observed. This result is partly driven by systematic differences between the simulated and observed gas fraction profiles. The simulated CC fractions with redshift show tentative agreement with the observed fractions, but linear fits demonstrate that the simulated evolution is steeper than observed. The conversion of CCs to NCCs appears to begin later and act more rapidly in the simulations. Examining the fraction of CCs and NCCs defined as relaxed we find no evidence that CCs are more relaxed, suggesting that mergers are not solely responsible for disrupting CCs. A comparison of the median thermodynamic profiles defined by different CC criteria shows that the extent to which they evolve in the cluster core is dependent on the CC criteria. We conclude that the thermodynamic structure of galaxy clusters in IllustrisTNG shares many similarities with observations, but achieving better agreement most likely requires modifications of the underlying galaxy formation model.

An off-axis galaxy cluster merger: Abell 0141

We present structural analysis results of Abell 0141 ($z$ = 0.23) based on X-ray data. The X-ray luminosity map demonstrates that Abell 0141 (A0141) is a bimodal galaxy cluster, which is separated on the sky by $\sim$ 0.65 Mpc with an elongation along the north-south direction. The optical galaxy density map also demonstrates this bimodality. We estimate sub-cluster ICM temperatures of 5.17$^{+0.20}_{-0.19}$ keV for A0141N and 5.23$^{+0.24}_{-0.23}$ keV for A0141S. We obtain X-ray morphological parameters w = 0.034$\pm{0.004}$, c = 0.113$\pm{0.004}$ and w = 0.039$\pm{0.004}$, c = 0.104$\pm{0.005}$ for A0141N and A0141S, respectively. The resulting X-ray morphological parameters indicate that both sub-clusters are moderately disturbed non-cool core structures. We find a slight brightness jump in the bridge region, and yet, there is still an absence of strong X-ray emitting gas between sub-clusters. We discover a significantly hotspot ($\sim$ 10 keV) between sub-clusters, and a Mach number $\textit{M}$ = 1.69$^{+0.40}_{-0.37}$ is obtained by using the temperature jump condition. However, we did not find direct evidence for shock-heating between sub-clusters. We estimate the sub-clusters' central entropies as $K_{0}$ > 100 keV cm$^{2}$, which indicates that the sub-clusters are not cool cores. We find some evidence that the system undergoes an off-axis collision; however, the cores of each sub-clusters have not yet been destroyed. Due to the orientation of X-ray tails of sub-clusters, we suggest that the northern sub-cluster moves through the south-west direction, and the southern cluster moves through the north-east direction. In conclusion, we are witnessing an earlier phase of close core passage between sub-clusters.

The uniformity and time-invariance of the intra-cluster metal distribution in galaxy clusters from the IllustrisTNG simulations

The distribution of metals in the intra-cluster medium encodes important information about the enrichment history and formation of galaxy clusters. Here we explore the metal content of clusters in IllustrisTNG - a new suite of galaxy formation simulations building on the Illustris project. Our cluster sample contains 20 objects in TNG100 - a ~(100 Mpc)^3 volume simulation with 2x1820^3 resolution elements, and 370 objects in TNG300 - a ~(300 Mpc)^3 volume simulation with 2x2500^3 resolution elements. The z=0 metallicity profiles agree with observations, and the enrichment history is consistent with observational data going beyond z~1, showing nearly no metallicity evolution. The abundance profiles vary only minimally within the cluster samples, especially in the outskirts with a relative scatter of ~15%. The average metallicity profile flattens towards the center, where we find a logarithmic slope of -0.1 compared to -0.5 in the outskirts. Cool core clusters have more centrally peaked metallicity profiles (~0.8 solar) compared to non-cool core systems (~0.5 solar), similar to observational trends. Si/Fe and O/Fe radial profiles follow positive gradients. The outer abundance profiles do not evolve below z~2, whereas the inner profiles flatten towards z=0. More than ~80% of the metals in the intra-cluster medium have been accreted from the proto-cluster environment, which has been enriched to ~0.1 solar already at z~2. We conclude that the intra-cluster metal distribution is uniform among our cluster sample, nearly time-invariant in the outskirts for more than 10 Gyr, and forms through a universal enrichment history.

Occurrence of Radio Minihalos in a Mass-limited Sample of Galaxy Clusters

Abstract We investigate the occurrence of radio minihalos—diffuse radio sources of unknown origin observed in the cores of some galaxy clusters—in a statistical sample of 58 clusters drawn from the Planck Sunyaev–Zel’dovich cluster catalog using a mass cut (M 500 &gt; 6 × 1014 M ⊙). We supplement our statistical sample with a similarly sized nonstatistical sample mostly consisting of clusters in the ACCEPT X-ray catalog with suitable X-ray and radio data, which includes lower-mass clusters. Where necessary (for nine clusters), we reanalyzed the Very Large Array archival radio data to determine whether a minihalo is present. Our total sample includes all 28 currently known and recently discovered radio minihalos, including six candidates. We classify clusters as cool-core or non-cool-core according to the value of the specific entropy floor in the cluster center, rederived or newly derived from the Chandra X-ray density and temperature profiles where necessary (for 27 clusters). Contrary to the common wisdom that minihalos are rare, we find that almost all cool cores—at least 12 out of 15 (80%)—in our complete sample of massive clusters exhibit minihalos. The supplementary sample shows that the occurrence of minihalos may be lower in lower-mass cool-core clusters. No minihalos are found in non-cool cores or “warm cores.” These findings will help test theories of the origin of minihalos and provide information on the physical processes and energetics of the cluster cores.

Testing the Large-scale Environments of Cool-core and Non-cool-core Clusters with Clustering Bias

Abstract There are well-observed differences between cool-core (CC) and non-cool-core (NCC) clusters, but the origin of this distinction is still largely unknown. Competing theories can be divided into internal (inside-out), in which internal physical processes transform or maintain the NCC phase, and external (outside-in), in which the cluster type is determined by its initial conditions, which in turn leads to different formation histories (i.e., assembly bias). We propose a new method that uses the relative assembly bias of CC to NCC clusters, as determined via the two-point cluster-galaxy cross-correlation function (CCF), to test whether formation history plays a role in determining their nature. We apply our method to 48 ACCEPT clusters, which have well resolved central entropies, and cross-correlate with the SDSS-III/BOSS LOWZ galaxy catalog. We find that the relative bias of NCC over CC clusters is b = 1.42 ± 0.35 (1.6σ different from unity). Our measurement is limited by the small number of clusters with core entropy information within the BOSS footprint, 14 CC and 34 NCC clusters. Future compilations of X-ray cluster samples, combined with deep all-sky redshift surveys, will be able to better constrain the relative assembly bias of CC and NCC clusters and determine the origin of the bimodality.