Cool Core Research Articles

The Relation between the Cool-core Radius and the Host Galaxy Clusters: Thermodynamic Properties and Cluster Mass

We present a detailed study of cool-core systems in a sample of four galaxy clusters (RXCJ1504.1-0248, A3112, A4059, and A478) using archival X-ray data from the Chandra X-ray Observatory. Cool cores are frequently observed at the centers of galaxy clusters and are considered to be formed by radiative cooling of the intracluster medium (ICM). Cool cores are characterized by a significant drop in the ICM temperature toward the cluster center. We extract and analyze X-ray spectra of the ICM to measure the radial profiles of the ICM thermodynamic properties, including temperature, density, pressure, entropy, and radiative cooling time. We define the cool-core radius as the turnover radius in the ICM temperature profile and investigate the relation between the cool-core radius and the properties of the host galaxy clusters. In our sample, we observe that the radiative cooling time of the ICM at the cool-core radius exceeds 10 Gyr, with RXCJ1504.1-0248 exhibiting a radiative cooling time of 32−11+5 Gyr at its cool-core radius. These results indicate that not only radiative cooling but also additional mechanisms, such as gas sloshing, may play an important role in determining the size of cool cores. Additionally, we might find that the best-fit relation between the cool-core radius and the cluster mass (M 500) is consistent with a linear relation. Our findings may suggest that cool cores are linked to the evolution of their host galaxy clusters.

Cool Cores in Clusters of Galaxies in the Dark Energy Survey

We search for the presence of cool cores in optically selected galaxy clusters from the Dark Energy Survey (DES) and investigate their prevalence as a function of redshift and cluster richness. Clusters were selected from the redMaPPer analysis of three years of DES observations that have archival Chandra X-ray observations, giving a sample of 99 clusters with a redshift range of 0.11 < z < 0.87 and a richness range of 25 < λ < 207. Using the X-ray data, the core temperature was compared to the outer temperature to identify clusters where the core temperature is a factor of 0.7 or less than the outer temperature. We found a cool core fraction of approximately 20% with no significant trend in the cool core fraction with either redshift or richness.

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 hot gas distribution, X-ray luminosity, and baryon budget in the L-Galaxies semi-analytic model of galaxy formation

ABSTRACT Hot ionized gas is important in the baryon cycle of galaxies and contributes the majority of their ‘missing baryons’. Until now, most semi-analytic models of galaxy formation have paid little attention to hot gaseous haloes and their X-ray emission. In this paper, we adopt the one-dimensional model from Sharma et al. instead of the isothermal sphere to describe the radial distribution of hot gas in the L-Galaxies semi-analytic model. The hot gas halo can be divided into two parts according to the ratio of the local thermal instability time-scale and the free-fall time-scale: a cool core with tTI/tff = 10 and a stable outer halo with tTI/tff &amp;gt; 10. We update the prescriptions of cooling, feedback, and stripping based on the new hot gas profiles, and then reproduce several X-ray observational results like the radial profiles of hot gas density, and the scaling relations of X-ray luminosity and temperature. We find: (1) Consistent with observations, flatter density profiles in halo centers produce lower X-ray emission than an isothermal sphere; (2) Cool core regions prone to precipitation have higher gas temperature than the virial temperature, and a larger TX/T200 ratio in smaller haloes leads to a steeper slope in the LX–TX relation; (3) The ionized gas in the unbounded reservoir and low-temperature intergalactic gas in low-mass haloes could be the main components of the halo ‘missing baryons’. Our model outputs can predict the observations of hot gas in the nearby universe and produce mock surveys of baryons probed by future X-ray telescopes.

Stability of Cool Cores during Galaxy Cluster Growth: A Joint Chandra/SPT Analysis of 67 Galaxy Clusters along a Common Evolutionary Track Spanning 9 Gyr

Abstract We present the results of a joint analysis of Chandra X-ray and South Pole Telescope (SPT) Sunyaev–Zel’dovich (SZ) observations targeting the first sample of galaxy clusters at 0.3 &lt; z &lt; 1.3, selected to be the progenitors of well-studied nearby clusters based on their expected accretion rate. We develop a new procedure in order to tackle the analysis challenge that is estimating the intracluster medium (ICM) properties of low-mass and high-redshift clusters with ∼150 X-ray counts. One of the dominant sources of uncertainty on the ICM density profile estimated with a standard X-ray analysis with such shallow X-ray data is due to the systematic uncertainty associated with the ICM temperature obtained through the analysis of the background-dominated X-ray spectrum. We show that we can decrease the uncertainty on the density profile by a factor varying between 2 and 8 with a joint deprojection of the X-ray surface brightness profile measured by Chandra and the SZ-integrated Compton parameter available in the SPT cluster catalog. We apply this technique to the whole sample of 67 clusters in order to track the evolution of the ICM core density during cluster growth. We confirm that the evolution of the gas density profile is well modeled by the combination of a fixed core and a self-similarly evolving non-cool-core profile. We show that the fraction of cool cores in this sample is remarkably stable with redshift although clusters have gained a factor of ∼4 in total mass over the past ∼9 Gyr.

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.

The First Astrophysical Result of Hisaki: A Search for the EUV He Lines in a Massive Cool Core Cluster at z = 0.7

Abstract Molecular cold gas and star formation have been observed at centers of cool core clusters, albeit at a level much smaller than expected from the classic cooling model. Feedback from the supermassive black hole is likely to have prevented hot gas from cooling. However, the exact cooling and heating processes are poorly understood. The missing key piece is the link between the hot gas (107 K) and cold gas (103 K). Using the extreme ultraviolet spectrometer on board Hisaki, we explore a distant galaxy cluster, RCS2 J232727.6-020437, one of the most massive cool core clusters with a cooling rate of 400 M ⊙ yr−1. We aim to detect gas at intermediate temperatures (3×104 K) emitting He i α and He i β at rest wavelengths of 58.4 nm and 53.7 nm, respectively. Our target resides at z = 0.6986, for which these He i lines shift away from the absorption of the Galaxy. Our findings show that the amount of 104–5 K gas at the center of this cluster is smaller than expected if cooling there was uninhibited, which demonstrates that feedback both operates and is efficient for massive clusters at these epochs.

Enhancing AGN efficiency and cool-core formation with anisotropic thermal conduction

ABSTRACT Understanding how baryonic processes shape the intracluster medium (ICM) is of critical importance to the next generation of galaxy cluster surveys. However, many models of structure formation neglect potentially important physical processes, like anisotropic thermal conduction (ATC). We explore the impact of ATC on the prevalence of cool-cores (CCs) via 12 pairs of magnetohydrodynamical galaxy cluster simulations, using the IllustrisTNG model with and without ATC. Examining their properties we find that the addition of ATC has a negligible impact on the median rotation measure, plasma β, the magnetic field-radial direction angle, and the effective Spitzer value. However, the scatter in the angle and effective Spitzer value is 50 per cent larger with ATC because the magnetic field aligns with the azimuthal direction to a greater extent in relaxed clusters. ATC’s impact varies from cluster to cluster and with CC criterion, but its inclusion produces a systematic shift to larger CC fractions at z = 0 for all CC criteria considered. Additionally, the inclusion of ATC flattens the CC fraction redshift evolution, helping to ease the tension with the observed evolution. With ATC, the energy required for the central black hole to self-regulate is reduced by 24 per cent and the gas fraction at $0.01\, r_{500}$ increases by 100 per cent, producing larger CC fractions. ATC makes the ICM unstable to perturbations and the increased efficiency of AGN feedback suggests that its inclusion results in a greater level of mixing in the ICM, demonstrated by the 10 per cent reduction in central metallicity for clusters with ATC.

Line-of-Sight Gas Sloshing in the Cool Core of Abell 907

Abstract We present line-of-sight (LOS) gas sloshing first found in a cool core in a galaxy cluster. The galaxy cluster Abell 907 is identified as a relaxed cluster owing to its global X-ray surface brightness taken by the Chandra X-ray Observatory. The X-ray residual image after removing the global emission of the intracluster medium (ICM), however, shows an arc-like positive excess and a negative excess surrounding the central positive excess in the cluster core, which in turn indicates a disturbance of the ICM. We analyze the X-ray spectra extracted from both regions and find that (1) the ICM temperature and the metal abundance in the positive excess are lower and higher than those in the negative excess, respectively, and (2) the ICM is nearly in pressure equilibrium. We also find a slight redshift difference between the positive and the negative excesses, which corresponds to the velocity shear of km s−1 (1σ). The X-ray residual image and the ICM properties are consistent with those expected by LOS gas sloshing. Assuming that the gas is moving inverse-parallel to each other along the LOS, the shear velocity is expected to be ∼800 km s−1. The velocity field of this level is able to provide nonthermal pressure support by ∼34% relative to the thermal one. The total kinetic energy inferred from the shear velocity corresponds to ∼30% of the bolometric luminosity of the sloshing ICM. Abell 907 is therefore complementary to galaxy clusters in which gas sloshing takes place in the plane of the sky, and is important for understanding gas dynamics driven by sloshing and its influence on the heating to prevent runaway cooling.

Physical properties of the X-ray gas as a dynamical diagnosis for galaxy clusters

We analysed XMM-{\it Newton} EPIC data for 53 galaxy clusters. Through 2D spectral maps, we provide the most detailed and extended view of the spatial distribution of temperature (kT), pressure (P), entropy (S) and metallicity (Z) of galaxy clusters to date with the aim of correlating the dynamical state of the system to six cool-core diagnoses from the literature. With the objective of building 2D maps and resolving structures in kT, P, S and Z, we divide the data in small regions from which spectra can be extracted. Our analysis shows that when clusters are spherically symmetric the cool-cores (CC) are preserved, the systems are relaxed with little signs of perturbation, and most of the CC criteria agree. The disturbed clusters are elongated, show clear signs of interaction in the 2D maps, and most do not have a cool-core. However, 16 well studied clusters classified as CC by at least four criteria show spectral maps that appear disturbed. All of these clusters but one show clear signs of recent mergers, with a complex structure and geometry but with a cool-core that remains preserved. Thus, although very useful for CC characterization, most diagnoses are too simplistic to reproduce the overall structure and dynamics of galaxy clusters, and therefore the selection of relaxed systems according to these criteria may affect mass estimates. The complex structure of galaxy clusters can be reliably assessed through the 2D maps presented here.

A Detailed Study of the Most Relaxed SPT-selected Galaxy Clusters: Properties of the Cool Core and Central Galaxy

Abstract We present a multi-wavelength analysis of the four most relaxed clusters in the South Pole Telescope 2500 deg2 survey, which lie at 0.55 &lt; z &lt; 0.75. This study, which utilizes new, deep data from the Chandra X-ray Observatory and Hubble Space Telescope, along with ground-based spectroscopy from Gemini and Magellan, improves significantly on previous studies in both depth and angular resolution, allowing us to directly compare to clusters at z ∼ 0. We find that the temperature, density, and entropy profiles of the intracluster medium (ICM) are very similar among the four clusters, and share similar shapes to those of clusters at z ∼ 0. Specifically, we find no evidence for deviations from self-similarity in the temperature profile over the radial range 10 kpc &lt; r &lt; 1 Mpc, implying that the processes responsible for preventing runaway cooling over the past ≳6 Gyr are, at least roughly, preserving self-similarity. We find typical metallicities of ∼0.3 Z ⊙ in the bulk of the ICM, rising to ∼0.5 Z ⊙ in the inner ∼100 kpc, and reaching ∼1 Z ⊙ at r &lt; 10 kpc. This central excess is similar in magnitude to what is observed in the most relaxed clusters at z ∼ 0, suggesting that both the global metallicity and the central excess that we see in cool core clusters at z ∼ 0 were in place very early in the cluster’s lifetime, and specifically that the central excess is not due to late-time enrichment by the central galaxy. Consistent with observations at z ∼ 0, we measure a diversity of stellar populations in the central brightest cluster galaxies of these four clusters, with star formation rates spanning a factor of ∼500, despite the similarities in cooling time, cooling rate, and central entropy. These data suggest that, while the details vary dramatically from system to system, runaway cooling has been broadly regulated in relaxed clusters over the past 6 Gyr.

A Cool Core Disturbed: Observational Evidence for the Coexistence of Subsonic Sloshing Gas and Stripped Shock-heated Gas around the Core of RX J1347.5–1145

Abstract RX J1347.5–1145 (z = 0.451) is one of the most luminous X-ray galaxy clusters; it hosts a prominent cool core and exhibits a signature of a major merger. We present the first direct observational evidence for the subsonic nature of the sloshing motion of the cool core. We find that a residual X-ray image from the Chandra X-ray Observatory after removing the global emission shows a clear dipolar pattern characteristic of gas sloshing, whereas we find no significant residual in the Sunyaev–Zel’dovich effect (SZE) image from the Atacama Large Millimeter/submillimeter Array (ALMA). We estimate the equation of state of perturbations in the gas from the X-ray and SZE residual images. The inferred velocity is km s−1, which is much lower than the adiabatic sound speed of the intracluster medium in the core. We thus conclude that the perturbation is nearly isobaric, and the gas sloshing motion is consistent with being in pressure equilibrium. Next, we report evidence for gas stripping of an infalling subcluster, which likely shock-heats the gas to a temperature well in excess of 20 keV. Using the mass distribution inferred from strong lensing images of the Hubble Space Telescope (HST), we find that the mass peak is located away from the peak position of the stripped gas with a statistical significance of &gt;5σ. Unlike for the gas sloshing, the velocity inferred from the equation of state of the excess hot gas is comparable to the adiabatic sound speed expected for the 20 keV intracluster medium. All of the results support that the southeast substructure is created by a merger. On the other hand, the positional offset between the mass and the gas limits the self-interaction cross section of dark matter to be less than 3.7 h −1 cm2 g−1 (95% CL).

Gas Perturbations in the Cool Cores of Galaxy Clusters: Effective Equation of State, Velocity Power Spectra, and Turbulent Heating

Abstract We present the statistical analysis of X-ray surface brightness and gas density fluctuations in the cool cores of 10 nearby, X-ray-bright galaxy clusters that have deep Chandra observations and show observational indications of radio-mechanical active galactic nucleus (AGN) feedback. Within the central parts of the cool cores, the total variance of fluctuations is dominated by isobaric and/or isothermal fluctuations on spatial scales ∼10–60 kpc, which are likely associated with slow gas motions and bubbles of relativistic plasma. Adiabatic fluctuations, associated with weak shocks and/or sonic turbulence, constitute less than 10% of the total variance in all clusters. The typical amplitude of density fluctuations is small, ∼10% or less on scales of ∼10–15 kpc. The observed subdominant contribution of adiabatic fluctuations and the small amplitude of density fluctuations support a model of gentle AGN feedback. The measured one-component velocities of gas motions are typically below 100–150 km s−1 on scales &lt;50 kpc and can be up to ∼300 km s−1 on ∼100 kpc scales. The nonthermal energy is &lt;12% of the thermal energy. Regardless of the source that drives these motions, the dissipation of the energy in such motions provides heat that is sufficient to balance radiative cooling on average, albeit with significant uncertainties. The results presented here support previous conclusions based on the analysis of the Virgo and Perseus Clusters and agree with the Hitomi measurements. With next-generation observatories like Athena and Lynx, these techniques will be yet more powerful.

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.

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.

The Role of Cerenkov Radiation in the Pressure Balance of Cool Core Clusters of Galaxies

Abstract Despite the substantial progress made recently in understanding the role of AGN feedback and associated non-thermal effects, the precise mechanism that prevents the core of some clusters of galaxies from collapsing catastrophically by radiative cooling remains unidentified. In this Letter, we demonstrate that the evolution of a cluster's cooling core, in terms of its density, temperature, and magnetic field strength, inevitably enables the plasma electrons there to quickly become Cerenkov loss dominated, with emission at the radio frequency of ≲350 Hz, and with a rate considerably exceeding free–free continuum and line emission. However, the same does not apply to the plasmas at the cluster's outskirts, which lacks such radiation. Owing to its low frequency, the radiation cannot escape, but because over the relevant scale size of a Cerenkov wavelength the energy of an electron in the gas cannot follow the Boltzmann distribution to the requisite precision to ensure reabsorption always occurs faster than stimulated emission, the emitting gas cools before it reheats. This leaves behind the radiation itself, trapped by the overlying reflective plasma, yet providing enough pressure to maintain quasi-hydrostatic equilibrium. The mass condensation then happens by Rayleigh–Taylor instability, at a rate determined by the outermost radius where Cerenkov radiation can occur. In this way, it is possible to estimate the rate at ≈2 M ⊙ year−1, consistent with observational inference. Thus, the process appears to provide a natural solution to the longstanding problem of “cooling flow” in clusters; at least it offers another line of defense against cooling and collapse should gas heating by AGN feedback be inadequate in some clusters.

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.

Pressure of the hot gas in simulations of galaxy clusters

We analyze the radial pressure profiles, the ICM clumping factor and the Sunyaev-Zel'dovich (SZ) scaling relations of a sample of simulated galaxy clusters and groups identified in a set of hydrodynamical simulations based on an updated version of the TreePM-SPH GADGET-3 code. Three different sets of simulations are performed: the first assumes non-radiative physics, the others include, among other processes, AGN and/or stellar feedback. Our results are analyzed as a function of redshift, ICM physics, cluster mass and cluster cool-coreness or dynamical state. In general, the mean pressure profiles obtained for our sample of groups and clusters show a good agreement with X-ray and SZ observations. Simulated cool-core (CC) and non-cool-core (NCC) clusters also show a good match with real data. We obtain in all cases a small (if any) redshift evolution of the pressure profiles of massive clusters, at least back to z=1. We find that the clumpiness of gas density and pressure increases with the distance from the cluster center and with the dynamical activity. The inclusion of AGN feedback in our simulations generates values for the gas clumping ($\sqrt C_{\rho}\sim 1.2$ at $R_{200}$) in good agreement with recent observational estimates. The simulated $Y_{SZ}-M$ scaling relations are in good accordance with several observed samples, especially for massive clusters. As for the scatter of these relations, we obtain a clear dependence on the cluster dynamical state, whereas this distinction is not so evident when looking at the subsamples of CC and NCC clusters.