Cool-core Galaxy Clusters Research Articles

The properties of magnetised cold filaments in a cool-core galaxy cluster

Filaments of cold gas ($T $ K) are found in the inner regions of many cool-core clusters. These structures are thought to play a major role in the regulation of feedback from active galactic nuclei (AGNs). We study the morphology of the filaments, their formation, and their impact on the propagation of the outflowing AGN jets. We present a set of GPU-accelerated 3D magnetohydrodynamic simulations of an idealised Perseus-like cluster using the performance portable code AthenaPK . We include radiative cooling and a self-regulated AGN feedback model that redistributes accreted material through kinetic, thermal, and magnetic feedback. We confirm that magnetic fields play an important role in both the formation and evolution of the cold material. These suppress the formation of massive cold discs and favour magnetically supported filaments over clumpy structures. Achieving resolutions of $25-50$ pc, we find that filaments are not monolithic as they contain numerous and complex magnetically supported sub-structures. We find that the mass distribution of these clumps follows a power law of slope of $ for all investigated filaments. Studying the evolution of individual filaments, we find that their formation pathways can be diverse. We find examples of filaments forming through a combination of gas uplifting and condensation, as well as systems of purely infalling clumps condensing out of the intracluster medium. The density contrast between the cold gas and the outflowing hot material leads to recurring deflections of the jets, favouring inflation of bubbles. Filaments in cool-core clusters are clumpy and contain numerous sub-structures, resulting from a complex interplay between magnetic fields, thermal instability, and jet-cloud interaction. Frequent deflections of the AGN outflows suppress jet collimation and favour the formation of large X-ray bubbles, and smaller off-axis cavities.

The first high-redshift cavity power measurements of cool-core galaxy clusters with the International LOFAR Telescope

Radio-mode feedback associated with the active galactic nuclei (AGNs) at the cores of galaxy clusters injects a large amount of energy into the intracluster medium (ICM), offsetting radiative losses through X-ray emission. This mechanism prevents the ICM from rapidly cooling down and fueling extreme starburst activity as it accretes onto the central galaxies, and it is therefore a key ingredient in the evolution of galaxy clusters. However, the influence and mode of feedback at high redshifts (z ∼ 1) remains largely unknown. Low-frequency sub-arcsecond-resolution radio observations taken with the International LOFAR Telescope have demonstrated their ability to assist X-ray observations with constraining the energy output from the AGNs (or “cavity power”) in galaxy clusters, thereby enabling research at higher redshifts than before. In this pilot project, we tested this hybrid method on a high-redshift (0.6 < z < 1.3) sample of 13 galaxy clusters for the first time with the aim of verifying the performance of this method at these redshifts and providing the first estimates of the cavity power associated with the central AGN for a sample of distant clusters. We were able to detect clear radio lobes in three out of 13 galaxy clusters at redshifts of 0.7 < z < 0.9, and we used these detections in combination with ICM pressures surrounding the radio lobes obtained from standard profiles to calculate the corresponding cavity powers of the AGNs. Combining our results with the literature, the current data appear to suggest that the average cavity power peaked at a redshift of z ∼ 0.4 and slowly decreases toward higher redshifts. However, we require more and tighter constraints on the cavity volume and a better understanding of our observational systematics to confirm any deviation of the cavity power trend from a constant level.

Sloshing and spiral structures breeding a putative radio mini-halo in the environment of a cool-core cluster, Abell 795

ABSTRACT Spiral structures and cold fronts in X-rays are frequently observed in cool-core galaxy clusters. However, studies on radio mini-haloes associated with such spirals and their physical connections are rare. Here, we present the detection of an extended diffuse radio emission entrained in the X-ray spiral structure in a known cool-core cluster, Abell 795. Though the cool core is a sign of the relaxed nature, our re-analysed 30-ks Chandra X-ray data of Abell 795 confirm the presence of an interesting log spiral structure of an X-ray deficit region complemented by an X-ray excess counter spiral in the residual map, exposing its dynamical activity. Our new analysis of 150- and 325-MHz Giant Metrewave Radio Telescope archival data confirms the detection of a ∼180-kpc ultra-steep (α ∼ −2.7) diffuse radio structure, previously reported as a candidate radio mini-halo from low-sensitive survey maps. This emission spans the entire spiral structure, enclosed by two previously reported cold fronts. Furthermore, optical spectra from the Sloan Digital Sky Survey Data Release 13 and far-ultraviolet data from the Galaxy Evolution Explorer show a considerably low total star formation rate of 2.52 M⊙ yr−1 with no significant variation in metallicity distribution. We argue that the two-phase (hot and cold) plasma at the core with differential velocity has plausibly caused the spiral formation and has redistributed the secondary electrons from the brightest cluster galaxy or the pre-accelerated electrons, which have been (re-)accelerated by the sloshing turbulence to form the observed candidate radio mini-halo structure. This is supported by a few previous studies indicating that spiral formation and sloshing turbulence quenches star formation and facilitates smooth metallicity distribution by mixing the gas in the core.

The case for hot-mode accretion in Abell 2029

ABSTRACT Radiative cooling and active galactic nucleus heating are thought to form a feedback loop that regulates the evolution of low-redshift cool-core galaxy clusters. Numerical simulations suggest that the formation of multiphase gas in the cluster core imposes a floor on the ratio of cooling time (tcool) to free-fall time (tff) at min(tcool/tff) ≈ 10. Observations of galaxy clusters show evidence for such a floor, and usually the cluster cores with min(tcool/tff) ≲ 30 contain abundant multiphase gas. However, there are important outliers. One of them is Abell 2029 (A2029), a massive galaxy cluster (M200 ≳ 1015 M⊙) with min(tcool/tff) ∼ 20, but little apparent multiphase gas. In this paper, we present high-resolution 3D hydrodynamic adaptive mesh refinement simulations of a cluster similar to A2029 and study how it evolves over a period of 1–2 Gyr. Those simulations suggest that A2029 self-regulates without producing multiphase gas because the mass of its central black hole (${\sim} 5 \times 10^{10} \, \mathrm{ M}_\odot$) is great enough for Bondi accretion of hot ambient gas to produce enough feedback energy to compensate for radiative cooling.

First evidence of a connection between cluster-scale diffuse radio emission in cool-core galaxy clusters and sloshing features

Radio observations of a few cool-core galaxy clusters have revealed the presence of diffuse emission on cluster scales, similar to what was found in merging clusters in the form of radio halos. These sources might suggest that a minor merger, while not sufficiently energetic to disrupt the cool core, could still trigger particle acceleration in the intracluster medium on scales of hundreds of kiloparsecs. We aim to verify the occurrence of cluster-scale diffuse radio emission in cool-core clusters and test the minor merger scenario. With the LOw Frequency ARray (LOFAR) at 144 MHz, we observed a sample of twelve cool-core galaxy clusters presenting some level of dynamical disturbances, according to X-ray data. We also performed a systematic search of cold fronts in these clusters, re-analysing archival Chandra observations. The clusters PSZ1G139.61+24, A1068 (new detection), MS 1455.0+2232, and RX J1720.1+2638 present diffuse radio emission on a cluster scale ($r $). This emission is characterised by a double component: a central mini-halo confined by cold fronts and diffuse emission on larger scales, whose radio power at 144 MHz is comparable to that of radio halos detected in merging systems with the same cluster mass. The cold fronts in A1068 are a new detection. We also found a candidate plasma depletion layer in this cluster. No sloshing features are found in the other eight clusters. Two of them present a mini-halo, with diffuse radio emission confined to the cluster core. We also found a new candidate mini-halo. Whereas, for the remaining five clusters, we did not detect halo-like emission. For clusters without cluster-scale halos, we derived upper limits to the radio halo power. We found that cluster-scale diffuse radio emission is not present in all cool-core clusters when observed at a low frequency, but it is correlated to the presence of cold fronts. The coexistence of cluster-scale diffuse radio emission and cold fronts in cool-core clusters requires a specific configuration of the merger and so it puts some constraints on the turbulence, which deserves to be investigated in the future with theoretical works.

“Beads-on-a-string” Star Formation Tied to One of the Most Powerful Active Galactic Nucleus Outbursts Observed in a Cool-core Galaxy Cluster

With two central galaxies engaged in a major merger and a remarkable chain of 19 young stellar superclusters wound around them in projection, the galaxy cluster SDSS J1531+3414 (z = 0.335) offers an excellent laboratory to study the interplay between mergers, active galactic nucleus (AGN) feedback, and star formation. New Chandra X-ray imaging reveals rapidly cooling hot (T ∼ 106 K) intracluster gas, with two “wings” forming a concave density discontinuity near the edge of the cool core. LOFAR 144 MHz observations uncover diffuse radio emission strikingly aligned with the “wings,” suggesting that the “wings” are actually the opening to a giant X-ray supercavity. The steep radio emission is likely an ancient relic of one of the most energetic AGN outbursts observed, with 4pV > 1061 erg. To the north of the supercavity, GMOS detects warm (T ∼ 104 K) ionized gas that enshrouds the stellar superclusters but is redshifted up to +800 km s−1 with respect to the southern central galaxy. The Atacama Large Millimeter/submillimeter Array detects a similarly redshifted ∼1010 M ⊙ reservoir of cold (T ∼ 102 K) molecular gas, but it is offset from the young stars by ∼1–3 kpc. We propose that the multiphase gas originated from low-entropy gas entrained by the X-ray supercavity, attribute the offset between the young stars and the molecular gas to turbulent intracluster gas motions, and suggest that tidal interactions stimulated the “beads-on-a-string” star formation morphology.

Gas rotation and dark matter halo shape in cool-core clusters of galaxies

Aims. We study the possibility that the gas in cool-core clusters of galaxies has non-negligible rotation support, the impact of gas rotation on mass estimates from current X-ray observations, and the ability of forthcoming X-ray observatories to detect such rotation. Methods. We present three representative models of massive cool-core clusters with a rotating intracluster medium (ICM) in equilibrium in cosmologically motivated spherical, oblate, or prolate dark matter halos, represented by physical density–potential pairs. In the models, the gas follows a composite-polytropic distribution, and has rotation velocity profiles consistent with current observational constraints and similar to those found in clusters formed in cosmological simulations. We show that the models are consistent with available measurements of the ICM properties of the massive cluster population: the thermodynamic profiles, the shape of the surface brightness distribution, the hydrostatic mass bias, and the broadening of X-ray emitting lines. Using the configuration for the microcalorimeter onboard the XRISM satellite, we generated a set of mock X-ray spectra for our cluster models, which we then analyzed to make predictions about the rotation speed that will be obtained with such an instrument. We then assessed what fraction of the hydrostatic mass bias of our models could be accounted for by detecting the rotation speed with XRISM spectroscopy over the range (0.1 − 1)r500, sampled with three nonoverlapping pointings. Results. Current data leave room for rotating ICM in cool-core clusters, with peaks in the rotation speed as high as 600 km s−1. We show that such rotation, if present, will be detected with upcoming X-ray facilities such as XRISM and that 60 − 70% of the hydrostatic mass bias due to rotation can be accounted for using the line-of-sight velocity measured from X-ray spectroscopy with XRISM, with a residual bias smaller than 3% at an overdensity of 500. In this way, XRISM will allow us to pin down any mass bias of a different origin from the rotation.

A three-component giant radio halo: The puzzling case of the galaxy cluster Abell 2142

Context. Turbulence introduced into the intracluster medium (ICM) through cluster-merger events transfers energy to non-thermal components, and can trigger the formation of diffuse synchrotron radio sources. Typical diffuse sources in the form of giant radio halos and mini-halos are found in merging and relaxed cool-core galaxy clusters, respectively. On the other hand, recent observations reveal an increasing complexity to the non-thermal phenomenology. Aims. Abell 2142 (A2142) is a mildly disturbed cluster that exhibits uncommon thermal and non-thermal properties. It is known to host a hybrid halo consisting of two components (H1 and H2), namely a mini-halo-like and an enigmatic elongated radio halo-like structure. We aim to investigate the properties, origin, and connections of each component. Methods. We present deep LOFAR observations of A2142 in the frequency ranges 30–78 MHz and 120 − 168 MHz. With complementary multi-frequency radio and X-ray data, we analysed the radio spectral properties of the halo and assessed the connection between the non-thermal and thermal components of the ICM. Results. We detect a third radio component (H3), which extends over the cluster volume on scales of ∼2 Mpc, embeds H1 and H2, and has a morphology that roughly follows the thermal ICM distribution. The radio spectral index is moderately steep in H1 (α = 1.09 ± 0.02) and H2 (α = 1.15 ± 0.02), but is steeper (α = 1.57 ± 0.20) in H3. Our analysis of the thermal and non-thermal properties allowed us to discuss possible formation scenarios for each radio component. Turbulence from sloshing motions of low-entropy gas on different scales may be responsible for the origin of H1 and H2. We classified H3 as a giant ultrasteep spectrum radio halo, and find that it may trace the residual activity from an old energetic merger and/or inefficient turbulent reacceleration induced by ongoing minor mergers.

Active galactic nucleus jet feedback in hydrostatic haloes

ABSTRACT Feedback driven by jets from active galactic nuclei is believed to be responsible for reducing cooling flows in cool-core galaxy clusters. We use simulations to model feedback from hydrodynamic jets in isolated haloes. While the jet propagation converges only after the diameter of the jet is well resolved, reliable predictions about the effects these jets have on the cooling time distribution function only require resolutions sufficient to keep the jet-inflated cavities stable. Comparing different model variations, as well as an independent jet model using a different hydrodynamics code, we show that the dominant uncertainties are the choices of jet properties within a given model. Independent of implementation, we find that light, thermal jets with low momentum flux tend to delay the onset of a cooling flow more efficiently on a 50 Myr time-scale than heavy, kinetic jets. The delay of the cooling flow originates from a displacement and boost in entropy of the central gas. If the jet kinetic luminosity depends on accretion rate, collimated, light, hydrodynamic jets are able to reduce cooling flows in haloes, without a need for jet precession or wide opening angles. Comparing the jet feedback with a ‘kinetic wind’ implementation shows that equal amounts of star formation rate reduction can be achieved by different interactions with the halo gas: the jet has a larger effect on the hot halo gas while leaving the denser, star-forming phase in place, while the wind acts more locally on the star-forming phase, which manifests itself in different time-variability properties.

Mining mini-halos with MeerKAT I. Calibration and imaging

ABSTRACT Radio mini-halos are clouds of diffuse, low-surface brightness synchrotron emission that surround the Brightest Cluster Galaxy (BCG) in massive cool-core galaxy clusters. In this paper, we use third generation calibration (3GC), also called direction-dependent (DD) calibration, and point source subtraction on MeerKAT extragalactic continuum data. We calibrate and image archival MeerKAT L-band observations of a sample of five galaxy clusters (ACO 1413, ACO 1795, ACO 3444, MACS J1115.8+0129, MACS J2140.2-2339). We use the CARACal pipeline for direction-independent (DI) calibration, DDFacet and killMS for 3GC, followed by visibility-plane point source subtraction to image the underlying mini-halo without bias from any embedded sources. Our 3GC process shows a drastic improvement in artefact removal, to the extent that the local noise around severely affected sources was halved and ultimately resulted in a 7 per cent improvement in global image noise. Thereafter, using these spectrally deconvolved Stokes I continuum images, we directly measure for four mini-halos the flux density, radio power, size, and in-band integrated spectra. Further to that, we show the in-band spectral index maps of the mini-halo (with point sources). We present a new mini-halo detection hosted by MACS J2140.2-2339, having flux density $S_{\rm 1.28\, GHz} = 2.61 \pm 0.31$ mJy, average diameter 296 kpc, and $\alpha ^{\rm 1.5\, GHz}_{\rm 1\, GHz} = 1.21 \pm 0.36$. We also found a ∼100 kpc southern extension to the ACO 3444 mini-halo which was not detected in previous VLA L-band observations. Our description of MeerKAT wide-field, wide-band data reduction will be instructive for conducting further mini-halo science.

Sub-kpc radio jets in the brightest central galaxy of the cool-core galaxy cluster RXJ1720.1+2638

ABSTRACT The cool-core galaxy cluster RXJ1720.1+2638 hosts extended radio emission near the cluster core, known as a minihalo. The origin of this emission is still debated and one piece of the puzzle has been the question of whether the supermassive black hole in the brightest central galaxy is actively powering jets. Here, we present high-resolution e-MERLIN observations clearly indicating the presence of sub-kpc jets; this may have implications for the proposed origin of the minihalo emission, providing an ongoing source of relativistic electrons rather than a single burst sometime in the past, as previously assumed in simulations attempting to reproduce observational characteristics of minihalo-hosting systems.

Extended radio emission in the galaxy cluster MS 0735.6+7421 detected with the Karl G. Jansky Very Large Array

ABSTRACT MS 0735.6+7421 (z = 0.216) is a massive cool core galaxy cluster hosting one of the most powerful active galactic nuclei (AGNs) outbursts known. The radio jets of the AGN have carved out an unusually large pair of X-ray cavities, each reaching a diameter of 200 kpc. This makes MS 0735.6+7421 a unique case to investigate active galactic nuclei feedback processes, as well as other cluster astrophysics at radio wavelengths. We present new low radiofrequency observations of MS 0735.6+7421 taken with the Karl G. Jansky Very Large Array (VLA): 5 h of P-band (224–480 MHz) and 5 h of L-band (1–2 GHz) observations, both in C configuration. Our VLA P-band (224–480 MHz) observations reveal the presence of a diffuse radio component reaching a scale of ≈ 900 kpc in the direction of the jets and of ≈ 500 kpc in the direction perpendicular to the jets. This component is centred on the cluster core and has a radio power scaled at 1.4 GHz of P1.4GHz = (4 ± 2) × 1024 W Hz−1. Its properties are consistent with those expected from a radio minihalo as seen in other massive cool core clusters, although it may also be associated with radio plasma that has diffused out of the X-ray cavities, or to a combination of these two hypotheses. Observations at higher spatial resolution are needed to fully characterize the properties and nature of this component. We also suggest that if radio minihaloes originate from jetted activity, we may be witnessing the early stages of this process.

Self-regulated AGN feedback of light jets in cool-core galaxy clusters

ABSTRACT Heating from active galactic nuclei (AGNs) is thought to stabilize cool-core clusters, limiting star formation and cooling flows. We employ radiative magnetohydrodynamic (MHD) simulations to model light AGN jet feedback with different accretion modes (Bondi–Hoyle–Lyttleton and cold accretion) in an idealized Perseus-like cluster. Independent of the probed accretion model, accretion efficiency, jet density and resolution, the cluster self-regulates with central entropies and cooling times consistent with observed cool-core clusters in this non-cosmological setting. We find that increased jet efficiencies lead to more intermittent jet powers and enhanced star formation rates. Our fiducial low-density jets can easily be deflected by orbiting cold gaseous filaments, which redistributes angular momentum and leads to more extended cold gas distributions and isotropic bubble distributions. In comparison to our fiducial low momentum-density jets, high momentum-density jets heat less efficiently and enable the formation of a persistent cold gas disc perpendicular to the jets that is centrally confined. Cavity luminosities measured from our simulations generally reflect the cooling luminosities of the intracluster medium and correspond to averaged jet powers that are relatively insensitive to short periods of low-luminosity jet injection. Cold gas structures in our MHD simulations with low momentum-density jets generally show a variety of morphologies ranging from discy to very extended filamentary structures. In particular, magnetic fields are crucial to inhibit the formation of unrealistically massive cold gas discs by redistributing angular momentum between the hot and cold phases and by fostering the formation of elongated cold filaments that are supported by magnetic pressure.

Velocity structure functions in multiphase turbulence: interpreting kinematics of Hα filaments in cool-core clusters

ABSTRACT The central regions of cool-core galaxy clusters harbour multiphase gas, with gas temperatures ranging from $10$ to $10^7\, \mathrm{K}$. Feedback from active galactic nuclei jets prevents the gas from undergoing a catastrophic cooling flow. However, the exact mechanism of this feedback energy input is unknown, mainly due to the lack of velocity measurements of the hot-phase gas. However, recent observations have measured the velocity structure functions (VSFs) of the cooler molecular (${\sim} 10\, \mathrm{K}$) and Hα filaments (${\sim} 10^4\, \mathrm{K}$) and used them to indirectly estimate the motions of the hot phase. In the first part of this study, we conduct high-resolution (3843–15363 resolution elements) simulations of homogeneous isotropic subsonic turbulence, without radiative cooling. We analyse the second-order velocity structure functions (VSF2) in these simulations and study the effects of varying spatial resolution, the introduction of magnetic fields, and the effect of projection along the line of sight (LOS) on it. In the second part of the study, we analyse high-resolution (7683 resolution elements) idealized simulations of multiphase turbulence in the intracluster medium from the companion study Mohapatra et al. We compare the VSF2 for both the hot ($T\sim 10^7\, \mathrm{K}$) and cold ($T\sim 10^4\, \mathrm{K}$) phases and find that their amplitude depends on the density contrast between the phases. They have similar scaling with separation, but introducing magnetic fields steepens the VSF2 of only the cold phase. We also find that projection along the LOS steepens the VSF2 for the hot phase and mostly flattens it for the cold phase.

A test of Radial Acceleration Relation for the Giles et al Chandra cluster sample

We carry out a test of the radial acceleration relation (RAR) for a sample of 10 dynamically relaxed and cool-core galaxy clusters imaged by the Chandra X-ray telescope, which was studied in Giles et al. For this sample, we observe that the best-fit RAR shows a very tight residual scatter equal to 0.09 dex. We obtain an acceleration scale of $1.59 \times 10^{-9} m/s^2$, which is about an order of magnitude higher than that obtained for galaxies. Furthermore, the best-fit RAR parameters differ from those estimated from some of the previously analyzed cluster samples, which indicates that the acceleration scale found from the RAR could be of an emergent nature, instead of a fundamental universal scale.

Raining in MKW 3 s: A Chandra-MUSE Analysis of X-Ray Cold Filaments around 3CR 318.1

Abstract We present the analysis of X-ray and optical observations of gas filaments observed in the radio source 3CR 318.1, associated with NGC 5920, the brightest cluster galaxy (BCG) of MKW 3 s, a nearby cool core galaxy cluster. This work is one of the first X-ray and optical analyses of filaments in cool core clusters carried out using MUSE observations. We aim at identifying the main excitation processes responsible for the emission arising from these filaments. We complemented the optical VLT/MUSE observations, tracing the colder gas phase, with X-ray Chandra observations of the hotter highly ionized gas phase. Using the MUSE observations, we studied the emission line intensity ratios along the filaments to constrain the physical processes driving the excitation, and, using the Chandra observations, we carried out a spectral analysis of the gas along these filaments. We found a spatial association between the X-ray and optical morphology of these filaments, which are colder and have lower metal abundance than the surrounding intracluster medium (ICM), as already seen in other BCGs. Comparing with previous results from the literature for other BCGs, we propose that the excitation process that is most likely responsible for these filaments emission is a combination of star formation and shocks, with a likely contribution from self-ionizing, cooling ICM. Additionally, we conclude that the filaments most likely originated from AGN-driven outflows in the direction of the radio jet.

A detailed study of X-ray cavities in the intracluster environment of the cool core cluster Abell 3017

ABSTRACT We present a detailed analysis of a cool-core galaxy cluster Abell 3017, at a redshift of z = 0.219, which has been identified to be merging with its companion cluster Abell 3016. This study has made use of X-ray (Chandra), ultraviolet (UV) [Galaxy Evolution Explorer(GALEX)], optical [European Southern Observatory (ESO)/very large telescope (VLT)], mid-infrared [(Wide-field Infrared Survey Explorer (WISE)], and radio upgraded Giant Metrewave radio telescope (uGMRT) observations of this cluster. Using various image processing techniques, such as unsharp masking, 2D fits using β models, contour binning and the use of surface brightness profiles, we show the existence of a pair of X-ray cavities, at a projected distance of ∼20 arcsec (70 kpc) and ∼16 arcsec (57 kpc), respectively, from the core of Abell 3017. We also detect an excess of X-ray emission located at ∼25 arcsec (88 kpc) south of the centre of Abell 3017, is likely due to the bulk motions in the intracluster medium either by gas sloshing or ram-pressure striping due to a merger. We find that the radio lobes are responsible for the observed X-ray cavities detected in this system. The lower values of mid-infrared WISE colour [W1–W2] and [W2–W3] imply that the central BCG of Abell 3017 is a star-forming galaxy. The current star formation rate of the central BCG, estimated from the H α and GALEX far-ultraviolet (FUV) luminosities, are equal to be ∼5.06 ± 0.78 and ∼9.20 ± 0.81 $\rm M_{\odot }$ yr−1, respectively. We detect, for the first time, a radio phoenix ∼150 kpc away from the radio core, with a spectral index of (α ≤ −1.8). We also report the detection of $\rm ~Pa\,\alpha$ emission in this cluster using ESO VLT SINFONI imaging data.

Tests of AGN Feedback Kernels in Simulated Galaxy Clusters

Abstract In cool-core galaxy clusters with central cooling times much shorter than a Hubble time, condensation of the ambient central gas is regulated by a heating mechanism, probably an active galactic nucleus. Previous analytical work has suggested that certain radial distributions of heat input may result in convergence to a quasi-steady global state that does not substantively change on the timescale for radiative cooling, even if the heating and cooling are not locally in balance. To test this hypothesis, we simulate idealized galaxy cluster halos using the ENZO code with an idealized, spherically symmetric heat input kernel intended to emulate. Thermal energy is distributed with radius according to a range of kernels, in which total heating is updated to match total cooling every 10 Myr. Some heating kernels can maintain quasi-steady global configurations, but no kernel we tested produces a quasi-steady state with central entropy as low as those observed in cool-core clusters. The general behavior of the simulations depends on the proportion of heating in the inner 10 kpc, with low central heating leading to central cooling catastrophes, high central heating creating a central convective zone with an inverted entropy gradient, and intermediate central heating resulting in a flat central entropy profile that exceeds observations. The timescale on which our simulated halos fall into an unsteady multiphase state is proportional to the square of the cooling time of the lowest-entropy gas, allowing more centrally concentrated heating to maintain a longer-lasting steady state.

Propagation of weak shocks in cool-core galaxy clusters in two-temperature magnetohydrodynamics with anisotropic thermal conduction

ABSTRACT We investigate how different magnetohydrodynamic models of propagation of a weak (Mach number ∼1.2) shock in the core of a galaxy cluster affect its observational appearance, using the Perseus cluster as our fiducial model. In particular, we study how thermal conduction, both isotropic and anisotropic, and ion–electron temperature equilibration modify a weak shock. Strong thermal conduction is expected to produce an electron temperature precursor. Less prominent pressure and density precursors are formed as well. A longer equilibration time largely reduces the density precursor but does not change the electron temperature precursor much. When thermal conduction becomes anisotropic, the intracluster magnetic field imprints its characteristic spatial scale on the distortions of the shock induced by heat fluxes.

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.