arXiv subjects

William R. Forman

Publications and source records attributed to William R. Forman.

At least 55 records · Page 3Linked to original sources

arXiv2018

The Recent Growth History of the Fornax Cluster Derived from Simultaneous Sloshing and Gas Stripping: Simulating the Infall of NGC 1404

We derive the recent growth history of the Fornax Cluster, in particular the recent infall of the giant elliptical galaxy NGC 1404. We show, using a simple cluster minor merger simulation tailored to Fornax and NGC 1404, that a second or more likely third encounter between the two reproduces all main merger features observed in both objects; we firmly exclude a first infall scenario. Our simulations reveal a consistent picture: NGC 1404 passed by NGC 1399 about 1.1 - 1.3 Gyrs ago from the NE to the SW and is now almost at the point of its next encounter from the S. This scenario explains the sloshing patterns observed in Fornax - a prominent northern cold front and an inner southern cold front. This scenario also explains the truncated atmosphere, the gas stripping radius of NGC 1404, and its faint gas tail. Independent of the exact history, we can make a number of predictions. A detached bow shock south of NGC 1404 should exist which is a remnant of the galaxy's previous infall at a distance from NGC 1404 between 450 - 750 kpc with an estimated Mach number between 1.3 and 1.5. The wake of NGC 1404 also lies S of the galaxy with enhanced turbulence and a slight enhancement in metallicity compared to the undisturbed regions of the cluster. SW of NGC 1404, there is likely evidence of old turbulence originating from the previous infall. No scenario predicts enhanced turbulence outside of the cold front north west of the cluster center.

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arXiv2018

A Galaxy-Scale Fountain of Cold Molecular Gas Pumped by a Black Hole

We present ALMA and MUSE observations of the Brightest Cluster Galaxy in Abell 2597, a nearby (z=0.0821) cool core cluster of galaxies. The data map the kinematics of a three billion solar mass filamentary nebula that spans the innermost 30 kpc of the galaxy's core. Its warm ionized and cold molecular components are both cospatial and comoving, consistent with the hypothesis that the optical nebula traces the warm envelopes of many cold molecular clouds that drift in the velocity field of the hot X-ray atmosphere. The clouds are not in dynamical equilibrium, and instead show evidence for inflow toward the central supermassive black hole, outflow along the jets it launches, and uplift by the buoyant hot bubbles those jets inflate. The entire scenario is therefore consistent with a galaxy-spanning "fountain", wherein cold gas clouds drain into the black hole accretion reservoir, powering jets and bubbles that uplift a cooling plume of low-entropy multiphase gas, which may stimulate additional cooling and accretion as part of a self-regulating feedback loop. All velocities are below the escape speed from the galaxy, and so these clouds should rain back toward the galaxy center from which they came, keeping the fountain long-lived. The data are consistent with major predictions of chaotic cold accretion, precipitation, and stimulated feedback models, and may trace processes fundamental to galaxy evolution at effectively all mass scales.

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arXiv2018

Deep Chandra observations of X-ray point sources in M87

We present a study of X-ray source populations in M87, the cD galaxy of the Virgo cluster, using 12 archival {\it Chandra} observations with a total exposure of $\sim$680 ks spanning about a decade. A total of 346 point-like sources are detected down to a limiting 0.5--8 keV luminosity of $4\times10^{37}{\rm~erg~s^{-1}}$ and out to a galactocentric radius of $\sim$40 kpc. We cross-correlate the X-ray sources with published catalogs of globular clusters (GCs), derived from the ACS Virgo Cluster Survey and the Next Generation Virgo Cluster Survey. This results in 122 matches, making it one of the largest samples of GC-hosting X-ray sources in an external galaxy. These sources, most likely low-mass X-ray binaries (LMXBs), correspond to $\sim$5\% of all known GCs within the {\it Chandra} field-of-view. Conversely, $\sim$50\% of the detected X-ray sources are found in a GC. Moreover, red (metal-rich) GCs are $\sim$2.2 times more likely to host an X-ray source than blue (metal-poor) GCs. We also examine 76 currently known ultra-compact dwarf galaxies around M87, but find no significant X-ray counterparts. After statistically accounting for the cosmic X-ray background, we identify $\sim$110 field-LMXBs. The GC-LMXBs and field-LMXBs differ in their luminosity function and radial distribution, which indicates that the latter cannot be primarily originated from GCs. Using another set of deep {\it Chandra} observations toward $\sim$100 kpc northwest of the M87 center, we statistically constrain the abundance of field-LMXBs in the stellar halo, which is consistent with that found in the central region. We also identify 40 variable X-ray sources, among which one source is likely a black hole binary residing in a GC.

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arXiv2017

Probing the Hot X-ray Corona around the Massive Spiral Galaxy, NGC 6753, Using Deep XMM-Newton observations

X-ray emitting gaseous coronae around massive galaxies are a basic prediction of galaxy formation models. Although the coronae around spiral galaxies offer a fundamental test of these models, observational constraints on their characteristics are still scarce. While the presence of extended hot coronae has been established around a handful of massive spiral galaxies, the short X-ray observations only allowed for measurements of the basic characteristics of the coronae. In this work, we utilize deep XMM-Newton observations of NGC 6753 to explore its extended X-ray corona in unprecedented detail. Specifically, we establish the isotropic morphology of the hot gas, suggesting that it resides in hydrostatic equilibrium. The temperature profile of the gas shows a decrease with increasing radius: it drops from $kT\approx0.7$ keV in the innermost parts to $kT\approx0.4$ keV at 50 kpc radius. The temperature map reveals the complex temperature structure of the gas. We study the metallicity distribution of the gas, which is uniform at $Z\approx0.1$ Solar. This value is about an order of magnitude lower than that obtained for elliptical galaxies with similar dark matter halo mass, hinting that the hot gas in spiral galaxies predominantly originates from external gas inflows rather than from internal sources. By extrapolating the density profile of the hot gas out to the virial radius, we estimate the total gas mass and derive the total baryon mass of NGC 6753. We conclude that the baryon mass fraction is $f_{\rm b} \approx 0.06$, implying that about half of the baryons are missing.

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arXiv2017

Gas sloshing regulates and records the evolution of the Fornax Cluster

We present results of a joint {\sl Chandra} and {\sl XMM-Newton} analysis of the Fornax Cluster, the nearest galaxy cluster in the southern sky. Signatures of merger-induced gas sloshing can be seen in the X-ray image. We identify four sloshing cold fronts in the intracluster medium, residing at radii of 3\,kpc (west), 10\,kpc (northeast), 30\,kpc (southwest) and 200\,kpc (east). Despite spanning over two orders of magnitude in radius, all four cold fronts fall onto the same spiral pattern that wraps around the BCG NGC~1399, likely all initiated by the infall of NGC~1404. The most evident front is to the northeast, 10\,kpc from the cluster center, which separates low-entropy high-metallicity gas and high-entropy low-metallicity gas. The metallicity map suggests that gas sloshing, rather than an AGN outburst, is the driving force behind the redistribution of the enriched gas in this cluster. The innermost cold front resides within the radius of the strong cool core. The sloshing time scale within the cooling radius, calculated from the Brunt-V$\ddot{\rm a}$s$\ddot{\rm a}$l$\ddot{\rm a}$ frequency, is an order of magnitude shorter than the cooling time. It is plausible that gas sloshing can contribute to the heating of the cool core, provided that gas of different entropies can be mixed effectively via Kelvin-Helmholtz instability. The estimated age of the outermost front suggests that this is not the first infall of NGC~1404.

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arXiv2017

Buoyant AGN bubbles in the quasi-isothermal potential of NGC 1399

The Fornax Cluster is a low-mass cool-core galaxy cluster. We present a deep {\sl Chandra} study of NGC 1399, the central dominant elliptical galaxy of Fornax. The cluster center harbors two symmetric X-ray cavities coincident with a pair of radio lobes fed by two collimated jets along a north-south axis. A temperature map reveals that the AGN outburst has created a channel filled with cooler gas out to a radius of 10 kpc. The cavities are surrounded by cool bright rims and filaments that may have been lifted from smaller radii by the buoyant bubbles. X-ray imaging suggests a potential ghost bubble of $\gtrsim$ 5\,kpc diameter to the northwest. We find that the amount of gas lifted by AGN bubbles is comparable to that which would otherwise cool, demonstrating that AGN driven outflow is effective in offsetting cooling in low-mass clusters. The cluster cooling time scale is $>30$ times longer than the dynamical time scale, which is consistent with the lack of cold molecular gas at the cluster center. The X-ray hydrostatic mass is consistent within 10\% with the total mass derived from the optical data. The observed entropy profile rises linearly, following a steeper slope than that observed at the centers of massive clusters; gas shed by stars in NGC 1399 may be incorporated in the hot phase. However, it is far-fetched for supernova-driven outflow to produce and maintain the thermal distribution in NGC 1399 and it is in tension with the metal content in the hot gas.

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arXiv2017

Probing the hot X-ray gas in the narrow-line region of Mrk 3

We study the prototypical Seyfert 2 galaxy, Markarian 3, based on imaging and high-resolution spectroscopy observations taken by the Chandra X-ray Observatory. We construct a deconvolved X-ray image, which reveals the S-shaped morphology of the hot gas in the narrow line region (NLR). While this morphology is similar to the radio and [O III] emission, the distribution of the X-ray gas is broader than that obtained at these other wavelengths. By mapping the density and temperature distribution of the hot gas in the NLR, we demonstrate the presence of shocks towards the west ($M=2.5^{+1.0}_{-0.6}$) and east ($M=1.5^{+1.0}_{-0.5}$). Moreover, we compute the flux ratios between the [O III] and $0.5-2$ keV band X-ray luminosity and show that it is non-uniform in the NLR with the western side of the NLR being more highly ionized. In addition, based on the Chandra grating data we investigate the line ratios of the Si XIII triplet, which are not consistent with pure photoionization. Based on these results, we suggest that in the NLR of Mrk 3 both photoionization and collisional ionization act as excitation mechanisms. We conclude that the canonical picture, in which photoionization is solely responsible for exciting the interstellar medium in the NLR of Seyfert galaxies, may be overly simplistic. Given that weak and small-scale radio jets are commonly detected in Seyfert galaxies, it is possible that shock heating plays a non-negligible role in the NLR of these galaxies.

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arXiv2017

The X-ray and Mid-Infrared luminosities in Luminous Type 1 Quasars

Several recent studies have reported different intrinsic correlations between the AGN mid-IR luminosity ($L_{MIR}$) and the rest-frame 2-10 keV luminosity ($L_{X}$) for luminous quasars. To understand the origin of the difference in the observed $L_{X}-L_{MIR}$ relations, we study a sample of 3,247 spectroscopically confirmed type 1 AGNs collected from Boötes, XMM-COSMOS, XMM-XXL-North, and the SDSS quasars in the Swift/XRT footprint spanning over four orders of magnitude in luminosity. We carefully examine how different observational constraints impact the observed $L_{X}-L_{MIR}$ relations, including the inclusion of X-ray non-detected objects, possible X-ray absorption in type 1 AGNs, X-ray flux limits, and star formation contamination. We find that the primary factor driving the different $L_{X}-L_{MIR}$ relations reported in the literature is the X-ray flux limits for different studies. When taking these effects into account, we find that the X-ray luminosity and mid-IR luminosity (measured at rest-frame $6μm$, or $L_{6μm}$) of our sample of type 1 AGNs follow a bilinear relation in the log-log plane: $\log L_X =(0.84\pm0.03)\times\log L_{6μm}/10^{45}{\rm erg\;s^{-1}} + (44.60\pm0.01)$ for $L_{6μm} < 10^{44.79}{\rm erg\;s^{-1}} $, and $\log L_X = (0.40\pm0.03)\times\log L_{6μm}/10^{45}{\rm erg\;s^{-1}} +(44.51\pm0.01)$ for $L_{6μm} \geq 10^{44.79}{\rm erg\;s^{-1}} $. This suggests that the luminous type 1 quasars have a shallower $L_{X}-L_{MIR}$ correlation than the approximately linear relations found in local Seyfert galaxies. This result is consistent with previous studies reporting a luminosity-dependent $L_{X}-L_{MIR}$ relation, and implies that assuming a linear $L_{X}-L_{MIR}$ relation to infer the neutral gas column density for X-ray absorption might overestimate the column densities in luminous quasars.

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arXiv2017

X-Ray morphological analysis of the Planck ESZ clusters

X-ray observations show that galaxy clusters have a very large range of morphologies. The most disturbed systems which are good to study how clusters form and grow and to test physical models, may potentially complicate cosmological studies because the cluster mass determination becomes more challenging. Thus, we need to understand the cluster properties of our samples to reduce possible biases. This is complicated by the fact that different experiments may detect different cluster populations. For example, SZ selected cluster samples have been found to include a greater fraction of disturbed systems than X-ray selected samples. In this paper we determined eight morphological parameters for the Planck Early Sunyaev-Zeldovich (ESZ) objects observed with XMM-Newton. We found that two parameters, concentration and centroid-shift, are the best to distinguish between relaxed and disturbed systems. For each parameter we provide the values that allow one to select the most relaxed or most disturbed objects from a sample. We found that there is no mass dependence on the cluster dynamical state. By comparing our results with what was obtained with REXCESS clusters, we also confirm that indeed the ESZ clusters tend to be more disturbed, as found by previous studies.

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arXiv2017

The fraction of cool-core clusters in X-ray vs. SZ samples using Chandra observations

We derive and compare the fractions of cool-core clusters in the {\em Planck} Early Sunyaev-Zel'dovich sample of 164 clusters with $z \leq 0.35$ and in a flux-limited X-ray sample of 100 clusters with $z \leq 0.30$, using {\em Chandra} observations. We use four metrics to identify cool-core clusters: 1) the concentration parameter: the ratio of the integrated emissivity profile within 0.15 $r_{500}$ to that within $r_{500}$, and 2) the ratio of the integrated emissivity profile within 40 kpc to that within 400 kpc, 3) the cuspiness of the gas density profile: the negative of the logarithmic derivative of the gas density with respect to the radius, measured at 0.04 $r_{500}$, and 4) the central gas density, measured at 0.01 $r_{500}$. We find that the sample of X-ray selected clusters, as characterized by each of these metrics, contains a significantly larger fraction of cool-core clusters compared to the sample of SZ selected clusters (44$\pm$7\% vs. 28$\pm$4\% using the concentration parameter in the 0.15--1.0 $r_{500}$ range, 61$\pm$8\% vs. 36$\pm$5\% using the concentration parameter in the 40--400 kpc range, 64$\pm$8\% vs. 38$\pm$5\% using the cuspiness, and 53$\pm$7\% vs. 39$\pm$5\% using the central gas density). Qualitatively, cool-core clusters are more X-ray luminous at fixed mass. Hence, our X-ray flux-limited sample, compared to the approximately mass-limited SZ sample, is over-represented with cool-core clusters. We describe a simple quantitative model that uses the excess luminosity of cool-core clusters compared to non-cool-core clusters at fixed mass to successfully predict the observed fraction of cool-core clusters in X-ray selected samples.

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arXiv2017

Galaxy-scale Bars in Late-type Sloan Digital Sky Survey Galaxies Do Not Influence the Average Accretion Rates of Supermassive Black Holes

Galaxy-scale bars are expected to provide an effective means for driving material towards the central region in spiral galaxies, and possibly feeding supermassive black holes (BHs). Here we present a statistically-complete study of the effect of bars on average BH accretion. From a well-selected sample of 50,794 spiral galaxies (with M* ~ 0.2-30 x 10^10 Msun) extracted from the Sloan Digital Sky Survey Galaxy Zoo 2 project, we separate those sources considered to contain galaxy-scale bars from those that do not. Using archival data taken by the Chandra X-ray Observatory, we identify X-ray luminous (L_X >~ 10^41 erg/s) active galactic nuclei (AGN) and perform an X-ray stacking analysis on the remaining X-ray undetected sources. Through X-ray stacking, we derive a time-averaged look at accretion for galaxies at fixed stellar mass and star formation rate, finding that the average nuclear accretion rates of galaxies with bar structures are fully consistent with those lacking bars (Mdot_acc ~ 3 x 10^-5 Msun/yr). Hence, we robustly conclude that large-scale bars have little or no effect on the average growth of BHs in nearby (z < 0.15) galaxies over gigayear timescales.

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arXiv2017

The Case for Electron Re-Acceleration at Galaxy Cluster Shocks

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-size radio sources have been found, so-called radio relics. These relics are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach number collisionless shocks generated by cluster-cluster merger events. A long-standing problem is how low-Mach number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here we report on the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.

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arXiv2017

The Double Galaxy Cluster Abell 2465 III. X-ray and Weak-lensing Observations

We report Chandra X-ray observations and optical weak-lensing measurements from Subaru/Suprime-Cam images of the double galaxy cluster Abell 2465 (z=0.245). The X-ray brightness data are fit to a beta-model to obtain the radial gas density profiles of the northeast (NE) and southwest (SW) sub-components, which are seen to differ in structure. We determine core radii, central temperatures, the gas masses within $r_{500c}$, and the total masses for the broader NE and sharper SW components assuming hydrostatic equilibrium. The central entropy of the NE clump is about two times higher than the SW. Along with its structural properties, this suggests that it has undergone merging on its own. The weak-lensing analysis gives virial masses for each substructure, which compare well with earlier dynamical results. The derived outer mass contours of the SW sub-component from weak lensing are more irregular and extended than those of the NE. Although there is a weak enhancement and small offsets between X-ray gas and mass centers from weak lensing, the lack of large amounts of gas between the two sub-clusters indicates that Abell 2465 is in a pre-merger state. A dynamical model that is consistent with the observed cluster data, based on the FLASH program and the radial infall model, is constructed, where the subclusters currently separated by ~1.2Mpc are approaching each other at ~2000km/s and will meet in ~0.4Gyr.

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arXiv2016

Relics in galaxy clusters at high radio frequencies

We observed three radio relics in galaxy clusters and one radio relic candidate at 4.85 and 8.35 GHz in total emission and linearly polarized emission with the Effelsberg 100-m telescope and one radio relic candidate in X-rays with the Chandra telescope. The radio spectra of the integrated emission below 8.35 GHz can be well fitted by single power laws for all four relics. The flat spectra (spectral indices of 0.9 and 1.0) for the "Sausage" relic in cluster CIZA J2242+53 and the "Toothbrush" relic in cluster 1RXS 06+42 indicate that models describing the origin of relics have to include effects beyond the assumptions of diffuse shock acceleration. The spectra of the radio relics in ZwCl 0008+52 and in Abell 1612 are steep, as expected from weak shocks (Mach number $\approx 2.4$). We find polarization degrees of more than 50 % in the two prominent Mpc-sized radio relics, the Sausage and the Toothbrush. The high degree of polarization indicates that the magnetic field vectors are almost perfectly aligned along the relic structure. The polarization degrees correspond to Mach numbers of $>2.2$. Polarized emission is also detected in the radio relics in ZwCl 0008+52 and, for the first time, in Abell 1612. Abell 1612 shows a complex X-ray surface brightness distribution, indicating a recent major merger. No Faraday depolarization is detected between 4.85 GHz and 8.35 GHz, except for one component of the Toothbrush relic. Faraday depolarization between 1.38 GHz and 8.35 GHz varies with distance from the center of the host cluster 1RXS 06+42, which can be explained by a decrease in electron density and/or in strength of a turbulent magnetic field. Faraday rotation measures show large-scale gradients along the relics, which cannot be explained by variations in the Milky Way foreground. Large-scale regular fields appear to be present in intergalactic space around galaxy clusters.

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arXiv2016

Deep Chandra observations of NGC 1404: cluster plasma physics revealed by an infalling early-type galaxy

The intracluster medium (ICM), as a magnetized and highly ionized fluid, provides an ideal laboratory to study plasma physics under extreme conditions that cannot yet be achieved on Earth. NGC 1404 is a bright elliptical galaxy that is being gas stripped as it falls through the ICM of the Fornax Cluster. We use the new {\sl Chandra} X-ray observations of NGC 1404 to study ICM microphysics. The interstellar medium (ISM) of NGC 1404 is characterized by a sharp leading edge, 8 kpc from the galaxy center, and a short downstream gaseous tail. Contact discontinuities are resolved on unprecedented spatial scales ($0\farcs5=45$\,pc) due to the combination of the proximity of NGC 1404, the superb spatial resolution of {\sl Chandra}, and the very deep (670 ksec) exposure. At the leading edge, we observe sub-kpc scale eddies generated by Kelvin-Helmholtz instability and put an upper limit of 5\% Spitzer on the isotropic viscosity of the hot cluster plasma. We also observe mixing between the hot cluster gas and the cooler galaxy gas in the downstream stripped tail, which provides further evidence of a low viscosity plasma. The assumed ordered magnetic fields in the ICM ought to be smaller than 5\,$μ$G to allow KHI to develop. The lack of evident magnetic draping layer just outside the contact edge is consistent with such an upper limit.

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arXiv2016

Capturing the 3D Motion of an Infalling Galaxy via Fluid Dynamics

The Fornax Cluster is the nearest galaxy cluster in the southern sky. NGC 1404 is a bright elliptical galaxy falling through the intracluster medium of the Fornax Cluster. The sharp leading edge of NGC 1404 forms a classical "cold front" that separates 0.6 keV dense interstellar medium and 1.5 keV diffuse intracluster medium. We measure the angular pressure variation along the cold front using a very deep (670\,ksec) {\sl Chandra} X-ray observation. We are taking the classical approach -- using stagnation pressure to determine a substructure's speed -- to the next level by not only deriving a general speed but also directionality which yields the complete velocity field as well as the distance of the substructure directly from the pressure distribution. We find a hydrodynamic model consistent with the pressure jump along NGC 1404's atmosphere measured in multiple directions. The best-fit model gives an inclination of 33$^{\circ}$ and a Mach number of 1.3 for the infall of NGC 1404, in agreement with complementary measurements of the motion of NGC 1404. Our study demonstrates the successful treatment of a highly ionized ICM as ideal fluid flow, in support of the hypothesis that magnetic pressure is not dynamically important over most of the virial region of galaxy clusters.

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arXiv2016

A Spectacular Bow Shock in the 11 keV Galaxy Cluster Around 3C 438

We present results of deep 153 ks Chandra observations of the hot, 11 keV, galaxy cluster associated with the radio galaxy 3C 438. By mapping the morphology of the hot gas and analyzing its surface brightness and temperature distributions, we demonstrate the presence of a merger bow shock. We identify the presence of two jumps in surface brightness and in density located at $\sim$400 kpc and $\sim$800 kpc from the cluster's core. At the position of the inner jump, we detect a factor of $2.3\pm 0.2$ density jump, while at the location of the outer jump, we detect a density drop of a factor of $3.5 \pm 0.7$. Combining this with the temperature distribution within the cluster, we establish that the pressure of the hot gas is continuous at the 400 kpc jump, while there is a factor of $6.2 \pm 2.8$ pressure discontinuity at 800 kpc jump. From the magnitude of the outer pressure discontinuity, using the Rankine-Hugoniot jump conditions, we determine that the sub-cluster is moving at $M = 2.3\pm 0.5$, or approximately $2600\pm 565$ km/s through the surrounding intracluster medium, creating the conditions for a bow shock. Based on these findings, we conclude that the pressure discontinuity is likely the result of an ongoing major merger between two massive clusters. Since few observations of bow shocks in clusters have been made, this detection can contribute to the study of the dynamics of cluster mergers, which offers insight on how the most massive clusters may have formed.

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arXiv2016

Binary Black Holes, Gas Sloshing, and Cold Fronts in the X-ray Halo Hosting 4C+37.11

We analyzed deep $Chandra$ ACIS-I exposures of the cluster-scale X-ray halo surrounding the radio source 4C+37.11. This remarkable system hosts the closest resolved pair of super-massive black hole and an exceptionally luminous elliptical galaxy, the likely product of a series of past mergers. We characterize the halo with $r_{500} = 0.95$ Mpc, $M_{500} = (2.5 \pm 0.2) \times 10^{14} \ M_{\rm{\odot}}$, $ kT = 4.6\pm 0.2$ keV, and a gas mass of $M_{\rm g,500} = (2.2 \pm 0.1) \times 10^{13} M_\odot$. The gas mass fraction within $r_{500}$ is $f_{\rm g} = 0.09 \pm 0.01$. The entropy profile shows large non-gravitational heating in the central regions. We see several surface brightness jumps, associated with substantial temperature and density changes, but approximate pressure equilibrium, implying that these are sloshing structures driven by a recent merger. A residual intensity image shows core spiral structure closely matching that seen for the Perseus cluster, although at $z=0.055$ the spiral pattern is less distinct. We infer the most recent merger occurred $1-2$ Gyr ago and that the event that brought the two observed super-massive black holes to the system core is even older. Under that interpretation, this black hole binary pair has, unusually, remained at pc-scale separation for more than 2 Gyr.

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