Intracluster Medium In Clusters Research Articles

The eROSITA view of the Abell 3391/95 field: Case study from the Magneticum cosmological simulation

Context.Clusters of galaxies reside at the nodes of the cosmic web, interconnected by filamentary structures that contain tenuous diffuse gas, especially in the warm-hot phase. Galaxy clusters grow by mergers of smaller objects and gas that are mainly accreted through these large-scale filaments. For the first time, the large-scale cosmic structure and a long gas-emission filament have been captured by eROSITA on board the Spectrum-Roentgen-Gamma mission in a direct X-ray observation of the A3391/95 field.Aims.We investigate the assembly history of an A3391/95-like system of clusters and the thermo-chemical properties of the diffuse gas in it by connecting simulation predictions to the eROSITA observations with the aim to constrain the origin and nature of the gas in the pair-interconnecting bridge.Methods.We analysed the properties of a system resembling A3391/95, extracted from the (352h−1cMpc)3volume of the Magneticum Pathfinder cosmological simulations atz= 0.07. We tracked the main progenitors of the pair clusters and of surrounding groups back in time to study the assembly history of the system and its evolution.Results.Similarly to the observed A3391/95 system, the simulated cluster pair is embedded in a complex network of gas filaments, with structures aligned over more than 20 projected Mpc, and the whole region collapses towards the central overdense node. The spheres of influence (3 ×R200) of the two main clusters already overlap atz= 0.07, but their virial boundaries are still physically separated. The diffuse gas located in the interconnecting bridge closely reflects the warm-hot intergalactic medium, with a typical temperature of ~1 keV and an overdensityδ ~100 with respect to the mean baryon density of the Universe, and a lower enrichment level compared to the intra-cluster medium in clusters. We find that most of the bridge gas collapsed from directions roughly orthogonal to the intra-cluster gas accretion directions, and its origin is mostly unrelated to the two cluster progenitors. We find clear signatures in the surrounding groups of infall motion towards the pair, such as significant radial velocities and a slowdown of gas compared to dark matter. These findings further support the hypothesis that the Northern Clump (MCXC J0621.7-5242) cluster infalls along a cosmic gas filament towards Abell 3391 and might be merging with it.Conclusions.We conclude that in this configuration, the pair clusters of the A3391/95-like system are in a pre-merger phase and have not yet interacted. The diffuse gas in the interconnecting bridge is mostly warm filament gas and not tidally stripped cluster gas.

Detection of a Double Relic in the Torpedo Cluster: SPT-CL J0245-5302

The Torpedo cluster, SPT-CL J0245-5302 (S0295) is a massive, merging cluster at a redshift of z = 0.300, which exhibits a strikingly similar morphology to the Bullet cluster 1E 0657-55.8 (z = 0.296), including a classic bow shock in the cluster's intra-cluster medium revealed by Chandra X-ray observations. We present Australia Telescope Compact Array data centred at 2.1 GHz and Murchison Widefield Array data at frequencies between 72 MHz and 231 MHz which we use to study the properties of the cluster. We characterise a number of discrete and diffuse radio sources in the cluster, including the detection of two previously unknown radio relics on the cluster periphery. The average spectral index of the diffuse emission between 70 MHz and 3.1 GHz is $\alpha=-1.63_{-0.10}^{+0.10}$ and a radio-derived Mach number for the shock in the west of the cluster is calculated as M = 2.04. The Torpedo cluster is thus as a double relic system at moderate redshift.

Connecting optical and X-ray tracers of galaxy cluster relaxation

Substantial effort has been devoted in determining the ideal proxy for quantifying the morphology of the hot intracluster medium in clusters of galaxies. These proxies, based on X-ray emission, typically require expensive, high-quality X-ray observations making them difficult to apply to large surveys of groups and clusters. Here, we compare optical relaxation proxies with X-ray asymmetries and centroid shifts for a sample of SDSS clusters with high-quality, archival X-ray data from Chandra and XMM-Newton. The three optical relaxation measures considered are: the shape of the member-galaxy projected velocity distribution -- measured by the Anderson-Darling (AD) statistic, the stellar mass gap between the most-massive and second-most-massive cluster galaxy, and the offset between the most-massive galaxy (MMG) position and the luminosity-weighted cluster centre. The AD statistic and stellar mass gap correlate significantly with X-ray relaxation proxies, with the AD statistic being the stronger correlator. Conversely, we find no evidence for a correlation between X-ray asymmetry or centroid shift and the MMG offset. High-mass clusters ($M_\mathrm{halo} > 10^{14.5}\,\mathrm{M_\odot}$) in this sample have X-ray asymmetries, centroid shifts, and Anderson-Darling statistics which are systematically larger than for low-mass systems. Finally, considering the dichotomy of Gaussian and non-Gaussian clusters (measured by the AD test), we show that the probability of being a non-Gaussian cluster correlates significantly with X-ray asymmetry but only shows a marginal correlation with centroid shift. These results confirm the shape of the radial velocity distribution as a useful proxy for cluster relaxation, which can then be applied to large redshift surveys lacking extensive X-ray coverage.

Stripped Elliptical Galaxies as Probes of ICM Physics. III. Deep Chandra Observations of NGC 4552: Measuring the Viscosity of the Intracluster Medium

Abstract We present results from a deep (200 ks) Chandra observation of the early-type galaxy NGC 4552 (M89), which is falling into the Virgo cluster. Previous shallower X-ray observations of this galaxy showed a remnant gas core, a tail to the South of the galaxy, and twin “horns” attached to the northern edge of the gas core. In our deeper data, we detect a diffuse, low surface brightness extension to the previously known tail, and measure the temperature structure within the tail. We combine the deep Chandra data with archival XMM-Newton observations to put a strong upper limit on the diffuse emission of the tail out to a large distance (10× the radius of the remnant core) from the galaxy center. In our two previous papers, we presented the results of hydrodynamical simulations of ram pressure stripping specifically for M89 falling into the Virgo cluster and investigated the effect of intracluster medium (ICM) viscosity. In this paper, we compare our deep data with our specifically tailored simulations and conclude that the observed morphology of the stripped tail in NGC 4552 is most similar to the inviscid models. We conclude that, to the extent the transport processes can be simply modeled as a hydrodynamic viscosity, the ICM viscosity is negligible. More generally, any micro-scale description of the transport processes in the high-β plasma of the cluster ICM must be consistent with the efficient mixing observed in the stripped tail on macroscopic scales.

Studying the ICM in clusters of galaxies via surface brightness fluctuations of the cosmic X-ray background

We study the surface brightness fluctuations of the cosmic X-ray background (CXB) using Chandra data of XBOOTES. After masking out resolved sources we compute the power spectrum of fluctuations of the unresolved CXB for angular scales from ~2 arcsec to ~3 deg. The non-trivial large-scale structure (LSS) signal dominates over the shot-noise of unresolved point sources at all scales above ~1 arcmin and is produced mainly by the intracluster medium (ICM) of unresolved clusters and groups of galaxies, as shown in our previous publication. The shot-noise-subtracted power spectrum of CXB fluctuations has a power-law shape with the slope of $\Gamma = 0.96 \pm 0.06$. Its energy spectrum is well described by the redshifted emission spectrum of optically-thin plasma with the best-fit temperature of $T \approx 1.3$ keV and the best-fit redshift of $z \approx 0.40$. They are in good agreement with theoretical expectations based on the X-ray luminosity function and scaling relations of clusters. From these values we estimate the typical mass and luminosity of the objects responsible for CXB fluctuations, $M_{500} \sim 10^{13.6}\,{\rm M}_{\odot}/h$ and $L_{0.5-2.0\,{\rm keV}} \sim 10^{42.5}$ erg/s. On the other hand, the flux-weighted mean temperature and redshift of resolved clusters are $T \approx 2.4$ keV and $z \approx 0.23$, confirming that fluctuations of unresolved CXB are caused by cooler (i.e. less massive) and more distant clusters, as expected. We show that the power spectrum shape is sensitive to the ICM structure all the way to the outskirts, out to $\sim{\rm few}\times R_{500}$. We also look for possible contribution of the warm-hot intergalactic medium (WHIM) to the observed CXB fluctuations. Our results underline the significant diagnostics potential of the CXB fluctuation analysis in studying the ICM structure in clusters.

The amazing diversity in the hot gas content of an X-ray unbiased massive galaxy clusters sample

We aim to determine the intrinsic variety, at a given mass, of the properties of the intracluster medium in clusters of galaxies. This requires a cluster sample selected independently of the intracluster medium content for which reliable masses and subsequent X-ray data can be obtained. We present one such sample, consisting of 34 galaxy clusters selected independently of their X-ray properties in the nearby ($0.050<z<0.135$) Universe and mostly with $14<\log M_{500}/M_\odot \lesssim 14.5$, where masses are dynamically estimated. We collected the available X-ray observations from the archives and then observed the remaining clusters with the low-background Swift X-ray telescope, which is extremely useful for sampling a cluster population expected to have low surface brightness. We found that clusters display a large range (up to a factor 50) in X-ray luminosities within $r_{500}$ at a given mass, whether or not the central emission ($r<0.15 r_{500}$) is excised, unveiling a wider cluster population than seen in Sunayev-Zeldovich surveys or inferred from the population seen in X-ray surveys. The measured dispersion is $0.5$ dex in $L_X$ at a given mass.

Globular cluster interstellar media: ionized and ejected by white dwarfs

UV radiation from white dwarfs can efficiently clear Galactic globular clusters (GCs) of their intra-cluster medium (ICM). This solves the problem of the missing ICM in clusters, which is otherwise expected to build up to easily observable quantities. To show this, we recreate the ionizing flux in 47 Tuc, following randomly generated stars through their AGB, post-AGB and white dwarf evolution. Each white dwarf can ionize all the material injected into the cluster by stellar winds for ~3 Myr of its evolution: ~40 such white dwarfs exist at any point. Every GC's ICM should be ionized. The neutral cloud in M15 should be caused by a temporary overdensity. A pressure-supported ICM will expand over the cluster's tidal radius, where it will be truncated, allowing Jeans escape. The modelled Jeans mass-loss rate approximates the total stellar mass-loss rate, allowing efficient clearing of ICM. Any cluster's ICM mass should equal the mass injected by its stars over the sound-travel time between the cluster core and tidal radius. We predict ~11.3 solar masses of ICM within 47 Tuc, cleared over ~4 Myr, compared to a dynamical timescale of 4.3 Myr. We present a new mass hierarchy, discussing the transition between globular clusters dwarf galaxies.

The flip side of galaxy formation: a combined model of galaxy formation and cluster heating

Only ~10 per cent of baryons in the Universe are in the form of stars, yet most models of luminous structure formation have concentrated on the properties of the luminous stellar matter. Such models are now largely successful at reproducing the observed properties of galaxies, including the galaxy luminosity function and the star formation history of the universe. In this paper we focus on the 'flip side' of galaxy formation and investigate the properties of the material that is not presently locked up in galaxies. This 'by-product' of galaxy formation can be observed as an X-ray emitting plasma [the intracluster medium (ICM)] in groups and clusters. Since much of this material has been processed through galaxies, observations of the ICM represent an orthogonal set of constraints on galaxy formation models. In this paper, we attempt to self-consistently model the formation of galaxies and the heating of the ICM. We set out the challenges for such a combined model and demonstrate a possible means of bringing the model into line with both sets of constraints. In this paper, we present a version of the Durham semi-analytic galaxy formation model GALFORM that allows us to investigate the properties of the ICM. As we would expect on the basis of gravitational scaling arguments, the previous model fails to reproduce even the most basic observed properties of the ICM. We present a simple modification to the model to allow for heat input into the ICM from the active galactic nucleus (AGN) 'radio-mode' feedback. This heating acts to expel gas from the X-ray luminous central regions of the host halo. With this modification, the model reproduces the observed gas mass fractions and luminosity–temperature (L–T) relation of groups and clusters. In contrast to simple 'pre-heating' models of the ICM, the model predicts mildly positive evolution of the L–T relation, particularly at low temperatures. The model is energetically plausible, but seems to exceed the observed heating rates of intermediate-temperature clusters. Introducing the heating process into the model requires changes to a number of model parameters in order to retain a good match to the observed galaxy properties. With the revised parameters, the best-fitting luminosity function is comparable to that presented in Bower et al. The new model makes a fundamental step forward, providing a unified model of galaxy and cluster ICM formation. However, the detailed comparison with the data is not completely satisfactory, and we highlight key areas for improvement.

Thermal Balance in the Intracluster Medium: Is AGN Feedback Necessary?

A variety of physical heating mechanisms are combined with radiative cooling to explore, via one dimensional hydrodynamic simulations, the expected thermal properties of the intracluster medium (ICM) in the context of the cooling flow problem. Energy injection from type Ia supernovae, thermal conduction, and dynamical friction (DF) from orbiting satellite galaxies are considered. The novel feature of this work is the exploration of a wide range of efficiencies of each heating process. While the latter two can provide a substantial amount of energy, neither mechanism operating alone can produce nor maintain an ICM in thermal balance over cosmological timescales, in stark contrast with observations. For simulated clusters with initially isothermal temperature profiles, both mechanisms acting in combination result in long-term thermal balance for a range of ICM temperatures and for central electron densities less than n_e~0.02 cm^-3; at greater densities catastrophic cooling invariably occurs. Furthermore, these heating mechanisms can neither produce nor maintain clusters with a declining temperature profile in the central regions, implying that the observed cooling-core'' clusters, which have such declining temperature profiles, cannot be maintained with these mechanisms alone. Thus, while there appears to be an abundant supply of energy capable of heating the ICM in clusters, it is extremely difficult for the energy deposition to occur in such a way that the ICM remains in thermal balance over cosmological time-scales. These results strongly suggest that a more dynamic heating process such as feedback from a central black hole is required to generate the properties of observed intracluster media. (ABRIDGED)

Smoothed Particle Inference: A Kilo‐Parametric Method for X‐Ray Galaxy Cluster Modeling

We propose an ambitious new method that models the intracluster medium in clusters of galaxies as a set of X-ray emitting smoothed particles of plasma. Each smoothed particle is described by a handful of parameters including temperature, location, size, and elemental abundances. Hundreds to thousands of these particles are used to construct a model cluster of galaxies, with the appropriate complexity estimated from the data quality. This model is then compared iteratively with X-ray data in the form of adaptively binned photon lists via a two-sample likelihood statistic and iterated via Markov Chain Monte Carlo. The complex cluster model is propagated through the X-ray instrument response using direct sampling Monte Carlo methods. Using this approach the method can reproduce many of the features observed in the X-ray emission in a less assumption-dependent way that traditional analyses, and it allows for a more detailed characterization of the density, temperature, and metal abundance structure of clusters. Multi-instrument X-ray analyses and simultaneous X-ray, Sunyaev-Zeldovich (SZ), and lensing analyses are a straight-forward extension of this methodology. Significant challenges still exist in understanding the degeneracy in these models and the statistical noise induced by the complexity of the models.

Energetics of Jet Interactions with the Intracluster Medium

An interpretation of X-ray observations of the (intracluster) gas in galaxy clusters, when combined with significant numerical work in simulating cluster formation, suggests that an additional heating mechanism is required to explain the dynamics and temperature profile in the intracluster medium (ICM). In addition to shock heating, star formation, and supernovae, a likely candidate for a heating mechanism for the ICM comes from the jets of material originating in the cores of active radio galaxies, themselves located in the cores of clusters. These large-scale jets propagate outward from the core of the galaxy into the intracluster medium. In this paper, we discuss the origin and energetics of these jets, the mechanisms of their propagation and energy loss, their constitution, and the likely consequences of their interactions with the intracluster medium in clusters of galaxies.

Observations of magnetic fields in regular and irregular clusters

AbstractThe existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. The most detailed evidence for the presence of cluster magnetic fields comes from radio observations. Magnetic fields can be investigated through the synchrotron emission of cluster‐wide diffuse sources and from studies of the rotation measure of polarized radio galaxies. I will review efforts to measure magnetic field strengths and power spectra and the main issues that have led to our knowledge on magnetic fields in regular and irregular clusters of galaxies. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

FARADAY ROTATION OBSERVATIONS OF MAGNETIC FIELDS IN GALAXY CLUSTERS

The presence of magnetic fields in the intracluster medium in clusters of galaxies has been revealed through several different observational techniques. These fields may be dynamically important in clusters as they will provide additional pressure support to the intracluster medium as well as inhibit transport mechanisms such as thermal conduction. Here, we review the current observational state of Faraday rotation measure studies of the cluster fields. The fields are generally found to be a few to 10 microG in non-cooling core clusters and ordered on scales of 10-20 kpc. Studies of sources at large impact parameters show that the magnetic fields extend from cluster cores to radii of at least 500 kpc. In central regions of cooling core systems the field strengths are often somewhat higher (10-40 microG) and appear to be ordered on smaller scales of a few to 10 kpc. We also review some of the recent work on interpreting Faraday rotation measure observations through theory and numerical simulations. These techniques allow us to build up a much more detailed view of the strength and topology of the fields.

Magnetic fields in clusters of galaxies

An important area of study of cosmic magnetic fields is on the largest scales, those of clusters of galaxies. In the last decade it has become clear that the intra-cluster medium in clusters of galaxies is magnetized and that magnetic fields play a critical role in the cluster formation and evolution. The observational evidence for the existence of cluster magnetic fields is obtained by the diffuse cluster-wide synchrotron radio emission and from rotation measure studies of extragalactic radio sources located within or behind the clusters. A significant breakthrough in the knowledge of the cluster magnetic fields will be reached through the SKA, owing to its capabilities, in particular the deep sensitivity and the polarization purity.

MAGNETIC FIELDS IN CLUSTERS OF GALAXIES

The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. Magnetic fields are investigated in the radio band from studies of the rotation measure of polarized radio galaxies and the synchrotron emission of cluster-wide diffuse sources. Other techniques include X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations. We review the main issues that have led to our knowledge on magnetic fields in clusters of galaxies. Observations show that cluster fields are at the μG level, with values up to tens of μG at the center of cooling core clusters. Estimates obtained from different observational approaches may differ by about an order of magnitude. However, the discrepancy may be alleviated by considering that the magnetic field is not constant throughout the cluster, and shows a complex structure. In particular, the magnetic field intensity declines with the cluster radius with a rough dependence on the thermal gas density. Moreover, cluster magnetic fields are likely to fluctuate over a wide range of spatial scales with values from a few kpc up to hundreds kpc. Important information on the cluster field are obtained by comparing the observational results with the prediction from numerical simulations. The origin of cluster magnetic fields is still debated. They might originate in the early Universe, either before or after the recombination, or they could have been deposited in the intracluster medium by normal galaxies, starburst galaxies, or AGN. In either case, magnetic fields undergo significant amplification during the cluster merger processes.

Infall of the Elliptical Galaxy NGC 1404 into the Fornax Cluster

We use three Chandra observations, totaling 134.3 ks, to constrain the dynamical motion of NGC 1404 falling toward the dominant elliptical NGC 1399 through the Fornax Cluster gas. The surface brightness profile of NGC 1404 shows a sharp edge at ~8 kpc from its center in the direction of NGC 1399, characteristic of jumplike temperature and density discontinuities from ram pressure stripping of the galaxy gas, caused by its motion through the surrounding intracluster medium (ICM). We find that the temperature of the galaxy gas inside the edge is ~2.8 times cooler (kT = 0.55 keV with abundance A = 0.73 Z☉) than the cluster gas (kT = 1.53 keV, A = 0.42 Z☉). We use the shape of the surface brightness profile across the edge to fit the position of the edge, the power-law behavior of NGC 1404's density distribution in the leading direction, and the density discontinuity at the edge. The electron density inside the edge (3.9-4.3) × 10-3 cm-3 depends strongly on the gas abundance, while the density of the ICM (7-8) × 10-4 cm-3 depends strongly on the assumed geometry (relative distance) between NGC 1404 and NGC 1399. The corresponding pressure jump of 1.7-2.1 across the leading edge of the galaxy and the cluster free-stream region implies near sonic motion (Mach number 0.83-1.03) for NGC 1404 with a velocity 531-657 km s-1 relative to the surrounding cluster gas. The inclination angle of the motion, inferred using the relative radial velocity between NGC 1404 and 1399 as representative of that between NGC 1404 and the cluster ICM, is uncomfortably large (40°) given the sharpness of the surface brightness edge, suggesting either a nonzero impact parameter between NGC 1404 and 1399 or that NGC 1399 is also moving radially with respect to the cluster ICM.

AGN Jet Interactions with the Intracluster Medium

Clusters of galaxies contain large ellipticals near their cores. Elliptical galaxies in the centers of these clusters are often found to be the source of large-scale jets that propagate outward into the intracluster medium. These jets are thought to be produced by accretion-powered processes in the active galactic nuclei (AGN) at the centers of some giant ellipticals. In this paper, we discuss the origin of these jets and the likely consequences of their interactions with the intracluster medium in clusters of galaxies.

ChandraObservations of A85: Merger of the South Subcluster

We present an analysis of a highly asymmetric cluster merger from a Chandra observation of A85. The merger shows significant disruption of the less massive subcluster from ram pressure effects. Nevertheless, a core, coincident with the cD galaxy, is observed to persist in the subcluster. We derive dynamical information from the motion of the core through the main cluster's intracluster medium. Multiple derivations of the velocity of the core suggest a Mach number of ≈ 1.4, or v ~ 2150 km s-1, although with substantial uncertainty. We construct a consistent kinematic model for the merger based on this dynamical analysis. As has been found for other such cold fronts, conduction appears to be suppressed across the front. Thermal conduction may be suppressed by a magnetic field with a significant component perpendicular to the subcluster's direction of motion. The effect of the merger interaction in creating and shaping the observed radio sources is also discussed. It appears most likely that the radio source is due to distorted and detached lobes from the subcluster cD galaxy, rather than being a radio halo.

On the supernova heating of the intergalactic medium

We present estimates of the energy input from supernovae (SNe) into the intergalactic medium using (i) recent measurements of Si and Fe abundances in the intracluster medium (ICM) and (ii) self-consistent gasdynamical galaxy formation simulations that include processes of cooling, star formation, SNe feedback, and a multi-phase model of the interstellar medium. We estimate the energy input from observed abundances using two different assumptions: (i) spatial uniformity of metal abundances in the ICM and (ii) radial abundance gradients. We show that these two cases lead to energy input estimates which are different by an order of magnitude, highlighting a need for observational data on large-scale abundance gradients in clusters. Analysis of galaxy formation results and estimates from observed Fe and Si abundances indicates that the SNe energy input can be important for heating of the entire ICM (providing energy of ~1 keV per particle) only if the ICM abundances are uniform and the efficiency of gas heating by SN explosions is close to 100% (implying that all of the initial kinetic energy of the explosion goes into heating of the ICM). We conclude that unless these most favorable conditions are met, SNe alone are unlikely to provide sufficient energy input to heat all of the cluster ICM and may need to be supplemented or even substituted by some other heating process(es). (Abridged)

The Properties of Poor Groups of Galaxies. II. X‐Ray and Optical Comparisons

We use ROSAT PSPC data to study the X-ray properties of a sample of 12 poor groups that have extensive membership information. Diffuse X-ray emission is detected in nine of these groups. In all but one of the X-ray-detected groups, the X-ray emission is centered on a luminous elliptical galaxy. Fits to the surface brightness profiles of the X-ray emission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy and is extended on scales of 20-40 h-1 kpc. The location and extent of this component, combined with its X-ray temperature (~0.7- 0.9 keV) and luminosity (~1041-42 h-2 ergs s-1), favor an origin in the interstellar medium of the central galaxy. Alternatively, the central component may be the result of a large-scale cooling flow. The second X-ray component is detected out to a radius of at least ~100-300 h-1 kpc. This component follows the same relationships found among the X-ray temperature (T), X-ray luminosity (LX), and optical velocity dispersion (σr) of rich clusters. This result suggests that the X-ray-detected groups are low-mass versions of clusters and that the extended gas component can properly be called the intragroup medium, by analogy to the intracluster medium in clusters. The failure to detect an intragroup medium in the three groups with very low velocity dispersions is consistent with their predicted X-ray luminosities and temperatures based on the relationships derived for clusters and X-ray-detected groups. The best-fit value of β derived from the σr-T relationship for groups and clusters is ~0.99 ± 0.08, implying that the galaxies and the hot gas trace the same potential with equal energy per unit mass and that the groups are dynamically relaxed. We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the position angle of the large-scale X-ray emission. This trend is consistent with that found for some rich clusters containing cD galaxies. The alignment of the central galaxy with the extended X-ray emission suggests that the formation and/or evolution of the central galaxy is linked to the shape of the global group potential. One possible scenario is that the central galaxy formed via galaxy-galaxy mergers early in the lifetime of the group and has not been subject to significant dynamical evolution recently.