Central Cluster Galaxies Research Articles

Discovery of Giant Radio Sources from TGSS Alternative Data Release 1: Radio, Optical, and Infrared Properties

Giant radio sources (GRSs) are the single largest astrophysical objects known in the Universe that have grown to megaparsec scales (≥0.7 Mpc). GRSs are much rarer compared with normal-sized radio galaxies. Still, the reason for the formation of their gigantic sizes is under debate. We systematically search for GRSs from the TIFR Giant Metrewave Radio Telescope Sky Survey Alternative Data Release 1 at 150 MHz. We have newly identified 34 GRSs from this study. We have also studied the multiwavelength properties (radio, optical, and infrared) of these GRSs. We have used the likelihood ratio method to identify highly reliable multiwavelength counterparts of GRSs from Pan-STARRS (optical) and Wide-field Infrared Survey Explorer (mid-IR) data. We have classified GRSs based on their accretion mode of the central black holes using optical and mid-IR data. For all sources, we also discuss the principal characteristic parameters (redshift distribution, angular and projected linear size, total integrated radio flux density, spectral index, and radio power). We show the radio evolution track and the location of the GRSs in the P–D diagram. Using a radio–optical luminosity diagram, we identify GRSs in the Fanaroff–Riley classification. Only two GRGs in our sample reside close to the centers of galaxy clusters.

Dark energy survey year 3 results: miscentring calibration and X-ray-richness scaling relations in redMaPPer clusters

ABSTRACT We use Dark Energy Survey Year 3 (DES Y3) clusters with archival XMM–Newton and Chandra X-ray data to assess the centring performance of the redMaPPer cluster finder and to measure key richness observable scaling relations. We find that 10–20 per cent of redMaPPer clusters are miscentred, both when comparing to the X-ray peak position and to the visually identified central cluster galaxy. We find no significant difference in miscentring in bins of low versus high richness or redshift. The dominant reasons for miscentring include masked or missing data and the presence of other bright galaxies in the cluster. For half of the miscentred clusters, the correct central was one of the possible centrals identified by redMaPPer, while for ∼40 per cent of miscentred clusters, the correct central is not a redMaPPer member mostly due to masking. Additionally, we fit scaling relations of X-ray temperature and luminosity with richness. We find a TX–λ scatter of $0.21\pm 0.01$. While the scatter in TX–λ is consistent in redshift bins, we find modestly different slopes, with high-redshift clusters displaying a somewhat shallower relation. Splitting based on richness, we find a marginally larger scatter for our lowest richness bin, 20 < λ < 40. We note that the robustness of the scaling relations at lower richnesses is limited by the unknown selection function, but at λ > 75, we detect nearly all of the clusters falling within existing X-ray pointings. The X-ray properties of detected, serendipitous clusters are generally consistent with those of targeted clusters.

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

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

Dark Energy Survey Year 6 results: Intra-cluster light from redshift 0.2 to 0.5

ABSTRACT Using the full 6 years of imaging data from the Dark Energy Survey, we study the surface brightness profiles of galaxy cluster central galaxies and intra-cluster light. We apply a ‘stacking’ method to over 4000 galaxy clusters identified by the redMaPPer cluster finder in the redshift range of 0.2−0.5. This yields high-signal-to-noise circularly averaged profile measurements of the central galaxy and intra-cluster light out to 1 Mpc from the cluster centre. Using redMaPPer richness as a cluster mass indicator, we find that the brightness of the intra-cluster light has a strong mass dependence throughout the 0.2−0.5 redshift range, and this dependence grows stronger at a larger radius. In terms of redshift evolution, we find some evidence that the central galaxy, as well as the diffuse light within the transition region between the cluster central galaxy and intra-cluster light within 80 kpc from the centre, may be growing over time. At larger radii, more than 80 kpc away from the cluster centre, we do not detect evidence of additional redshift evolution beyond the cluster mass dependence, consistent with the findings from the IllustrisTNG hydrodynamic simulation. We speculate that the major driver of intra-cluster light growth, especially at large radii, is associated with cluster mass growth. Finally, we find that the colour of the cluster central galaxy and intra-cluster light displays a radial gradient that becomes bluer at a larger radius, which is consistent with a stellar stripping and disruption origin of intra-cluster light as suggested by simulation studies.

Characterizing tidal features around galaxies in cosmological simulations

ABSTRACT Tidal features provide signatures of recent mergers and offer a unique insight into the assembly history of galaxies. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will enable an unprecedentedly large survey of tidal features around millions of galaxies. To decipher the contributions of mergers to galaxy evolution it will be necessary to compare the observed tidal features with theoretical predictions. Therefore, we use cosmological hydrodynamical simulations NewHorizon, eagle, IllustrisTNG, and Magneticum to produce LSST-like mock images of z ∼ 0 galaxies (z ∼ 0.2 for NewHorizon) with $M_{\scriptstyle \star ,\text{ 30 pkpc}}\ge 10^{9.5}$ M$_{\scriptstyle \odot }$. We perform a visual classification to identify tidal features and classify their morphology. We find broadly good agreement between the simulations regarding their overall tidal feature fractions: $f_{{\small NewHorizon}}=0.40\pm 0.06$, $f_{{\small EAGLE}}=0.37\pm 0.01$, $f_{{\small TNG}}=0.32\pm 0.01$, and $f_{{\small Magneticum}}=0.32\pm 0.01$, and their specific tidal feature fractions. Furthermore, we find excellent agreement regarding the trends of tidal feature fraction with stellar and halo mass. All simulations agree in predicting that the majority of central galaxies of groups and clusters exhibit at least one tidal feature, while the satellite members rarely show such features. This agreement suggests that gravity is the primary driver of the occurrence of visually identifiable tidal features in cosmological simulations, rather than subgrid physics or hydrodynamics. All predictions can be verified directly with LSST observations.

Noema formIng Cluster survEy (NICE): Discovery of a starbursting galaxy group with a radio-luminous core at z = 3.95

The study of distant galaxy groups and clusters at the peak epoch of star formation is limited by the lack of a statistically and homogeneously selected and spectroscopically confirmed sample. Recent discoveries of concentrated starburst activities in cluster cores have opened a new window to hunt for these structures based on their integrated IR luminosities. Here, we carry out a large NOEMA (NOrthern Extended Millimeter Array) program targeting a statistical sample of infrared-luminous sources associated with overdensities of massive galaxies at z > 2, the Noema formIng Cluster survEy (NICE). We present the first result from the ongoing NICE survey, a compact group at z = 3.95 in the Lockman Hole field (LH-SBC3), confirmed via four massive (M⋆ ≳ 1010.5 M⊙) galaxies detected in the CO(4–3) and [CI](1–0) lines. The four CO-detected members of LH-SBC3 are distributed over a 180 kpc physical scale and the entire structure has an estimated halo mass of ∼1013 M⊙ and total star formation rate of ∼4000 M⊙ yr−1. In addition, the most massive galaxy hosts a radio-loud active galactic nucleus with L1.4 GHz, rest = 3.0 × 1025 W Hz−1. The discovery of LH-SBC3 demonstrates the feasibility of our method to efficiently identify high-z compact groups or cluster cores undergoing formation. The existence of these starbursting cluster cores up to z ∼ 4 provides critical insights into the mass assembly history of the central massive galaxies in clusters.

Lopsided Galaxy Distributions in Massive TNG300 Clusters

Studies of isolated central galaxies and their satellites have shown that the spatial distributions of the satellites are distinctly “lopsided” with respect to the locations of the central galaxies. Here we extend this type of analysis to larger systems by analyzing the lopsidedness of 280 massive galaxy clusters in the IllustrisTNG300 Λ Cold Dark Matter simulation. Using a pairwise clustering statistic, we compute the probability that pairs of cluster galaxies are separated by a given polar angle difference, Δϕ, in the plane of the sky. Relative to the location of the central cluster galaxy, we find a statistically significant excess of galaxy pairs that are located on the same side of the cluster. The lopsidedness of the galaxy distribution is most pronounced for large clusters and for pairs of intrinsically red galaxies. The results summarized here were presented at the 243rd meeting of the American Astronomical Society.

Examining the self-interaction of dark matter through central cluster galaxy offsets

ABSTRACT While collisionless cold dark matter models have been largely successful in explaining a wide range of observational data, some tensions still exist, and it remains possible that dark matter possesses a non-negligible level of self-interactions. In this paper, we investigate a possible observable consequence of self-interacting dark matter: offsets between the central galaxy and the centre of mass of its parent halo. We examine 23 relaxed galaxy clusters in a redshift range of 0.1–0.3 drawn from clusters in the Dark Energy Survey and the Sloan Digital Sky Survey which have archival Chandra X-ray data of sufficient depth for centre and relaxation determination. We find that most clusters in our sample show non-zero offsets between the X-ray centre, taken to be the centroid within the cluster core, and the central galaxy position. All of the measured offsets are larger, typically by an order of magnitude, than the uncertainty in the X-ray position due to Poisson noise. In all but six clusters, the measured offsets are also larger than the estimated, combined astrometric uncertainties in the X-ray and optical positions. A more conservative cut on concentration to select relaxed clusters marginally reduces but does not eliminate the observed offset. With our more conservative sample, we find an estimated median X-ray to central galaxy offset of $\mu = 6.0 ^{+ 1.4}_{- 1.5}$ kpc. Comparing to recent simulations, this distribution of offsets is consistent with some level of dark matter self-interaction, though further simulation work is needed to place constraints.

Massive dark matter haloes at high redshift: implications for observations in the JWST era

ABSTRACT The presence of massive galaxies at high z as recently observed by JWST appears to contradict the current ΛCDM cosmology. Here, we aim to alleviate this tension by incorporating uncertainties from three sources in counting galaxies: cosmic variance, error in stellar mass estimation, and backsplash enhancement. Each of these factors significantly increases the cumulative stellar mass density ρ*(>M*) at the high-mass end, and their combined effect can boost the density by more than one order of magnitude. Assuming a star formation efficiency of ϵ* ∼ 0.5, cosmic variance alone reduces the tension to a 2σ level, except for the most massive galaxy at z = 8. Additionally, incorporating a 0.3 dex lognormal dispersion in the stellar mass estimation brings the observed ρ*(>M*) at z ∼ 7–10 within 2σ. The tension is completely eliminated when we account for the gas stripped from backsplash haloes. These results highlight the importance of fully modelling uncertainties when interpreting observational data of rare objects. We use the constrained simulation, ELUCID, to investigate the descendants of high-z massive galaxies. Our findings reveal that a significant portion of these galaxies ultimately resides in massive haloes at z = 0 with $M_{\rm halo} \gt 10^{13}\, h^{-1}{\rm M_\odot }$. Moreover, a large fraction of local central galaxies in $M_{\rm halo} \geqslant 10^{14.5} \, h^{-1}{\rm M_\odot }$ haloes are predicted to contain substantial amounts of ancient stars formed in massive galaxies at z ∼ 8. This prediction can be tested by studying the structure and stellar population of central galaxies in present-day massive clusters.

Central radio galaxies in galaxy clusters: Joint surveys by eROSITA and ASKAP

Contact. The extended ROentgen Survey with an Imaging Telescope Array (eROSITA) telescope on board the Spectrum-Roentgen-Gamma (SRG) mission has completed the first eROSITA All-Sky Survey (eRASS:1). It detected ∼104 galaxy clusters in the western Galactic hemisphere. In the radio band, the Australian Square Kilometre Array Pathfinder (ASKAP) telescope completed its pilot 1 phase of the project Evolutionary Map of the Universe (EMU) with 220 000 sources in a 270 deg2 field overlapping with eRASS:1. These two surveys are used to study radio-mode active galactic nuclei in clusters. Aims. In order to understand the efficiency of radio-mode feedback at the centers of galaxy clusters, we relate the radio properties of the brightest cluster galaxies to the X-ray properties of the host clusters. Methods. We identified the central radio sources in eRASS:1 clusters or calculated corresponding upper limits on the radio luminosity. Then, we derived relations between the X-ray properties of the clusters and the radio properties of the corresponding central radio source. Results. In total, we investigated a sample of 75 clusters. We find a statistically significant correlation between the X-ray luminosity of the cluster and the 944 MHz radio luminosity of the corresponding central radio galaxy. There is also a positive trend between the radio power and the largest linear size of the radio source. The density and the largest linear size are not correlated. We find that the kinetic luminosity of the radio jets in high-luminosity clusters with LX > 1043 erg s−1 is no longer correlated with the X-ray luminosity, and we discuss various reasons. We find an anticorrelation between the central cooling time tcool and the radio luminosity LR, indicating a need for more powerful active galactic nuclei in clusters with short central cooling times.

The correlation between the 500 pc scale molecular gas masses and AGN powers for massive elliptical galaxies

Abstract Massive molecular clouds have been discovered in massive elliptical galaxies at the center of galaxy clusters. Some of this cold gas is expected to flow in the central supermassive black holes and activate galactic nucleus (AGN) feedback. In this study, we analyze archival Atacama Large Millimeter Array (ALMA) data of nine massive elliptical galaxies, focusing on CO line emissions, to explore the circumnuclear gas. We show that the mass of the molecular gas within a fixed radius (500 pc) from the AGNs (Mmol ∼ 107–108 M⊙) is correlated with the jet power estimated from X-ray cavities (Pcav ∼ 1042–1045 erg s−1). The mass accretion rate of the circumnuclear gas $\dot{M}$ also has a correlation with Pcav. On the other hand, the continuum luminosities at ∼1.4 GHz and ∼100–300 GHz have no correlation with Mmol. These results indicate that the circumnuclear gas is sustaining the long-term AGN activities (∼107 yr) rather than the current ones. The circumnuclear gas mass is a better indicator of the jet power than the continuum luminosity, which probably changes on a shorter time scale. We also study the origin of the continuum emission from the AGNs at ∼100–300 GHz and find that it is mostly synchrotron radiation. For low-luminosity AGNs, however, dust emission appears to contaminate the continuum.

The nature of the motions of multiphase filaments in the centers of galaxy clusters

The intracluster medium (ICM) in the centers of galaxy clusters is heavily influenced by the “feedback” from supermassive black holes (SMBHs). Feedback can drive turbulence in the ICM and turbulent dissipation can potentially be an important source of heating. Due to the limited spatial and spectral resolutions of X-ray telescopes, direct observations of turbulence in the hot ICM have been challenging. Recently, we developed a new method to measure turbulence in the ICM using multiphase filaments as tracers. These filaments are ubiquitous in cluster centers and can be observed at very high resolution using optical and radio telescopes. We study the kinematics of the filaments by measuring their velocity structure functions (VSFs) over a wide range of scales in the centers of ∼10 galaxy clusters. We find features of the VSFs that correlate with the SMBHs activities, suggesting that SMBHs are the main driver of gas motions in the centers of galaxy clusters. In all systems, the VSF is steeper than the classical Kolmogorov expectation and the slopes vary from system to system. One theoretical explanation is that the VSFs we have measured so far mostly reflect the motion of the driver (jets and bubbles) rather than the cascade of turbulence. We show that in Abell 1795, the VSF of the outer filaments far from the SMBH flattens on small scales to a Kolmogorov slope, suggesting that the cascade is only detectable farther out with the current telescope resolution. The level of turbulent heating computed at small scales is typically an order of magnitude lower than that estimated at the driving scale. Even though SMBH feedback heavily influences the kinematics of the ICM in cluster centers, the level of turbulence it drives is rather low, and turbulent heating can only offset ≲ 10% of the cooling loss, consistent with the findings of numerical simulations.

The first quiescent galaxies in TNG300

ABSTRACT We identify the first quiescent galaxies in TNG300, the largest volume of the IllustrisTNG cosmological simulation suite, and explore their quenching processes and time evolution to z = 0. We find that the first quiescent galaxies with stellar masses M* > 3 × 1010 M⊙ and specific star formation rates sSFR < 10−11 yr−1 emerge at z ∼ 4.2 in TNG300. Suppression of star formation in these galaxies begins with a thermal mode of active galactic nucleus feedback at z ∼ 6, and a kinetic feedback mode acts in each galaxy by z ∼ 4.7 to complete the quenching process, which occurs on a time-scale of ∼0.35 Gyr. Surprisingly, we find that the majority of these galaxies are not the main progenitors of their z = 0 descendants; instead, four of the five galaxies fall into more massive galaxies in subsequent mergers at a range of redshifts 2.5 < z < 0.2. By z = 0, these descendants are the centres of galaxy clusters with average stellar masses of 8 × 1011 M⊙. We make predictions for the first quenched galaxies to be located by the JWST.

SPT-CL J2215−3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster

We present the discovery of the most distant, dynamically relaxed cool core cluster, SPT-CL J2215−3537 (SPT2215), and its central brightest cluster galaxy (BCG) at z = 1.16. Using new X-ray observations, we demonstrate that SPT2215 harbors a strong cool core with a central cooling time of 200 Myr (at 10 kpc) and a maximal intracluster medium cooling rate of 1900 ± 400 M ⊙ yr−1. This prodigious cooling may be responsible for fueling the extended, star-forming filaments observed in Hubble Space Telescope imaging. Based on new spectrophotometric data, we detect bright [O ii] emission in the BCG, implying an unobscured star formation rate (SFR) of M ⊙ yr−1. The detection of a weak radio source (2.0 ± 0.8 mJy at 0.8 GHz) suggests ongoing feedback from an active galactic nucleus (AGN), though the implied jet power is less than half the cooling luminosity of the hot gas, consistent with cooling overpowering heating. The extreme cooling and SFR of SPT2215 are rare among known cool core clusters, and it is even more remarkable that we observe these at such high redshift, when most clusters are still dynamically disturbed. The high mass of this cluster, coupled with the fact that it is dynamically relaxed with a highly isolated BCG, suggests that it is an exceptionally rare system that must have formed very rapidly in the early universe. Combined with the high SFR, SPT2215 may be a high-z analog of the Phoenix cluster, potentially providing insight into the limits of AGN feedback and star formation in the most massive galaxies.

Atmospheric Pressure and Molecular Cloud Formation in Early-type Galaxies

A strong correlation between atmospheric pressure and molecular gas mass is found in central cluster galaxies and early-type galaxies. This trend and a similar trend with atmospheric gas density would naturally arise if the molecular clouds condensed from hot atmospheres. Limits on the ratio of molecular to atomic hydrogen in these systems exceed unity. The data are consistent with ambient pressure being a significant factor in the rapid conversion of atomic hydrogen into molecules as found in normal spiral galaxies.

Death at watersheds: Galaxy quenching in low-density environments

Context.The evolution of galaxies is influenced by their local and global environment in the cosmic web. Galaxies with very old stellar populations (VO galaxies withDn(4000) index ≥1.75) mostly lie in the centres of galaxy clusters, where they evolve under the influence of processes characteristic of high-density cluster environments. However, VO galaxies have also been found in poor groups in global low-density environments between superclusters, which we call watershed regions.Aims.Our aim is to analyse the properties of galaxies in various cosmic environments with a focus on VO galaxies in the watershed regions to understand their evolution, and the origin of the large-scale morphology–density relation.Methods.We employ the Sloan Digital Sky Survey DR10 MAIN spectroscopic galaxy sample in the redshift range 0.009 ≤ z ≤ 0.200 to calculate the luminosity–density field of galaxies, to determine groups and filaments in the galaxy distribution, and to obtain data on galaxy properties. The luminosity–density field with smoothing length 8h−1Mpc,D8, characterises the global environment of galaxies. We analyse the group and galaxy contents of regions with variousD8 thresholds. We divide groups into low- and high-luminosity groups based on the highest luminosity of groups in the watershed region,Lgr ≤ 15 × 1012h−2L⊙. We compare the stellar masses, the concentration index, and the stellar velocity dispersions of quenched and star-forming galaxies among single galaxies, satellite galaxies, and the brightest group galaxies (BGGs) in various environments.Results.We show that the global density is most strongly related to the richness of galaxy groups. Its influence on the overall star formation quenching in galaxies is less strong. Correlations between the morphological properties of galaxies and the global density field are the weakest. The watershed regions withD8 < 1 are populated mostly by single galaxies, constituting 70% of all galaxies there, and by low-luminosity groups. Still, approximately one-third of all galaxies in the watershed regions are VO galaxies. They have lower stellar masses, smaller stellar velocity dispersions, and stellar populations that are up to 2 Gyr younger than those of VO galaxies in other global environments. In higher density global environments (D8 > 1), the morphological properties of galaxies are very similar. Differences in galaxy properties are the largest between satellites and BGGs in groups.Conclusions.Our results suggest that galaxy evolution is determined by the birthplace of galaxies in the cosmic web, and mainly by internal processes which lead to the present-day properties of galaxies. This may explain the similarity of (VO) galaxies in extremely different environments.

AGN jets do not prevent the suppression of conduction by the heat buoyancy instability in simulated galaxy clusters

Centres of galaxy clusters must be efficiently reheated to avoid a cooling catastrophe. One potential reheating mechanism is anisotropic thermal conduction, which could transport thermal energy from intermediate radii to the cluster centre. However, if fields are not re-randomised, anisotropic thermal conduction drives the heat buoyancy instability (HBI) which re-orients magnetic field lines and shuts off radial heat fluxes. We revisit the efficiency of thermal conduction under the influence of spin-driven active galactic nuclei (AGN) jets in idealised magneto-hydrodynamical simulations with anisotropic thermal conduction. Despite the black hole spin’s ability to regularly re-orientate the jet so that the jet-induced turbulence is driven in a quasi-isotropic fashion, the HBI remains efficient outside the central 50 kpc of the cluster, where the reservoir of heat is the largest. As a result, conduction plays no significant role in regulating the cooling of the intracluster medium if central AGN are the sole source of turbulence. Whistler-wave-driven saturation of thermal conduction reduces the magnitude of the HBI, but does not prevent it.

High-resolution, High-sensitivity, Low-frequency uGMRT View of Coma Cluster of Galaxies

We present high-resolution, high-sensitivity upgraded Giant Metrewave Radio Telescope observations of the Coma cluster (A1656) at 250–500 MHz and 550–850 MHz. At 250–500 MHz, 135 sources have extensions >0.′45 (with peak-to-local-noise ratio >4). Of these, 24 sources are associated with Coma-member galaxies. In addition, we supplement this sample of 24 galaxies with 20 ram pressure stripped (RPS) galaxies from (Chen et al. 2020, eight are included in the original extended radio source sample) and an additional five are detected and extended. We present radio morphologies, radio spectra, spectral index maps, and equipartition properties for these two samples. In general, we find the equipartition properties lie within a narrow range (e.g., = 1–3 × 10− 13 dynes cm−2). Only NGC 4874, one of the two brightest central Coma cluster galaxies, has a central energy density and pressure about five times higher and a radio source age about 50% lower than that of the other Coma galaxies. We find a diffuse tail of radio emission trailing the dominant galaxy of the merging NGC 4839 group that coincides with the slingshot tail seen in X-rays. The southwestern radio relic, B1253+275, has a large extent ≈32′ × 10′ (≃1.08 × 0.34 Mpc2). For NGC 4789, whose long radio tails merge into the relic and may be a source of its relativistic seed electrons, we find a transverse radio spectral gradient, a steepening from southwest to northeast across the width of the radio source. Finally, radio morphologies of the extended and RPS samples suggest that these galaxies are on their first infall into Coma on (predominantly) radial orbits.

A Candle in the Wind: A Radio Filament in the Core of the A3562 Galaxy Cluster

Using a MeerKAT observation of the galaxy cluster A3562 (a member of the Shapley supercluster), we have discovered a narrow, long and straight, very faint radio filament, which branches out at a straight angle from the tail of a radio galaxy located in projection near the core of the cluster. The radio filament spans 200 kpc and aligns with a sloshing cold front seen in the X-rays, staying inside the front in projection. The radio spectral index along the filament appears uniform (within large uncertainties) at α ≃ −1.5. We propose that the radio galaxy is located outside the cold front but dips its tail under the front. The tangential wind that blows there may stretch the radio plasma from the radio galaxy into a filamentary structure. Some reacceleration is needed in this scenario to keep the radio spectrum uniform. Alternatively, the cosmic-ray electrons from that spot in the tail can spread along the cluster magnetic field lines, straightened by that same tangential flow, via anomalously fast diffusion. Our radio filament can provide constraints on this process. We also uncover a compact radio source at the brightest cluster galaxy that is 2–3 orders of magnitude less luminous than those in typical cluster central galaxies—probably an example of a brightest cluster galaxy starved of accretion fuel by gas sloshing.

Detection of a Superluminous Spiral Galaxy in the Heart of a Massive Galaxy Cluster

It is well established that brightest cluster galaxies (BCGs), residing in the centers of galaxy clusters, are typically massive and quenched galaxies with cD or elliptical morphology. An optical survey suggested that an exotic galaxy population, superluminous spiral and lenticular galaxies, could be the BCGs of some galaxy clusters. Because the cluster membership and the centroid of a cluster cannot be accurately determined based solely on optical data, we followed up a sample of superluminous disk galaxies and their environments using XMM-Newton X-ray observations. Specifically, we explored seven superluminous spiral and lenticular galaxies that are candidate BCGs. We detected massive galaxy clusters around five superluminous disk galaxies and established that one superluminous spiral, 2MASX J16273931+3002239, is the central BCG of a galaxy cluster. The temperature and total mass of the cluster are keV and M 500 = (2.39 ± 0.19) × 1014 M ⊙. We identified the central galaxies of the four clusters that do not host superluminous disk galaxies at their cores, and established that the centrals are massive elliptical galaxies. However, for two of the clusters, the offset superluminous spirals are brighter than the central galaxies, implying that the superluminous disk galaxies are the brightest cluster galaxies. Our results demonstrate that superluminous disk galaxies are rarely the central systems of galaxy clusters. This is likely because galactic disks are destroyed by major mergers, which are more frequent in high-density environments. We speculate that the disks of superluminous disk galaxies in cluster cores may have been reformed due to mergers with gas-rich satellites.