[en] Identification of Abell 3120 as a galaxy cluster has recently been questioned with alternative suggestions including a fossil remnant of a group merger, non-thermal emission from a radio galaxy, and projected emission from a filamentary string of galaxies. We report on our analysis of the Chandra observation and evaluate these hypotheses based on our results. Abell 3120 shows X-ray emission extending 158 kpc, well beyond the central galaxy. The spatial distribution of X-rays in the core more closely follows the radio emission showing a jet-like structure extending to the north that is misaligned with the stellar light distribution of the central galaxy. At larger radii, the X-ray emission is aligned with the SE-NW running axis of the galaxy distribution in the cluster core. Modeling the X-ray spectrum excludes purely non-thermal emission. The emission-weighted temperature is 1.93-2.19 keV and the 0.3-10 keV luminosity is 1.23 x 10⁴³ erg s^-1. Abell 3120 appears to be a poor cluster with Virgo and MKW 4 as peers. The best-fitting model consists of a thermal component and a second component that may be either thermal or non-thermal, with luminosity ∼25% of the total X-ray luminosity. While a more detailed spatial-spectral search failed to detect a central active galactic nucleus, there is some evidence for an extended hard X-ray component. Cooler gas, 1.28-1.80 keV, was detected in the central 20 kpc. The second thermal component marginally requires a higher redshift, >0.12, which may be due to a second cluster in the rich surrounding environment consisting of nearly a thousand cataloged galaxies.

[en] We report on a confirmed galaxy cluster at z = 1.62. We discovered two concentrations of galaxies at z ∼ 1.6 in the Subaru/XMM-Newton deep field based on deep multi-band photometric data. We made a near-IR spectroscopic follow-up observation of them and confirmed several massive galaxies at z = 1.62. One of the two is associated with an extended X-ray emission at 4.5σ on a scale of 0.'5, which is typical of high-z clusters. The X-ray detection suggests that it is a gravitationally bound system. The other one shows a hint of an X-ray signal, but only at 1.5σ, and we obtained only one secure redshift at z = 1.62. We are not yet sure if this is a collapsed system. The possible twins exhibit a clear red sequence at K < 22 and seem to host relatively few number of faint red galaxies. Massive red galaxies are likely old galaxies-they have colors consistent with the formation redshift of z_f = 3, and a spectral fit of the brightest confirmed member yields an age of 1.8^+0.1_-0.2 Gyr with a mass of 2.5^+0.2_-0.1 x 10¹¹ M_sun. Our results show that it is feasible to detect clusters at z > 1.5 in X-rays and also to perform detailed analysis of galaxies in them with the existing near-IR facilities on large telescopes.

[en] Mahdavi et al. find that the degree of agreement between weak lensing and X-ray mass measurements is a function of cluster radius. Numerical simulations also point out that X-ray mass proxies do not work equally well at all radii. The origin of the effect is thought to be associated with cluster mergers. Recent work presenting the cluster maps showed an ability of X-ray maps to reveal and study cluster mergers in detail. Here, we present a first attempt to use the study of substructure in assessing the systematics of the hydrostatic mass measurements using two-dimensional (2D) X-ray diagnostics. The temperature map is uniquely able to identify the substructure in an almost relaxed cluster which would be unnoticed in the intracluster medium electron number density and pressure maps. We describe the radial fluctuations in the 2D maps by a cumulative/differential scatter profile relative to the mean profile within/at a given radius. The amplitude indicates ∼10% fluctuations in the temperature, electron number density, and entropy maps, and ∼15% fluctuations in the pressure map. The amplitude of and the discontinuity in the scatter complement 2D substructure diagnostics, e.g., indicating the most disturbed radial range. There is a tantalizing link between the substructure identified using the scatter of the entropy and pressure fluctuations and the hydrostatic mass bias relative to the expected mass based on the M-Y_X and M-M_gas relations particularly at r₅₀₀. XMM-Newton observations with ∼120,000 source photons from the cluster are sufficient to apply our substructure diagnostics via the spectrally measured 2D temperature, electron number density, entropy, and pressure maps.

[en] The brightest cluster radio halo known resides in the Coma cluster of galaxies. The relativistic electrons producing this diffuse synchrotron emission should also produce inverse Compton emission that becomes competitive with thermal emission from the intracluster medium (ICM) at hard X-ray energies. Thus far, claimed detections of this emission in Coma are controversial. We present a Suzaku HXD-PIN observation of the Coma cluster in order to nail down its nonthermal hard X-ray content. The contribution of thermal emission to the HXD-PIN spectrum is constrained by simultaneously fitting thermal and nonthermal models to it and a spatially equivalent spectrum derived from an XMM-Newton mosaic of the Coma field. We fail to find statistically significant evidence for nonthermal emission in the spectra which are better described by only a single- or multitemperature model for the ICM. Including systematic uncertainties, we derive a 90% upper limit on the flux of nonthermal emission of 6.0 x 10^-12 erg s^-1 cm^-2 (20-80 keV, for Γ = 2.0), which implies a lower limit on the cluster-averaged magnetic field of B>0.15 μG. Our flux upper limit is 2.5 times lower than the detected nonthermal flux from RXTE and BeppoSAX. However, if the nonthermal hard X-ray emission in Coma is more spatially extended than the observed radio halo, the Suzaku HXD-PIN may miss some fraction of the emission. A detailed investigation indicates that ∼50%-67% of the emission might go undetected, which could make our limit consistent with that of Rephaeli and Gruber and Fusco-Femiano et al. The thermal interpretation of the hard Coma spectrum is consistent with recent analyses of INTEGRAL and Swift data.

[en] We present the direct detection of the splashback feature using the sample of massive galaxy clusters from the Local Cluster Substructure Survey (LoCuSS). This feature is clearly detected (above 5σ) in the stacked luminosity density profile obtained using the K-band magnitudes of spectroscopically confirmed cluster members. We obtained the best-fit model by means of Bayesian inference, which ranked models including the splashback feature as more descriptive of the data with respect to models that do not allow for this transition. In addition, we have assessed the impact of the cluster dynamical state on the occurrence of the splashback feature. We exploited the extensive multiwavelength LoCuSS data set to test a wide range of proxies for the cluster formation history, finding the most significant dependence of the splashback feature location and scale according to the presence or absence of X-ray emitting galaxy groups in the cluster infall regions. In particular, we report for the first time that clusters that do not show massive infalling groups present the splashback feature at a smaller clustercentric radius r _sp/r _200,m = 1.158 ± 0.071 than clusters that are actively accreting groups r _sp/r _200,m = 1.291 ± 0.062. The difference between these two subsamples is significant at 4.2σ, suggesting a correlation between the properties of the cluster potential and its accretion rate and merger history. Similarly, clusters that are classified as old and dynamically inactive present stronger signatures of the splashback feature, with respect to younger, more active clusters. We are directly observing how fundamental dynamical properties of clusters reverberate across vastly different physical scales.

[en] We present the results of a pilot XMM-Newton and Chandra program aimed at studying the diffuse intragroup medium (IGM) of optically selected nearby groups from the Zurich ENvironmental Study (ZENS) catalog. The groups are in a narrow mass range about ${10}^{13}{M}_{\odot <!--\; ???\; -->}$, a mass scale at which the interplay between the IGM and the group member galaxies is still largely unprobed. X-ray emission from the IGM is detected in the energy band 0.5–2 keV with flux $⩽<!--\; ???\; -->{10}^{-<!--\; ???\; -->13}$ erg s⁻¹ cm⁻², which is one order of magnitude fainter than for typical ROSAT groups (RASS). For many groups, we set upper limits on the X-ray luminosity, indicating that the detections are likely probing the upper envelope of the X-ray emitting groups. We find that weighting the group halo mass by the fraction of the total stellar mass locked in the bulge galaxy components might reduce the bias of mass estimates based on the total optical luminosity with respect to the X-ray mass estimates, (consistent with Andreon, at larger mass scales). We measure a stellar mass fraction with a median value of about 1%, with a contribution from the most massive galaxies between 30% and 50%. Optical and X-ray data often give complementary answers concerning the dynamical state of the groups, and are essential for a complete picture of the group system. Extending this pilot program to a larger sample of groups is necessary to unveil any imprint of interaction between member galaxies and IGM in halo potentials of key importance for environmentally driven galactic evolution.

[en] The Coma Cluster of galaxies hosts the brightest radio halo known and has therefore been the target of numerous searches for associated inverse Compton (IC) emission, particularly at hard X-ray energies where the IC signal must eventually dominate over thermal emission. The most recent search with the Suzaku Hard X-ray Detector failed to confirm previous IC detections with RXTE and BeppoSAX, instead setting an upper limit 2.5 times below their non-thermal flux. However, this discrepancy can be resolved if the IC emission is very extended, beyond the scale of the cluster radio halo. Using reconstructed sky images from the 58-month Swift Burst Alert Telescope (BAT) all-sky survey, the feasibility of such a solution is investigated. Building on Renaud et al., we test and implement a method for extracting the fluxes of extended sources, assuming specified spatial distributions. BAT spectra are jointly fit with an XMM-Newton EPIC-pn spectrum derived from mosaic observations. We find no evidence for large-scale IC emission at the level expected from the previously detected non-thermal fluxes. For all non-thermal spatial distributions considered, which span the gamut of physically reasonable IC models, we determine upper limits for which the largest (most conservative) limit is ∼<4.2 x 10^-12 erg s^-1 cm^-2 (20-80 keV), which corresponds to a lower limit on the magnetic field B > 0.2 μ G. A nominal flux upper limit of <2.7 x 10^-12 erg s^-1 cm^-2, with corresponding B > 0.25 μ G, is derived for the most probable IC distribution given the size of the radio halo and likely magnetic field radial profile.

[en] A3667 is the archetype of a merging cluster with radio relics. The northwest (NW) radio relic is the brightest cluster relic or halo known and is believed to be due to a strong merger shock. We have observed the NW relic for ∼40 ks of net XMM-Newton time. We observe a global decline of temperature across the relic from 6 to 1 keV, similar to the Suzaku results. Our new observations reveal a sharp change of both temperature and surface brightness near the position of the relic. The increased X-ray emission on the relic can be equivalently well described by either a thermal or nonthermal spectral model. The parameters of the thermal model are consistent with a Mach number M∼2 shock and a shock speed of ∼1200 km s^-1. The energy content of the relativistic particles in the radio relic can be explained if they are (re)-accelerated by the shock with an efficiency of ∼0.2%. Comparing the limit on the inverse Compton X-ray emission with the measured radio synchrotron emission, we set a lower limit to the magnetic field in the relic of 3 μG. If the emission from the relic is nonthermal, this lower limit is in fact the required magnetic field.

[en] Weak lensing is commonly measured using shear through galaxy ellipticities or using the effect of magnification bias on galaxy number densities. Here, we report on the first detection of weak-lensing magnification with a new, independent technique using the distribution of galaxy sizes and magnitudes. These data come for free in galaxy surveys designed for measuring shear. We present the magnification estimator and apply it to an X-ray-selected sample of galaxy groups in the COSMOS Hubble Space Telescope survey. The measurement of the projected surface density Σ(r) is consistent with the shear measurements within the uncertainties and has roughly 40% of the signal to noise of the latter. We discuss systematic issues and challenges to realizing the potential of this new probe of weak lensing.

[en] The Zurich Environmental Study (ZENS) is based on a sample of ∼1500 galaxy members of 141 groups in the mass range ∼10^12.5-14.5 M_☉ within the narrow redshift range 0.05 < z < 0.0585. ZENS adopts novel approaches, described here, to quantify four different galactic environments, namely: (1) the mass of the host group halo; (2) the projected halo-centric distance; (3) the rank of galaxies as central or satellites within their group halos; and (4) the filamentary large-scale structure density. No self-consistent identification of a central galaxy is found in ∼40% of <10^13.5 M_☉ groups, from which we estimate that ∼15% of groups at these masses are dynamically unrelaxed systems. Central galaxies in relaxed and unrelaxed groups generally have similar properties, suggesting that centrals are regulated by their mass and not by their environment. Centrals in relaxed groups have, however, ∼30% larger sizes than in unrelaxed groups, possibly due to accretion of small satellites in virialized group halos. At M > 10¹⁰ M_☉, satellite galaxies in relaxed and unrelaxed groups have similar size, color, and (specific) star formation rate distributions; at lower galaxy masses, satellites are marginally redder in relaxed relative to unrelaxed groups, suggesting quenching of star formation in low-mass satellites by physical processes active in relaxed halos. Overall, relaxed and unrelaxed groups show similar stellar mass populations, likely indicating similar stellar mass conversion efficiencies. In the enclosed ZENS catalog, we publish all environmental diagnostics as well as the galaxy structural and photometric measurements described in companion ZENS papers II and III