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Cosmological simulations of structure formation predict that galaxy clusters continue to grow and evolve through ongoing mergers with group-scale systems. During these merging events, the ram pressure applied by the intracluster medium acts to strip the gas from the infalling groups, forming large tails of stripped gas, which eventually become part of the main cluster. In this work, we present a detailed analysis of our new deep Chandra observations of the NGC 4839 group falling into the nearby Coma cluster, providing a unique opportunity to explore the way galaxy clusters in the local universe continue to grow. Our analysis reveals a cold front feature at the leading head of the group, preceded by a bow shock of hot gas in front with a Mach number of $\sim\! 1.5$. The power spectrum of surface brightness fluctuations in the tail shows that the slope gets less steep as the distance from the leading head increases, changing from $-2.35_{-0.06}^{+0.07}$ at the inner part of the tail to $-1.37_{-0.07}^{+0.09}$ at the outermost part of the tail. These values are shallower than the slope of the Kolmogorov 2D power spectrum, indicating that thermal conduction is being suppressed throughout the tail, enabling long-lived small-scale turbulence, which would typically be washed out if thermal conduction was not inhibited. The characteristic amplitude of surface brightness fluctuations in the tail suggests a mild level of turbulence with a Mach number in the range of 0.1-0.5, agreeing with that found for the infalling group in Abell 2142., 10 pages, 10 figures, 3 tables. Accepted for publication in MNRAS

Non-thermal emission from clusters of galaxies at the high-energy X-ray regime has been searched with various instruments, but the detection significance of this emission has yet been found to be either marginal or controversial. Taking advantage of NuSTAR's unique capability to focus X-rays in the hard energy band, we present a detailed analysis of 238 ks NuSTAR observations of the merging galaxy cluster SPT-CL J2031-4037, searching for non-thermal inverse Compton emission. Our spectral analysis of SPT-CL J2031-4037 shows a possibility that the hard X-ray emission of the cluster can be described by a non-thermal component, though we cannot completely rule out a purely thermal origin for this hard emission. Including the statistical and systematic uncertainties, our best model fit yields a 20-80 keV non-thermal flux of $3.93_{-1.10}^{+1.24} \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$. The estimated non-thermal flux is comparable to those found in other galaxy clusters using NuSTAR and other X-ray instruments. Using this non-thermal flux with the existing radio data of the cluster, we estimate a volume-averaged magnetic field strength in the range of around 0.1-0.2 $\mu$G., Comment: 10 pages, 10 figures, 4 tables. Accepted for publication in MNRAS

Observations of the ICM in the outskirts of the Virgo cluster with Suzaku have found the gas mass fraction in the northern direction to be significantly above the expected level, indicating that there may be a very high level of gas clumping on small scales in this direction. Here, we explore the XMM-Newton data in the outskirts of Virgo, dividing it into a Voronoi tessellation to separate the bulk ICM component from the clumped ICM component. As the nearest galaxy cluster, Virgo's large angular extent allows the spatial scale of the tessellation to be much smaller than has been achieved using the same technique on intermediate redshift clusters, allowing us to probe gas clumping on the scales down to 5$\times$5 kpc. We find that the level of gas clumping in the outskirts to the north is relatively mild, ($\sqrt{C} < 1.1$), suggesting that our point-source detection procedure may have excluded a significant fraction of clumps. While correcting for clumping brings the gas mass fraction at $r_{200}$ into agreement with the universal gas mass fraction, the values outside $r_{200}$ remain significantly above it. This may suggest that non-thermal pressure support in the outskirts to the north is significant, and we find that a non-thermal pressure support at the level of 20 per cent of the total pressure outside $r_{200}$ can explain the high gas mass fraction to the north, which is in agreement with the range expected from simulations., 10 pages, 12 figures, 1 table. Accepted for publication in MNRAS

We probe the formation scenarios and the active galactic nuclei (AGN) occupation fraction of ultra-diffuse galaxies (UDGs) in the nearby Coma cluster by utilizing XMM-Newton observations of 779 out of 854 UDG candidates identified by Subaru survey. Their origin is probed by measuring the dark matter halo mass of the stacked sample of UDGs and the population of low-mass X-ray binaries residing in globular clusters. Our measurements suggest that the average UDG population does not have a substantial amount of hot gas or a large number of globular clusters. This supports the formation scenario, in which UDGs are puffed-up dwarf galaxies, agreeing with that obtained for 404 Coma cluster UDGs using Chandra. We also determine AGN occupation fraction of UDGs by cross-correlating the position of UDGs with the detected point sources in Coma. We detect three X-ray sources with detection significance $\sigma \geq 5$ that could be off-centre AGN within 5 arcsec from the centre of the UDG 317, UDG 432, and UDG 535. We identify an optical counterpart for the X-ray source associated with the UDG 317, suggesting that this source is more likely an off-centre AGN. Based on the current data, however, we cannot conclusively constrain whether the detected AGN is residing in the Coma cluster or not., Comment: 10 pages, 5 figures, 1 table. Accepted for publication in MNRAS

We investigate the properties of the dark matter (DM) halo surrounding the nearby galaxy group NGC 1600. Through the use of deep (252 ks) Chandra observations and 64.3 ks of XMM-Newton observations, we construct surface brightness profiles in multiple energy bands in order to perform hydrostatic equilibrium analysis of the hot plasma within NGC 1600. Regardless of the DM model profile assumed, we measure a halo concentration (c$_{200}$) that is an extreme, positive outlier of the $\Lambda$CDM c$_{200}$-M$_{200}$ relation. For a typical NFW DM profile, we measure c$_{200}\!=\!26.7\pm1.4$ and M$_{200}\!=\!(2.0\pm0.2)\times10^{13}$ M$_\odot$; assuming a similar halo mass, the average concentration expected is c$_{200}=6-7$ for the theoretical $\Lambda$CDM c-M relation. Such a high concentration is similar to that of well-known fossil groups MRK 1216 and NGC 6482. While NGC 1600 exhibits some properties of a fossil group, it fails to meet the X-ray luminosity threshold of L$_X>5\times10^{41}$ erg s$^{-1}$. Whether or not it is considered a fossil group, the high concentration value makes it part of a select group of galaxy groups., Comment: 7 pages, 5 figures. Accepted for publication in MNRAS

We present new XMM-Newton observations extending the mosaic of the Perseus cluster out to the virial radius to the west. Previous studies with ROSAT have reported a large excess in surface brightness to the west, possibly the result of large scale gas sloshing, but lacked the spatial resolution and depth to determine if this excess lay behind a cold front. In our new XMM observations we have found that there is a sharp edge in X-ray surface brightness near the cluster virial radius (1.7Mpc) to the west, with a width comparable to the mean free path. The temperature measurements obtained with Suzaku data across this edge show that the temperature increases sharply outside this edge. All of these properties are consistent with this edge being the largest cold front ever observed in a galaxy cluster. We also find a second edge to the west, 1.2Mpc from the core, which also appears to be a cold front. Our results indicate that magnetic fields are able to stabilize the cold fronts against turbulence all the way out to the cluster virial radius. By comparing with numerical simulations, we find that these large cold fronts require large impact parameter, low mass ratio mergers which can produce fast gas motions without destroying the cluster core., 10 pages, 7 figures, submitted to ApJ

We present a joint Suzaku and XMM-Newton analysis of the outskirts of the nearby galaxy cluster Abell 2199, the only nearby galaxy cluster to be observed with near complete azimuthal coverage with Suzaku. Using the XMM-Newton observations to correct for the effects of gas clumping, we find that the azimuthally averaged entropy profile in the outskirts follows a power law with a slope of $1.20 \pm 0.23$, statistically consistent with a slope of 1.1 predicted by non-radiative simulations for purely gravitational hierarchical structure formation. However, when divided into 10 sectors, the entropy shows significant azimuthal variation, with some sectors lying below the baseline level. The azimuthally averaged gas mass fraction is found to agree with the cosmic mean baryon fraction. The metal abundance in the outskirts is found to be consistent with being uniform in all directions and it has an average value of $0.29_{-0.03}^{+0.03}\,Z_{\odot}$, consistent with the gas accreting onto clusters being pre-enriched with metals., 11 pages, 13 figures, 2 tables. Accepted for publication in MNRAS

I investigate an analytical model of galaxy clusters based on the assumptions that the intracluster medium plasma is polytropic and is in hydrostatic equilibrium. The Einasto profile is adopted as a model for the spatial-density distribution of dark matter halos. This model has sufficient degrees of freedom to simultaneously fit X-ray surface brightness and temperature profiles, with five parameters to describe the global cluster properties and three additional parameters to describe the cluster's cool-core feature. The model is tested with Chandra X-ray data for seven galaxy clusters, including three polytropic clusters and four cool-core clusters. It is found that the model accurately reproduces the X-ray data over most of the radial range. For all galaxy clusters, the data allows one to show that the model is essentially as good as that of Vikhlinin et al. and Bulbul et al., as inferred by the reduced χ 2.

We investigate the gas mass distribution in the high redshift cluster MS 1054-0321 using Chandra X-ray and OCRA SZ effect data. We use a superposition of offset $\beta$-type models to describe the composite structure of MS 1054-0321. We find gas mass fractions $f_{gas}^\rm{X\mbox{-}ray} = 0.087_{-0.001}^{+0.005}$ and $f_{gas}^\rm{SZ} = 0.094_{-0.001}^{+0.003}$ for the (main) eastern component of MS 1054-0321 using X-ray or SZ data, but $f_{gas}^\rm{X\mbox{-}ray} = 0.030 _{-0.014}^{+0.010}$ for the western component. The gas mass fraction for the eastern component is in agreement with some results reported in the literature, but inconsistent with the cosmic baryon fraction. The low gas mass fraction for the western component is likely to be a consequence of gas stripping during the ongoing merger. The gas mass fraction of the integrated system is $0.060_{-0.009}^{+0.004}$: we suggest that the missing baryons from the western component are present as hot diffuse gas which is poorly represented in existing X-ray images. The missing gas could appear in sensitive SZ maps., Comment: 12 pages, 4 figures, and 5 tables