Turbulent magnetic fields in merging clusters: a case study of Abell 2146

Kelvin-Helmholtz instabilities (KHI) along contact discontinuities in galaxy clusters have been used to constrain the strength of magnetic fields in galaxy clusters, following the assumption that, as magnetic field lines drape around the interface between the cold and hot phases, their magnetic tension resists the growth of perturbations. This has been observed in simulations of rigid objects moving through magnetized media and sloshing galaxy clusters, and then applied in interpreting observations of merger cold fronts. Using a suite of magnetohydrodynamic (MHD) simulations of binary cluster mergers, we show that even magnetic field strengths stronger than yet observed (β = Pth/PB = 50) show visible KHI features. This is because our initial magnetic field is tangled, producing AlfvCrossed D sig

A Systematic Search for X-ray Cavities in the Hot Gas of Galaxy Groups

We have performed a systematic search for X-ray cavities in the hot gas of 51 galaxy groups with Chandra archival data. The cavities are identified based on two methods: subtracting an elliptical beta model fitted to the X-ray surface brightness, and performing unsharp masking. 13 groups in the sample 25% are identified as clearly containing cavities, with another 13 systems showing tentative evidence for such structures. We find tight correlations between the radial and tangential radii of the cavities, and between their size and projected distance from the group center, in quantitative agreement with the case for more massive clusters. This suggests that similar physical processes are responsible for cavity evolution and disruption in systems covering a large range in total mass. We see no clear association between the detection of cavities and the current 1.4 GHz radio luminosity of the central brightest group galaxy, but there is a clear tendency for systems with a cool core to be more likely to harbor detectable cavities. To test the efficiency of the adopted cavity detection procedures, we employ a set of mock images designed to mimic typical Chandra data of our sample, and find that the model-fitting approach is generally more reliable than unsharp masking for recovering cavity properties. Finally, we find that the detectability of cavities is strongly influenced by a few factors, particularly the signal-to-noise ratio of the data, and that the real fraction of X-ray groups with prominent cavities could be substantially larger than the 25--50% suggested by our analysis.Comment: 19 pages, 16 figures, 2 tables, accepted by Ap

Is Thermal Instability Significant in Turbulent Galactic Gas?

We investigate numerically the role of thermal instability (TI) as a generator of density structures in the interstellar medium (ISM), both by itself and in the context of a globally turbulent medium. Simulations of the instability alone show that the condenstion process which forms a dense phase (``clouds'') is highly dynamical, and that the boundaries of the clouds are accretion shocks, rather than static density discontinuities. The density histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak at low densities plus a slope change at high densities. Final static situations may be established, but the equilibrium is very fragile: small density fluctuations in the warm phase require large variations in the density of the cold phase, probably inducing shocks into the clouds. This result suggests that such configurations are highly unlikely. Simulations including turbulent forcing show that large- scale forcing is incapable of erasing the signature of the TI in the density PDFs, but small-scale, stellar-like forcing causes erasure of the signature of the instability. However, these simulations do not reach stationary regimes, TI driving an ever-increasing star formation rate. Simulations including magnetic fields, self-gravity and the Coriolis force show no significant difference between the PDFs of stable and unstable cases, and reach stationary regimes, suggesting that the combination of the stellar forcing and the extra effective pressure provided by the magnetic field and the Coriolis force overwhelm TI as a density-structure generator in the ISM. We emphasize that a multi-modal temperature PDF is not necessarily an indication of a multi-phase medium, which must contain clearly distinct thermal equilibrium phases.Comment: 18 pages, 11 figures. Submitted to Ap

Gas sloshing regulates and records the evolution of the Fornax Cluster

We present results of a joint Chandra and 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 timescale within the cooling radius, calculated from the Brunt–Väsälä frequency, is an order of magnitude shorter than the cooling time. It is plausible that gas sloshing is contributing 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

Supermassive Black Hole Feedback

Understanding the processes that drive galaxy formation and shape the observed properties of galaxies is one of the most interesting and challenging frontier problems of modern astrophysics. We now know that the evolution of galaxies is critically shaped by the energy injection from accreting supermassive black holes (SMBHs). However, it is unclear how exactly the physics of this feedback process affects galaxy formation and evolution. In particular, a major challenge is unraveling how the energy released near the SMBHs is distributed over nine orders of magnitude in distance throughout galaxies and their immediate environments. The best place to study the impact of SMBH feedback is in the hot atmospheres of massive galaxies, groups, and galaxy clusters, which host the most massive black holes in the Universe, and where we can directly image the impact of black holes on their surroundings. We identify critical questions and potential measurements that will likely transform our understanding of the physics of SMBH feedback and how it shapes galaxies, through detailed measurements of (i) the thermodynamic and velocity fluctuations in the intracluster medium (ICM) as well as (ii) the composition of the bubbles inflated by SMBHs in the centers of galaxy clusters, and their influence on the cluster gas and galaxy growth, using the next generation of high spectral and spatial resolution X-ray and microwave telescopes.Comment: 10 pages, submitted to the Astro2020 decada

A new class of x-ray tails of early-type galaxies and subclusters in galaxy clusters: Slingshot tails versus ram pressure stripped tails

© 2019. The American Astronomical Society. All rights reserved.. We show that there is a new class of gas tails - slingshot tails - that form as a subhalo (i.e., a subcluster or early-type cluster galaxy) moves away from the cluster center toward the apocenter of its orbit. These tails can point perpendicular or even opposite to the subhalo direction of motion, not tracing the recent orbital path. Thus, the observed tail direction can be misleading, and we caution against naive conclusions regarding the subhalo's direction of motion based on the tail direction. A head-tail morphology of a galaxy's or subcluster's gaseous atmosphere is usually attributed to ram pressure stripping, and the widely applied conclusion is that gas stripped tail traces the most recent orbit. However, during the slingshot tail stage, the subhalo is not being ram pressure stripped (RPS) and the tail is shaped by tidal forces more than just the ram pressure. Thus, applying a classic RPS scenario to a slingshot tail leads not only to an incorrect conclusion regarding the direction of motion but also to incorrect conclusions regarding the subhalo velocity, expected locations of shear flows, instabilities, and mixing. We describe the genesis and morphology of slingshot tails using data from binary cluster merger simulations and discuss their observable features and how to distinguish them from classic RPS tails. We identify three examples from the literature that are not RPS tails but slingshot tails and discuss other potential candidates

The Properties of Poor Groups of Galaxies: I. Spectroscopic Survey and Results

We use multi-fiber spectroscopy of 12 nearby, poor groups of galaxies to address whether the groups are bound systems or chance projections of galaxies along the line-of-sight, why the members of each group have not already merged to form a single galaxy, despite the groups' high galaxy densities, short crossing times, and likely environments for galaxy-galaxy mergers, and how galaxies might evolve in these groups, where the collisional effects of the intra-group gas and the tidal influences of the global potential are weaker than in rich clusters. We conclude the following. (1) The nine groups with diffuse X-ray emission (cf. Paper II) are bound systems with at least 20-50 group members. (2) Galaxies in each X-ray-detected group have not all merged together, because a significant fraction of the group mass lies outside of the galaxies and in a common halo. (3) Unlike cD galaxies in some rich clusters, the giant, brightest elliptical in each X-ray group lies in the center of the group potential, suggesting that such galaxies may form first in poor groups. (4) In some groups, the fraction and recent star formation histories of the early types are consistent with those in rich clusters, suggesting that the effects of cluster environment on these galaxies are relatively unimportant at the current epoch. (Abridged)Comment: 58 pages, AASLaTeX with 9 figures. Table 1 available on request. To appear with Paper II in Ap

The Interaction of 3C401 with the Surrounding Intracluster Medium

We present an observation of the radio-galaxy 3C401 and the surrounding intracluster medium (ICM) of its host galaxy cluster by the Chandra X-ray Observatory. This luminous radio-galaxy is notable in that it has characteristics intermediate between the FRI and FRII morphologies. We clearly detect point-like emission coincident with the radio-core of 3C401, although the spatial resolution of even Chandra is only 2kpc at the distance of 3C401 (z=0.201) and so the possibility remains that this is a dense (and rapidly cooling) thermal gaseous core in the center of the ICM atmosphere. Strong departures from spherical symmetry in the central 10-20kpc of the ICM clearly suggest interaction between the ICM and the radio-lobes of 3C401. A central X-ray bar probably results from the evacuation of two ICM cavities by the expanding radio lobes. Beyond these central regions, the cluster possesses a flatter profile than many clusters of comparable mass suggesting the importance of ICM heating and entropy injection by 3C401. We detect an interesting cross-like structure in the ICM on 100kpc scales. We speculate that this could be a radio-galaxy induced disturbance corresponding to a time when 3C401 was substantially more powerful. A particularly exciting possibility is that this cross-like structure corresponds to a large scale global g-mode oscillation excited by a past outburst of 3C401.Comment: 7 pages, 5 postscript figures. Accepted for publication in MNRA

Non-thermal intracluster medium: a simultaneous interpretation of the central soft X-ray excess and WMAP's detection of reduced Sunyaev-Zel'dovich Effect

WMAP's detection of the Sunyaev-Zel'dovich effect at a much reduced level among several large samples of rich clusters is interpreted in terms of conventional physics. It has been suggested that the central soft X-ray and EUV excess found in some clusters cannot be of thermal origin, due to problems with rapid gas cooling and the persistent non-detection of the O VII line, but may arise from inverse-Compton scattering between intracluster relativistic electrons and the cosmic microwave background. In fact, recent XMM observations of the soft X-rays from Coma and Abell 3112 are equally well fitted by a power law or a thermal virialized gas. Therefore the missing Sunyaev-Zel'dovich flux could partly be due to an overestimate of the central density of virialized electrons which scatter the CMB. Synchrotron radiation in an intracluster magnetic field of strength of a few $\mu$G is responsible for significant additional electron energy loss. Equipartition between relativistic particle and magnetic field energy densities is a realistic possibility. GHz radiation data from a Coma cluster halo yields information on the high energy steepening of the cluster relativistic electron spectrum. Cluster microwave emission in the WMAP passbands by higher energy cosmic ray electrons and gamma ray emission from an accompanying cosmic ray proton flux are also considered. The energetic electrons could originate from AGN jet injection, then distributed cluster-wide by Alfven wave sweeping, with accompanying {\it in situ} Fermi acceleration.Comment: ApJ in pres

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 the 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 Gyr ago from the northeast to the southwest and is now almost at the point of its next encounter from the south. 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 and 750 kpc with an estimated Mach number between 1.3 and 1.5. The wake of NGC 1404 also lies south of the galaxy with enhanced turbulence and a slight enhancement in metallicity compared to the undisturbed regions of the cluster. Southwest 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 northwest of the cluster center