Keywords

NGC 1275, the central galaxy in the Perseus cluster, is the host of gigantic hot bipolar bubbles inflated by active galactic nucleus (AGN) jets observed in the radio as Perseus A. It presents a spectacular Hα-emitting nebulosity surrounding NGC 1275, with loops and filaments of gas extending to over 50 kpc. The origin of the filaments is still unknown, but probably correlates with the mechanism responsible for the giant buoyant bubbles. We present 2.5 and three-dimensional magnetohydrodynamical (MHD) simulations of the central region of the cluster in which turbulent energy, possibly triggered by star formation and supernovae (SNe) explosions, is introduced. The simulations reveal that the turbulence injected by massive stars could be responsible for the nearly isotropic distribution of filaments and loops that drag magnetic fields upward as indicated by recent observations. Weak shell-like shock fronts propagating into the intracluster medium (ICM) with velocities of 100–500 km s⁻¹ are found, also resembling the observations. The isotropic outflow momentum of the turbulence slows the infall of the ICM, thus limiting further starburst activity in NGC 1275. As the turbulence is subsonic over most of the simulated volume, the turbulent kinetic energy is not efficiently converted into heat and additional heating is required to suppress the cooling flow at the core of the cluster. Simulations combining the MHD turbulence with the AGN outflow can reproduce the temperature radial profile observed around NGC 1275. While the AGN mechanism is the main heating source, the SNe are crucial to isotropize the energy distribution.

We describe two-dimensional gasdynamical computations of the X-ray emitting gas in the rotating elliptical galaxy NGC 4649 that indicate an inflow of ∼1 M_☉ yr⁻¹ at every radius. Such a large instantaneous inflow cannot have persisted over a Hubble time. The central constant-entropy temperature peak recently observed in the innermost 150 pc is explained by compressive heating as gas flows toward the central massive black hole. Since the cooling time of this gas is only a few million years, NGC 4649 provides the most acutely concentrated known example of the cooling flow problem in which the time-integrated apparent mass that has flowed into the galactic core exceeds the total mass observed there. This paradox can be resolved by intermittent outflows of energy or mass driven by accretion energy released near the black hole. Inflowing gas is also required at intermediate kpc radii to explain the ellipticity of X-ray isophotes due to spin-up by mass ejected by stars that rotate with the galaxy and to explain local density and temperature profiles. We provide evidence that many luminous elliptical galaxies undergo similar inflow spin-up. A small turbulent viscosity is required in NGC 4649 to avoid forming large X-ray luminous disks that are not observed, but the turbulent pressure is small and does not interfere with mass determinations that assume hydrostatic equilibrium.

A Chandra observation of the X-ray bright group NGC 5044 shows that the X-ray emitting gas has been strongly perturbed by recent outbursts from the central active galactic nucleus (AGN) and also by motion of the central dominant galaxy relative to the group gas. The NGC 5044 group hosts many small radio-quiet cavities with a nearly isotropic distribution, cool filaments, a semi-circular cold front, and a two-armed spiral shaped feature of cool gas. A Giant Metrewave Radio Telescope (GMRT) observation of NGC 5044 at 610 MHz shows the presence of extended radio emission with a "torus-shaped" morphology. The largest X-ray filament appears to thread the radio torus, suggesting that the lower entropy gas within the filament is material being uplifted from the center of the group. The radio emission at 235 MHz is much more extended than the emission at 610 MHz, with little overlap between the two frequencies. One component of the 235 MHz emission passes through the largest X-ray cavity and is then deflected just behind the cold front. A second detached radio lobe is also detected at 235 MHz beyond the cold front. All of the smaller X-ray cavities in the center of NGC 5044 are undetected in the GMRT observations. Since the smaller bubbles are probably no longer momentum driven by the central AGN, their motion will be affected by the group "weather" as they buoyantly rise outward. Hence, most of the enthalpy within the smaller bubbles will likely be deposited near the group center and isotropized by the group weather. The total mechanical power of the smaller radio quiet cavities is P_c = 9.2 × 10⁴¹ erg s⁻¹ which is sufficient to suppress about one-half of the total radiative cooling within the central 10 kpc. This is consistent with the presence of Hα emission within this region which shows that at least some of the gas is able to cool. The mechanical heating power of the larger southern cavity, located between 10 and 20 kpc, is six times greater than the combined mechanical power of the smaller radio-quiet cavities and could suppress all radiative cooling within the central 25 kpc if the energy were deposited and isotropized within this region. Within the central 20 kpc, emission from low-mass X-ray binaries (LMXBs) is a significant component of the X-ray emission above 2 keV. The presence of hard X-ray emission from unresolved LMXBs makes it difficult to place strong constraints on the amount of shock heated gas within the X-ray cavities.

The radio active galactic nucleus (AGN) feedback in X-ray cool cores has been proposed as a crucial ingredient in the evolution of baryonic structures. However, it has long been known that strong radio AGNs also exist in "noncool core" clusters, which brings up the question whether an X-ray cool core is always required for the radio feedback. In this work, we present a systematic analysis of brightest cluster galaxies (BCGs) and strong radio AGNs in 152 groups and clusters from the Chandra archive. All 69 BCGs with radio AGN more luminous than 2 × 10²³ W Hz⁻¹ at 1.4 GHz are found to have X-ray cool cores. BCG cool cores can be divided into two classes: the large cool core (LCC) class and the corona class. Small coronae, easily overlooked at z > 0.1, can trigger strong heating episodes in groups and clusters, long before LCCs are formed. Strong radio outbursts triggered by coronae may destroy embryonic LCCs and thus provide another mechanism to prevent the formation of LCCs. However, it is unclear whether coronae are decoupled from the radio feedback cycles as they have to be largely immune to strong radio outbursts. Our sample study also shows the absence of groups with a luminous cool core while hosting a strong radio AGN, which is not observed in clusters. This points to a greater impact of radio heating on low-mass systems than clusters. Few L_1.4 GHz > 10²⁴ W Hz⁻¹ radio AGNs (∼16%) host an L_0.5-10 keV > 10⁴² erg s⁻¹ X-ray AGN, while above these thresholds, all X-ray AGNs in BCGs are also radio AGNs. As examples of the corona class, we also present detailed analyses of a BCG corona associated with a strong radio AGN (ESO 137-006 in A3627) and one of the faintest coronae known (NGC 4709 in the Centaurus cluster). Our results suggest that the traditional cool core/noncool core dichotomy is too simple. A better alternative is the cool core distribution function, with the enclosed X-ray luminosity or gas mass.

Recent observations by Chandra and XMM-Newton indicate that there are complex structures at the cores of galaxy clusters, such as cavities and filaments. One plausible model for the formation of such structures is the interaction of radio jets with the intracluster medium (ICM). To investigate this idea, we use three-dimensional magnetohydrodynamic simulations including anisotropic (Braginskii) viscosity to study the effect of magnetic fields on the evolution and morphology of buoyant bubbles in the ICM. We investigate a range of different initial magnetic field geometries and strengths, and study the resulting X-ray surface brightness distribution for comparison to observed clusters. Magnetic tension forces and viscous transport along field lines tend to suppress instabilities parallel, but not perpendicular, to field lines. Thus, the evolution of the bubble depends strongly on the initial field geometry. We find that toroidal field loops initially confined to the interior of the bubble are best able to reproduce the observed cavity structures.

We present hydrodynamical N-body simulations of clusters of galaxies with feedback taken from semianalytic models of galaxy formation. The advantage of this technique is that the source of feedback in our simulations is a population of galaxies that closely resembles that found in the real universe. We demonstrate that, to achieve the high entropy levels found in clusters, active galactic nuclei must inject a large fraction of their energy into the intergalactic/intracluster media throughout the growth period of the central black hole. These simulations reinforce the argument of Bower et al., who arrived at the same conclusion on the basis of purely semianalytic reasoning.

We have obtained deep, high spatial resolution images of the central region of Abell 1795 at Hα and [N ii] λ6583 with the Maryland-Magellan Tunable Filter (MMTF), and in the far-ultraviolet (FUV) with the Advanced Camera for Surveys solar blind channel on the Hubble Space Telescope (HST). The superb image quality of the MMTF data has made it possible to resolve the known SE filament into a pair of thin, intertwined filaments extending for ∼50 kpc, with a width <1 kpc. The presence of these thin, tangled strands is suggestive of a cooling wake where runaway cooling is taking place, perhaps aided by an enhanced magnetic field in this region. The HST data further resolve these strands into chains of FUV-bright stellar clusters, indicating that these filaments are indeed sites of ongoing star formation, but at a rate ∼2 orders of magnitude smaller than the mass-deposition rates predicted from the X-ray data. The elevated [N ii]/Hα ratio and large spatial variations of the FUV/Hα flux ratio across the filaments indicate that O-star photoionization is not solely responsible for the ionization. The data favor collisional heating by cosmic rays either produced in situ by magnetohydrodynamical processes or conducted from the surrounding intracluster medium.

Fossil groups are systems with one single central elliptical galaxy and an unusual lack of luminous galaxies in the inner regions. The standard explanation for the formation of these systems suggests that the lack of bright galaxies is due to galactic cannibalism. In this study, we show the results of an optical and X-ray analysis of RX J1340.6+4018, the prototype fossil group. The data indicate that RX J1340.6+4018 is similar to clusters in almost every sense (dynamical mass, X-ray luminosity, M/L, and luminosity function) except for the lack of L* galaxies. There are claims in the literature that fossil systems have a lack of small mass halos, compared to predictions based on the lambda cold dark matter scenario. The observational data gathered on this and other fossil groups so far offer no support for this idea. Analysis of the SN Ia/SN II ejecta ratio in the inner and outer regions shows a marginally significant central dominance of SN Ia material. This suggests that either the merger which originated in the central galaxy was dry or the group has been formed at early epochs, although better data are needed to confirm this result.

Early-type galaxies possess a dilute hot ((2–10) × 10⁶ K) gas that is probably the thermalized ejecta of the mass loss from evolving stars. We investigate the processes by which the mass loss from orbiting stars interacts with the stationary hot gas for the case of the mass ejected in a planetary nebula event. Numerical hydrodynamic simulations show that at first, the ejecta expands nearly symmetrically, with an upstream bow shock in the hot ambient gas. At later times, the flow past the ejecta creates fluid instabilities that cause about half of the ejecta to separate and the other half to flow more slowly downstream in a narrow wake. When radiative cooling is included, most of the material in the wake (>80%) remains below 10⁵ K while the separated ejecta is hotter (10⁵–10⁶ K). The separated ejecta is still less than one-quarter the temperature of the ambient medium and the only way it will reach the temperature of the ambient medium is through turbulent mixing (after the material has left the grid). These calculations suggest that a significant fraction of the planetary nebula ejecta may not become part of the hot ambient material. This is in contrast to our previous calculations for continuous mass loss from giant stars in which most of the mass loss became hot gas. We speculate that detectable O vi emission may be produced, but more sophisticated calculations will be required to determine the emission spectrum and to better define the fraction of cooled material.

Using a linear stability analysis and two- and three-dimensional nonlinear simulations, we study the physics of buoyancy instabilities in a combined thermal and relativistic (cosmic ray) plasma, motivated by the application to clusters of galaxies. We argue that the cosmic-ray diffusion time is likely to be long compared to the buoyancy time on large length scales, so that cosmic rays are effectively adiabatic. If the cosmic-ray pressure p_cr is ≳25% of the thermal pressure, and the cosmic-ray "entropy" p_cr/ρ^4/3 (where ρ is the thermal-plasma density) decreases outward, cosmic rays drive an adiabatic convective instability analogous to Schwarzschild convection in stars. Global simulations of galaxy cluster cores show that this instability saturates by reducing the cosmic-ray entropy gradient and driving efficient convection and turbulent mixing. At larger radii in cluster cores where cosmic-ray pressure is negligible, the thermal plasma is unstable to the heat-flux-driven buoyancy instability (HBI), a convective instability generated by anisotropic thermal conduction and a background conductive heat flux. The HBI saturates by rearranging the magnetic field lines to become largely perpendicular to the local gravitational field; the resulting turbulence also primarily mixes plasma in the perpendicular plane. Cosmic-ray-driven convection and the HBI may contribute to redistributing metals produced by Type Ia supernovae in clusters. Our calculations demonstrate that adiabatic simulations of galaxy clusters can artificially suppress the mixing of thermal plasma. When anisotropic thermal conduction is included, the buoyant response of the thermal plasma is not governed by the stable entropy gradient, and mixing (driven by mergers, cosmic ray buoyancy, etc.) is more effective. Such mixing may contribute to cosmic rays being distributed throughout the cluster volume.