Recent observations show that the cooling flows in the central regions of galaxy clusters are highly suppressed. Observed active galactic nuclei (AGN)-induced cavities/bubbles are a leading candidate for suppressing cooling, usually via some form of mechanical heating. At the same time, observed X-ray cavities and synchrotron emission point towards a significant nonthermal particle population. Previous studies have focused on the dynamical effects of cosmic ray pressure support, but none has built successful models in which cosmic ray heating is significant. Here, we investigate a new model of AGN heating, in which the intracluster medium is efficiently heated by cosmic rays, which are injected into the intra-cluster medium (ICM) through diffusion or the shredding of the bubbles by Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal conduction as well. Using numerical simulations, we show that the cooling catastrophe is efficiently suppressed. The cluster quickly relaxes to a quasiequilibrium state with a highly reduced accretion rate and temperature and density profiles which match observations. Unlike the conduction-only case, no fine-tuning of the Spitzer conduction suppression factor f is needed. The cosmic ray pressure, P c /P g 0.1 and ∇P c 0.1ρg, is well within observational bounds. Cosmic ray heating is a very attractive alternative to mechanical heating, and may become particularly compelling if Gamma-ray Large Array Space Telescope (GLAST) detects the γ -ray signature of cosmic rays in clusters.

The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in the Galaxy, which extend about 50• (∼10 kpc) above and below the Galactic center (GC) and are symmetric about the Galactic plane. Using axisymmetric hydrodynamic simulations with a self-consistent treatment of the dynamical cosmic ray (CR)-gas interaction, we show that the bubbles can be created with a recent active galactic nucleus (AGN) jet activity about 1-3 Myr ago, which was active for a duration of ∼0.1-0.5 Myr. The bipolar jets were ejected into the Galactic halo along the rotation axis of the Galaxy. Near the GC, the jets must be moderately light with a typical density contrast 0.001 η 0.1 relative to the ambient hot gas. The jets are energetically dominated by kinetic energy, and overpressured with either CR or thermal pressure which induces lateral jet expansion, creating fat CR bubbles as observed. The sharp edges of the bubbles imply that CR diffusion across the bubble surface is strongly suppressed. The jet activity induces a strong shock, which heats and compresses the ambient gas in the Galactic halo, potentially explaining the ROSAT X-ray shell features surrounding the bubbles. The Fermi bubbles provide plausible evidence for a recent powerful AGN jet activity in our Galaxy, providing new insights into the origin of the halo CR population and the channel through which massive black holes in disk galaxies release feedback energy during their growth.

The observed bimodality in radio luminosity in galaxy clusters is puzzling. We investigate the possibility that cosmic-ray (CR) streaming in the intra-cluster medium can 'switch off' hadronically induced radio and gamma-ray emission. For self-confined CRs, this depends on the source of MHD wave damping: if only non-linear Landau damping operates, then CRs stream on the slow Alfvénic timescale, but if turbulent wave damping operates, super-Alfvénic streaming is possible. As turbulence increases, it promotes outward streaming more than it enables inward turbulent advection. Curiously, the CR flux is independent of ∇f (as long as it is non-zero) and depends only on plasma parameters; this enables radio halos with flat inferred CR profiles to turn off. We perform 1D time-dependent calculations of a radio mini-halo (Perseus) and giant radio halo (Coma) and find that both diminish in radio luminosity by an order of magnitude in several hundred Myr, given plausible estimates for the magnetic field in the outskirts of the cluster. Due to the energy dependence of CR streaming, spectral curvature develops, and radio halos turn off more slowly at low frequencies -properties consistent with observations. Similarly, CR streaming rapidly turns off gamma-ray emission at the high-energies probed by Cherenkov telescopes, but not at the low energies probed by Fermi. CR mediated wave-heating of the ICM is unaffected, as it is dominated by ∼GeV CRs which stream Alfvénically.

We investigate the effects of AGN feedback on the cosmological evolution of an isolated elliptical galaxy by performing two-dimensional high-resolution hydrodynamical numerical simulations. The inner boundary of the simulation is chosen so that the Bondi radius is resolved. Compared to previous works, the two accretion modes, namely hot and cold, which correspond to different accretion rates and have different radiation and wind outputs, are carefully discriminated and the feedback effects by radiation and wind in each mode are taken into account. The most updated AGN physics, including the descriptions of radiation and wind from the hot accretion flows and wind from cold accretion disks, are adopted. Physical processes like star formation, Type Ia and Type II supernovae are taken into account. We study the AGN light curve, typical AGN lifetime, growth of the black hole mass, AGN duty-cycle, star formation, and the X-ray surface brightness of the galaxy. We compare our simulation results with observations and find general consistency. Comparisons with previous simulation works find significant differences, indicating the importance of AGN physics. The respective roles of radiation and wind feedbacks are examined and it is found that they are different for different problems of interest such as AGN luminosity and star formation. We find that it is hard to neglect any of them, so we suggest to use the names of "cold feedback mode" and "hot feedback mode" to replace the currently used ones.

The Crab Nebula is a bright source of gamma-rays powered by the Crab Pulsar's rotational energy, through the formation and termination of a relativistic electron-positron wind. We report the detection of γ-rays from this source with energies from 5 × 10−4 to 1.1 petaelectronvolts (PeV), with a spectrum showing gradual steepening over three energy decades. The ultra-high-energy photons imply the presence of a PeV electron accelerator (a pevatron) in the nebula, with an acceleration rate exceeding 15% of the theoretical limit. We constrain the pevatron's size between 0.025 and 0.1 pc, and magnetic field ≈110 μG. The production rate of PeV electrons, 2.5 × 1036 erg s−1, constitutes 0.5% of the pulsar spin-down luminosity, although we cannot exclude a contribution of PeV protons to the production of the highest energy γ-rays.