Parker/buoyancy instabilities with anisotropic thermal conduction, cosmic rays, and arbitrary magnetic field strength

We report the results of a local stability analysis for a magnetized, gravitationally stratified plasma containing cosmic rays. We account for cosmic-ray diffusion and thermal conduction parallel to the magnetic field and allow beta to take any value, where p is the plasma pressure and B is the magnetic field strength. We take the gravitational acceleration to be in the -z-direction and the equilibrium magnetic field to be in the y-direction, and we derive the dispersion relation for small-amplitude instabilities and waves in the large-|k_x| limit. We use the Routh-Hurwitz criterion to show analytically that the necessary and sufficient criterion for stability in this limit is n k_B dT/dz + dp_cr/dz + (1/8pi)dB^2/dz > 0, where T is the temperature, n is the number density of thermal particles, and p_cr is the cosmic-ray pressure. We present approximate analytical solutions for the normal modes in the low- and high-diffusivity limits, show that they are consistent with the derived stability criterion, and compare them to numerical results obtained from the full, unapproximated, dispersion relation. Our results extend earlier analyses of buoyancy instabilities in galaxy-cluster plasmas to the beta <= 1 regime. Our results also extend earlier analyses of the Parker instability to account for anisotropic thermal conduction, and show that the interstellar medium is more unstable to the Parker instability than was predicted by previous studies in which the thermal plasma was treated as adiabatic.Comment: 36 pages, 2 figures, Accepted for publication in Ap

Models of the ICM with Heating and Cooling: Explaining the Global and Structural X-ray Properties of Clusters

(Abridged) Theoretical models that include only gravitationally-driven processes fail to match the observed mean X-ray properties of clusters. As a result, there has recently been increased interest in models in which either radiative cooling or entropy injection play a central role in mediating the properties of the intracluster medium. Both sets of models give reasonable fits to the mean properties of clusters, but cooling only models result in fractions of cold baryons in excess of observationally established limits and the simplest entropy injection models do not treat the "cooling core" structure present in many clusters and cannot account for entropy profiles revealed by recent X-ray observations. We consider models that marry radiative cooling with entropy injection, and confront model predictions for the global and structural properties of massive clusters with the latest X-ray data. The models successfully and simultaneously reproduce the observed L-T and L-M relations, yield detailed entropy, surface brightness, and temperature profiles in excellent agreement with observations, and predict a cooled gas fraction that is consistent with observational constraints. The model also provides a possible explanation for the significant intrinsic scatter present in the L-T and L-M relations and provides a natural way of distinguishing between clusters classically identified as "cooling flow" clusters and dynamically relaxed "non-cooling flow" clusters. The former correspond to systems that had only mild levels (< 300 keV cm^2) of entropy injection, while the latter are identified as systems that had much higher entropy injection. This is borne out by the entropy profiles derived from Chandra and XMM-Newton.Comment: 20 pages, 15 figures, accepted for publication in the Astrophysical Journa

On viscosity, conduction and sound waves in the intracluster medium

Recent X-ray and optical observations of the Perseus cluster indicate that the viscous and conductive dissipation of sound waves is the mechanism responsible for heating the intracluster medium and thus balancing radiative cooling of cluster cores. We discuss this mechanism more generally and show how the specific heating and cooling rates vary with temperature and radius. It appears that the heating mechanism is most effective above 10^7K, which allows for radiative cooling to proceed within normal galaxy formation but will stifle the growth of very massive galaxies. The scaling of the wavelength of sound waves with cluster temperature and feedback in the system are investigated.Comment: 5 pages, 4 figures, MNRAS accepte

X-ray iron line variability for the model of an orbiting flare above a black hole accretion disc

The broad X-ray iron line, detected in many active galactic nuclei, is likely to be produced by fluorescence from the X-ray illuminated central parts of an accretion disc close to a supermassive black hole. The time-averaged shape of the line can be explained most naturally by a combination of special and general relativistic effects. Such line profiles contain information about the black hole spin and the accretion disc as well as the geometry of the emitting region and may help to test general relativity in the strong gravity regime. In this paper we embark on the computation of the temporal response of the line to the illuminating flux. Previous studies concentrated on the calculation of reverberation signatures from static sources illuminating the disc. In this paper we focus on the more physically justified case of flares located above the accretion disc and corotating with it. We compute the time dependent iron line taking into account all general relativistic effects and show that its shape is of very complex nature, and also present light curves accompanying the iron line variability. We suggest that future X-ray satellites like XMM or Constellation-X may be capable of detecting features present in the computed reverberation maps.Comment: Accepted for publication in MNRAS, 11 pages, 12 figure

Impact of Systematic Errors in Sunyaev-Zel'dovich Surveys of Galaxy Clusters

Future high-resolution microwave background measurements hold the promise of detecting galaxy clusters throughout our Hubble volume through their Sunyaev-Zel'dovich (SZ) signature, down to a given limiting flux. The number density of galaxy clusters is highly sensitive to cluster mass through fluctuations in the matter power spectrum, as well as redshift through the comoving volume and the growth factor. This sensitivity in principle allows tight constraints on such quantities as the equation of state of dark energy and the neutrino mass. We evaluate the ability of future cluster surveys to measure these quantities simultaneously when combined with PLANCK-like CMB data. Using a simple effective model for uncertainties in the cluster mass-SZ flux relation, we evaluate systematic shifts in cosmological constraints from cluster SZ surveys. We find that a systematic bias of 10% in cluster mass measurements can give rise to shifts in cosmological parameter estimates at levels larger than the $1\sigma$ statistical errors. Systematic errors are unlikely to be detected from the mass and redshift dependence of cluster number counts alone; increasing survey size has only a marginal effect. Implications for upcoming experiments are discussed.Comment: 12 pages, 6 figures; accepted to JCAP; revised to match submitted versio

XMM-Newton observations of the merging galaxy cluster CIZA J2242.8+5301

We studied the intracluster medium of the galaxy cluster CIZA J2242.8+5301 using deep XMM-Newton observations. The cluster hosts a remarkable 2-Mpc long, ~50-kpc wide radio relic that has been nicknamed the "Sausage". A smaller, more irregular counter-relic is also present, along with a faint giant radio halo. We analysed the distribution of the ICM physical properties, and searched for shocks by trying to identify density and temperature discontinuities. East of the southern relic, we find evidence of shock compression corresponding to a Mach number of 1.3, and speculate that the shock extends beyond the length of the radio structure. The ICM temperature increases at the northern relic. More puzzling, we find a "wall" of hot gas east of the cluster centre. A partial elliptical ring of hot plasma appears to be present around the merger. While radio observations and numerical simulations predict a simple merger geometry, the X-ray results point towards a more complex merger scenario.Comment: Extensively revised and expanded, with 18 pages and 17 figure

High brightness temperatures and circular polarisation in extra-galactic radio sources

Some rapidly variable extra-galactic radio sources show very high brightness temperatures T_B>10^{12}K and high degrees of circular polarisation (1%). Standard synchrotron models that assume a power-law electron distribution cannot produce such high temperatures and have much lower degrees of intrinsic circular polarisation. We examine the synchrotron and inverse Compton radiation from a monoenergetic electron distribution using standard synchrotron theory. Constraints on the source parameters are found by formulating the results as functions of the source size, Doppler boosting factor, optical depth to synchrotron self-absorption, maximum frequency of synchrotron emission, and the strength of the inverse Compton radiation. The model gives brightness temperatures T_B=10^{13}K to 10^{14}K for moderate (<10) Doppler boosting factors and intrinsic degrees of circular polarisation at the percent level. It predicts a spectrum I_\nu\propto\nu^{1/3} between the radio and the infra-red as well as emission in the MeV to GeV range. We find the conditions under which electrons do not cool within the source, enabling the GHz emission to emerge without absorption and the potentially catastrophic energy losses by inverse Compton scattering to be avoided. We suggest that sources such as PKS 1519 -273, PKS 0405 -385 and J 1819 +3845 can be understood within this scenario without invoking high Doppler boosting factors, coherent emission mechanisms, or the dominance of proton synchrotron radiation.Comment: 4 pages, to appear in A&A Letter

HST/ACS Emission Line Imaging of Low Redshift 3CR Radio Galaxies I: The Data

We present 19 nearby (z<0.3) 3CR radio galaxies imaged at low- and high-excitation as part of a Cycle 15 Hubble Space Telescope snapshot survey with the Advanced Camera for Surveys. These images consist of exposures of the H-alpha (6563 \AA, plus [NII] contamination) and [OIII] 5007 \AA emission lines using narrow-band linear ramp filters adjusted according to the redshift of the target. To facilitate continuum subtraction, a single-pointing 60 s line-free exposure was taken with a medium-band filter appropriate for the target's redshift. We discuss the steps taken to reduce these images independently of the automated recalibration pipeline so as to use more recent ACS flat-field data as well as to better reject cosmic rays. We describe the method used to produce continuum-free (pure line-emission) images, and present these images along with qualitative descriptions of the narrow-line region morphologies we observe. We present H-alpha+[NII] and [OIII] line fluxes from aperture photometry, finding the values to fall expectedly on the redshift-luminosity trend from a past HST/WFPC2 emission line study of a larger, generally higher redshift subset of the 3CR. We also find expected trends between emission line luminosity and total radio power, as well as a positive correlation between the size of the emission line region and redshift. We discuss the associated interpretation of these results, and conclude with a summary of future work enabled by this dataset.Comment: 18 pages, 12 figures, accepted for publication in ApJ

The impact of mergers on relaxed X-ray clusters - III. Effects on compact cool cores

(Abridged) We use the simulations presented in Poole et al. 2006 to examine the effects of mergers on compact cool cores in X-ray clusters. We propose a scheme for classifying the morphology of clusters based on their surface brightness and entropy profiles. Three dominant morphologies emerge: two hosting compact cores and central temperatures which are cool (CCC systems) or warm (CWC systems) and one hosting extended cores which are warm (EWC systems). We find that CCC states are disrupted only after direct collisions between cluster cores in head-on collisions or during second pericentric passage in off-axis mergers. By the time they relax, our remnant cores have generally been heated to warm core (CWC or EWC) states but subsequently recover CCC states. The only case resulting in a long-lived EWC state is a slightly off-axis 3:1 merger for which the majority of shock heating occurs during the accretion of a low-entropy stream formed from the disruption of the secondary's core. Compression prevents core temperatures from falling until after relaxation thus explaining the observed population of relaxed CWC systems with no need to invoke AGN feedback. The morphological segregation observed in the L_x-T_x and beta-r_c scaling relations is reflected in our simulations as well. However, none of the cases we have studied produce sufficiently high remnant central entropies to account for the most under-luminous EWC systems observed. Lastly, systems which initially host central metallicity gradients do not yield merger remnants with flat metallicity profiles. Taken together, these results suggest that once formed, compact core systems are remarkably stable against disruption from mergers. It remains to be demonstrated exactly how the sizable observed population of extended core systems was formed.Comment: 19 pages, 8 figures, submitted for publication in MNRA

Little change in the sizes of the most massive galaxies since z = 1

Recent reports suggest that elliptical galaxies have increased their size dramatically over the last ~8 Gyr. This result points to a major re-think of the processes dominating the latetime evolution of galaxies. In this paper we present the first estimates for the scale sizes of brightest cluster galaxies (BCGs) in the redshift range 0.8 < z < 1.3 from an analysis of deep Hubble Space Telescope imaging, comparing to a well matched local sample taken from the Local Cluster Substructure Survey at z ~ 0.2. For a small sample of 5 high redshift BCGs we measure half-light radii ranging from 14 - 53 kpc using de Vaucuoleurs profile fits, with an average determined from stacking of 32.1 \pm 2.5 kpc compared to a value 43.2 \pm 1.0 kpc for the low redshift comparison sample. This implies that the scale sizes of BCGs at z = 1 are ~ 30% smaller than at z = 0.25. Analyses comparing either Sersic or Petrosian radii also indicate little or no evolution between the two samples. The detection of only modest evolution at most out to z = 1 argues against BCGs having undergone the large increase in size reported for massive galaxies since z = 2 and in fact the scale-size evolution of BCGs appears closer to that reported for radio galaxies over a similar epoch. We conclude that this lack of size evolution, particularly when coupled with recent results on the lack of BCG stellar mass evolution, demonstrates that major merging is not an important process in the late time evolution of these systems. The homogeneity and maturity of BCGs at z = 1 continues to challenge galaxy evolution models.Comment: Accepted for publication in MNRA