I. Zhuravleva

X-ray surface brightness and gas density fluctuations in the Coma cluster

X-ray surface brightness fluctuations in the core (650 × 650 kpc) region of the Coma cluster observed with XMM-Newton and Chandra are analyzed using a 2D power spectrum approach. The resulting 2D spectra are converted to 3D power spectra of gas density fluctuations. The characteristic amplitude of the volume filling density fluctuations relative to the smooth underlying density distribution varies from 7-10% on scales of �500 kpc down to �5% at scales � 30 kpc. On smaller spatial scales, projection effects smear the density fluctuations by a large factor, precluding strong limits on the fluctuations in 3D. On the largest scales probed (hundreds of kpc), the dominant contributions to the observed fluctuations most likely arise from perturbations of the gravitational potential by the two most massive galaxies in Coma, NGC4874 and NGC4889, and the low entropy gas brought to the cluster by an infalling group. Other plausible sources of density fluctuations are discussed, including turbulence, metal abundance variations, and unresolved sources.

Constraints on turbulent pressure in the X-ray haloes of giant elliptical galaxies from resonant scattering

The dense cores of X-ray emitting gaseous halos of large elliptical galaxies with temperatures kT . 0.8 keV show two prominent Fexvii emission features, which provide a sensitive diagnostic tool to measure the effects of resonant scattering. We present here high-resoluti on spectra of five bright nearby elliptical galaxies, obtained with the Reflection Grating Spectrometers (RGS) on the XMM-Newtonsatellite. The spectra for the cores of four of the galaxies s how the Fexvii line at 15.01 A being suppressed by resonant scattering. The data for NGC 4636 in particular allow the effects of resonant scattering to be studied in detail and to pro ve that the 15.01 A line is suppressed only in the dense core and not in the surrounding regions. Using deprojected density and temperature profiles for this ga laxy obtained with the Chandra satellite, we model the radial intensity profiles of the stro ngest resonance lines, accounting for the effects of resonant scattering, for different values of the characteristic turbulent velocity. Comparing the model to the data, we find that the isotropic tur bulent velocities on spatial scales smaller than≈1 kpc are less than 100 km s −1 and the turbulent pressure support in the galaxy core is smaller than 5% of the thermal pressure at the 9 0% confidence level, and less than 20% at 95% confidence. Neglecting the effects of resonant scattering in spectral fitting of the inner 2 kpc core of NGC 4636 will lead to underestimates of the chemical abundances of Fe and O by∼10‐20%.

Quantifying properties of ICM inhomogeneities

We present a new method to identify and characterize the structure of the intracluster medium (ICM) in simulated galaxy clusters. The method uses the median of gas properties, such as density and pressure, which we show to be very robust to the presence of gas inhomogeneities. In particular, we show that the radial proles of median gas properties in cosmological simulations of clusters are smooth and do not exhibit uctuations at locations of massive clumps in contrast to mean and mode properties. Analysis of simulations shows that distribution of gas properties in a given radial shell can be well described by a log-normal PDF and a tail. The former corresponds to a nearly hydrostatic bulk component, accounting for 99 per cent of the volume, while the tail corresponds to high density inhomogeneities. The clumps can thus be easily identied with the volume elements corresponding to the tail of the distribution. We show that this results in a simple and robust separation of the diuse and clumpy components of the ICM. The full width half maximum of the density distribution in simulated clusters is a growing function of radius and varies from 0.15 dex in cluster centre to 0.5 dex at 2r500 in relaxed clusters. The small scatter in the width between relaxed clusters suggests that the degree of inhomogeneity is a robust characteristic of the ICM. It broadly agrees with the amplitude of density perturbations found in the Coma cluster core. We discuss the origin of ICM density variations in spherical shells and show that less than 20 per cent of the width can be attributed to the triaxiality of the cluster gravitational potential. As a link to X-ray observations of real clusters we evaluated the ICM clumping factor, weighted with the temperature dependent X-ray emissivity, with and without high density inhomogeneities. We argue that these two cases represent upper and lower limits on the departure of the observed X-ray emissivity from the median value. We nd that the typical value of the clumping factor in the bulk component of relaxed clusters varies from 1:1 1:2 at r500 up to 1:3 1:4 at r200 , in broad agreement with recent observations.

Azimuthally Resolved X-Ray Spectroscopy to the Edge of the Perseus Cluster

We present the results from extensive, new observations of the Perseus Cluster of galaxies, obtained as a Suzaku Key Project. The 85 pointings analyzed span eight azimuthal directions out to 2 degrees = 2.6 Mpc, to and beyond the virial radius r_200 ~ 1.8 Mpc, offering the most detailed X-ray observation of the intracluster medium (ICM) at large radii in any cluster to date. The azimuthally averaged density profile for r>0.4r_200 is relatively flat, with a best-fit power-law index of 1.69+/-0.13 significantly smaller than expected from numerical simulations. The entropy profile in the outskirts lies systematically below the power-law behavior expected from large-scale structure formation models which include only the heating associated with gravitational collapse. The pressure profile beyond ~0.6r_200 shows an excess with respect to the best-fit model describing the SZ measurements for a sample of clusters observed with Planck. The inconsistency between the expected and measured density, entropy, and pressure profiles can be explained primarily by an overestimation of the density due to inhomogeneous gas distribution in the outskirts; there is no evidence for a bias in the temperature measurements within the virial radius. We find significant differences in thermodynamic properties of the ICM at large radii along the different arms. Along the cluster minor axis, we find a flattening of the entropy profiles outside ~0.6r_200, while along the major axis, the entropy rises all the way to the outskirts. Correspondingly, the inferred gas clumping factor is typically larger along the minor than along the major axis.

Steepening mass profiles, dark matter and environment of X-ray bright elliptical galaxies

We use a new non-parametric Bayesian approach to obtain the most probable mass distributions and circular velocity curves along with their confidence ranges, given deprojected density and temperature profiles of the hot gas surrounding X-ray bright elliptical galaxies. For a sample of six X-ray bright ellipticals, we find that all circular velocity curves are rising in the outer parts due to a combination of a rising temperature profile and a logarithmic pressure gradient that increases in magnitude. Therefore at large radii, mass density profiles rise more steeply than isothermal profiles, implying that we are probing the more massive group-sized haloes in which these galaxies are embedded. Comparing the circular velocity curves we obtain from X-rays to those obtained from dynamical models, we find that the former are often lower in the central � 10 kpc. This is probably due to a combination of: i) Non-thermal contributions of up to � 35% in the pressure (with stronger effects in NGC 4486), ii) multiple-temperature components in the hot gas, iii) incomplete kinematic spatial coverage in the dynamical models, and iv) mass profiles that are insufficiently general in the dynamical modelling. Complementing the total mass information from the Xrays with photometry and stellar population models to infer the dark matter content, we find evidence for massive dark matter haloes with dark matter mass fractions of � 35–80% at 2Re, rising to a maximum of 80–90% at the outermost radii. We also find that the six galaxies follow a Tully-Fisher relation with slope � 4 and that their circular velocities at 1Re correlate strongly with the velocity dispersion of the local environment. As a result, the galaxy luminosity at 1Re also correlates with the velocity dispersion of the environment. These relations suggest a close link between the properties of central X-ray bright elliptical galaxies and their environments.

The relation between gas density and velocity power spectra in galaxy clusters: High-resolution hydrodynamic simulations and the role of conduction

Exploring the power spectrum of fluctuations and velocities in the intracluster medium (ICM) can help us to probe the gas physics of galaxy clusters. Using high-resolution 3D plasma simulations, we study the statistics of the velocity field and its inti mate relation with the ICM thermodynamic perturbations. The normalization of the ICM spectrum (related to density, entropy, or pressure fl uctuations) is linearly tied to the level of large-scale motions, which e xcite both gravity and sound waves due to stratification. For low 3D Mach number M ∼ 0.25, gravity waves mainly drive entropy perturbations, traced by preferentially tangential turbu lence. For M > 0.5, sound waves start to significantly contribute, passing th e leading role to compressive pressure fluctuations, associated with isotropic turbulence (or a slight radial bias). Densit y and temperature fluctuations are then characterized by the dominant process: isobaric (low M), adiabatic (high M), or isothermal (strong conduction). Most clusters reside in the intermediate regime, showing a mixture of gravity and sound waves, hence drifting towards isotropic velocities. Remarkably, regardless of t he regime, the variance of density perturbations is comparable to the 1D Mach number, M1D ∼δρ/ρ. This linear relation allows to easily convert between gas motions and ICM perturbations (δρ/ρ 0.1 in massive clusters), allowing to calibrate the linear relation and to constrain relative perturbations down to just a few per cent .

Estimating turbulent velocities in the elliptical galaxies NGC 5044 and NGC 5813

Context. The interstellar and intra-cluster medium (ICM) in giant elliptical galaxies and clusters of galaxies is often assumed to be in hydrostatic equilibrium. Numerical simulations, however, show that about 5-30% of the pressure in a cluster is provided by turbulence induced by, for example, the central active galactic nucleus (AGN) and merger activity. Aims. We aim to put constraints on the turbulent velocities and the turbulent pressure in the ICM of the giant elliptical galaxies NGC 5044 and NGC 5813 using XMM-Newton reflection grating spectrometer (RGS) observations. Methods. The magnitude of the turbulence is estimated using the Fe xvii lines at 15.01 A, 17.05 A, and 17.10 A in the RGS spectra. At low turbulent velocities, the gas becomes optically thick in the 15.01 A line due to resonant scattering, while the 17 A lines remain optically thin. By comparing the (I17.05 + I17.10)/I15.01 line ratio from RGS with simulated line ratios for different Mach numbers, the level of turbulence is constrained. The measurement is, however, limited by the systematic uncertainty in the line ratio for an optically thin plasma, which is about 20-30%. Results. We find that the (I17.05 + I17.10)/I15.01 line ratio in NGC 5813 is significantly higher than in NGC 5044. This difference can be explained by a higher level of turbulence in NGC 5044. The best estimates for the turbulent velocities using resonant scattering and upper limits from the line widths, are 320 40% in NGC 5044, assuming isotropic turbulence, confirm that it is a highly disturbed system, probably due to an off-axis merger. The turbulent pressure support in NGC 5813 is more modest at 15-45%. The (I17.05 + I17.10)/I15.01 line ratio in an optically thin plasma, calculated using AtomDB v2.0.1, is 2σ above the ratio measured in NGC 5044, which cannot be explained by resonant scattering. This shows that the discrepancies between theoretical, laboratory, and astrophysical data on Fe xvii lines need to be reduced to improve the accuracy of the determination of turbulent velocities using resonant scattering.

Gas density fluctuations in the Perseus Cluster: clumping factor and velocity power spectrum

X-ray surface brightness fluctuations in the core of the Perseus Cluster are analyzed, using deep observations with the Chandra observatory. The amplitude of gas density fluctuations on different scales is measured in a set of radial annuli. It varies from 8 to 12 per cent on scales of ~10-30 kpc within radii of 30-160 kpc from the cluster center and from 9 to 7 per cent on scales of ~20-30 kpc in an outer, 60-220 kpc annulus. Using a statistical linear relation between the observed amplitude of density fluctuations and predicted velocity, the characteristic velocity of gas motions on each scale is calculated. The typical amplitudes of the velocity outside the central 30 kpc region are 90-140 km/s on ~20-30 kpc scales and 70-100 km/s on smaller scales ~7-10 kpc. The velocity power spectrum is consistent with cascade of turbulence and its slope is in a broad agreement with the slope for canonical Kolmogorov turbulence. The gas clumping factor estimated from the power spectrum of the density fluctuations is lower than 7-8 per cent for radii ~30-220 kpc from the center, leading to a density bias of less than 3-4 per cent in the cluster core. Uncertainties of the analysis are examined and discussed. Future measurements of the gas velocities with the Astro-H, Athena and Smart-X observatories will directly measure the gas density-velocity perturbation relation and further reduce systematic uncertainties in these quantities.

Constraints on the ICM velocity power spectrum from the X-ray lines width and shift

Future X-ray observations of galaxy clusters by high spectral resolution missions will provide spatially resolved measurements of the energy and width for the brightest emission lines in the intracluster medium (ICM) spectrum. In this paper we discuss various ways of using these high resolution data to constrain velocity power spectrum in galaxy clusters. We argue that variations of these quantities with the projected distance R in cool core clusters contain important information on the velocity field length scales (i.e. the size of energy-containing eddies) in the ICM. The effective length leff along the line of sight, which provides dominant contribution to the line flux, increases with R, allowing one to probe the amplitude of the velocity variations at different spatial scales. In particular, we show that the width of the line as a function of R is closely linked to the structure function of the 3D velocity field. Yet another easily obtainable proxy of the velocity field length scales is the ratio of the amplitude of the projected velocity field (line energy) variations to the dispersion of the velocity along the line of sight (line width). Finally the projected velocity field can be easily converted into 3D velocity field, especially for clusters like Coma with an extended flat core in the surface brightness. Under assumption of a homogeneous isotropic Gaussian 3D velocity field we derived simple expressions relating the power spectrum of the 3D velocity field (or structure function) and the observables. We illustrate the sensitivity of these proxies to changes in the characteristics of the power spectrum for a simple isothermal β-model of a cluster. The uncertainties in the observables, caused by stochastic nature of the velocity field, are estimated by making multiple realizations of the random Gaussian velocity field and evaluating the scatter in observables. If large scale motions are present in the ICM these uncertainties may dominate the statistical errors of line width and shift measurements.

EXPLORING THE UNUSUALLY HIGH BLACK-HOLE-TO-BULGE MASS RATIOS IN NGC 4342 AND NGC 4291: THE ASYNCHRONOUS GROWTH OF BULGES AND BLACK HOLES

ABSTRACTWe study two nearby early-type galaxies, NGC 4342 and NGC 4291, that host unusually massive black holesrelative to their low stellar mass. The observed black-hole-to-bulge mass ratios of NGC 4342 and NGC 4291are 6 . 9 +3 . 8−2 . 3 % and 1 . 9% ±0 . 6%, respectively, which significantly exceed the typical observed ratio of ∼0 . 2%. Asa consequence of the exceedingly large black-hole-to-bulge mass ratios, NGC 4342 and NGC 4291 are ≈5 . 1 σ and ≈3 . 4 σ outliers from the M • – M bulge scaling relation, respectively. In this paper, we explore the origin of theunusually high black-hole-to-bulge mass ratio. Based on Chandra X-ray observations of the hot gas content ofNGC 4342 and NGC 4291, we compute gravitating mass profiles, and conclude that both galaxies reside in massivedark matter halos, which extend well beyond the stellar light. The presence of dark matter halos around NGC 4342and NGC 4291 and a deep optical image of the environment of NGC 4342 indicate that tidal stripping, in which 90% of the stellar mass was lost, cannot explain the observed high black-hole-to-bulge mass ratios. Therefore,we conclude that these galaxies formed with low stellar masses, implying that the bulge and black hole did notgrow in tandem. We also find that the black hole mass correlates well with the properties of the dark matter halo,suggesting that dark matter halos may play a major role in regulating the growth of the supermassive black holes.