arXiv subjects

J. S. Sanders

Publications and source records attributed to J. S. Sanders.

At least 91 records · Page 5Linked to original sources

arXiv2016

HST imaging of the dusty filaments and nucleus swirl in NGC4696 at the centre of the Centaurus Cluster

Narrow-band HST imaging has resolved the detailed internal structure of the 10 kpc diameter H alpha+[NII] emission line nebulosity in NGC4696, the central galaxy in the nearby Centaurus cluster, showing that the dusty, molecular, filaments have a width of about 60pc. Optical morphology and velocity measurements indicate that the filaments are dragged out by the bubbling action of the radio source as part of the AGN feedback cycle. Using the drag force we find that the magnetic field in the filaments is in approximate pressure equipartition with the hot gas. The filamentary nature of the cold gas continues inward, swirling around and within the Bondi accretion radius of the central black hole, revealing the magnetic nature of the gas flows in massive elliptical galaxies. HST imaging resolves the magnetic, dusty, molecular filaments at the centre of the Centaurus cluster to a swirl around and within the Bondi radius.

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arXiv2016

Applications for edge detection techniques using Chandra and XMM-Newton data: galaxy clusters and beyond

The unrivalled spatial resolution of the Chandra X-ray observatory has allowed many breakthroughs to be made in high energy astrophysics. Here we explore applications of Gaussian Gradient Magnitude (GGM) filtering to X-ray data, which dramatically improves the clarity of surface brightness edges in X-ray observations, and maps gradients in X-ray surface brightness over a range of spatial scales. In galaxy clusters, we find that this method is able to reveal remarkable substructure behind the cold fronts in Abell 2142 and Abell 496, possibly the result of Kelvin Helmholtz instabilities. In Abell 2319 and Abell 3667, we demonstrate that the GGM filter can provide a straightforward way of mapping variations in the widths and jump ratios along the lengths of cold fronts. We present results from our ongoing programme of analysing the Chandra and XMM-Newton archives with the GGM filter. In the Perseus cluster we identify a previously unseen edge around 850 kpc from the core to the east, lying outside a known large scale cold front, which is possibly a bow shock. In MKW 3s we find an unusual 'V' shape surface brightness enhancement starting at the cluster core, which may be linked to the AGN jet. In the Crab nebula a new, moving feature in the outer part of the torus is identified which moves across the plane of the sky at a speed of ~0.1c, and lies much further from the central pulsar than the previous motions seen by Chandra.

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arXiv2016

Detecting edges in the X-ray surface brightness of galaxy clusters

The effects of many physical processes in the intracluster medium of galaxy clusters imprint themselves in X-ray surface brightness images. It is therefore important to choose optimal methods for extracting information from and enhancing the interpretability of such images. We describe in detail a gradient filtering edge detection method that we previously applied to images of the Centaurus cluster of galaxies. The Gaussian gradient filter measures the gradient in the surface brightness distribution on particular spatial scales. We apply this filter on different scales to Chandra X-ray observatory images of two clusters with AGN feedback, the Perseus cluster and M87, and a merging system, A3667. By combining filtered images on different scales using radial filters spectacular images of the edges in a cluster are produced. We describe how to assess the significance of features in filtered images. We find the gradient filtering technique to have significant advantages for detecting many kinds of features compared to other analysis techniques, such as unsharp-masking. Filtering cluster images in this way in a hard energy band allows shocks to be detected.

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arXiv2016

Deep Chandra study of the truncated cool core of the Ophiuchus cluster

We present the results of a deep (280 ks) Chandra observation of the Ophiuchus cluster, the second-brightest galaxy cluster in the X-ray sky. The cluster hosts a truncated cool core, with a temperature increasing from kT~1 keV in the core to kT~9 keV at r~30 kpc. Beyond r~30 kpc the intra-cluster medium (ICM) appears remarkably isothermal. The core is dynamically disturbed with multiple sloshing induced cold fronts, with indications for both Rayleigh-Taylor and Kelvin-Helmholtz instabilities. The sloshing is the result of the strongly perturbed gravitational potential in the cluster core, with the central brightest cluster galaxy (BCG) being displaced southward from the global center of mass. The residual image reveals a likely subcluster south of the core at the projected distance of r~280 kpc. The cluster also harbors a likely radio phoenix, a source revived by adiabatic compression by gas motions in the ICM. Even though the Ophiuchus cluster is strongly dynamically active, the amplitude of density fluctuations outside of the cooling core is low, indicating velocities smaller than ~100 km/s. The density fluctuations might be damped by thermal conduction in the hot and remarkably isothermal ICM, resulting in our underestimate of gas velocities. We find a surprising, sharp surface brightness discontinuity, that is curved away from the core, at r~120 kpc to the southeast of the cluster center. We conclude that this feature is most likely due to gas dynamics associated with a merger and not a result of an extraordinary active galactic nucleus (AGN) outburst. The cooling core lacks any observable X-ray cavities and the AGN only displays weak, point-like radio emission, lacking lobes or jets, indicating that currently it may be largely dormant. The lack of strong AGN activity may be due to the bulk of the cooling taking place offset from the central supermassive black hole.

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arXiv2016

ALMA observations of cold molecular gas filaments trailing rising radio bubbles in PKS0745-191

We present ALMA observations of the CO(1-0) and CO(3-2) line emission tracing filaments of cold molecular gas in the central galaxy of the cluster PKS0745-191. The total molecular gas mass of 4.6 +/- 0.3 x 10^9 solar masses, assuming a Galactic X_{CO} factor, is divided roughly equally between three filaments each extending radially 3-5 kpc from the galaxy centre. The emission peak is located in the SE filament roughly 1 arcsec (2 kpc) from the nucleus. The velocities of the molecular clouds in the filaments are low, lying within +/-100 km/s of the galaxy's systemic velocity. Their FWHMs are less than 150 km/s, which is significantly below the stellar velocity dispersion. Although the molecular mass of each filament is comparable to a rich spiral galaxy, such low velocities show that the filaments are transient and the clouds would disperse on <10^7 yr timescales unless supported, likely by the indirect effect of magnetic fields. The velocity structure is inconsistent with a merger origin or gravitational free-fall of cooling gas in this massive central galaxy. If the molecular clouds originated in gas cooling even a few kpc from their current locations their velocities would exceed those observed. Instead, the projection of the N and SE filaments underneath X-ray cavities suggests they formed in the updraft behind bubbles buoyantly rising through the cluster atmosphere. Direct uplift of the dense gas by the radio bubbles appears to require an implausibly high coupling efficiency. The filaments are coincident with low temperature X-ray gas, bright optical line emission and dust lanes indicating that the molecular gas could have formed from lifted warmer gas that cooled in situ.

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arXiv2016

A very deep Chandra view of metals, sloshing and feedback in the Centaurus cluster of galaxies

We examine deep Chandra X-ray observations of the Centaurus cluster of galaxies, Abell 3526. Applying a gradient magnitude filter reveals a wealth of structure, from filamentary soft emission on 100pc (0.5 arcsec) scales close to the nucleus to features 10s of kpc in size at larger radii. The cluster contains multiple high-metallicity regions with sharp edges. Relative to an azimuthal average, the deviations of metallicity and surface brightness are correlated, and the temperature is inversely correlated, as expected if the larger scale asymmetries in the cluster are dominated by sloshing motions. Around the western cold front are a series of ~7 kpc 'notches', suggestive of Kelvin-Helmholtz instabilities. The cold front width varies from 4 kpc down to close to the electron mean free path. Inside the front are multiple metallicity blobs on scales of 5-10 kpc, which could have been uplifted by AGN activity, also explaining the central metallicity drop and flat inner metallicity profile. Close to the nucleus are multiple shocks, including a 1.9-kpc-radius inner shell-like structure and a weak 1.1-1.4 Mach number shock around the central cavities. Within a 10 kpc radius are 9 depressions in surface brightness, several of which appear to be associated with radio emission. The shocks and cavities imply that the nucleus has been repeatedly active on 5-10 Myr timescales, indicating a tight balance between heating and cooling. We confirm the presence of a series of linear quasi-periodic structures. If they are sound waves, the ~5 kpc spacing implies a period of 6 Myr, similar to the ages of the shocks and cavities. Alternatively, these structures may be Kelvin-Helmholtz instabilities, their associated turbulence or amplified magnetic field layers.

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arXiv2015

Deep Chandra observation and numerical studies of the nearest cluster cold front in the sky

We present the results of a very deep (500 ks) Chandra observation, along with tailored numerical simulations, of the nearest, best resolved cluster cold front in the sky, which lies 90 kpc (19 arcmin) to the north-west of M 87. The northern part of the front appears the sharpest, with a width smaller than 2.5 kpc (1.5 Coulomb mean free paths; at 99 per cent confidence). Everywhere along the front, the temperature discontinuity is narrower than 4-8 kpc and the metallicity gradient is narrower than 6 kpc, indicating that diffusion, conduction and mixing are suppressed across the interface. Such transport processes can be naturally suppressed by magnetic fields aligned with the cold front. Interestingly, comparison to magnetohydrodynamic simulations indicates that in order to maintain the observed sharp density and temperature discontinuities, conduction must also be suppressed along the magnetic field lines. However, the northwestern part of the cold front is observed to have a non-zero width. While other explanations are possible, the broadening is consistent with the presence of Kelvin-Helmholtz instabilities (KHI) on length-scales of a few kpc. Based on comparison with simulations, the presence of KHI would imply that the effective viscosity of the intracluster medium is suppressed by more than an order of magnitude with respect to the isotropic Spitzer-like temperature dependent viscosity. Underneath the cold front, we observe quasi-linear features that are ~10 per cent brighter than the surrounding gas and are separated by ~15 kpc from each other in projection. Comparison to tailored numerical simulations suggests that the observed phenomena may be due to the amplification of magnetic fields by gas sloshing in wide layers below the cold front, where the magnetic pressure reaches ~5-10 per cent of the thermal pressure, reducing the gas density between the bright features.

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arXiv2015

Thermodynamic perturbations in the X-ray halo of 33 clusters of galaxies observed with Chandra ACIS

In high-resolution X-ray observations of the hot plasma in clusters of galaxies significant structures caused by AGN feedback, mergers, and turbulence can be detected. Many clusters have been observed by Chandra in great depth and at high resolution. Using archival data taken with the Chandra ACIS instrument the aim was to study thermodynamic perturbations of the X-ray emitting plasma and to apply this to better understand the thermodynamic and dynamic state of the intra cluster medium (ICM). We analysed deep observations for a sample of 33 clusters with more than 100 ks of Chandra exposure each at distances between redshift 0.025 and 0.45. The combined exposure of the sample is 8 Ms. Fitting emission models to different regions of the extended X-ray emission we searched for perturbations in density, temperature, pressure, and entropy of the hot plasma. For individual clusters we mapped the thermodynamic properties of the ICM and measured their spread in circular concentric annuli. Comparing the spread of different gas quantities to high-resolution 3D hydrodynamic simulations, we constrain the average Mach number regime of the sample to Mach1D ~ 0.16 +- 0.07. In addition we found a tight correlation between metallicity, temperature and redshift with an average metallicity of Z ~ 0.3 +- 0.1 Z(solar). This study provides detailed perturbation measurements for a large sample of clusters which can be used to study turbulence and make predictions for future X-ray observatories like eROSITA, Astro-H, and Athena.

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arXiv2015

Investigating the cores of fossil systems with Chandra

We investigate the cores of fossil galaxy groups and clusters (`fossil systems') using archival Chandra data for a sample of 17 fossil systems. We determined the cool-core fraction for fossils via three observable diagnostics, the central cooling time, cuspiness, and concentration parameter. We quantified the dynamical state of the fossils by the X-ray peak/brightest cluster galaxy (BCG), and the X-ray peak/emission weighted centre separations. We studied the X-ray emission coincident with the BCG to detect the presence of potential thermal coronae. A deprojection analysis was performed for z < 0.05 fossils to obtain cooling time and entropy profiles, and to resolve subtle temperature structures. We investigated the Lx-T relation for fossils from the 400d catalogue to see if the scaling relation deviates from that of other groups. Most fossils are identified as cool-core objects via at least two cool-core diagnostics. All fossils have their dominant elliptical galaxy within 50 kpc of the X-ray peak, and most also have the emission weighted centre within that distance. We do not see clear indications of a X-ray corona associated with the BCG unlike that has been observed for some other objects. Fossils do not have universal temperature profiles, with some low-temperature objects lacking features that are expected for ostensibly relaxed objects with a cool-core. The entropy profiles of the z < 0.05 fossil systems can be well-described by a power law model, albeit with indices smaller than 1. The 400d fossils Lx-T relation shows indications of an elevated normalisation with respect to other groups, which seems to persist even after factoring in selection effects.

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arXiv2015

Constraining gas motions in the Centaurus cluster using X-ray surface brightness fluctuations and metal diffusion

We compare two different methods of constraining the characteristic velocity and spatial scales of gas motions in the X-ray bright, nearby Centaurus cluster, using new deep (760ks) Chandra observations. The power spectrum of excess surface brightness fluctuations in the 0.5-6.0 keV band in a sector to the west is measured and compared to theoretical expectations for Kolmogorov index fluctuations. The observed power spectrum is flatter than these expectations, and the surface brightness fluctuations are around the 8 percent level on length scales of 2 kpc. We convert the 2D power spectrum of fluctuations into a 3D power spectrum using the method of Churazov et al., and then convert this into constraints on the one-component velocity of the gas motions as a function of their length scale. We find one-component velocities in the range 100-150 km/s on spatial scales of 4-10 kpc. An independent constraint on the characteristic velocity and length scales of the gas motions is then found by considering the diffusion coefficient needed to explain the distribution of metals in the Centaurus cluster, combined with the need to balance the rate of gas cooling with the rate of heat dissipated by the gas motions. We find that these two methods of constraining the velocity and length scales of the gas motions are in good agreement.

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arXiv2015

Dynamical analysis of galaxy cluster merger Abell 2146

We present a dynamical analysis of the merging galaxy cluster system Abell 2146 using spectroscopy obtained with the Gemini Multi-Object Spectrograph on the Gemini North telescope. As revealed by the Chandra X-ray Observatory, the system is undergoing a major merger and has a gas structure indicative of a recent first core passage. The system presents two large shock fronts, making it unique amongst these rare systems. The hot gas structure indicates that the merger axis must be close to the plane of the sky and that the two merging clusters are relatively close in mass, from the observation of two shock fronts. Using 63 spectroscopically determined cluster members, we apply various statistical tests to establish the presence of two distinct massive structures. With the caveat that the system has recently undergone a major merger, the virial mass estimate is M_vir = 8.5 +4.3 -4.7 x 10 ^14 M_sol for the whole system, consistent with the mass determination in a previous study using the Sunyaev-Zeldovich signal. The newly calculated redshift for the system is z = 0.2323. A two-body dynamical model gives an angle of 13-19 degrees between the merger axis and the plane of the sky, and a timescale after first core passage of 0.24-0.28 Gyr.

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arXiv2015

X-ray Analysis of Filaments in Galaxy Clusters

We perform a detailed X-ray study of the filaments surrounding the brightest cluster galaxies in a sample of nearby galaxy clusters using deep Chandra observations, namely the Perseus, Centaurus and Virgo clusters, and Abell 1795. We compare the X-ray properties and spectra of the filaments in all of these systems, and find that their Chandra X-ray spectra are all broadly consistent with an absorbed two temperature thermal model, with temperature components at 0.75 and 1.7 keV. We find that it is also possible to model the Chandra ACIS filament spectra with a charge exchange model provided a thermal component is also present, and the abundance of oxygen is suppressed relative to the abundance of Fe. In this model, charge exchange provides the dominant contribution to the spectrum in the 0.5-1.0 keV band. However, when we study the high spectral resolution RGS spectrum of the filamentary plume seen in X-rays in Centaurus, the opposite appears to be the case. The properties of the filaments in our sample of clusters are also compared to the X-ray tails of galaxies in the Coma cluster and Abell 3627. In the Perseus cluster, we search for signs of absorption by a prominent region of molecular gas in the filamentary structure around NGC 1275. We do find a decrement in the X-ray spectrum below 2 keV, indicative of absorption. However the spectral shape is inconsistent with this decrement being caused by simply adding an additional absorbing component. We find that the spectrum can be well fit (with physically sensible parameters) with a model that includes both absorption by molecular gas and X-ray emission from the filament, which partially counteracts the absorption.

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arXiv2015

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.

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arXiv2014

Turbulent Heating in Galaxy Clusters Brightest in X-rays

The hot, X-ray-emitting intracluster medium (ICM) is the dominant baryonic constituent of clusters of galaxies. In the cores of many clusters, radiative energy losses from the ICM occur on timescales significantly shorter than the age of the system. Unchecked, this cooling would lead to massive accumulations of cold gas and vigorous star formation, in contradiction to observations. Various sources of energy capable of compensating these cooling losses have been proposed, the most promising being heating by the supermassive black holes in the central galaxies through inflation of bubbles of relativistic plasma. Regardless of the original source of energy, the question of how this energy is transferred to the ICM has remained open. Here we present a plausible solution to this question based on deep Chandra X-ray observatory data and a new data-analysis method that enables us to evaluate directly the ICM heating rate due to the dissipation of turbulence. We find that turbulent heating is sufficient to offset radiative cooling and indeed appears to balance it locally at each radius - it might therefore be the key element in resolving the gas cooling problem in cluster cores and, more universally, in atmospheres of X-ray gas-rich systems.

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arXiv2014

Bayes-X: a Bayesian inference tool for the analysis of X-ray observations of galaxy clusters

We present the first public release of our Bayesian inference tool, Bayes-X, for the analysis of X-ray observations of galaxy clusters. We illustrate the use of Bayes-X by analysing a set of four simulated clusters at z=0.2-0.9 as they would be observed by a Chandra-like X-ray observatory. In both the simulations and the analysis pipeline we assume that the dark matter density follows a spherically-symmetric Navarro, Frenk and White (NFW) profile and that the gas pressure is described by a generalised NFW (GNFW) profile. We then perform four sets of analyses. By numerically exploring the joint probability distribution of the cluster parameters given simulated Chandra-like data, we show that the model and analysis technique can robustly return the simulated cluster input quantities, constrain the cluster physical parameters and reveal the degeneracies among the model parameters and cluster physical parameters. We then analyse Chandra data on the nearby cluster, A262, and derive the cluster physical profiles. To illustrate the performance of the Bayesian model selection, we also carried out analyses assuming an Einasto profile for the matter density and calculated the Bayes factor. The results of the model selection analyses for the simulated data favour the NFW model as expected. However, we find that the Einasto profile is preferred in the analysis of A262. The Bayes-X software, which is implemented in Fortran 90, is available at http://www.mrao.cam.ac.uk/facilities/software/bayesx/.

astro-ph.CO↗

arXiv2014

Implications of Coronal Line Emission in NGC 4696

We announce a new facility in the spectral code CLOUDY that enables tracking the evolution of a cooling parcel of gas with time. For gas cooling from temperatures relevant to galaxy clusters, earlier calculations estimated the [Fe XIV] λ5303 / [Fe X] λ6375 luminosity ratio, a critical diagnostic of a cooling plasma, to slightly less than unity. By contrast, our calculations predict a ratio ~3. We revisit recent optical coronal line observations along the X-ray cool arc around NGC 4696 by Canning et al. (2011), which detected [Fe X] λ6375, but not [Fe XIV] λ5303. We show that these observations are not consistent with predictions of cooling flow models. Differential extinction could in principle account for the observations, but it requires extinction levels (A_V > 3.625) incompatible with previous observations. The non-detection of [Fe XIV] implies a temperature ceiling of 2.1 million K. Assuming cylindrical geometry and transonic turbulent pressure support, we estimate the gas mass at ~1 million solar masses. The coronal gas is cooling isochorically. We propose that the coronal gas has not condensed out of the intracluster medium, but instead is the conductive or mixing interface between the X-ray plume and the optical filaments. We present a number of emission lines that may be pursued to test this hypothesis and constrain the amount of intermediate temperature gas in the system.

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arXiv2014

Feedback, scatter and structure in the core of the PKS 0745-191 galaxy cluster

We present Chandra X-ray Observatory observations of the core of the galaxy cluster PKS 0745-191. Its centre shows X-ray cavities caused by AGN feedback and cold fronts with an associated spiral structure. The cavity energetics imply they are powerful enough to compensate for cooling. Despite the evidence for AGN feedback, the Chandra and XMM-RGS X-ray spectra are consistent with a few hundred solar masses per year cooling out of the X-ray phase, sufficient to power the emission line nebula. The coolest X-ray emitting gas and brightest nebula emission is offset by around 5 kpc from the radio and X-ray nucleus. Although the cluster has a regular appearance, its core shows density, temperature and pressure deviations over the inner 100 kpc, likely associated with the cold fronts. After correcting for ellipticity and projection effects, we estimate density fluctuations of ~4 per cent, while temperature, pressure and entropy have variations of 10-12 per cent. We describe a new code, MBPROJ, able to accurately obtain thermodynamical cluster profiles, under the assumptions of hydrostatic equilibrium and spherical symmetry. The forward-fitting code compares model to observed profiles using Markov Chain Monte Carlo and is applicable to surveys, operating on 1000 or fewer counts. In PKS0745 a very low gravitational acceleration is preferred within 40 kpc radius from the core, indicating a lack of hydrostatic equilibrium, deviations from spherical symmetry or non-thermal sources of pressure.

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arXiv2014

A volume-limited sample of X-ray galaxy groups and clusters - II. X-ray cavity dynamics

We present the results of our study of a volume-limited sample (z <= 0.071) of 101 X-ray galaxy groups and clusters, in which we explore the X-ray cavity energetics. Out of the 101 sources in our parent sample, X-ray cavities are found in 30 of them, all of which have a central cooling time of less than3 Gyr. New X-ray cavities are detected in three sources. We focus on the subset of sources that have a central cooling time of less than 3 Gyr, whose active galactic nucleus (AGN) duty cycle is approximately 61 percent (30/49). This rises to over 80 percent for a central cooling time of less than 0.5 Gyr. When projection effects and central radio source detection rates are considered, the actual duty cycle is probably much higher. In addition, we show that data quality strongly affects the detection rates of X-ray cavities. After calculating the cooling luminosity and cavity powers of each source with cavities, it is evident that the bubbling process induced by the central AGN has to be, on average, continuous, to offset cooling. We find that the radius of the cavities, r, loosely depends on the ambient gas temperature as T^0.5, above about 1.5 keV, with much more scatter below that temperature. Finally, we show that, at a given location in a group or cluster, larger bubbles travel faster than smaller ones. This means that the bubbles seen at larger distances from cluster cores could be the result of the merging of several smaller bubbles, produced in separate AGN cycles.

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