arXiv subjects

N. Werner

Publications and source records attributed to N. Werner.

At least 91 records · Page 5Linked to original sources

arXiv2013

Thermodynamics of the Coma Cluster Outskirts

We present results from a large mosaic of Suzaku observations of the Coma Cluster, the nearest and X-ray brightest hot, dynamically active, non-cool core system, focusing on the thermodynamic properties of the ICM on large scales. For azimuths not aligned with an infalling subcluster towards the southwest, our measured temperature and X-ray brightness profiles exhibit broadly consistent radial trends, with the temperature decreasing from about 8.5 keV at the cluster center to about 2 keV at a radius of 2 Mpc, which is the edge of our detection limit. The SW merger significantly boosts the surface brightness, allowing us to detect X-ray emission out to ~2.2 Mpc along this direction. Apart from the southwestern infalling subcluster, the surface brightness profiles show multiple edges around radii of 30-40 arcmin. The azimuthally averaged temperature profile, as well as the deprojected density and pressure profiles, all show a sharp drop consistent with an outward propagating shock front located at 40 arcmin, corresponding to the outermost edge of the giant radio halo observed at 352 MHz with the WSRT. The shock front may be powering this radio emission. A clear entropy excess inside of r_500 reflects the violent merging events linked with these morphological features. Beyond r_500, the entropy profiles of the Coma Cluster along the relatively relaxed directions are consistent with the power-law behavior expected from simple models of gravitational large-scale structure formation. The pressure is also in agreement at these radii with the expected values measured from SZ data from the Planck satellite. However, due to the large uncertainties associated with the Coma Cluster measurements, we cannot yet exclude an entropy flattening in this system consistent with that seen in more relaxed cool core clusters.

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arXiv2013

The nature of filamentary cold gas in the core of the Virgo Cluster

We present a multi-wavelength study of the emission-line nebulae located southeast of the nucleus of M87, the central dominant galaxy of the Virgo Cluster. We report the detection of far-infrared (FIR) [CII] line emission from the nebulae using observations made with Herschel PACS. The infrared line emission is extended and cospatial with optical Hα+[NII], far-ultraviolet CIV lines, and soft X-ray emission. The filamentary nebulae evidently contain multi-phase material spanning a temperature range of at least 5 orders of magnitude, from ~100 K to ~10^7 K. This material has most likely been uplifted by the AGN from the center of M87. The thermal pressure of the 10^4 K phase appears to be significantly lower than that of the surrounding hot intra-cluster medium (ICM) indicating the presence of additional turbulent and magnetic pressure in the filaments. If the turbulence in the filaments is subsonic then the magnetic field strength required to balance the pressure of the surrounding ICM is B~30-70 μG. The spectral properties of the soft X-ray emission from the filaments indicate that it is due to thermal plasma with kT~0.5-1 keV, which is cooling by mixing with the cold gas and/or radiatively. Charge exchange can be ruled out as a significant source of soft X-rays. Both cooling and mixing scenarios predict gas with a range of temperatures. This is at first glance inconsistent with the apparent lack of X-ray emitting gas with kT<0.5 keV. However, we show that the missing very soft X-ray emission could be absorbed by the cold gas in the filaments with an integrated hydrogen column density of ~1.6x10^21 cm^-2, providing a natural explanation for the apparent temperature floor to the X-ray emission at kT~0.5 keV. The FIR through ultra-violet line emission is most likely primarily powered by the ICM particles penetrating the cold gas following a shearing induced mixing process.

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arXiv2013

Giant Lobes of Centaurus A Radio Galaxy Observed with the Suzaku X-ray Satellite

[abridged] We report on Suzaku observations of selected regions within the Southern giant lobe of the radio galaxy Centaurus A. We focus on distinct X-ray features likely associated with fine radio structure of the lobe. We find that the spectral properties of the detected X-ray features are equally consistent with thermal emission from hot gas, or with a power-law radiation continuum. However, the plasma parameters implied by these different models favor a synchrotron origin for the analyzed X-ray spots, indicating that a very efficient acceleration of electrons is taking place within the giant structure of Centaurus A, albeit only in isolated and compact regions. We also present a detailed analysis of the diffuse X-ray emission, resulting in a tentative detection of a soft excess component best fitted by a thermal model with a temperature of 0.5 keV. The exact origin of the observed excess remains uncertain, although energetic considerations point to thermal gas filling the bulk of the volume of the lobe and mixed with the non-thermal plasma. The corresponding pressure of the thermal gas in such a case appears to be in almost exact equipartition with the pressure provided by the radio-emitting electrons and the magnetic field. Although tentative, our findings potentially imply that the structure of the extended lobes in active galaxies is likely to be highly inhomogeneous, with magnetic reconnection processes continuously converting magnetic energy to internal energy of the plasma particles, leading to spatial and temporal variations in the plasma parameters around the equilibrium condition.

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arXiv2012

Ripping Apart at the Seams: The Network of Stripped Gas Surrounding M86

We present a new study of the Virgo Cluster galaxies M86, M84, NGC 4338, and NGC 4438 using a mosaic of five separate pointings with XMM-Newton. Our observations allow for robust measurements of the temperature and metallicity structure of each galaxy along with the entire ~ 1 degree region between these galaxies. When combined with multiwavelength observations, the data suggest that all four of these galaxies are undergoing ram pressure stripping by the Intracluster Medium (ICM). The manner in which the stripped gas trailing the galaxies interacts with the ICM, however, is observably distinct. Consistent with previous observations, M86 is observed to have a long tail of ~ 1 keV gas trailing to the north-west for distances of ~ 100-150 kpc. However, a new site of ~ 0.6 keV thermal emission is observed to span to the east of M86 in the direction of the disturbed spiral galaxy NGC 4438. This region is spatially coincident with filaments of H-alpha emission, likely originating in a recent collision between the two galaxies. We also resolve the thermodynamic structure of stripped ~ 0.6 keV gas to the south of M84, suggesting that this galaxy is undergoing both AGN feedback and ram pressure stripping simultaneously. These four sites of stripped X-ray gas demonstrate that the nature of ram pressure stripping can vary significantly from site to site.

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arXiv2012

Large-scale Motions in the Perseus Galaxy Cluster

By combining large-scale mosaics of ROSAT PSPC, XMM-Newton, and Suzaku X-ray observations, we present evidence for large-scale motions in the intracluster medium of the nearby, X-ray bright Perseus Cluster. These motions are suggested by several alternating and interleaved X-ray bright, low-temperature, low-entropy arcs located along the east-west axis, at radii ranging from ~10 kpc to over a Mpc. Thermodynamic features qualitatively similar to these have previously been observed in the centers of cool core clusters, and were successfully modeled as a consequence of the gas sloshing/swirling motions induced by minor mergers. Our observations indicate that such sloshing/swirling can extend out to larger radii than previously thought, on scales approaching the virial radius.

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arXiv2012

On the thermodynamic self-similarity of the nearest, most relaxed, giant ellipticals

We present detailed spatially resolved measurements of the thermodynamic properties of the X-ray emitting gas in the inner regions of the five nearest, X-ray and optically brightest, and most X-ray morphologically relaxed giant elliptical galaxies known. Beyond the innermost region at r > 1 kpc, and out to r ~ 6 kpc, the density, pressure, entropy, and cooling time distributions for the X-ray emitting gas follow remarkably similar, simple, power-law like distributions. Notably, the entropy profiles follow a power-law form, with an index 0.92-1.07. The cumulative hot X-ray emitting gas mass profiles and the gas-mass to stellar-light ratios of all five galaxies are also similar. Overall the observed similarity of the thermodynamic profiles in this radial range argues that, in these systems, relativistic jets heat the gas at a similar rate averaged over time scales longer than the cooling time of 10^8 yr. These jets are powered by accretion from the hot gas, or material entrained within it, onto the central super-massive black hole. This jet heating creates an energy balance where heating and cooling are in equilibrium, keeping the hot galactic atmospheres in a `steady-state'. Within r < 1 kpc, this similarity breaks down: the observed entropy profiles show well resolved flattening and the values differ from system to system substantially. The accretion rate onto the black hole and the AGN activity, heating the interstellar medium, must therefore vary significantly on time scales shorter than the cooling time of 10^7 - 10^8 yr.

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arXiv2012

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

The interstellar and intra-cluster medium 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 AGN and merger activity. We aim to put constraints on the turbulent velocities and turbulent pressure in the ICM of the giant elliptical galaxies NGC 5044 and NGC 5813 using XMM-Newton RGS observations. 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 (I(17.05)+I(17.10))/I(15.01) line ratio from RGS with simulated line ratios for different Mach numbers, the level of turbulence is constrained. The measurement is limited by systematic uncertainties in the atomic data, which are at the 20-30% level. We find that the 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 high turbulent velocities and the fraction of the turbulent pressure support of >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 (I(17.05)+I(17.10))/I(15.01) line ratio in an optically thin plasma, calculated using AtomDB v2.0.1, is 2 sigma 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.

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arXiv2011

Baryons in the outskirts of the X-ray brightest galaxy cluster

Studies of the diffuse X-ray emitting gas in galaxy clusters have provided powerful constraints on cosmological parameters and insights into plasma astrophysics. However, measurements of the faint cluster outskirts have become possible only over the last few years. Here, we present results from Suzaku observations of the Perseus Cluster, which provide our best measurements of the thermodynamic properties of the ICM at large radii to date. In particular, we focus on the details of the data analysis procedure and discuss the evidence for a clumpy distribution of the gas in the outskirts, which is important for understanding the physics of the ongoing growth of clusters from the surrounding cosmic web.

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arXiv2011

Core-collapse supernova enrichment in the core of the Virgo Cluster

Using a deep (574 ks) Chandra observation of M87, the dominant galaxy of the nearby Virgo Cluster, we present the best measurements to date of the radial distribution of metals in the central intracluster medium (ICM). Our measurements, made in 36 independent annuli with $\sim$250,000 counts each, extend out to a radius r$\sim$40 kpc and show that the abundance profiles of Fe, Si, S, Ar, Ca, Ne, Mg, and Ni are all centrally peaked. Interestingly, the abundance profiles of Si and S - which are measured robustly and to high precision - are even more centrally peaked than Fe, while the Si/S ratio is relatively flat. These measurements challenge the standard picture of chemical enrichment in galaxy clusters, wherein type Ia supernovae (SN Ia) from an evolved stellar population are thought to dominate the central enrichment. The observed abundance patterns are most likely due to one or more of the following processes: continuing enrichment by winds of a stellar population pre-enriched by SNCC products; intermittent formation of massive stars in the central cooling core; early enrichment of the low entropy gas. We also discuss other processes that might have contributed to the observed radial profiles, such as a stellar initial mass function that changes with radius; changes in the pre-enrichment of core-collapse supernova progenitors; and a diversity in the elemental yields of SN Ia. Although systematic uncertainties prevent us from measuring the O abundance robustly, indications are that it is about 2 times lower than predicted by the enrichment models.

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arXiv2011

The metal contents of two groups of galaxies

The hot gas in clusters and groups of galaxies is continuously being enriched with metals from supernovae and stars. It is well established that the enrichment of the gas with elements from oxygen to iron is mainly caused by supernova explosions. The origins of nitrogen and carbon are still being debated. Possible candidates include massive, metal-rich stars, early generations of massive stars, intermediate or low mass stars and Asymptotic Giant Branch (AGB) stars. In this paper we accurately determine the metal abundances of the gas in the groups of galaxies NGC 5044 and NGC 5813, and discuss the nature of the objects that create these metals. We mainly focus on carbon and nitrogen. We use spatially-resolved high-resolution X-ray spectroscopy from XMM-Newton. For the spectral fitting, multi-temperature hot gas models are used. The abundance ratios of carbon over oxygen and nitrogen over oxygen that we find are high compared to the ratios in the stars in the disk of our Galaxy. The oxygen and nitrogen abundances we derive are similar to what was found in earlier work on other giant ellipticals in comparable environments. We show that the iron abundances in both our sources have a gradient along the cross-dispersion direction of the Reflection Grating Spectrometer (RGS). We conclude that it is unlikely that the creation of nitrogen and carbon takes place in massive stars, which end their lives as core-collapse supernovae, enriching the medium with oxygen because oxygen should then also be enhanced. Therefore we favour low-and intermediate mass stars as sources of these elements. The abundances in the hot gas can best be explained by a 30-40% contribution of type Ia supernovae based on the measured oxygen and iron abundances and under the assumption of a Salpeter Initial Mass Function (IMF).

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arXiv2011

Violent interaction between the AGN and the hot gas in the core of the galaxy cluster Sersic 159-03

We present a multi-wavelength study of the energetic interaction between the central active galactic nucleus (AGN), the intra-cluster medium, and the optical emission line nebula in the galaxy cluster Sersic 159-03. We use X-ray data from Chandra, high resolution X-ray spectra and UV images from XMM-Newton, Halpha images from the SOAR telescope, HST optical imaging, and VLA and GMRT radio data. The cluster center displays signs of powerful AGN feedback, which has cleared the central regions (r<7.5 kpc) of dense, X-ray emitting ICM. X-ray spectral maps reveal a high pressure ring surrounding the central AGN at a radius of r~15 kpc, indicating an AGN driven weak shock. The cluster harbors a bright, 44 kpc long Halpha+[NII] filament extending from the centre of the cD galaxy to the north. Along the filament, we see low entropy, high metallicity, cooling X-ray gas. The gas in the filament has most likely been uplifted by `radio mode' AGN activity and subsequently stripped from the galaxy due to its relative southward motion. Because this X-ray gas has been removed from the direct influence of the AGN jets, part of it cools and forms stars as indicated by the observed dust lanes, molecular and ionized emission line nebulae, and the excess UV emission.

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arXiv2011

X-ray Spectroscopy of the Virgo Cluster out to the Virial Radius

We present results from the analysis of a mosaic of thirteen XMM-Newton pointings covering the Virgo Cluster from its center northwards out to a radius r~1.2 Mpc (~4.5 degrees), reaching the virial radius and beyond. This is the first time that the properties of a modestly sized (M_vir~1.4e14 M_sun, kT~2.3 keV), dynamically young cluster have been studied out to the virial radius. The density profile of the cluster can be described by a surprisingly shallow power-law with index 1.21+/-0.12. In the radial range of 0.3r_vir<r<r_vir, the best fit temperature drops by roughly 60 per cent. Within a radius r<450 kpc, the entropy profile has an approximate power-law form with index 1.1, as expected for gravitationally collapsed gas in hydrostatic equilibrium. Beyond r~450 kpc, however, the temperature and metallicity drop abruptly, and the entropy profile becomes flatter, staying consistently below the expected value by a factor of 2-2.5. The most likely explanation for the unusually shallow density profile and the flattening of entropy at large radius is clumping in the ICM. Our data provide direct observational evidence that the ICM is enriched by metals all the way to r_200 to at least Z=0.1 Solar.

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arXiv2011

ICM Signatures from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN Feedback

We present results from Chandra observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. We focus on three main results. 1) The diffuse gas shows clear signatures from three distinct outbursts of the central AGN, with three pairs of roughly collinear cavities. The inner two cavity pairs are associated with unambiguous elliptical shock fronts, with Mach numbers M~1.7 and M~1.5 for the inner and outer shocks, respectively. 2) The mean power of the two most recent outbursts differs by a factor of six, indicating that the mean jet power varies over long (~10^7 yr) time scales. 3) The heating from the shocks alone is sufficient to balance radiative cooling of the gas within at least the central 10 kpc, allowing feedback to operate isotropically at small radii.

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arXiv2010

Cold fronts and multi-temperature structures in the core of Abell 2052

The physics of the coolest phases in the hot Intra-Cluster Medium (ICM) of clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the central Active Galactic Nucleus (AGN) contain enough energy to heat the surrounding gas and stop cooling, but locally blobs or filaments of gas appear to be able to cool to low temperatures of 10^4 K. In X-rays, however, gas with temperatures lower than 0.5 keV is not observed. Using a deep XMM-Newton observation of the cluster of galaxies Abell 2052, we derive 2D maps of the temperature, entropy, and iron abundance in the core region. About 130 kpc South-West of the central galaxy, we discover a discontinuity in the surface brightness of the hot gas which is consistent with a cold front. Interestingly, the iron abundance jumps from ~0.75 to ~0.5 across the front. In a smaller region to the North-West of the central galaxy we find a relatively high contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below that temperature. However, the region appears to be associated with much cooler H-alpha filaments in the optical waveband. The elliptical shape of the cold front in the SW of the cluster suggests that the front is caused by sloshing of the hot gas in the clusters gravitational potential. This effect is probably an important mechanism to transport metals from the core region to the outer parts of the cluster. The smooth temperature profile across the sharp jump in the metalicity indicates the presence of heat conduction and the lack of mixing across the discontinuity. The cool blob of gas NW of the central galaxy was probably pushed away from the core and squeezed by the adjacent bubble, where it can cool efficiently and relatively undisturbed by the AGN. Shock induced mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively and explain our non-detection of gas below 0.5 keV.

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arXiv2010

Shocks and Cavities from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN Feedback

We present results from new Chandra, GMRT, and SOAR observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. The system shows three pairs of collinear cavities at 1 kpc, 8 kpc, and 20 kpc from the central source, from three distinct outbursts of the central AGN, which occurred 3x10^6, 2x10^7, and 9x10^7 yr ago. The H-alpha and X-ray observations reveal filaments of cool gas that has been uplifted by the X-ray cavities. The inner two cavity pairs are filled with radio emitting plasma, and each pair is associated with an elliptical surface brightness edge, which we unambiguously identify as shocks (with measured temperature jumps) with Mach numbers of M~1.7 and M~1.5 for the inner and outer shocks, respectively. Such clear signatures from three distinct AGN outbursts in an otherwise dynamically relaxed system provide a unique opportunity to study AGN feedback and outburst history. The mean power of the two most recent outbursts differs by a factor of six, from 1.5--10x10^42 erg/s, indicating that the mean jet power changes significantly over long (~10^7 yr) timescales. The total energy output of the most recent outburst is also more than an order of magnitude less than the total energy of the previous outburst (1.5x10^56 erg versus 4x10^57 erg), which may be a result of the lower mean power, or may indicate that the most recent outburst is ongoing. The outburst interval implied by both the shock and cavity ages (~10^7 yr) indicates that, in this system, shock heating alone is sufficient to balance radiative cooling close to the central AGN, which is the relevant region for regulating feedback between the ICM and the central SMBH.

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arXiv2010

Feedback under the microscope: thermodynamic structure and AGN driven shocks in M87

(abridged) Using a deep Chandra exposure (574 ks), we present high-resolution thermodynamic maps created from the spectra of $\sim$16,000 independent regions, each with $\sim$1,000 net counts. The excellent spatial resolution of the thermodynamic maps reveals the dramatic and complex temperature, pressure, entropy and metallicity structure of the system. Excluding the 'X-ray arms', the diffuse cluster gas at a given radius is strikingly isothermal. This suggests either that the ambient cluster gas, beyond the arms, remains relatively undisturbed by AGN uplift, or that conduction in the intracluster medium (ICM) is efficient along azimuthal directions. We confirm the presence of a thick ($\sim$40 arcsec or $\sim$3 kpc) ring of high pressure gas at a radius of $\sim$180 arcsec ($\sim$14 kpc) from the central AGN. We verify that this feature is associated with a classical shock front, with an average Mach number M = 1.25. Another, younger shock-like feature is observed at a radius of $\sim$40 arcsec ($\sim$3 kpc) surrounding the central AGN, with an estimated Mach number M > 1.2. As shown previously, if repeated shocks occur every $\sim$10 Myrs, as suggested by these observations, then AGN driven weak shocks could produce enough energy to offset radiative cooling of the ICM. A high significance enhancement of Fe abundance is observed at radii 350 - 400 arcsec (27 - 31 kpc). This ridge is likely formed in the wake of the rising bubbles filled with radio-emitting plasma that drag cool, metal-rich gas out of the central galaxy. We estimate that at least $\sim1.0\times10^6$ solar masses of Fe has been lifted and deposited at a radius of 350-400 arcsec; approximately the same mass of Fe is measured in the X-ray bright arms, suggesting that a single generation of buoyant radio bubbles may be responsible for the observed Fe excess at 350 - 400 arcsec.

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arXiv2010

Feedback under the microscope II: heating, gas uplift, and mixing in the nearest cluster core

Using a combination of deep 574ks Chandra data, XMM-Newton high-resolution spectra, and optical Halpha+NII images, we study the nature and spatial distribution of the multiphase plasma in M87. Our results provide direct observational evidence of `radio mode' AGN feedback in action, stripping the central galaxy of its lowest entropy gas and preventing star-formation. This low entropy gas was entrained with and uplifted by the buoyantly rising relativistic plasma, forming long "arms". These arms are likely oriented within 15-30 degrees of our line-of-sight. The mass of the uplifted gas in the arms is comparable to the gas mass in the approximately spherically symmetric 3.8 kpc core, demonstrating that the AGN has a profound effect on its immediate surroundings. The coolest X-ray emitting gas in M87 has a temperature of ~0.5 keV and is spatially coincident with Halpha+NII nebulae, forming a multiphase medium where the cooler gas phases are arranged in magnetized filaments. We place strong upper limits of 0.06 Msun/yr on the amount of plasma cooling radiatively from 0.5 keV and show that a uniform, volume-averaged heating mechanism could not be preventing the cool gas from further cooling. All of the bright Halpha filaments appear in the downstream region of the <3 Myr old shock front, at smaller radii than ~0.6'. We suggest that shocks induce shearing around the filaments, thereby promoting mixing of the cold gas with the ambient hot ICM via instabilities. By bringing hot thermal particles into contact with the cool, line-emitting gas, mixing can supply the power and ionizing particles needed to explain the observed optical spectra. Mixing of the coolest X-ray emitting plasma with the cold optical line emitting filamentary gas promotes efficient conduction between the two phases, allowing non-radiative cooling which could explain the lack of X-ray gas with temperatures under 0.5 keV.

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arXiv2010

Central galaxy growth and feedback in the most massive nearby cool core cluster

We present multi-wavelength observations of the centre of RXCJ1504.1-0248 - the galaxy cluster with the most luminous and relatively nearby cool core at z~0.2. Although there are several galaxies within 100 kpc of the cluster core, only the brightest cluster galaxy (BCG), which lies at the peak of the X-ray emission, has blue colours and strong line-emission. Approximately 80 Msun/yr of intracluster gas is cooling below X-ray emitting temperatures, similar to the observed UV star formation rate of ~140 Msun/yr. Most star formation occurs in the core of the BCG and in a 42 kpc long filament of blue continuum, line emission, and X-ray emission, that extends southwest of the galaxy. The surrounding filamentary nebula is the most luminous around any observed BCG. The number of ionizing stars in the BCG is barely sufficient to ionize and heat the nebula, and the line ratios indicate an additional heat source is needed. This heat source can contribute to the Hα-deduced star formation rates (SFRs) in BCGs and therefore the derived SFRs should only be considered upper limits. AGN feedback can slow down the cooling flow to the observed mass deposition rate if the black hole accretion rate is of the order of 0.5 Msun/yr at 10% energy output efficiency. The average turbulent velocity of the nebula is vturb ~325 km/s which, if shared by the hot gas, limits the ratio of turbulent to thermal energy of the intracluster medium to less than 6%.

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