Feedback under the microscope II: heating, gas uplift, and mixing in the nearest cluster core
N. Werner, A. Simionescu, E. T. Million, S. W. Allen, P. E. J. Nulsen, A. von der Linden, S. M. Hansen, H. Boehringer, E. Churazov, A. C. Fabian, W. R. Forman, C. Jones, J. S. Sanders, G. B. Taylor
MMon. Not. R. Astron. Soc. , 000–000 (0000) Printed 27 October 2018 (MN L A TEX style file v2.2)
Feedback under the microscope II: heating, gas uplift, and mixing inthe nearest cluster core
N. Werner (cid:63) , A. Simionescu † , E. T. Million , S. W. Allen , P. E. J. Nulsen ,A. von der Linden , S. M. Hansen , H. B ¨ohringer , E. Churazov , , A. C. Fabian ,W. R. Forman , C. Jones , J. S. Sanders , and G. B. Taylor Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305-4060, USAand SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA University of California Observatories & Department of Astronomy, University of California, Santa Cruz, CA 95064, USA Max-Planck-Institut f¨ur extraterrestrische Physik, Giessenbachstr, 85748 Garching, Germany Max-Planck-Institut f¨ur Astrophysik, Karl-Schwarzschild-Strasse 1, 85741 Garching, Germany Space Research Institute (IKI), Profsoyznaya 84 /
32, Moscow 117810, Russia Institute of Astronomy, Madingley Road, Cambridge CB3 0HA Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131, USA
27 October 2018
ABSTRACT
Using a combination of deep (574 ks)
Chandra data,
XMM-Newton high-resolution spectra,and optical H α + [N ii ] images, we study the nature and spatial distribution of the multiphaseplasma in M 87. Our results provide direct observational evidence of ‘radio mode’ AGN feed-back in action, stripping the central galaxy of its lowest entropy gas and therefore preventingstar-formation. This low entropy gas was entrained with and uplifted by the buoyantly ris-ing relativistic plasma, forming long “arms”. A number of arguments suggest that these armsare oriented within 15 ◦ –30 ◦ of our line-of-sight. The mass of the uplifted gas in the arms iscomparable to the gas mass in the approximately spherically symmetric 3.8 kpc core, demon-strating that the AGN has a profound e ff ect on its immediate surroundings. The coolest X-rayemitting gas in M 87 has a temperature of ∼ α + [N ii ]nebulae, forming a multiphase medium where the cooler gas phases are arranged in magne-tized filaments. We place strong upper limits of 0.06 M (cid:12) / yr (at 95 per cent confidence) on theamount of plasma cooling radiatively from 0.5 to 0.25 keV and show that a uniform, volume-averaged heating mechanism could not be preventing the cool gas from further cooling. Allof the bright H α filaments in M 87 appear in the downstream region of the < ∼ (cid:48) . We suggest that shocks induce shearing around thefilaments, thereby promoting mixing of the cold gas with the ambient hot ICM via instabil-ities. By bringing hot thermal particles into contact with the cool, line-emitting gas, mixingcan supply the power and ionizing particles needed to explain the observed optical spectra.Furthermore, mixing of the coolest X-ray emitting plasma with the cold optical line emittingfilamentary gas promotes e ffi cient conduction between the two phases, allowing non-radiativecooling which could explain the lack of X-ray gas with temperatures under 0.5 keV. Key words:
X-rays: galaxies: clusters – galaxies: individual: M 87 – galaxies: intergalacticmedium – cooling flows
If the energy radiated away at the centres of so-called “cool-core”clusters of galaxies, which show sharp X-ray surface brightness (cid:63)
Chandra / Einstein fellow, E-mail: [email protected] † Einstein fellow peaks and central temperature dips, came only from the thermal en-ergy of the hot, di ff use intra-cluster medium (ICM), the ICM wouldcool and form stars at rates orders of magnitude above what theobservations suggest (see Peterson & Fabian 2006, for a review).It is currently believed that the energy which o ff sets the coolingis provided by the interaction between the active galactic nuclei c (cid:13) a r X i v : . [ a s t r o - ph . C O ] M a r N. Werner et al. (AGN) in the central dominant galaxies and the ICM (see Chu-razov et al. 2000, 2001, 2002; McNamara & Nulsen 2007, for areview). Through a tight feedback loop, it is thought that the AGNcan provide enough energy to prevent catastrophic cooling. Recentdeep observations of nearby bright cooling core clusters such asPerseus, M 87, Centaurus, and Hydra A revealed AGN inducedweak shocks and sound waves with su ffi cient energy flux to coun-terbalance the radiative cooling (Sanders & Fabian 2007; Formanet al. 2005; Sanders & Fabian 2008; Nulsen et al. 2005; Simionescuet al. 2009a). To the first order it is thus understood where the en-ergy comes from and how it gets transported. However, the detailedphysics of the AGN / ICM interaction is not yet clear.M 87, the central dominant galaxy of the Virgo cluster, at adistance of only 16.1 Mpc (Tonry et al. 2001) is the nearest, X-raybright cool core. It is an ideal system for detailed studies of the en-ergy input from the AGN to the hot, cooling ICM. After the Perseuscluster, M 87 is the second brightest extended extragalactic objectin soft X-rays. The most striking X-ray features in M 87 are its two“X-ray arms”, first detected by Feigelson et al. (1987) using
Ein-stein Observatory data and later studied in detail by B¨ohringer et al.(1995) using
ROSAT and VLA radio observations. The X-ray armsextend to the East and Southwest from the centre of the galaxy andthey spatially correlate with the radio emission. B¨ohringer et al.(1995) found that the X-ray arms are due to cooler gas, possiblyuplifted from the centre of the galaxy. These results and the sub-sequent high-quality radio data by Owen et al. (2000) led Chura-zov et al. (2001) to argue that the X-ray and radio morphology canbe explained by bubbles of radio-emitting plasma rising buoyantlythrough the hot ICM. These rising bubbles entrain and uplift coolergas from the centre of the galaxy, which then further adiabaticallycools. Churazov et al. (2001) suggested that the buoyantly risingbubbles dissipate energy into sound waves, internal waves, turbu-lent motion in the wake, and potential energy of the uplifted gas,all of which could provide heating to the cooling core region (seealso e.g. Br¨uggen & Kaiser 2002; Kaiser 2003; Br¨uggen 2003; DeYoung 2003; Ruszkowski et al. 2004a,b; Heinz & Churazov 2005).Observations with
Chandra and
XMM-Newton greatly en-hanced our view of M 87. Using
XMM-Newton observations, Bel-sole et al. (2001), Matsushita et al. (2002), and Molendi (2002)showed that the regions associated with radio arms have two-temperature (in the ranges of kT ∼ . kT ∼ . Chandra observation which showed cavities and edges in the hotplasma. They compared the X-ray morphology with the 6 cm ra-dio (Hines et al. 1989) and H α + [N ii ] emission (Sparks et al. 1993)and showed that the optical filaments lie outside of X-ray cavitiesfilled with radio plasma; some are along the edges of the cavities,and several optical filaments coincide with knots of cooler X-raygas. Using a longer ( ∼
150 ks)
Chandra observation, Sparks et al.(2004) studied the relation of the X-ray and optical filaments andconcluded that electron conduction from the hot X-ray emittingplasma to the cooler phase provides a quantitatively acceptableenergy source for the optical filaments. Using the same
Chandra data, Forman et al. (2005) identified shock fronts associated withan AGN outburst about 1–2 × yr ago. They argued that shockfronts may be the most significant channel for heating (i.e., entropyincrease) of the ICM near to the AGN.Subsequent Chandra observations increased the total expo-sure time to 500 ks, which allowed Forman et al. (2007) to resolvea web of filamentary structure in both X-ray arms. Both arms show
Table 1.
Summary of the
Chandra observations. Colums list the observationID, detector, observation mode, exposure after cleaning, and observationdate.Obs. ID Detector Mode Exposure (ks) Obs. date2707 ACIS-S FAINT 82.9 Jul. 6 20023717 ACIS-S FAINT 11.1 Jul. 5 20025826 ACIS-I VFAINT 126.8 Mar. 3 20055827 ACIS-I VFAINT 156.2 May 5 20055828 ACIS-I VFAINT 33.0 Nov. 17 20056186 ACIS-I VFAINT 51.5 Jan. 31 20057210 ACIS-I VFAINT 30.7 Nov. 16 20057211 ACIS-I VFAINT 16.6 Nov. 16 20057212 ACIS-I VFAINT 65.2 Nov. 14 2005 narrow filaments, with a length-to-width ratio of up to ∼
50. How-ever, while the Southwestern arm has only a single set of filaments,the Eastern arm shows multiple sets of filaments and bubbles, in-creasing in scale with the distance from the centre. The high spatialresolution of
Chandra also highlights the fact that while the East-ern X-ray arm is cospatial with the radio arm, the radio emissionbends around and avoids the Southwestern X-ray arm. A residualimage of M 87 showing the inner cavities and the X-ray arms ob-tained using all
Chandra
ACIS-I data is shown in Fig. 1 (from Mil-lion et al. 2010), together with the 90 cm radio image by Owenet al. (2000). The temperature structure and metallicity of the X-ray arms was studied in detail by Simionescu et al. (2008) usingdeep (120 ks)
XMM-Newton data. These authors confirmed that thearms are multiphase with a temperature distribution between 0.6–3.2 keV. This temperature distribution is also consistent with thatfound by spectral fits to the
XMM-Newton
Reflection Grating Spec-trometer (RGS) data (Werner et al. 2006), which show the presenceof weak Fe xvii lines. Simionescu et al. (2008) estimate the totalmass of gas below 1.5 keV, most of which was uplifted by the AGN,as 5 × M (cid:12) .This is the second in a series of papers (following the workby Million et al. 2010, hereafter Paper I) presenting detailed spa-tially resolved spectroscopy of M 87, exploiting the superb spa-tial resolution and excellent photon statistics of the deep (574 ks) Chandra observation, to study AGN feedback and ICM physics. Inthis paper, we study for the first time the detailed nature and thespatial distribution of the multiphase plasma associated with theX-ray arms.
Chandra allows us to study detailed structure in thecooler phases at a much higher spatial resolution than is possiblewith
XMM-Newton . Our data allow us to sample well the smallsubstructure seen in the
Chandra images, map the spatial distribu-tion of the individual temperature components and compare it withnew H α images obtained at the Lick Observatory , and archival datafrom the
Hubble Space Telescope (HST) . In Section 2, we describethe data reduction and analysis; in Section 3, we present the resultsof the observations; in Section 4, we determine the physical prop-erties of the cooler plasma phases and discuss the uplift of the lowentropy gas from the bottom of the gravitational potential well, ande ff ects of shocks and mixing. Finally, in Section 5, we summarizeour conclusions.A distance to M 87 of 16.1 Mpc (Tonry et al. 2001) implies alinear scale of 4.7 kpc arcmin − . All errors are quoted at the 68 percent confidence level for one interesting parameter. c (cid:13) , 000–000 GN heating and gas uplift in M 87 Right ascension D ec li n a t i on
10 kpc 10.0 12:31:00.0 50.0 40.0 30.0 30:20.030:00.028:00.026:00.024:00.022:00.012:20:00.018:00.0
Right ascension D ec li n a t i on
10 kpc
Figure 1.
Left panel:
Relative deviations of the surface brightness from a double beta model fit to the
Chandra
XMM-Newton
RGSextraction region is overplotted.
Right panel:
The 90 cm radio image by Owen et al. (2000). Two flows (radio arms) emerge from the inner-jet region, onedirected Eastward (spatially coincident with the X-ray arm) and the other directed to the Southwest. While the Eastern radio arm ends in edge-brightened torus-like vortex rings, the Southwestern radio arm develops a S-shaped Southward twist. Both radio arms are immersed in a pair of large, partially overlappingradio lobes. Both panels show the same 14.6 (cid:48) × (cid:48) field. Chandra data
The
Chandra observations of M 87 were taken between July 2002and November 2005 using the Advanced CCD Imaging Spectrom-eter (ACIS). The observations are listed in Table 1. The total netexposure time after cleaning is 574 ks. We follow the data reduc-tion procedure described in Paper I (see also Million & Allen 2009;Million et al. 2009).The individual regions for the spectral analysis were deter-mined using the Contour Binning algorithm (Sanders 2006), whichgroups neighboring pixels of similar surface brightness until a de-sired signal-to-noise threshold is met. In order to have small enoughregions to resolve substructure and still have enough counts to fit asimple multi-temperature model, we adopted a signal-to-noise ratioof 50. The total number of regions in this analysis is ∼ ∼ . −
12 keV band (the statisticaluncertainty in the observed 9 . −
12 keV flux is less than 5 per centin all cases). The background level in these observations is low andrepresents only a marginal source of systematic uncertainty in thedetermined quantities.Spectral modeling has been performed with the SPEX pack-age (Kaastra et al. 1996, SPEX uses an updated version of theMEKAL plasma model with respect to XSPEC). We analyze datain the 0.6–7.0 keV band. To investigate the multi-temperature struc-ture and map the spatial distribution of plasmas at di ff erent temper- atures, we fit to each bin a model consisting of collisionally ion-ized equilibrium plasmas at four fixed temperatures, with variablenormalizations and common metallicity. This method was intro-duced by Sanders et al. (2004) in the analysis of deep Chandra observations of the Perseus cluster. For M 87, the temperaturesare fixed at 0.5, 1.0, 2.0, and 3.0 keV. The temperatures of thecoolest and hottest components were selected based on previousmulti-temperature analysis of
XMM-Newton
RGS and EPIC data(Werner et al. 2006; Simionescu et al. 2008). We also searched fora 0.25 keV component in our spectral fits, but did not detect anyemission at this temperature. The temperatures of the individualspectral components are su ffi ciently far apart that the spectral anal-ysis can constrain their emission measures if they are simultane-ously present in the same extraction region. The overall metallicityin each region is free to vary. The O / Fe ratio is fixed at 0.59 Solar,Ne / Fe at 1.20 Solar, and the Mg / Fe at 0.60 Solar, as determinedfrom
XMM-Newton
RGS spectra (Werner et al. 2006). The Galac-tic absorption toward M 87 is modelled as neutral gas with Solarabundances with a column density fixed at N H = . × cm − ,the value determined by the Leiden / Argentine / Bonn (LAB) Sur-vey of Galactic H i (Kalberla et al. 2005). Throughout the paper,abundances are given with respect to the “proto-solar values” byLodders (2003), which are for O, Ne, and Fe approximately 30 percent lower than the values given by Anders & Grevesse (1989). XMM-Newton
RGS data
The multi-temperature structure of the hot plasma in and aroundM 87 exhibits a complex set of spectral lines that cannot be mod-elled by a single-temperature spectral model (Sakelliou et al. 2002; c (cid:13) , 000–000 N. Werner et al.
50 100 150 200 D ec li n a t i on
10 kpc
200 400 600 D ec li n a t i on
10 kpc
200 600 1000 D ec li n a t i on
10 kpc D ec li n a t i on
10 kpc
Figure 2.
Spatial distribution of the emission measure, Y = (cid:82) n H n e d V , (in 10 cm − arcsec − ) of the 0.5 keV (upper left), 1.0 keV (upper right), 2.0 keV(bottom left), and 3.0 keV (bottom right) plasma detected at 99.7 per cent confidence. On the upper right panel, we over-plotted the contours of the 90 cmradio image (Owen et al. 2000). All the panels show the same 10.45 (cid:48) × (cid:48) field. For a zoomed in version of the spatial distribution of 0.5 keV plasma in thecore see the right panel of Fig. 4. Werner et al. 2006). In order to confirm that our simple four-temperature model describes the temperature structure of M 87well, we fit the same model to the
XMM-Newton
RGS spectra.The
XMM-Newton
RGS data were obtained in January 2005,with a net exposure time of 84 ks. The data were processed as de-scribed in Werner et al. (2006). Spectra were extracted from a 1.1 (cid:48) wide extraction region, centred at the core of the galaxy. Becausethe RGS operates without a slit, it collects all photons from withinthe 1.1 (cid:48) × ∼ (cid:48) field of view. Line photons originating at angle ∆ θ (in arcminutes) along the dispersion direction are shifted in wave-length by ∆ λ = . ∆ θ Å. Therefore, every line is broadened bythe spatial extent of the source. To account for this spatial broad- ening in our spectral model, we produce a predicted line spreadfunction (LSF) by convolving the RGS response with the surfacebrightness profile of the galaxy derived from the EPIC / MOS1 im-age along the dispersion direction. Because the radial profile of aparticular spectral line can be di ff erent from the overall radial sur-face brightness profile, the line profile is multiplied by a scale factor s , which is the ratio of the observed LSF to the expected LSF. Thisscale factor is a free parameter in the spectral fit.We fit the spectra in the 8–38 Å band with the same four-temperature model fitted to the Chandra data, with an additional0.25 keV component included to determine an upper limit on theamount of such cool plasma. We have also separately fitted the RGS c (cid:13) , 000–000 GN heating and gas uplift in M 87 data with a model consisting of four isobaric cooling flows withelemental abundances tied between the models (similar to one ofthe models used for the Centaurus cluster in Sanders et al. 2008).In this case, we model separately gas cooling from 3 keV to 2 keV,from 2 keV to 1 keV, from 1 keV to 0.5 keV, and from 0.5 keVto 0.25 keV. The central AGN cannot be excluded from the RGSdata of the core of the galaxy and also needs to be accounted for.We model the AGN spectrum with a power-law of a photon index γ = .
95 and a 2–10 keV luminosity of L X = . × erg s − (Werner et al. 2006). α data Narrow-band H α imaging (central wavelength 6606 Å, FWHM75 Å) of M 87 was carried out using the Shane
Lick Observatory on March 26th, 2009. The total integration timewas 24 ks, split into 20 exposures using a dither pattern with ∼ (cid:48)(cid:48) o ff sets. We also acquired 1200 seconds of R band imaging, alsosplit into 20 exposures. The data were processed using the pipelineof Erben et al. (2005). A constant background was subtracted, es-timated from the region of the image with the lowest counts. Theimages were registered with scamp (Bertin 2006), and resampledand combined with swarp (Bertin et al. 2002). The seeing of thecoadded images is about 2 (cid:48)(cid:48) .To study the morphology of the brighter H α filaments in thecluster core, we have also analyzed two H α images available inthe HST archive. Both images were taken with WFPC2, placingthe core onto the Planetary Camera. The Southeastern region wasimaged with the Wide Field Camera for only 2700 seconds (pro-posal ID: 5122, PI Ford), and is not as deep as our ground-basedimage. The Northwestern region was imaged significantly longer(13900 seconds, proposal ID: 6296, PI Ford).
Fig. 2 shows the spatial distribution of the emission measures forthe four individual temperature components (0.5, 1.0, 2.0, and3.0 keV) obtained from the
Chandra data. The top left panel showsan interesting result: the map reveals the presence of gas with a tem-perature of kT ∼ σ is presentwithin the Eastern radio arm. While all the detected 0.5 keV plasmain the core and in the Southeastern horseshoe is within the RGS ex-traction region (see the left panel of Fig. 1), the Southwestern armis outside of the area covered by the present RGS data.The spatial distribution of the ∼ α + [N ii ]emission (see below). Based on two-temperature spectral fits to XMM-Newton data it has been shown that the X-ray arms are rela-tively isothermal at ∼ ∼ . . . k T ( k e V ) Radius (arcmin)
Figure 3.
The temperature profile of the Southwestern X-arm fitted witha two-temperature model. While the temperature of the hotter componentshows a slight radial increase (black data points) the radial distribution ofthe cooler temperature component (red data points) looks remarkably flat ataround 1 keV. If the uplift were adiabatic, the temperature of the cooler gaswithin the arm would be radially decreasing. which is also present to the South of the “stem of the radio mush-room” where the 0.5 keV horseshoe is observed. The brightest partof the Southwestern radio arm is not spatially coincident with the0.5 keV and 1 keV plasma, but it bends around the cooler X-raygas. On the Southwest, just outside of the core region, the 1 keVplasma forms a narrow and remarkably straight and smooth fila-ment, which broadens with increasing distance from the core. Ataround 3 (cid:48) , which is the approximate distance of the circular shockfront described in Paper I and by Forman et al. (2007), the distri-bution of 1 keV gas broadens, and at 4.5 (cid:48) it starts to bend to theSoutheast.The spatial distribution of the plasma at ∼ ∼ (cid:48) (Forman et al. 2007, Paper I, seealso the right panel of Fig. 5).The fraction of the emission measure of cooler gas (1 keV and0.5 keV) in the X-ray arms with respect to the total emission mea-sure is 10–35 per cent, consistent with the results obtained by fittinga di ff erential emission measure distribution to XMM-Newton databy Simionescu et al. (2008). The spatial distribution of the coolestgas phases is also in good agreement with that found in the
XMM-Newton data analysis (Simionescu et al. 2008), but is revealed by
Chandra with a strikingly better spatial resolution.The elemental abundances obtained from the fits are likely tobe biased due to the limitations of our model, which has its tem-peratures fixed at specific values (detailed study of the metallicitywill be presented in Million et al. in prep.; see also Paper I andSimionescu et al. 2008). c (cid:13) , 000–000 N. Werner et al.
Right ascension D ec li n a t i on Ηα+[ΝΙΙ]
Right ascension D ec li n a t i on Figure 4.
The H α emission (left panel) and plasma at ∼ (cid:48) × (cid:48) field. D ec li n a t i on Ηα+[ΝΙΙ]
Pressure1 kpc
Figure 5.
Left panel: H α image of the innermost 2 (cid:48) × (cid:48) region of M 87, divided by the best fit De Vaucouleurs profile, obtained by HST . Right panel: Chandra pressure map (in units of keV cm − × (cid:16) (cid:17) − / , see Paper I for details on producing the pressure map) of the same inner 2 (cid:48) × (cid:48) region overplotted with the6 cm radio emission contours from Hines et al. (1989). The pressure map clearly shows a discontinuity, a likely shock front, at a radius of 0.6 (cid:48) . The H α imageshows that all the bright H α filaments appear in the downstream region of the shock. The arrows identify the bright H α features furthest from the centre, at theedges of apparently underpressured X-ray cavities, and a bright H α “knee” on the Southeast. α nebulae: new clues about theirpowering The left panel of Fig. 4 shows an excerpt of the H α + [N ii ] imageobtained at the Lick Observatory after subtraction of the scaled R -band continuum image. Owing to the large depth, our data reveala previously undiscovered horseshoe-like filament of H α + [N ii ]emission to the Southeast of the core. The feature is detected at amean level of 5 σ above the sky. Fig. 4 shows that the spatial distri-bution of the H α + [N ii ] emission is remarkably similar to that of the0.5 keV plasma, both in the core and in the Southeastern horseshoelike region. Except for a small knot of H α + [N ii ] emission in the“cap of the mushroom” (see Gavazzi et al. 2000), the Eastern radioarm is devoid of observable H α filaments. As mentioned above, theEastern radio arm is also devoid of any detectable 0.5 keV emis- sion. Our Lick Observatory data (Fig. 4) and the
HST data (Fig. 5)unfortunately do not cover the Southwestern X-ray arm.Fig. 5 shows an H α image for the innermost 2 (cid:48) × (cid:48) region ofM 87 from HST next to a pressure map obtained in Paper I byusing
Chandra data. In the pressure map we see a discontinuity,likely shock front, at 0.6 (cid:48) and several underpressured regions filledwith radio emitting plasma, as shown by the contours of 6 cm radioemission from Hines et al. (1989). Assuming that this inner shockpropagates at an approximately constant velocity of 1000 km s − ,it originated in an AGN outburst ∼ α emission is within the inner higher pressure regionsurrounded by the shock front at 0.6 (cid:48) . Some filaments appear to beparallel to the shock front, others seem to lie at the edges of cavi-ties, while the filament to the Southeast of the core is perpendicular c (cid:13) , 000–000 GN heating and gas uplift in M 87 to the shock front, bright in the downstream region and faint in theupstream. Upstream of the shock front, the Southeastern H α fila-ment continues at a much lower surface brightness and forms theprominent horseshoe. On the H α image, we identify with arrowsthe bright features furthest from the centre, at the edges of X-raycavities, and a bright H α “knee” on the Southeast. The same ar-rows are plotted in the pressure map. XMM-Newton
RGS spectra
To confirm the presence of the coolest gas phases indicated by fit-ting the
Chandra data and to verify that the simple four-temperaturemodel fits the data su ffi ciently well, we also analyze the XMM-Newton
RGS high-resolution line spectra. The results of the five-temperature fit to the RGS data are listed in Table 2 and the best fitmodel to the spectra is shown in Fig. 6.The emission measures for the 2 keV and 1 keV phases ob-tained by the RGS are consistent with those from the
Chandra mapsfor the area covered by the gratings. The emission measure of the0.5 keV phase obtained using the RGS is higher by a factor of 3than the value from the
Chandra map. The sensitivity of
Chandra in the 0.6–7 keV band to 0.5 keV emission in these moderate S / Nspectra is limited and if we only consider regions where the 0.5keV component was detected at larger than the 99.7% confidencelevel then we miss the cool emission in some regions. By includ-ing all regions detected at the 68% confidence the 0.5 keV featuresin the map become wider and the agreement between the emissionmeasures from
Chandra and RGS improves.The lack of O vii lines provides strong constraints on theamount of 0.25 keV plasma. The 95 per cent upper limit for itsemission measure is lower by a factor of 11 than the emission mea-sure of the 0.5 keV plasma.To get constraints on the amount of radiative cooling, we alsofit the RGS spectra with a set of cooling flow models (see Table 2).If the plasma would cool isobarically in the absence of heating,the best fit mass deposition rates would be the same in the di ff er-ent temperature ranges. However, the mass deposition rates are de-creasing as a function of temperature and for the gas cooling from0.5 keV to 0.25 keV we determine a tight 95 per cent upper limit of˙ M = . M (cid:12) / yr. The inferred properties of the X-ray arms depend critically on theirorientation with respect to our line-of-sight. Based on
HST obser-vations of the superluminal motion in the jet of M 87, Biretta et al.(1999) concluded that the position angle of the jet is < ◦ fromour line-of-sight. The two large, partially overlapping, outer radiolobes of M 87 (the di ff use structures which embed the more colli-mated, brighter radio arms seen in the right panel of Fig. 1 and inOwen et al. 2000) are also probably oriented close to our line-of-sight. It is likely that the Southern radio lobe is positioned towardsus and the Northern lobe is oriented away from us. This picture issupported by the observation of polarized flux from the Southernlobe (Andernach et al. 1979), which shows that the Faraday depthtowards it is likely to be small. The radius of the large lobes / bubblesis ∼
22 kpc and their approximate centres are in projection 26 kpcapart. Assuming that the physical distance between the centres ofthe bubbles is at least twice their radius, a conservative limit for the
Table 2.
Best fit parameters for a five-temperature fit and a four-cooling-flow model fit to the high-resolution RGS spectra extracted from a 1.1 (cid:48) wideregion centred on the core of M 87. Emission measures, Y = (cid:82) n H n e d V , aregiven in 10 cm − . The scale factor s is the ratio of the observed LSF tothe expected LSF based on the overall radial surface brightness profile. Theupper limits are quoted at their 95 per cent confidence level. Abundancesare quoted with respect to the proto-solar values of Lodders (2003).Parameter 5T-model 4-c.f. model Y . < .
07 – Y . . ± .
05 –˙ M . − . keV – < . Y . . ± . M . − . keV – 0 . ± . Y . . ± . M . − . keV – 3 . ± . Y . < . M . − . keV – 6 . ± .
21s 2 . ± .
10 2 . ± .
11C 0 . ± .
21 0 . ± .
22N 1 . ± . . ± .
3O 0 . ± .
04 0 . ± . . ± .
13 1 . ± . . ± .
15 1 . ± . . ± .
04 1 . ± . . ± . . ± .
10 20 30 . . C oun t s / s / Å wavelength (Å) O V III L α F e XV II N e X F e XV III/ O V III L β F e XV II FeXXIIIFeXXII/ F e XX F e X I X F e XV III
Figure 6.
The first order RGS spectrum extracted from a 1.1 (cid:48) wide regioncentred on the core of M 87. The continuous line represents the best fitmodel to the spectrum. orientation of the axis along which the bubbles rise is < ◦ fromour line-of-sight. The Southwestern radio arm appears to “merge”into the Southern lobe indicating that its orientation is similar tothat of the two large bubbles. Although the relation of the Easternradio arm to the outer lobe is less clear, the most likely orientationof both X-ray and radio arms is approximately anti-parallel and farfrom the plane of the sky. c (cid:13) , 000–000 N. Werner et al.
Figure 7.
Cumulative gas mass, entropy ( S = kT / n / ), and radiative cool-ing time of the 0.5 keV phase (red lines), 1.0 keV phase (blue lines), andthe ambient ICM (black lines) as a function of projected radius for orien-tation angles of 15 ◦ (dashed lines), 30 ◦ (dotted lines), and 90 ◦ (full lines)from our line-of-sight. For the deprojected density and temperature profilesof the ambient ICM we assume the profiles parametrized by equations (6)and (7) in Churazov et al. (2008). The cooler gas phases are assumed tobe in pressure equilibrium with the ambient medium (see text for details).The vertical dotted lines denote the radii of the outer edge of the ridge ofmultiphase gas North of the jet, the end of the Southeastern horseshoe, andthe radius of the shock at ∼ (cid:48)(cid:48) which corresponds to the inner edge ofthe “cap of the mushroom” of the Eastern arm. We determine the properties of the X-ray arms for three di ff erentorientation angles (15 ◦ , 30 ◦ , and 90 ◦ ) from our line-of-sight. Foreach orientation, we determine the radial profiles of the gas mass,entropy, and cooling time for the di ff erent phases (see Fig. 7). Weassume that all the gas phases are in equilibrium with the ambi-ent pressure at the given distance from the centre. The radial dis-tribution of the ambient pressure is determined using the depro-jected temperature and electron density profiles parametrized byequations (6) and (7) in Churazov et al. (2008). Using this pres-sure profile p ( r ), we calculate the electron number density pro-file n e ( r ) = p ( r ) / ( kT ) of the kT = . Y = (cid:82) n H n e d V , for each extraction region, we determine thevolumes which the cooler phases occupy: V = Y / ( n H n e ), where n H = n e / . l , of the emitting volumes, V , shownin Fig. 8, are then calculated as l = V / A , where A is the area on thesky of the given extraction region. The corresponding gas massesare calculated as M = . n H m p V (where m p is the proton mass),only in the spatial regions where their presence was determinedwith better than 3 σ significance. The cumulative radial gas massprofiles are shown in the bottom panel of Fig. 7. The middle panel of this figure shows the radial entropy profiles calculated as S = kT / n / . The top panel shows the cooling time profiles calculatedas the gas enthalpy divided by the energy lost per unit volume ofthe plasma: t cool = ( n e + n i ) kTn e n i Λ ( T ) , (1)where the ion number density n i = . n e and Λ ( T ) is the cool-ing function for Solar metallicity tabulated by Sutherland & Dopita(1993). Cooling functions based on more up to date plasma codes(Schure et al. 2009) predict for the 0.5 keV plasma a 9 per centhigher cooling rate.If the arms lie in the plane of the sky, then the small line-of-sight depths of the emitting volumes in Fig. 8 indicate that the1 keV gas is not volume filling at most radii, including the core ofthe galaxy. The 1 keV gas is then made of small blobs and filamentswith an entropy smaller than the lowest entropy of the ambientmedium in the centre of the galaxy (see Fig. 7). This would mean,as pointed out by Molendi (2002), that the cool blobs / filaments canhardly originate from the adiabatic evolution of the hot-phase gasentrained by buoyant radio bubbles, as suggested by Churazov et al.(2001). However, if the arms are oriented close to our line-of-sight,the physical picture changes.For the likely angles of 15 ◦ –30 ◦ from our line-of-sight, boththe radial distribution of the gas entropy and the distribution ofthe depths of the emitting volumes of the 1 keV component give amore consistent picture with other observables. The 1 keV plasmaappears to be mostly volume filling, except in regions where theX-ray images reveal cavities. Its cooling time is > . × yr,which is comparable to or longer than the uplift time. Moreover,by taking the projection e ff ects properly into account, the appar-ent large angle between the current direction of the jet and theSouthwestern arm translates into a much smaller physical angle.The inferred spatial distribution of the depths of the emitting vol-umes (see Fig. 8) indicates that the orientation angle of the filamentis not constant and at certain positions the arms are folded alongour line-of-sight. The Southwestern arm appears to be folded at theprojected radius of 3.4 (cid:48) , where the arm divides into two filaments(see Fig. 1). The Eastern arm appears to have folds at ∼ (cid:48) and itsline-of-sight depth increases in the cap of the mushroom. We cau-tion that because of the strongly simplified assumptions the valuesfor the depths quoted in the figures are approximate numbers only.If the uplift of the low entropy plasma were strictly adiabatic,then the temperature of the arms would decrease as a function ofradius. However, as shown by Molendi (2002), Simionescu et al.(2008), and in Fig. 3 using a two-temperature model, the averagetemperature of the arms is remarkably constant. The constant tem-perature implies rising entropy with radius and suggests that con-duction plays an important role in the thermal evolution of the arms.The estimated total mass of 1 keV gas in the arms is between6–9 × M (cid:12) , which given the uncertainties is remarkably sim-ilar to the total gas mass of 6 . × M (cid:12) within the innermost3.8 kpc region (0.8 (cid:48) in the plane of the sky), where the X-ray emis-sion is relatively spherically symmetric. Outside of this radius, allthe ∼ × M (cid:12) of ∼ . × M (cid:12) (Simionescu et al. c (cid:13) , 000–000 GN heating and gas uplift in M 87 Figure 8.
Maps of the line-of-sight depths in kpc for the emitting volumes of 1 keV plasma for orientation angles of 90 ◦ (top left panel), 30 ◦ (top right panel),and 15 ◦ (bottom left panel) from our line-of-sight. The map on the bottom right panel shows the line-of-sight depth distribution of the 0.5 keV component foran orientation angle of 30 ◦ from our line-of-sight. All the panels show the same 10.36 (cid:48) × (cid:48) field. ffi cient in evacuating theirlowest entropy plasma during AGN outbursts.By assuming that the outer radio halo is a spherical bubble ofhot plasma undergoing a steady energy input from the jet, Owenet al. (2000) estimated its age to ∼ yr. However, if the radio haloconsists of two bubbles rising buoyantly in the hot ICM, then theouter radio lobes are likely to be older. The age of the radio and X-ray arms estimated by Churazov et al. (2001) is a few times 10 yr.However, if the whole Southwestern arm originated in a single up- lift following an AGN outburst, then for an orientation angle of < ◦ from our line-of-sight, its length is >
60 kpc, which assumingan uplift velocity of 400 km s − implies an age of > . × yr.The age of the observed structures is thus greater than previouslythought. The NW edge of the Southwestern X-ray arm is remarkably smoothout to the radius of ∼ (cid:48) and the filament is surprisingly straightfor a distance of over 2 (cid:48) . This strongly suggests that the gas mo- c (cid:13) , 000–000 N. Werner et al. tions cannot be very turbulent in either phase. The lack of strongturbulence in the ICM has been inferred previously based on thestraightness of the H α filaments in the Perseus cluster (Fabian et al.2003), and strong upper limits on turbulent velocities were placedbased on resonance scattering in NGC 4636 (Werner et al. 2009)and turbulent line broadening in Abell 1835 (Sanders et al. 2010b).The bending of the X-ray arms of M 87 at larger radii might becaused by gas sloshing, which creates a spiral like shearing motionpattern in the ICM which is the most obvious at the two oppositeand staggered cold fronts seen at 7 (cid:48) to the Southeast (Simionescuet al. 2007) and at 19 (cid:48) to the North (Simionescu et al. 2010). Thesloshing induced shearing motions might also generate mixing ofthe ∼ The total mass of ∼ × M (cid:12) .Because we calculate the mass of the 0.5 keV component only inspatial regions where it was detected at higher than the 99.7 percent confidence, this value is a conservative lower limit. The in-ferred small line-of-sight depths of the emitting volumes of the0.5 keV component imply that the coolest X-ray emitting gas isnot volume filling, but forms a multi-phase medium together withboth the hotter X-ray emitting and the cold H α emitting gas phases.Soft band X-ray images (Forman et al. 2007) reveal that this coolerphase forms a network of filaments, which spatially coincide with H α filaments (Sparks et al. 2004, see also Fig. 4). Relatively cool( ∼ . α filaments has also beenobserved in the Perseus cluster and in 2A 0335 +
096 (Sanders &Fabian 2007; Sanders et al. 2009).One of the most prominent regions in M 87 occupied by multi-phase plasma is a ridge to the North of the jet, which contains 2 keV,1 keV, 0.5 keV, and H α gas. This region is just outside of the innerradio cocoon and possibly contributes to the confinement of the ra-dio plasma on the Northern side and to the depolarization seen thereby Owen et al. (1990). The depolarization of the radio emission inthis region overlapping with the filaments indicates that the mag-netic fields are disordered on scales less than 0.1 kpc. Therefore, itis likely that the H α filaments consist of many narrow magnetizedthreads, which are not perfectly aligned and are small comparedto the observed beam. Depolarization of the radio emission at re-gions containing H α and soft X-ray emission has been previouslyobserved in the Centaurus cluster (Taylor et al. 2007). Even though the cooling time of the ∼ vii lines with RGS is Y = (cid:82) n H n e d V = × cm − ,which is only 9 per cent of the emission measure of the 0.5 keVplasma. Therefore, assuming pressure equilibrium between di ff er-ent gas phases, the mass of the 0.25 keV phase is at least 22 timeslower than the mass of the 0.5 keV plasma. Assuming the 1 keV gasis isobarically cooling to 0.5 keV, its best fit mass deposition rateis ˙ M = . M (cid:12) yr − , which is 15 times larger than the 95 per centconfidence upper limit for the mass deposition rate from 0.5 keV to0.25 keV. The upper limits on gas cooling below 0.5 keV rely onthe O vii lines and thus on the O abundance of the cool gas, which we assumed to be the same as that of the hotter phase which isfairly constant across the core (Werner et al. 2006). For significantamounts of gas cooling radiatively below 0.5 keV to be present inthe core, without detectable O vii emission, the O abundance of thecooling plasma would have to be unrealistically low. The lack oflarger amounts of gas radiatively cooling below 0.5 keV impliesthe presence of heating. A similar temperature floor at ∼ + ∼ ffi cientconduction between the two phases (Fabian et al. 2002; Soker et al.2004). As indicated by the fact that all the bright H α + [N ii ] fila-ments end at the innermost shock front, this mixing might be pro-moted by the shock, which induces enhanced shearing motions, andmight also power the detected optical and UV line emission (see thenext section). α filaments The optical emission line filaments in M 87 are likely due to cooledgas, enriched by dust from stellar mass loss (Sparks et al. 1993),which has been uplifted by the buoyant relativistic plasma, togetherwith the coolest X-ray emitting phase. The total mass of the ∼ KH α emitting gas in the filament system in M 87 is estimated to be ofthe order of 10 –10 M (cid:12) (Sparks et al. 1993), which is comparableto the mass of ∼ α emitting regions is only a few times 10 M (cid:12) (Salom´e & Combes 2008). This implies that the H α emitting gas isnot a thin skin on the underlying clouds of neutral and moleculargas as in the Perseus cluster, where the inferred total mass of thecold gas is as high as ∼ × M (cid:12) (Salom´e et al. 2006). Recently,Sparks et al. (2009) discovered filaments of C iv emission in M 87,with a total power of 1 . × erg s − , arising from gas at a temper-ature of ∼ K, which spatially coincides with the H α filaments onall scales. This detection indicates that the cool and hot gas phasesare in thermal communication. The emission strengths are consis-tent with thermal conduction, which would create this intermediatetemperature phase at the interface of the hot ICM and the coolergas. However, even though conduction can quantitatively providethe energy seen in the form of line emission (Sparks et al. 2004),it would have to proceed at close to the Spitzer or saturated levelsto power the filaments. The very thin and long H α filaments aremagnetized (see Sect. 4.2 and Fabian et al. 2008) and the likely ori-entation of the magnetic fields parallel to the interface of the coldand hot phase would largely suppress the level of conduction. Thus c (cid:13)000
096 (Sanders &Fabian 2007; Sanders et al. 2009).One of the most prominent regions in M 87 occupied by multi-phase plasma is a ridge to the North of the jet, which contains 2 keV,1 keV, 0.5 keV, and H α gas. This region is just outside of the innerradio cocoon and possibly contributes to the confinement of the ra-dio plasma on the Northern side and to the depolarization seen thereby Owen et al. (1990). The depolarization of the radio emission inthis region overlapping with the filaments indicates that the mag-netic fields are disordered on scales less than 0.1 kpc. Therefore, itis likely that the H α filaments consist of many narrow magnetizedthreads, which are not perfectly aligned and are small comparedto the observed beam. Depolarization of the radio emission at re-gions containing H α and soft X-ray emission has been previouslyobserved in the Centaurus cluster (Taylor et al. 2007). Even though the cooling time of the ∼ vii lines with RGS is Y = (cid:82) n H n e d V = × cm − ,which is only 9 per cent of the emission measure of the 0.5 keVplasma. Therefore, assuming pressure equilibrium between di ff er-ent gas phases, the mass of the 0.25 keV phase is at least 22 timeslower than the mass of the 0.5 keV plasma. Assuming the 1 keV gasis isobarically cooling to 0.5 keV, its best fit mass deposition rateis ˙ M = . M (cid:12) yr − , which is 15 times larger than the 95 per centconfidence upper limit for the mass deposition rate from 0.5 keV to0.25 keV. The upper limits on gas cooling below 0.5 keV rely onthe O vii lines and thus on the O abundance of the cool gas, which we assumed to be the same as that of the hotter phase which isfairly constant across the core (Werner et al. 2006). For significantamounts of gas cooling radiatively below 0.5 keV to be present inthe core, without detectable O vii emission, the O abundance of thecooling plasma would have to be unrealistically low. The lack oflarger amounts of gas radiatively cooling below 0.5 keV impliesthe presence of heating. A similar temperature floor at ∼ + ∼ ffi cientconduction between the two phases (Fabian et al. 2002; Soker et al.2004). As indicated by the fact that all the bright H α + [N ii ] fila-ments end at the innermost shock front, this mixing might be pro-moted by the shock, which induces enhanced shearing motions, andmight also power the detected optical and UV line emission (see thenext section). α filaments The optical emission line filaments in M 87 are likely due to cooledgas, enriched by dust from stellar mass loss (Sparks et al. 1993),which has been uplifted by the buoyant relativistic plasma, togetherwith the coolest X-ray emitting phase. The total mass of the ∼ KH α emitting gas in the filament system in M 87 is estimated to be ofthe order of 10 –10 M (cid:12) (Sparks et al. 1993), which is comparableto the mass of ∼ α emitting regions is only a few times 10 M (cid:12) (Salom´e & Combes 2008). This implies that the H α emitting gas isnot a thin skin on the underlying clouds of neutral and moleculargas as in the Perseus cluster, where the inferred total mass of thecold gas is as high as ∼ × M (cid:12) (Salom´e et al. 2006). Recently,Sparks et al. (2009) discovered filaments of C iv emission in M 87,with a total power of 1 . × erg s − , arising from gas at a temper-ature of ∼ K, which spatially coincides with the H α filaments onall scales. This detection indicates that the cool and hot gas phasesare in thermal communication. The emission strengths are consis-tent with thermal conduction, which would create this intermediatetemperature phase at the interface of the hot ICM and the coolergas. However, even though conduction can quantitatively providethe energy seen in the form of line emission (Sparks et al. 2004),it would have to proceed at close to the Spitzer or saturated levelsto power the filaments. The very thin and long H α filaments aremagnetized (see Sect. 4.2 and Fabian et al. 2008) and the likely ori-entation of the magnetic fields parallel to the interface of the coldand hot phase would largely suppress the level of conduction. Thus c (cid:13)000 , 000–000 GN heating and gas uplift in M 87 it seems unlikely that thermal conduction alone can explain the op-tical line emission.All of the bright H α emission in M 87 is observed in the down-stream region of the innermost < ∼ . (cid:48) . This suggests that the H α emission in M 87 issomehow related to shocks in the ICM. The passage of a shock ac-celerates the intra-cluster medium, but because of the large densitycontrast the relatively cool H α emitting gas is left behind, result-ing in a sudden strong shear around the filaments. It is likely thatsuch shear promotes mixing of the cold gas with the ambient hotICM via instabilities. Ferland et al. (2009) recently showed that thespectra of optical filaments observed around the central galaxies ofcooling core clusters can be explained if the filaments are heatedby ionizing particles, either conducted from the surrounding re-gions or produced in situ by MHD waves. By facilitating contactbetween the hot thermal particles and the cold gas, mixing can sup-ply the power and the ionizing particles needed to explain the opti-cal line emission spectrum. Mixing of X-ray emitting plasma withthe cold gas also naturally explains the presence of the ∼ K in-termediate temperature gas phase. If the coolest X-ray emitting gascools by mixing with the filamentary gas behind the shock frontand conductively connects with it (e.g. Begelman & Fabian 1990;Soker et al. 2004), then its thermal energy will be radiated away inthe UV band, in the optical H α + [N ii ], and most likely at infraredwavelengths in the presence of dust. The series of recent ‘radio’ or ‘jet’-mode AGN outbursts in M 87,revealed in exquisite detail by the
Chandra data, by no means rep-resent a unique or extreme phenomenon. For example, a complete,flux-limited study of eighteen optically and X-ray bright, nearbygalaxies, including M 87, by Dunn et al. (2010) shows that 17 / ffi ciencywith which the accretion of hot gas powers jets in giant ellipticals,including M 87, is remarkably uniform. Our results for the nearest,brightest cluster core show that even the current, relatively modestlevel of radio mode AGN activity has been su ffi cient to remove asizeable fraction of the coolest, lowest entropy gas from the centralgalaxy. This is the material that would have cooled to form stars(Peterson & Fabian 2006). Our results therefore provide direct ev-idence that radio mode AGN input has a profound e ff ect on theability of large galaxies to form stars from their surrounding X-rayemitting halos. Using a combination of deep (574 ks)
Chandra data,
XMM-Newton high-resolution spectra, and optical H α + [N ii ] images, we have per-formed an in-depth study of AGN feedback in M 87. Accountingfor the fact that the long X-ray and radio “arms” are seen in projec-tion with likely angles of ∼ ◦ –30 ◦ from our line-of-sight, we findthat: • The mass of the uplifted low entropy gas in the arms is com-parable to the gas mass in the approximately spherically symmetric3.8 kpc core, demonstrating that the AGN has a profound e ff ect onits immediate surroundings. This result has important implicationsfor understanding AGN feedback in galaxy formation models (e.g.Croton et al. 2006; De Lucia & Blaizot 2007; Sijacki et al. 2007). • The coolest detected X-ray plasma, with a temperature of ∼ α + [N ii ] emitting gas, and the UV emitting gas arecospatial, and appear to be arranged in magnetized filaments withinthe hot ambient atmosphere, forming a multi-phase medium. • We do not detect X-ray emitting gas with temperatures be-low ∼ • All of the bright H α and UV filaments are seen in the down-stream region of the < ∼ (cid:48) . This argues that the generation of H α and UV emission inM 87 is related to shocks in the ICM.Based on these observations we propose that shocks induceshearing around the denser gas filaments, thereby promoting mix-ing of the cold gas with the ambient hot ICM via instabilities. Thisprocess may both provide a mechanism for powering the opticalline emission and explain the lack of plasma with temperature un-der 0.5 keV: • By helping to get hot thermal particles into contact with thecool, line-emitting gas, mixing can supply the power and the ioniz-ing particles needed to explain the optical spectra of the H α + [N ii ]nebulae (see Ferland et al. 2009). • Mixing of the coolest X-ray emitting plasma with the coldoptical line emitting filamentary gas promotes e ffi cient conduc-tion between the two phases, allowing the X-ray gas to cool non-radiatively, which may explain the lack of gas with temperaturesunder 0.5 keV. ACKNOWLEDGMENTS
Support for this work was provided by the National Aeronauticsand Space Administration through Chandra / Einstein PostdoctoralFellowship Award Number PF8-90056 and PF9-00070 issued bythe Chandra X- ray Observatory Center, which is operated bythe Smithsonian Astrophysical Observatory for and on behalf ofthe National Aeronautics and Space Administration under contractNAS8-03060. This work was supported in part by the US De-partment of Energy under contract number DE-AC02-76SF00515.SMH is supported by the National Science Foundation PostdoctoralFellowship program under award number AST-0902010.
REFERENCES
Allen, S. W., Dunn, R. J. H., Fabian, A. C., Taylor, G. B., &Reynolds, C. S. 2006, MNRAS, 372, 21Andernach, H., Baker, J. R., von Kap-Herr, A., & Wielebinski, R.1979, A&A, 74, 93Anders, E. & Grevesse, N. 1989, Geochim. Cosmochim. Acta, 53,197Begelman, M. C. & Fabian, A. C. 1990, MNRAS, 244, 26PBelsole, E., Sauvageot, J. L., B¨ohringer, H., et al. 2001, A&A,365, L188Bertin, E. 2006, in Astronomical Society of the Pacific Confer-ence Series, Vol. 351, Astronomical Data Analysis Software andSystems XV, ed. C. Gabriel, C. Arviset, D. Ponz, & S. Enrique,112– + c (cid:13) , 000–000 N. Werner et al.
Bertin, E., Mellier, Y., Radovich, M., et al. 2002, in AstronomicalSociety of the Pacific Conference Series, Vol. 281, AstronomicalData Analysis Software and Systems XI, ed. D. A. Bohlender,D. Durand, & T. H. Handley, 228– + Best, P. N., Kau ff mann, G., Heckman, T. M., et al. 2005, MNRAS,362, 25Biretta, J. A., Sparks, W. B., & Macchetto, F. 1999, ApJ, 520, 621B¨ohringer, H., Matsushita, K., Churazov, E., Ikebe, Y., & Chen,Y. 2002, A&A, 382, 804B¨ohringer, H., Nulsen, P. E. J., Braun, R., & Fabian, A. C. 1995,MNRAS, 274, L67Br¨uggen, M. 2003, ApJ, 592, 839Br¨uggen, M. & Kaiser, C. R. 2002, Nature, 418, 301Churazov, E., Br¨uggen, M., Kaiser, C. R., B¨ohringer, H., & For-man, W. 2001, ApJ, 554, 261Churazov, E., Forman, W., Jones, C., & B¨ohringer, H. 2000,A&A, 356, 788Churazov, E., Forman, W., Vikhlinin, A., et al. 2008, MNRAS,388, 1062Churazov, E., Sunyaev, R., Forman, W., & B¨ohringer, H. 2002,MNRAS, 332, 729Croton, D. J., Springel, V., White, S. D. M., et al. 2006, MNRAS,365, 11De Lucia, G. & Blaizot, J. 2007, MNRAS, 375, 2de Vaucouleurs, G. & Nieto, J.-L. 1978, ApJ, 220, 449De Young, D. S. 2003, MNRAS, 343, 719Dunn, R. J. H., Allen, S. W., Taylor, G. B., et al. 2010, MNRASDunn, R. J. H. & Fabian, A. C. 2006, MNRAS, 373, 959Dunn, R. J. H. & Fabian, A. C. 2008, MNRAS, 385, 757Erben, T., Schirmer, M., Dietrich, J. P., et al. 2005, AstronomischeNachrichten, 326, 432Fabian, A. C., Allen, S. W., Crawford, C. S., et al. 2002, MNRAS,332, L50Fabian, A. C., Johnstone, R. M., Sanders, J. S., et al. 2008, Nature,454, 968Fabian, A. C., Sanders, J. S., Crawford, C. S., et al. 2003, MN-RAS, 344, L48Feigelson, E. D., Wood, P. A. D., Schreier, E. J., Harris, D. E., &Reid, M. J. 1987, ApJ, 312, 101Ferland, G. J., Fabian, A. C., Hatch, N. A., et al. 2009, MNRAS,392, 1475Forman, W., Jones, C., Churazov, E., et al. 2007, ApJ, 665, 1057Forman, W., Nulsen, P., Heinz, S., et al. 2005, ApJ, 635, 894Gavazzi, G., Boselli, A., V´ılchez, J. M., Iglesias-Paramo, J., &Bonfanti, C. 2000, A&A, 361, 1Heinz, S. & Churazov, E. 2005, ApJ, 634, L141Hines, D. C., Eilek, J. A., & Owen, F. N. 1989, ApJ, 347, 713Kaastra, J. S., Mewe, R., & Nieuwenhuijzen, H. 1996, in UVand X-ray Spectroscopy of Astrophysical and Laboratory Plas-mas p.411, K. Yamashita and T. Watanabe. Tokyo : UniversalAcademy PressKaiser, C. R. 2003, MNRAS, 343, 1319Kalberla, P. M. W., Burton, W. B., Hartmann, D., et al. 2005,A&A, 440, 775Lodders, K. 2003, ApJ, 591, 1220Matsushita, K., Belsole, E., Finoguenov, A., & B¨ohringer, H.2002, A&A, 386, 77McNamara, B. R. & Nulsen, P. E. J. 2007, ARA&A, 45, 117Million, E. T. & Allen, S. W. 2009, MNRAS, 399, 1307Million, E. T., Allen, S. W., Werner, N., & Taylor, G. B. 2009,ArXiv:[0910.0025]Million, E. T., Werner, N., Simionescu, A., et al. 2010, MNRAS Molendi, S. 2002, ApJ, 580, 815Nulsen, P. E. J., McNamara, B. R., Wise, M. W., & David, L. P.2005, ApJ, 628, 629Owen, F. N., Eilek, J. A., & Kassim, N. E. 2000, ApJ, 543, 611Owen, F. N., Eilek, J. A., & Keel, W. C. 1990, ApJ, 362, 449Peterson, J. R. & Fabian, A. C. 2006, Phys. Rep., 427, 1Ruszkowski, M., Br¨uggen, M., & Begelman, M. C. 2004a, ApJ,611, 158Ruszkowski, M., Br¨uggen, M., & Begelman, M. C. 2004b, ApJ,615, 675Sakelliou, I., Peterson, J. R., Tamura, T., et al. 2002, A&A, 391,903Salom´e, P. & Combes, F. 2008, A&A, 489, 101Salom´e, P., Combes, F., Edge, A. C., et al. 2006, A&A, 454, 437Sanders, J. S. 2006, MNRAS, 371, 829Sanders, J. S. & Fabian, A. C. 2007, MNRAS, 381, 1381Sanders, J. S. & Fabian, A. C. 2008, MNRAS, 390, L93Sanders, J. S., Fabian, A. C., Allen, S. W., et al. 2008, MNRAS,385, 1186Sanders, J. S., Fabian, A. C., Allen, S. W., & Schmidt, R. W. 2004,MNRAS, 349, 952Sanders, J. S., Fabian, A. C., Frank, K. A., Peterson, J. R., & Rus-sell, H. R. 2010a, MNRAS, 402, 127Sanders, J. S., Fabian, A. C., Smith, R. K., & Peterson, J. R.2010b, MNRAS, 402, L11Sanders, J. S., Fabian, A. C., & Taylor, G. B. 2009, MNRAS, 396,1449Schure, K. M., Kosenko, D., Kaastra, J. S., Keppens, R., & Vink,J. 2009, A&A, 508, 751Sijacki, D., Springel, V., Di Matteo, T., & Hernquist, L. 2007,MNRAS, 380, 877Simionescu, A., B¨ohringer, H., Br¨uggen, M., & Finoguenov, A.2007, A&A, 465, 749Simionescu, A., Roediger, E., Nulsen, P. E. J., et al. 2009a, A&A,495, 721Simionescu, A., Werner, N., B¨ohringer, H., et al. 2009b, A&A,493, 409Simionescu, A., Werner, N., Finoguenov, A., B¨ohringer, H., &Br¨uggen, M. 2008, A&A, 482, 97Simionescu, A., Werner, N., Forman, W. R., et al. 2010, MNRASin press, ArXiv:[1002.0395]Soker, N., Blanton, E. L., & Sarazin, C. L. 2004, A&A, 422, 445Sparks, W. B., Donahue, M., Jord´an, A., Ferrarese, L., & Cˆot´e, P.2004, ApJ, 607, 294Sparks, W. B., Ford, H. C., & Kinney, A. L. 1993, ApJ, 413, 531Sparks, W. B., Pringle, J. E., Donahue, M., et al. 2009, ApJ, 704,L20Sun, M. 2009, ApJ, 704, 1586Sutherland, R. S. & Dopita, M. A. 1993, ApJS, 88, 253Taylor, G. B., Fabian, A. C., Gentile, G., et al. 2007, MNRAS,382, 67Tonry, J. L., Dressler, A., Blakeslee, J. P., et al. 2001, ApJ, 546,681Werner, N., B¨ohringer, H., Kaastra, J. S., et al. 2006, A&A, 459,353Werner, N., Zhuravleva, I., Churazov, E., et al. 2009, MNRAS,398, 23Young, A. J., Wilson, A. S., & Mundell, C. G. 2002, ApJ, 579,560 c (cid:13)000