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Astronomy&Astrophysicsmanuscriptno.AA˙2006˙6650 (cid:13)c ESO2008 February5,2008 The gaseous atmosphere of M87 seen with XMM-Newton A.Simionescu1,H.Bo¨hringer1,M.Bru¨ggen2,andA.Finoguenov1,3 1 MaxPlanckInstituteforExtraterrestialPhysics,Giessenbachstr,85748Garching,Germany 2 InternationalUniversityBremen,CampusRing1,28759Bremen,Germany 3 UniversityofMaryland,BaltimoreCounty,1000HilltopCircle,Baltimore,MD21250,USA 7 Received27October2006/Accepted29December2006 0 0 ABSTRACT 2 Aims.M87isakeyobjectwhosestudycanrevealthecomplexphenomenaincoolingcores.WeuseadeepXMM-Newtonobservation n ofM87toproducedetailedtemperature,pressureandentropymapsinordertoanalyzethephysicalprocessesofcoolingcoresandof a theirheatingmechanisms. J Methods.Weemployedbothbroad-bandfittingandfullspectroscopicalone-temperaturemodelanalysistoderivetemperatureand 9 surface brightness maps, from which the pseudo-deprojected entropy and pressure were calculated. We discuss possible physical 2 interpretationsofsmalldeviationsfromradialandellipticalsymmetryinthesemaps. Results.ThemostprominentfeaturesobservedaretheEandSWX-rayarmsthatcoincidewithpowerfulradiolobes,aweakshock 2 ataradiusof3′,anoverallellipticityinthepressuremapandaNW/SEasymmetryintheentropymapwhichweassociatewiththe v motionof thegalaxytowards theNW.Forthefirsttimewefindevidence that cold, metal-richgasisbeingtransported out of the 4 center,possiblythroughbubble-inducedmixing.Severaledgesintheabundancemapindicateanoscillationofthegalaxyalongthe 7 NW/SEdirection.Furthermore,theradiolobesappeartorisealongtheshortaxisoftheellipticalpressuredistribution,followingthe 8 steepestgradientofthegravitationalpotential,andseemtocontainanonthermalpressurecomponent. 0 Keywords.galaxies:individual:M87–galaxies:intergalacticmedium–coolingflows–X-rays:galaxies:clusters 1 6 0 / 1. Introduction Davidetal.(2001),Churazovetal.(2001),Fabianetal.(2003), h Bru¨ggenetal.(2005),Heinzetal.(2006)). p As earlyas 1973,the first detailedX-ray observationswith the - M87,thecentraldominantgalaxyinthenearbyVirgoclus- o UhuruandCopernicussatellitesrevealedthattheX-rayluminos- ter,isanidealcandidatewhosestudymayshedlightonthecom- r ity at the center of some galaxyclusters and elliptical galaxies t hasaverysharppeak,implyingahighenoughcentralgasden- plexphenomenaincoolingcores.M87wasrecognizedearlyon s as having a peaked X-ray surface brightness corresponding,in a sitythatthecoolingtimefallsbelowtheHubbletime(Leaetal. : 1973).Consequently,the so-calledcoolingflow modelwasde- the absence of heating, to a cooling flow with a mass deposi- v tion rate of 10 M /year (Stewartetal. 1984). However, using i velopedwhichassertedthatthe gas,in losingenergybymeans ⊙ X the spectral information from XMM-Newton, Bo¨hringeretal. ofradiationandintheabsenceofotherheatsources,coolsdown (2001)putanupperlimitonthemassdepositionrateatleastone r to temperatures so low as to become undetectable in X-rays, a order of magnitude below this value (see also Matsushitaetal. andsinkstowardstheclustercore(e.g.Fabian&Nulsen(1977), (2002), Molendi(2002)). The energyinputwhich may prevent Cowie&Binney(1977)). The modelhoweverfailed to explain the cooling of the gas could be supplied by the central AGN, the ultimate fate of the cooling gas, and the evidence in wave- poweredbythegalaxy’scentralsupermassiveblackholewhich bandsotherthanX-raysshowedthatmuchlessgaswascondens- has a mass of 3.2×109 M (Harmsetal. 1994). Based on the ing into a cooler state than the ”coolingflow” modelpredicted ⊙ observedcomplexsystemofradiolobes,believedto beassoci- (e.g.McNamara&O’Connell(1989)). atedwiththeAGNjetandunseencounterjet,Owenetal.(2000) ThelatestgenerationofX-raysatellites,inparticularXMM- showedthatthemechanicalpowerinputfromthisAGNismore NewtonandChandra,providedmoredetailedspectralinforma- thansufficienttocompensatefortheenergylossthroughX-ray tionshowingthatindeedmuchlessgasundergoessucha”cool- radiation. ing flow” than the initial model based on the X-ray luminos- itypeakpredicted(Petersonetal.(2001),Petersonetal.(2003), TheradioplasmainM87clearlyinteractswiththeX-raygas. Bo¨hringeretal.(2001),Matsushitaetal.(2002)).Thisledtothe The best proof of this is the fact that the two most prominent conclusionthataheatingsourcemustexistwhichinteractswith radiolobesextendingtotheeastandsouth-westoftheM87cen- the gas in a fine-tuned way, stopping the cooling at just the tercorrespondtoregionsofenhancedX-raysurfacebrightness, right level to reproduce the observations (e.g. Bo¨hringeretal. alsoknownastheEandSWX-rayarms.Moreover,theseEand (2002)). Active galactic nuclei (AGN) are found in most clus- SWarmsshowamorecomplexmulti-phasetemperaturestruc- ters previously believed to host a ”cooling flow” (Burns 1990) ture (Molendi 2002) while a previousobservationwith XMM- and represent the most favoured candidates for providing such NewtonshowedthattherestoftheICMislocallywellapprox- heat input. The main challenge remains the understanding of imatedassingle-phase(Matsushitaetal. 2002).Thismotivated the exact mechanisms by which the heating takes place (e.g. Churazovetal. (2001) to develop a model according to which thebubblesseenintheradiomapbuoyantlyrisethroughthehot Sendoffprintrequeststo:A.Simionescu,[email protected] plasmaupliftingcoolergasfromthecentralregion. 2 A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton thiscleaning,theneteffectiveexposureis∼62ks,∼79.3ksand ∼80.5ksforpn,MOS1andMOS2,recordingintotal13.2,4.7 and4.9millionphotonsrespectively.Fordatareductionweused the6.5.0versionoftheXMM-NewtonScienceAnalysisSystem (SAS);thestandardanalysismethodsusingthissoftwarearede- scribedinWatsonetal.(2001).Avignetting-correctedfluxmap oftheobservationforallthreedetectorscombinedintheenergy band0.5-7.5keVisshowninFigure1. For the background subtraction, owing to the extended nature of our source, which covers the entire field of view, we used a collection of blank-sky maps from which point sources have been excised (Read&Ponman 2003). We transposed the blanksky-mapstoapositionintheskycorrespondingtotheori- entationofXMM-NewtonduringourobservationofM87.The backgroundswerescaled accordingtothe correspondingexpo- suretimesforeachdetector,afterhavingremovedthetimeperi- odsaffectedbyprotonflares.Thescalingfactorsinthiscaseare calculatedastheratiosbetweentheobservationexposuretimes and the backgroundexposure times. An alternative method for calculatingbackgroundscalingfactorswasemployedbyrequir- ingthatinthehigherenergyregion(10-12keVforMOS,12-14 Fig.1.M87surfacebrightnessmap,fromallthreeEPICdetec- keVforPN)theemissionintheoutskirtsoftheM87observation tors vignetting correctedand combined,in the 0.5-7.5keV en- shouldhavethesame levelas thebackground,since we expect ergyrange littleornosuchhigh-energyemissionfromthegashaloaround M87. The backgroundscaling factors were in this second case calculated as the ratios between the total number of observa- FurtherfeaturesintheX-raysurfacebrightnessmapofM87 tiontobackgroundcountsinthehigherenergyregionspecified described by Formanetal. (2005) from Chandra observations above,in a 7′-9′ ring. The valuesfor the scaling factors calcu- include a ring of enhanced emission with a radius of 14 kpc, lated from the two methodsagree to within less than 5%. This possibly associated with a weak shock which may be one of justifiesconfidenceinhavingchosenacorrectscaling. the mechanisms contributing to the heating of the ICM, addi- Where a stable minimum signal-to-noise ratio was needed tional arc-like features at 17 and 37 kpc, and excess emission for our analysis, in particular for determining the temperature, regions NW and SE of the core. Using deeper Chandra data, we employed an adaptive binning method based on weighted Formanetal.(2006)confirmedthepresenceofaweakshockat Voronoitessellations(Diehl&Statler2006),whichisageneral- 14kpcanddetermineditsMachnumberasM∼1.2. izationofthealgorithmpresentedinCappellari&Copin(2003). Detailed temperature,entropyand pressuremapsare a pre- The advantage of this algorithm is that it produces smoothly requisite for the physical understanding of the phenomena be- varying binningshapes that are geometricallyunbiased and do hindthesefeatures.Thisisthemotivationforthesecond,deeper not introduce artificially-looking structures, as can be the case observationofM87withXMM-Newton,theresultsofwhichare for e.g. rectangular bins. Binning the dataset from the second presentedinthispaper. XMM-Newton observation alone to 104 counts per spatial bin Thepaperislaidoutasfollows.InSection2wepresentthe in the 0.5-7.5 keV energy range, which should yield tempera- datasetanddataanalysismethods.Thefeaturesseeninthere- turevaluesaccuratetolessthan 5%forone-temperaturemodel sultingtemperature,entropyandpressuremapsaredescribedin spectralfits,weobtainbinsroughlycorrespondinginsizetothe Section3.InSection4wepresentmorequantitativeanalysesof extent of the XMM point-spread function (PSF). Therefore, in these features and possible physical explanations. Our conclu- thispaperweonlydiscusstheresultsfromthesecondobserva- sionsaresummarizedinSection5.WeadoptaredshiftforM87 tionaldataset,whichcontainsenoughcountsforthepurposeof ofz=0.00436andaluminositydistanceof16Mpc(Tonryetal. thecurrentanalysis.Moredetailedspectralmodelsandspectral 2001),whichyieldsascaleof4.65kpcperarcminute. fittingwhicharethescopeoffutureworkwillusedatafromboth observations. 2. Observationsanddatareduction Out-of-timeeventswere subtractedfromthe PN datausing thestandardSASprescription. M87wasfirstobservedwithXMM-Newtonintheperformance verification (PV) phase on June 19, 2000, for 60 kiloseconds (ksec). A subsequent 109 ksec observation was performed on 2.1.Broad-bandfitting January10th,2005.Wewillfocusinthisworkprimarilyonthe EPICdatafromthesecondobservation,inwhichthePNdetector Inordertoobtainafirstimpressionofthetemperatureandabun- wasoperatedintheextendedfullframewindowmodewhilethe dancedistributionofthehotgasaroundM87,werestrictedthe fullframemodewasemployedfortheMOSdetector.Theresults analysisofthesurfacebrightnesstofourdifferentenergybands fromthe RGS dataanalysisforthetwo combinedobservations selectedasfollows: canbefoundinWerneretal.(2006). Weextractedalightcurveforeachofthethreedetectorssep- 1. Lowenergyband(0.4-0.9keV) arately and excluded the time periods in the observation when 2. Fe-Llinecomplexenergyband(0.9-1.1keV) thecountratedeviatedfromthemeanbymorethan3σinorder 3. Intermediateenergyband(1.1-2.0keV) toremoveflaringfromsoftprotons(Pratt&Arnaud2002).After 4. Highenergyband(3.0-7.5keV) A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton 3 The principleis to comparethe trendof fluxesin each pixelin easwherefewerphotonsarecollectedandtoensureanapproxi- each of the four bands to fluxes expected in these bands from matelyhomogeneousaccuracyinthetemperaturedetermination model-spectraforarangeoftemperaturesT={0.8,1.0,1.5,1.7, acrossthe image.A SNR of 100shouldgivea temperatureac- 2.0,2.2,2.5,2.7,3.0,3.3,3.6,4.0}keVandmetallicitiesz={0.1, curacy better than 5% (for a one-temperature fit) and a metal 0.3, 0.5, 0.7, 1.0, 1.3} solar (using the relative abundance val- abundanceaccuracyof10-15%. uesofAnders&Grevesse(1989)).Subsequently,thebest-fitting Forthespectralanalysiswechosethemethoddescribede.g. model was determined using a least-chi-square fit. In all mod- byArnaudetal.(2001)inwhichthevignettingcorrectionisac- els, theabsorbingcolumndensityn wasset to1.5×1020cm−2 countedforbyaddingaweightcolumntotheeventfile,allowing H (Lieuetal.1996),andtheredshiftto0.00436. theuseofasingleon-axisancillaryresponsefile(ARF)contain- We merged the two MOS detectors, which have very simi- inginformationaboutthetelesope-specificeffectivearea,quan- larspectralproperties,andproducedbackground-correctedflux tum efficiency attenuation and filter transmission. The redistri- maps in each of the four selected bands for the PN and com- butionmatrixfilesontheotherhandwerecomputedseparately binedMOSdetectors.Eachmapwasadaptivelybinnedusinga foreachbinandeachdetector.Detectorgapswereaccountedfor binningschemebasedonthecombinedrawcountsimagefrom bydividingthefluxineachbinthroughthecorrespondingratio the three detectorsin the 0.5-7.5keV band.We performedone between the average unvignetted exposure time in the bin and fitwithasignaltonoiseratio(SNR)of50andonewithaSNR themaximumunvignettedexposuretimeofthedetector. of100whichensuredaminimumnumberofcountsof25...100 The spectra in each bin were fitted in XSPEC using a perbin in eachof the fourconsideredbandsand especiallythe MEKALone-temperaturemodel.Thespectrafromallthethree highenergy3.0-7.5keVbandwhichhastheleastcounts. detectorswere groupedtogetherand fitted simultaneouslywith This method allowed us to clearly identify the eastern thesamebest-fitmodelinordertohavegoodstatistics.Weused and southwestern arms of M87 as lower temperature features. the0.5-7.5keVrangetodeterminethebestfit.However,inthe Furthermore,alowertemperaturecomponentcouldbedetected rare bins (approximately 10 bins out of a total of about 1500) in the directionof the M87jet (north-westof the core),almost where one of the EPIC detectors had an effective average ex- in continuationof each of the arms, spatially connectingthem. posuretime ofbelow10%ofthe maximum,the datafromthat However, beyond a radius of around 11 arcmin, both the SN detectorwasomittedsincethenumberofcountswastoolowto 50andSN 100temperaturemapsbecameverynoisy.Thiswas provideanyreliablespectralinformation.Thiscorrectionisnec- likelyduetothefactthatourchosenenergybandsweretunedto essary,asthegapsarenotincludedin thevignettingcorrection determineratherlowtemperaturesaccurately;foragooddeter- informationstoredintheweightcolumn. minationofhighertemperaturesitwouldbenecessarytosample This method confirms the results obtained from the broad thehigherenergyregime(above3.0keV)withmorebands.The band fitting in that we can clearly identify the same substruc- abundancepatternwasalsorathernoisythroughouttheproduced tureasnotedbefore:theeasternandsouthwesternarmsofM87 maps;itseemedtobeinfluencedsignificantlybythePNgappat- as lower temperature features, the lower temperature compo- tern,anddidnotshowa clear distributionrelatingtothe X-ray nent NW of the nucleus in the direction of the M87 jet, and arms. This was likely due to the factthat our energybandsare the nucleus of M87 distinguished as a high-temperature com- verywide, leadingto a washing-outofanyemission lineswith ponent.Wealsoobservethepositivetemperaturegradientinthe theexceptionoftheFe-Lcomplex,whichisnotsufficientforan region outside the arms, in agreement with the radial analysis accurateabundancedetermination. of Matsushitaetal. (2002). The outer parts of the gas halo are In summary, this method was a very fast, computationally significantly less noisy than derived from the broad band fit- cheapwaytogaininsightintothespatialdistributionoftemper- ting. However, at smaller radii the values of the temperatures aturesintheobservation.However,theresultseemedtoberather determinedwiththetwomethodsagreeverywell.Ourtemper- sensitive to thechoice ofthe energybands,andthe insufficient ature map is also in goodagreementwith the temperaturemap sampling of the higher-energyregime caused a noisy fit in the obtainedfromthepreviousXMM-NewtonobservationofM87, hotter regions of the gas halo. Moreover, it was an insensitive computed following the method of Churazovetal. (1996) and methodforobtainingmetallicitymaps. presentedinFormanetal.(2005). The abundance pattern is much better defined with this method compared to the broad-band fitting, however some 2.2.Spectralanalysis noise can still be seen. We note that the statistics in each Amorereliablemethodforcreatingtemperaturemapsisacom- bin were not sufficient to determine individual element abun- prehensive fit to the spectra from each bin using the XSPEC dances,thereforetheabundanceratioswereassumedtobesolar code,takingintoconsiderationa largenumberofenergychan- (Anders&Grevesse 1989). Detailed radialprofiles of the indi- nels and allowing the temperature and metallicity to vary con- vidualelements(Matsushitaetal.2003)howevershowthatthis tinuously until a best fit is found. But, given the large number is not generally the case. Given our signal-to-noise ratios, the ofspatialbinsforthisobservation,thismethodcanbecomputa- obtainedabundancevaluesaremostlikelydrivenmainlybythe tionallyverydemanding. iron content whose L-line complex is the most prominentfea- Point sources were found using the source detection algo- ture in the spectrum. We find that the abundance in the arms rithm implemented in SAS and excised from the observation is in generallowerthan in the surroundingmedium(Figure3), after a visual check to eliminate spurious detections. The cen- which is mostlikely due to the fact that we are using a single- tralAGNwasnotconsidereda point-sourcecontaminationand temperature fit in a region which has been shown to have at was thus kept in the dataset throughout the analysis. We then leasttwocomponentswithdifferenttemperatures(Belsoleetal. adaptivelybinnedthecombinedcountsimageinthe0.5-7.5keV (2001),Matsushitaetal.(2002),Molendi(2002)).Astheseau- rangeusingtheweighedVoronoitessellationsmethodtoatarget thors have already noted, a one-temperaturefit severely under- signal-to-noiseratioof100andextractedthespectraofeachbin estimates the metallicity in this case. Moreover, by creating a definedinthisway.Thereasonforbinningtherawcountsmap map(notshown)oftheabsorbinghydrogencolumndensity(n ) H andnotthefluxmapisto obtainlargerbinsaroundthegapar- which was left free in the fit, we find elevated values in the 4 A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton Fig.3.Abundancemapobtainedfromspectralanalysis,usinga Fig.2.Temperaturemapobtainedfromspectralanalysis,using binningtoSNRof100. abinningtoSNRof100. 2004).Thus,thismethodtakesintoaccountanapproximateesti- regions corresponding to the arms, which may also show that mationofthethree-dimensionalextentofeachbin,butassumes a single-temperaturefit is not sufficient to appropriatelymodel aconstanttemperaturealongthelineofsight,andcantherefore thedata.Weperformedasecondfitfixingtheabsorbinghydro- be viewed as a quasi deprojection.Since most of the emission gencolumndensityto2.0×1020cm−2whichdidnotsignificantly in each bin originates from the densest gas which is found at change the temperature map. Therefore, any uncertainty in the the smallest effective 3D radii, the maps can be interpreted as n seems to have little effect on the temperatureresults; in the H measuresofthepressureandentropyinatwodimensionalslice followingwewillbaseourdiscussionontheinitialtemperature through the middle of the cluster, perpendicular to the line of resultsobtainedwithanunconstrainedn inthefit. H sight. Wefittedtheradialpressureandentropyprofilesusingnon- 2.3.Pressureandentropymaps parametric, locally weighted, linear regression smoothing. The datapointsandthecorrespondingsmoothradialmodelareover- Weusedthetemperatureandspectrumnormalizationineachbin plotted in Figure 4. The pressure and entropy maps were di- todetermineaquasi-deprojectedmeasureofthepressureanden- videdbytheresultingradialmodelinordertorevealsmall-scale tropy,whichwe calculatedas p = nekT andS = kTn−e2/3. The fluctuations(Figures5 and6). We can identifyseveralfeatures entropyasdefinedhereissomewhatdifferentfromthethermo- pointed out by Formanetal. (2005), such as the 3′ (14 kpc) dynamic quantity defined as S ∼ (3/2)kln(Tρ−2/3) but is nev- ring of enhanced emission corresponding to an increase in the ertheless a measure of adiabaticity and has become commonly pressure.Thisconfirmstheassociationofthissubstructurewith usedinclusterastrophysics. a shock, while the NW/SE brightness enhancementsseen with The electron density ne is determined from the spectrum Chandra (Formanetal. 2005) appear as low-entropy features. normalization,whichisdefinedandimplementedinXSPECas The X-ray arms are clearly low-entropy structures, which also K =10−14/[4πD2(1+z)2] n2dV,fromwhichfollows seemstoconfirmthehypothesisthatthegasintheseregionswas A e R upliftedfrommorecentralregionswhereweexpectthesmallest K entropyin hydrostaticequilibriumconditions.Thenextsection ne ∝ rVDA(1+z) presentsamoredetailedanalysisofthesefeatures. whereD istheangulardiameterdistanceandzistheredshift. A 3. SubstructureintheM87gashalo Ascanbeseenfromtheaboveformula,weneedtoalsocalculate thevolumeValongthelineofsightcorrespondingtoeachbin. 3.1.TemperatureandEntropy ThiswasdeterminedasV ≈(4/3)D3Ω(θ2 −θ2)1/2,whereΩis A out in thesolidanglesubtendedbythebinandθ ,θ aretheangles Thetemperatureandentropymapsbothshowsimilarsubstruc- in out correspondingtothesmallestandrespectivelylargestdistances turedetails,sothatwechosetodiscusstheminparallel.Byfar betweenanyofthebinpixelsandtheM87nucleus(Henryetal. themoststrikingfeaturesin bothmapsarethe EandSWarms A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton 5 Fig.5. Entropy deviations from a smooth radially symmetric Fig.6. Pressure deviations from a smooth radially symmetric model.TheEandSWarms,aswellasthelow-entropyfeature model.ArelativepressureincreasetowardstheSEandNWsug- close to theNW ofthe coreare seen indarkblue.The entropy gestsanoverallellipticityinthepressure.Apressuredecreaseis edgetotheSWismarkedbythesharpgreentoyellowtransition. foundinthedirectionoftheEandSWarms,whichriseperpen- ANW/SEasymmetryiseasilyseeninthemap. dicular to the SE/NW ellipticity long axis. A ring of enhanced pressurewitharadiusofroughly3′isalsoseen. characterizedby lower temperatureand lower entropy with re- specttothesurroundings.Thedescriptionofthethermalstruc- Finally,afeatureeasilyseenintheentropymapbutwhichis ture of the arms will be the subject of an upcoming paper in notreadilyapparentinthetemperaturemapisaNW/SEasym- whichamoredetailedspectralanalysiswillbeemployedinor- metry. Within a radius of 6′ the entropy values to the NW are der to understandthe complexmulti-temperaturestructure that clearlymoreelevatedthanintheSE.TheedgetotheNWwhere several authors have already found in these regions (Molendi theentropydecreasescorrespondsspatiallyverywelltotheedge (2002),Matsushitaetal.(2002),Belsoleetal.(2001)).Wepoint of theNW large-scaleradiobubble.Possible physicalexplana- outthatinthetemperatureandentropymapsbotharmsareseen tionsforthisarealsopresentedinthenextsection. tocurveclockwise,therebyconnectingtothelarger-scaleradio halos north and south of the nucleus. This was already known 3.2.Pressure fortheSWarm(seeforexampleFormanetal.(2005)),buthad notpreviouslybeenobservedfortheeasternarm. A map of the pressure deviations from a radially symmetric, Apart from the E and SW arms, another low-temperature smoothmodelshowsthreenoteworthyfeatures.Thefirstofthese and low-entropy feature can be identified close to the nucleus isarelativepressureincreasetowardstheNWandSE.Sincewe towards the NW, almost at the intersection of the lines defin- subtracted a radially symmetric model, this would suggest an ing the generaldirectionsof the arms. This regioncorresponds overallellipticity of the pressure distribution and consequently to high X-ray luminosity and low entropy, implying a locally oftheunderlyingdarkmatterdistributionundertheassumption highgasdensity.Additionally,enhancedHαemissionhasbeen of hydrostatic equilibrium. Secondly, in the direction of the E observedhere (Sparksetal. 2004), which makesthis regionan andSWX-rayarms(coincidingwiththeprominentradiolobes attractivecandidateforhostinga classical coolingflow and as- and orthogonalto the regionsof enhancedpressure), we find a sociatedmassdeposition.However,detailedspectralanalysisof pressuredecrease.Thirdly,thereis aringofenhancedpressure theregion,asdescribedinthenextsection,doesnotprovideevi- with a radiusof roughly3′, coincidingwith the positionof the denceforcoolingtothelowestX-raydetectabletemperaturesat weakshockproposedbyFormanetal.(2005). theratespredictedintheabsenceofanyheatingsources. Wetriedtoconfirmandquantifytheellipticityintheinitial Anotherfeaturefoundin bothmapsis anedgeto theSE at pressure map by fitting to it elliptical isocontourswith the ”el- aradiusofroughly6′.Asonemovesoutwardsacrossthisedge, lipse” task in IRAF. The position angle and ellipticity were in- boththetemperatureandentropyincrease,thereforetheproper- dependentlyfittedforeachpressurecontourandthemodelneed tiesatthejumparemoreconsistentwiththecold-frontinterpre- notbesmoothinintensityasafunctionofthesemi-majoraxis. tation of Markevitchetal. (2001) than with the characteristics Wemaskedtheregionscorrespondingtotheradiolobesfromthe ofashock.Amoredetaileddiscussionofthisispresentedinthe fit,sincehereitislikelythatthepressuredeviatesfromanellip- nextsection. ticalmodel.Dividingtheinitialpressuremapbythefittedmodel 6 A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton Fig.8. Data and best-fit vmcflow model with fixed low- temperature cutoff for the small-scale low-temperature feature Fig.7. Pressure deviations from an elliptical model (ratio). NWofthecore.Thedisagreementofthemodelwiththedatais Contourlevelsaredrawnat0.9,0.95,1.05and1.1.Tworegions easilyvisible,indicatingtheabsenceofaclassicalcoolingflow. roughly correspondingto the end of the E arm and to the SW arm are seen to have lower than average values, indicating the possiblepresenceofnonthermalpressuresupport. obtainedwithIRAF, we areableto identifydeviationsfroman ellipticalratherthanaradiallysymmetricmodel(Figure7).We find that, with respect to the elliptical model, all substructure seenpreviouslynolongerappear,withtheexceptionofthelower pressurein regionsthatcoincidewith the radiolobes.Here the decreaseinobservedpressurewithrespecttotheellipticalmodel isonaverage5-10%.Assuminganoverallpressurebalance,this decreasesuggeststhatintheradiolobesthereshouldbeanaddi- tionalcontributiontothepressure.Thispressuresourcemaybe providedbyrelativisticelectronsinjectedbytheAGN.Thering at 3′ is no longer visible since we did not require the pressure intensityinthemodeltobesmoothwithincreasingsemi-major axis,whilethehigherpressuresNWandSEofthenucleuswhich appearedasdeviationsfromaradiallysymmetricmodelcanbe entirelyexplainedbyanellipticalpressuredistribution. Fig.9. Data and best-fit two-temperaturevmekalmodelfor the small-scalelow-temperaturefeatureNWofthecore.Thismodel 4. Discussion is clearly in better agreement with the data than the classical coolingflowmodel. 4.1.Absenceofaclassicalcooling-flowonsmallerscale Whilealarge-scalecoolingflowhasbeenruledout,coolingand condensation could still happen locally, since it is not easy to ×10−2 M⊙/year. According to a classical cooling flow analysis exactlybalanceheatingandcoolingeverywherethroughoutthe donebyBo¨hringer(1999),themassdepositionrateinthisregion coolingcoreregion.Aprimecandidatetargettolookforsucha in the absence of a heating source should be 0.1-0.2 M⊙/year, local cooling flow region is the small low-entropyfeature NW whichisafactorofabout5higherthanwhatwefindfromspec- ofthenucleuswherealsodiffuseopticalemissionlinesareob- tral fitting. A two-temperature vmekal model with T1 = 1.016 served. ± 0.022 and T2 = 1.855 ± 0.030 also describes the data very We extracted a spectrum from this region and fitted it in well.Therefore,althoughthedensityinthissmaller-scaleregion XSPECwithavmcflowmodelfixingthelowtemperaturecutoff isveryhighandalthoughtheregionisassociatedwithHαemis- attheminimumlevelavailableinthemodel(81eV)inorderto sion,wefindthatitdoesnotexhibitspectralevidenceofaclas- probetheexistanceofaclassicalcoolingflowspectrum.Wefind sicalcoolingflowandsufficientmassdepositionrate. a very poor fit, especially around the Fe-L complex where the dataliebelowthespectralmodelbetween0.6-1keV(Figure8). 4.2.MotionofthegalaxytotheNW A vmekal+vmcflowmodelwith a fixed low-temperaturecutoff at81eVprovidesagoodfitforanambienttemperature(vmekal) To investigatethe NW/SE asymmetryseen inthe entropymap, of1.508±0.027keVandamassdepositionrateof3.18±0.16 we extracted spectra from concentric ring sectors towards the A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton 7 NW(-25to135degreescounterclockwiseofWest)andSE(200 to280degreescounterclockwiseofWest).Theanglesarechosen suchthattheEandSWarmsareexcluded.Thespectrawerefit- tedwithaone-temperaturevmekalmodel,sinceweexpectfrom the results of Matsushitaetal. (2002) and Molendi (2002) that the gas outside the arm regionsis single-phase locally. The re- sultsplottedinFigure11showthattheradialtemperatureprofile towardstheSEliesbelowtheprofiletowardstheNW,whilethe radialabundanceprofilerevealshighervaluesintheSEthanin theNW.ThesimplestphenomenonthatexplainssuchaNW/SE temperatureandabundanceasymmetryisamotionofM87tothe NWthroughthehotgashalo,oralternatively,alarge-scalebulk motionofthehotgashalotowardstheSEwhilethegalaxyitself remains in place. In either case, there are two possible mecha- nismsrelatedtothemotionwhichwouldgeneratetheobserved asymmetry. Thefirstpossibilityisthatcolder,highly-abundantgasnear thecenterisram-pressurestrippedduringthemotion.Inthecase of a large-scale bulk motion of the gas halo, the stripped gas is then carried towards the SE along with the bulk flow, while if the galaxy is moving towards the NW it is left behind in a wake.However,thepressuremapissymmetricalandshowsno increase to the NW as one would expect if ram-pressure were important. Thesecondpossibility,whichisfavouredbyourentropyand pressure maps, is that, due to the relative velocity between the nucleusandthegashalo,bubblesemittedinitiallytothenorth- west are advected downstream. This effect is important if the relativebulk velocityis at least as high as, and of the orderof, thebubblerisingvelocityandwouldimplythatalargepartofthe bubblesemittedbytheAGNeventuallyrisetotheSEindepen- dentlyoftheirinitialdirectionofemission.Muchmoremixing is therefore induced towards the SE, favoring the transport of metals, which explains the relative iron and silicon abundance increase in this direction (Figure 11 (b) and (c)). Recent simulations Roedigeretal. (2006) indeed show that bubble-induced metaltransportresultsinelongatedabundanceprofilesalongthe direction of propagationof the bubbles. The observed NW/SE temperature asymmetry can be explained using this bubble- advectionscenarioifwe considerthatthebubble-inducedmix- ingnotonlyfavorsthetransportofmetalsintheatmospherebut canalsobringcoldergasfromthecenterouttolargerradii.Ithas beenalreadynoted(e.g.Nulsenetal.(2002))thatradiobubbles areoftensurroundedbyrimsofcold,densegaswhichtheyuplift duringtheir inflation and rising. Thereforeit is probablethat a preferentialpropagationof a series of bubblesto one direction mayresultinloweringthedownstreamaveragetemperature. Further indicationthat bubblesare advected downstreamis provided by Chandra data in Formanetal. (2006); the authors describe a series of four consecutive bubbles to the SSE while nocounterpartofsuchstructureisseenintheoppositedirection. Finally, one other observational feature leads us to believe that M87 is moving to the NW. If we consider the analogy of M87 with a typical wide-angle tailed (WAT) radio source, the motionthroughtheintraclustermediummayhelptoexplainwhy Fig.11. Temperature and abundance NW/SE asymmetry. The theEandSWradiolobes,andthecorrespondingX-rayarms,do solidlinerepresentstheprofileNWofthe core,thedashedred notliealongdirectionsexactlyoppositetoeachother.However, lineSEofthecore. thewideanglebetweenthelobesandespeciallytheradio”ear” seenattheendoftheeasternarmsuggestthatthebuoyantveloc- ity is at least comparableto the bulk flow velocity.Thisagrees qualitatively with our previous statement that the relative bulk the bubble enthalpy is in most cases enough to halt the cool- velocityshouldbeontheorderofthebubblerisingvelocity. ingflow.However,thepicturedrawnbyNulsenetal.(2002)for LetusreconsiderthemodelofICMheatingthroughradio-jet Hydra A and in this work also for M87 is much more com- inflated bubbles. Bˆırzanetal. (2004), among others, show that plex. While the AGN does inject energy into the ICM in the 8 A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton form of bubble enthalpy, bubble formation is likely often associated with the uplift and mixing of cool gas from the center. A well knownexampleforthe upliftare the X-rayarmsof M87 (Churazovetal. 2001). In this paper we also find for the first time evidence of this phenomenon at smaller temperature anddensitycontrastswithrespecttotheambientmediumwithin thesamegalaxy,contrastswhichcouldthereforenothavebeen observedwithoutthe high-qualitystatistics of the dataset. This suggeststhatcool-gasupliftingbybuoyantbubblesmaybemore wide-spreadthanpreviouslythought,andhasnotbeendetected sofarsolelyowingtoinsufficientstatistics.Whethertheuplifted cool negatively buoyant gas will fall back towards the cluster centerthermalizingits potentialenergy(Nulsenetal. 2002), or whetheritisheatedbythermalconductiononceitreacheslarger radii,isamatterforfuturemodels;however,itseemsthatbub- blesdogenerallytransportcoolgasoutofthecenter. 4.3.Possiblecoreoscillations While theabundanceandtemperatureprofilesoftheX-raygas relativelyclosetoM87(withintheinner∼6′)arewellexplained by a simple relative motion between the gas and the galaxy, a look at the featuresseen furthertowards the outskirtsrevealsa morecomplexnatureoftheongoingphysicalphenomenon.The Fig.12.Entropyandtemperatureradialprofiles,calculatedasan model-dividedentropymaprevealsanedgeatabout6′SEofthe averageofthevaluesinthebinswhosecenterfallsinsidecircular corewhichissuggestiveofacoldfront,withthepressureacross annulisectorswithanopeningangleof90degreesNWandSE the front staying roughlyconstant and the temperature and en- ofthecore.Theerrorbarsrepresentthecorrespondingstandard tropybeingloweronthemoreX-rayluminousside.Thisfeature deviationineachannulussector. correspondsinthemetallicitymap(Figure3)toavisiblesharp edgeintheabundancedistribution,emphasizingthepresenceof acontactdiscontinuityhere.Anotherrelativelysharpedgeinthe metallicitycanalsobeseenatabout3′NWofthecore,although itisnotassociatedwithanyvisibleentropyortemperaturefea- 4.4.Large-scalecorrelationoftheradioemissionand tures.Moreover,alsoNWofthecore,wefindaregionoflower entropymap entropybeyond6′. Theoppositeandstaggeredplacementoftheselow-entropy One ofthe striking large-scalefeaturesin Figure 10is the spa- regions and abundance edges suggests that the current relative tial coincidence of the northern outermost radio contour-edge motionofthegasandthegalaxy,proposedintheprevioussec- and a decrease in the entropyby 10-15%,which cannotbe ex- tion, may be part of a longer process of oscillatory motions plainedeitherbyadirectedmotionofM87totheNW,orbythe along the NW/SE direction. Churazovetal. (2003) observed a possiblecoreoscillations.Owenetal.(2000)notethattheradio similar E/W asymmetry and a set of sharp edges in entropy, contour-edgerepresentsasharpdecreaseinradiointensityover temperature and surface brightness in the Perseus cluster and arangeofwavelengths,thereforeindicatinginteractionbetween performed simulations which suggest that minor mergers may theradio-loudplasmaandthehotgashalo. trigger oscillations of the cluster gas, thus generating multi- Toillustratetheentropyvariationacrosstheradioedge,we ple sharp edges on opposite sides of the central galaxy along averagedthe entropyin circularannulisectorswith anopening thedirectionofthemerger.Similaroppositeandstaggeredfea- angleof90degreesintheNWandSEquadrantrespectively.We tures generated by oscillations of the intracluster gas are seen comparetheradialprofilesoftheentropyinthesetwoquadrants alsoinnumericalsimulationsbyTittley&Henriksen(2005)and inFigure12,whichalsoshowsradialtrendsofthetemperature Ascasibar&Markevitch(2006). inthecorrespondingregions.Uptoaradiusofroughly6′-7′the Furtherevidenceofapossibleoscillatorymotionofthegas entropyin the NW quadrantis higherthan in the SE quadrant. in the M87 halo comes from an analysis of the pressure map. Beyondthisradius,whichcorrespondsroughlytotheradioedge, Fittinganellipticalmodeltotherawpressuremapasdescribed theentropyandtemperatureNWofthecoreshowamuchshal- in Section 4.5 and allowing the centers of the elliptical isobars lower increase with radiusthan in the SE, the temperaturedis- to vary, we find that these centers systematically shift towards playingevenasmallnegativegradient.Thespatialcoincidence theSEforhighervaluesofthesemimajoraxis.Thusthegalaxy betweenthenorthradioedgeandthepointwheretheentropyand potential located at smaller scales is not exactly centered with temperaturegradientsNW of the core decrease suggests a link the large-scale halo potential, which would lead to the oscilla- betweentheinjectionofradio-loudplasmaintotheICMandthe tionsuggestedhere.Inamoredetailedfutureanalysis,thiscan processesleadingtoentropygenerationandgasheating.Sucha be used to revealinformationaboutthe amplitude,energy,and linkisatthebasisofthefeedbackmodelsinwhichmechanical time-scaleoftheseoscillations,whichmayhelpinunderstand- input, usually by a radio source, offsets the cooling flow, heating how this phenomenoncontributes over time to the heating ingthegasattheclustercenterandflatteningthecentralentropy vs. cooling balance, and how it competes with other proposed profiles.Theexactdetailsoftheentropy-generationmechanism methodsforenergyinputintotheICM. remainunclear. A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton 9 4.5.Ellipticityoftheunderlyingdarkmatterpotential As mentioned above, we fitted the raw pressure map with an elliptical model in IRAF in order to quantify the eccentricity and position angle-dependency with semi-major axis. The re- sultsoftheobtainedfitshowlargervariationsinthecentralpart butoutside3′ aratherconstantpositionangleandellipticityare reached.Thepositionanglevaluesoutside3′rangebetween140 and 150 degrees, measured counterclockwise of north. These values are somewhat smaller than, but comparable to, results fromtheanalysisofanopticalimageofM87byCarter&Dixon (1978),whoobtainedvaluesfortheellipticalisophote-fitof160 ±5degreesforthepositionangle.Ourresultsagreealsowiththe workofBo¨hringeretal.(1997)onROSATPSPCdata,givingan ellipticity ofthe X-raysurfacebrightnesswith a positionangle of158±10degrees.Outsideof3′,theellipticityvariesgenerally between0.08and 0.17,highervaluesof0.13-0.17beingfound intheregionsfromwhichtheradiolobeshadbeencutout.This isinagreementwithBo¨hringeretal.(1997),whofoundanellip- ticityof0.1to0.16.TheellipticityintheX-raysthusseemstobe significantlysmallerthanintheopticalregime,whereitranges from0.3-0.5.Thiscanbeexplainedbythefactthatanisotropic velocity dispersions of the stars can sustain a higher ellipticity ofthegalaxyintheopticaldomainwhiletheisotropicnatureof the X-rayemittinggashalo makesitmoreround.Surprisingly, despitetheevidenceofcoreoscillationsseenintheentropymap, thepositionanglesoftheX-rayisobarsandtheopticalisophotes agreeverywell,andtheopticalellipticityisasexpectedroughly 3timeshigherthantheX-rayvalue(seealsoBuote&Canizares (1996)).Thereforethedeviationsofthehotgasatmospherefrom hydrostaticequilibriumcannotbeverylargeoutsidethearmre- Fig.13. Projected temperature and relative pressure jumps gions. around the 3′ shock, corresponding to Mach numbers of 1.05 A further look at the position angle values found in the fit and1.04respectively. reveals the fact that the radio lobes rise in directions orthogo- nal to the semimajor axis of the elliptical dark matter profile, therebyfollowingthesteepestdark-matterpotentialgradientas tentwiththeMachnumberM≈1.2calculatedfromdeprojection onewouldexpect. analysis of Chandra data (Formanetal. 2006) considering that theMachnumbersderivedfromthisanalysisarelowerlimitsto therealvaluesduetoprojectioneffectsandthesmoothingeffect 4.6.HeatingoftheICMbyweakshocks oftheXMMPSF.Accuratespectraldeprojectionanalysisisthe scopeofupcomingwork. AsseeninFigure6,weareabletospectroscopicallyconfirmthe 3′shockseenbyFormanetal.(2005),Formanetal.(2006)with Chandra.Weakshocksarebecomingwidelyacceptedascandi- 5. Summaryandconclusions dates for heating mechanismsof the X-ray gas, thereforea de- tailedunderstandingoftheirpropertiesis required.To describe Wepresenttheresultsfroma109ksecXMM-Newtonobserva- theM873′shockmorequantitatively,weusedtheellipticaliso- tion of the hot gas halo surroundingthe giant elliptical galaxy barsfromthefitdescribedabovetodefineeightspectralextrac- M87. We use two methods, namely broad-bandfitting and full tionregionswithsemi-majoraxesbetween2and4arcminutes. spectroscopy,tocreatespatiallyresolvedtemperaturemapsfrom The temperature and spectrum normalization corresponding to the data, andpresentthe advantagesanddisadvantagesofeach the regions contained between each two consecutive elliptical method.Theresultsofthespectroscopicanalysisareusedtopro- isobars in the chosen semi-major axis range were determined duce entropy and pressure maps. We describe and discuss the usingavmekalone-temperaturespectralfit.Thecorresponding featuresseeninthesemaps,themostimportantofwhichbeing pressure was then determined for each region as described in the cool E and SW X-ray arms, which coincide with powerful Section2.3.TheresultsareplottedinFigure13.Intheplot,the radiolobes,aweakshockringwitharadiusof3′,anoverallel- pressurewasdividedbyasmoothmodeltoemphasizethejump. lipticityinthepressuremapandaNW/SEasymmetryintheen- tropymap.Forthe3′weakshockwefindjumpsinboththepres- The temperatureshows a jump of ∼0.1 keV, which relative sure and temperature corresponding to Mach numbers of 1.04 to the overall temperature of ∼2 keV, represents ∼5%. Using and1.05respectively,whichwe interpretaslower limitsto the the Rankine-Hugoniotjump conditionsassumingγ = 5/3,this realMachnumbervalues.Thepressuremapshowsanoverallel- amountstoaMachnumberof1.05.Thepressurejumpamounts lipticityofbetween0.08and0.17,withpositionanglesingood to 8-10% with respect to the smooth model, which implies a agreement with the optical data. Under the assumption of hy- Machnumberofabout1.04,inagreementwiththeMachnum- drostaticequilibrium,theellipticityinthepressuremapimplies ber we derived from the temperature jump. This confirms the an elliptical underlyingdark matter potential. Furthermore, we supersonic nature of the weak shock at 3′, and seems consis- findthattheX-rayarmsandassociatedradiolobesriseroughly 10 A.Simionescuetal.:ThegaseousatmosphereofM87seenwithXMM-Newton perpendicular to the semimajor axis of the pressure ellipticity Forman,W.,Nulsen,P.,Heinz,S.,etal.2005,ApJ,635,894 followingthesteepestgradientofthedarkmatterpotential. Harms,R.J.,Ford,H.C.,Tsvetanov,Z.I.,etal.1994,ApJ,435, The NW/SE entropyasymmetryis indicativeof the motion L35 ofM87throughthesurroundinghotgas,associatedwithdown- Heinz, S., Brueggen, M., Young, A., & Levesque, E. 2006, stream advectionof thebubblesinjectedby theAGN. We con- ArXivAstrophysicse-prints[astro-ph/0606664] cludethatbubble-inducedcoldgasmixingfromthecentermay Henry,J.P.,Finoguenov,A.,&Briel,U.G.2004,ApJ,615,181 appearonawidevarietyofscalesandshouldbeconsideredinto Lea,S. M., Silk,J., Kellogg,E.,& Murray,S. 1973,ApJ, 184, thecooling-flowenergybalancecalculations.NWandSEedges L105 intheentropymapmoreoversuggestthatthepossiblemotionof Lieu,R.,Mittaz,J.P.D.,Bowyer,S.,etal.1996,ApJ,458,L5 M87totheNWispartofalongercycleofcoreoscillations. Markevitch,M., Vikhlinin,A., & Mazzotta,P. 2001,ApJ, 562, L153 Acknowledgements. WewouldliketothankW.Forman,P.Nulsen,E.Churazov Matsushita, K., Belsole, E., Finoguenov, A., & Bo¨hringer, H. and G. W. Pratt for helpful discussion. We acknowledge the support by the 2002,A&A,386,77 DFG grant BR 2026/3 within the Priority Programme ”Witnesses of Cosmic Matsushita, K., Finoguenov,A., & Bo¨hringer, H. 2003, A&A, History”. TheXMM-Newton project is anESA Science Mission with instru- 401,443 mentsandcontributions directlyfundedbyESAMemberStates andtheUSA (NASA). The XMM-Newton project is supported by the Bundesministerium McNamara,B.R.&O’Connell,R.W.1989,AJ,98,2018 fuerWirtschaftundTechnologie/Deutsches ZentrumfuerLuft-undRaumfahrt Molendi,S.2002,ApJ,580,815 (BMWI/DLR,FKZ50OX0001),theMax-PlanckSocietyandtheHeidenhain- Nulsen,P.E.J.,David,L.P.,McNamara,B.R.,etal.2002,ApJ, Stiftung,andalsobyPPARC,CEA,CNES,andASI.AFacknowledgessupport 568,163 fromBMBF/DLRundergrant50OR0207andMPG.Thisprojectispartially supportedbyaNASAgrantNNG05GH40GtoUMBC.ASalsothanksSteven Owen,F.N.,Eilek,J.A.,&Kassim,N.E.2000,ApJ,543,611 DiehlandThomasStatlerformakingtheirWeightedVoronoibinningalgorithm Peterson,J. R.,Kahn,S. M.,Paerels,F. B. 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