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

5 MB·

by  J. S. Sanders

MNRAS000,000–000(0000) Preprint8January2016 CompiledusingMNRASLATEXstylefilev3.0
A very deep Chandra view of metals, sloshing and feedback in the
Centaurus cluster of galaxies
J. S. Sanders1, A. C. Fabian2, G. B. Taylor3, H. R. Russell2, K. M. Blundell4,
R. E. A. Canning5,6, J. Hlavacek-Larrondo7,5,6, S. A. Walker2 and C. K. Grimes3
1Max-Planck-Institutfu¨rextraterrestrischePhysik,Giessenbachstrasse1,85748Garching,Germany
2InstituteofAstronomy,MadingleyRoad,CambridgeCB30FT
3DepartmentofPhysicsandAstronomy,UniversityofNewMexico,Albuquerque,NM87131,USA
6 4UniversityofOxford,Astrophysics,KebleRoad,OxfordOX13RH
1 5KavliInstituteforParticleAstrophysicsandCosmology,StanfordUniversity,452LomitaMall,Stanford,CA94305,USA
0 6DepartmentofPhysics,StanfordUniversity,382ViaPuebloMall,Stanford,CA94305,USA
2 7De´partementdePhysique,Universite´deMontre´al,C.P.6128,Succ.Centre-Ville,Montreal,QuebecH3C3J7,Canada

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A ABSTRACT
We examine deep Chandra X-ray observations of the Centaurus cluster of galaxies, Abell
G
3526. Applying a gradient magnitude filter reveals a wealth of structure, from filamentary
h. softemissionon100pc(0.5arcsec)scalesclosetothenucleustofeatures10sofkpcinsizeat
p largerradii.Theclustercontainsmultiplehigh-metallicityregionswithsharpedges.Relative
- to an azimuthal average, the deviations of metallicity and surface brightness are correlated,
o
and the temperature is inversely correlated, as expected if the larger scale asymmetries in
r
t theclusteraredominatedbysloshingmotions.Aroundthewesterncoldfrontareaseriesof
s
∼7kpc‘notches’,suggestiveofKelvin-Helmholtzinstabilities.Thecoldfrontwidthvaries
a
[ from4kpcdowntoclosetotheelectronmeanfreepath.Insidethefrontaremultiplemetal-
licity blobs on scales of 5–10 kpc, which could have been uplifted by AGN activity, also
1
explainingthecentralmetallicitydropandflatinnermetallicityprofile.Closetothenucleus
v
aremultipleshocks,includinga1.9-kpc-radiusinnershell-likestructureandaweak1.1–1.4
9
Machnumbershockaroundthecentralcavities.Withina10kpcradiusare9depressionsin
8
surfacebrightness,severalofwhichappeartobeassociatedwithradioemission.Theshocks
4
1 andcavitiesimplythatthenucleushasbeenrepeatedlyactiveon5−10Myrtimescales,in-
0 dicatingatightbalancebetweenheatingandcooling.Weconfirmthepresenceofaseriesof
. linearquasi-periodicstructures.Iftheyaresoundwaves,the∼5kpcspacingimpliesaperiod
1
of6Myr,similartotheagesoftheshocksandcavities.Alternatively,thesestructuresmaybe
0
Kelvin-Helmholtzinstabilities,theirassociatedturbulenceoramplifiedmagneticfieldlayers.
6
1
Keywords: galaxies:clusters:individual:Abell3526—X-rays:galaxies:clusters
:
v
i
X
r
a 1 INTRODUCTION that thecentral region contains multiple innercavities, where the
radio plasma displaces the X-ray emitting material (Taylor et al.
The Centaurus cluster, Abell 3526, is one of the X-ray bright- 2002).Theplumewasresolvedintowispyfilamentsofsoft,cool,
est and nearest galaxy clusters in the sky, with a flux of 2.7× X-rayemittinggas.Thedataalsoshowedtwosemicircularedges
10−10ergcm−2s−1 inthe0.1to2.4keVband,correspondingtoa insurfacebrightness(SB)totheeastandwest,wherethetempera-
bolometricluminosityof1.1×1044ergs−1 (Reiprich&Bo¨hringer turealsochanges,likelytobecoldfronts(Markevitch&Vikhlinin
2002).Theclusterhastwosub-components,amainclusterCen30, 2007), discontinuities where the temperature and density change
with a redshift of 0.0104 and a subcluster, Cen 45, 15 arcmin butthegasisinpressureequilibrium.Comparisonwithsimulations
(∼200kpc)totheeastwithavelocity1500kms−1 higher(Lucey indicatesthatthegasissloshingaboutinthepotentialwell.
etal.1986).
XMM-NewtonReflectionGratingSpectrometer(RGS)obser-
The first Chandra observations (Sanders & Fabian 2002) of vations(Sandersetal.2008)oftheclustercorerevealedaspectrum
the cluster revealed a plume-like soft X-ray structure extending withemissionfromFeXVIItoXXIV,butnotOVII,showingthereis
from the nucleus and multiphase X-ray material on small spatial X-rayemittingmaterialdowntotemperaturesbetween0.3to0.45
scales.DeeperChandraobservations(Fabianetal.2005)showed keV.Above2keVthespectraareconsistentwith40M(cid:12)yr−1 of
(cid:13)c 0000TheAuthors
2 J.S.Sandersetal.
radiativecooling,butat1keVthisreducesto4M(cid:12)yr−1 andless cluster,PKS1246–410,appearstobeinteractingwithanddisplac-
than0.8M(cid:12)yr−1 appearstobecoolingbelow0.4keV.Fromthe ingtheICM(Tayloretal.2002;Rudnick&Blundell2003),most
outskirtstothecentre,thetemperatureoftheintraclustermedium obviouslyseenbythepresenceoftwocentralcavitiesintheICM.
(ICM)decreasesbyafactorof∼10.Themeanradiativecooling The most extended radio emission appears to go beyond to re-
timeoftheICMdropsto∼107yrinthecentre. gionsoflowthermalpressure,indicatingtheremaybefurtherX-
ThesoftX-rayemissioninthecoreoftheclusteriscorrelated ray cavities there (Crawford et al. 2005). The optical nucleus is
with atomic and molecular material at much lower temperatures. double (Laine et al. 2003). The radio nucleus is associated with
The central galaxy, NGC 4696, contains a dust lane (Shobbrook compactlow-luminosityX-rayemission,butisoffsetfromtheX-
1963)andhasbrightfilamentarycentralHαemission(Fabianetal. ray-brightest region of the cluster (Taylor et al. 2006). If the nu-
1982), both of which have similar morphologies (Sparks et al. cleuswereaccretingattheBondirate,itwouldoverproduceby3.5
1989).Thesecomponentsarethemselvescorrelatedwiththecool ordersofmagnitudeitsobservedX-rayradiativeluminosity.Very
X-rayemittingfilaments(Crawfordetal.2005),andappeartobe long baseline interferometry (VLBI) observations show a broad
drawn up by buoyant bubbles rising in the ICM (Crawford et al. one-sidedjet(Tayloretal.2006).TheclustershowsFaradayrota-
2005).Thismodelissupportedbydeepintegral-fieldspectroscopy tionmeasures,indicatingmagneticfieldsof25µGon1kpcscales.
(Canning et al. 2011b), who find no evidence for fast shock ex- Thethermalpressureappearsdominantoverthemagneticpressure.
citation and whose spectra support the particle heating excitation Theline-emittinggas,softX-raymaterial,regionswithanexcess
modelofFerlandetal.(2009). ofrotation-measureanddepolarisedregionsappeartobespatially
UsingSpitzer,Johnstoneetal.(2007)foundevidenceforpure- correlated(Rudnick&Blundell2003;Tayloretal.2007).
rotationallinesfrommolecularhydrogen,indicatinganexcitation Sanders & Fabian (2008) found X-ray SB ripples in several
temperature of 300−400 K. The flux in the 0-0 S(1) molecular directionswithintheinner40kpc,postulatingthattheyaresound
hydrogenlinecorrelateswellwiththestrengthoftheopticallines. wavesintheICMgeneratedbytheactivityofthecentralnucleus,
Molecular material cooler than 400 K dominates the mass of the similar to those found in Perseus (Fabian et al. 2006) and Abell
outer filaments. With Herschel, Mittal et al. (2011) found two of 2052(Blantonetal.2011).Suchsoundwavescouldtransporten-
the strongest cooling lines in the interstellar medium, [CII] and ergyfromthenucleustoprovidethedistributedheatingrequiredto
[OI]. The [CII] emission has a similar morphology and velocity preventtheexceptedhighcoolingrates.
structurecomparedtotheHαemission,suggestingthatthesoftX- In this paper we present new results from 760 ks of Chan-
rays,opticallinesandfar-infraredhavethesameenergysource.In dra observations, including 480 ks of new data. We assume the
additionHerschelfound1.6×106M(cid:12)ofdustat19K,althoughthe galaxyclusterliesataredshiftof0.0104(Luceyetal.1986).Using
FIRluminositysuggestsalow0.13M(cid:12)yr−1 starformationrate. H0=70kms−1Mpc−1,1arcsecontheskycorrespondsto0.213
Canningetal.(2011a)detectedopticalcoronalemissionfrom kpc.Imagesarealignedwithnorthtothetopandeasttotheleft.
106Kgasinthecoreofthecluster.Thelowesttemperatureprobe Fordetailsofthedatareduction,seeAppendixA.Wediscussthe
[FeX]λ6374istwiceasbrightasexpected.Chatzikosetal.(2015) largerscalestructureinSection2,includingindetailsloshinggas
suggestthatthecoronalspectrumisnotindicativeofcoolingma- inthepotentialwell(§2.3),thewesterncoldfront(§2.4),thelin-
terial,butinsteadcomesfromaconductiveormixinginterfacebe- earstructuresinsidethecoldfront(§2.5)andthehighmetallicity
tweentheX-rayplumeandopticalfilaments. blobs(§2.6).InSection3weexaminetheinnerregionaroundthe
The Chandra observations confirmed that the cluster has a nucleusindetail,includingtheinnershock(§3.2),theinteraction
high (1.5−2Z(cid:12)) metallicity flat core with a sharp edge, previ- oftheradiosourceandX-rayplasma(§3.3),thecavities(§3.4),the
ously seen using ROSAT and ASCA data (Allen & Fabian 1994; shocksurroundingthecavities(§3.5),theassociationbetweensoft
Fukazawa et al. 1994; Ikebe et al. 1999; Allen et al. 2001). The X-raysanddust(§3.6),theoriginoftheplume(§3.7)andthero-
boxymetallicityprofilerequiresalargeeffectivediffusioncoeffi- tation measures and magnetic fields (§3.8). We discuss the inner
cient which drops very rapidly with radius (Graham et al. 2006). multiphase structure in Section 4 (also Appendix B), the central
Themetallicityratiosinthecoreoftheclusterareroughlyconsis- abundancedropinSection5andconcludeinSection6.
tentwithsolarvalues(Sanders&Fabian2006b),indicatingenrich-
mentbybothTypeIaandIIsupernovae,althoughOandMgare
∼40percentlessabundant(Sakumaetal.2011).Themetallicity
profileandratiosshowthatthecentreoftheclusterappearstohave 2 LARGERSCALESTRUCTURE
notsufferedmajordisruptionoverthepast8Gyrorlonger.
2.1 X-rayimages
In the centre of the cluster there is a drop in the observed
metallicity (Sanders & Fabian 2002), which appears not to be Fig.1(toppanel)isanX-rayimageofthecentral130×100kpcof
caused by resonance scattering (Sanders & Fabian 2006a) or in- thecluster.Thereare8.1×106 countsinthedatasetinsidea5ar-
adequate spectral modelling (Fabian et al. 2005). The decrement cminradius.Prominentintheimageisthecentralfeedbackregion
is consistent with the deposition of metals onto grains which are associatedwithNGC4696,whichweexamineindepthinSection
incorporatedintodustyfilaments(Panagouliaetal.2015). 3.ThereareSBedges,previouslyidentifiedascoldfronts(Sanders
TheCen45subclusterhasatemperatureexcesssurrounding &Fabian2002;Fabianetal.2005),1.5arcmintotheeastand3.3
it,interpretedashavingbeenheatedbyitsinteractionwithCen30 arcmintothewest.Weexaminethewesterncoldfrontindetailin
(Churazovetal.1999).ThisexcesswasconfirmedusingXMMand Section2.4.Thedepression3.3arcmintothesouth-eastisduetoan
can be explained by a simple shock heating model (Walker et al. edge-ondiscgalaxy,ESO322-93,inthemainCen30clusterwhich
2013).Apressurejumpisalsoseeninthedirectionofthemerger absorbssomesoftclusterX-rayemission(Fabianetal.2005).
andthemergingsubclusterappearstohaveretaineditsmetals.No Tobetterexaminethefeaturesintheimageweappliedagra-
bulkvelocitiesgreaterthan1400kms−1 intheICMweredetected dient filter to highlight sharp and flat regions. Gradient filtering
inthecluster(Otaetal.2007). has previously been used to examine cluster simulations (Roedi-
The extended component of the central radio source in the geretal.2013).TheGaussiangradientmagnitude(GGM)filteris
MNRAS000,000–000(0000)
AverydeepviewoftheCentauruscluster 3
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192.300 192.250 192.200 192.150 192.100
1.0 1.4 2.1 3.6 6.5 12.3 23.9 47.1 93.7 185.9 369.6
Figure1.(Toppanel)X-rayimageofthecentral130×100kpcofthecluster.The0.5to7keVimageisbackground-subtractedandexposure-corrected.To
thesouth-eastcanbeseentheX-rayshadowofadiscgalaxy.ThecolourbarshowstheSB,scaledtoshowthecentralnumberofcountsper0.492arcsecpixel.
(Bottompanel)ThesameregionafterapplyingGGMgradientfilterswithdifferentscalesandsummingwithradialscaling.Notethatthescaleisnotconsistent
acrosstheimageanddoesnotaccuratelyreflectthemagnitudeofjumps.ItemphasisestheregionswithlargestSBgradientsafterpointsourceremoval.
MNRAS000,000–000(0000)
4 J.S.Sandersetal.
1 2 4
Inner edge
Nucleus
Bay
Plume
Soft filaments
Hook
Depressions F,E,H and I
8 Linear structures 16 X1 northern extension 32
Eastern cold front
E3 edge E4 edge
ESO 322-93
Western cold front
Figure2.GGMgradient-filteredimagesusingscalesfrom1to32pixels(1pixelis0.492arcsec).Thewhitebarhasalengthof20kpc(∼94arcsec).Labelled
featuresarereferredtointhetext.
similartotheSobelfilter,calculatingthemagnitudeoftheimage central radio nucleus) and reduce the contribution of large scales
gradientassumingGaussianderivatives.ByvaryingtheGaussian there. We experimented with scaling factors, finding linear sum-
width,σ,thefilterissensitivetogradientsondifferentscales.GGM mations of the different filtered images do not introduce features
filteringhastheadvantageofbeingasimpleconvolutionappliedto which are not present in the individual filtered images. We note
thedataandisunlikelytointroduceartifacts. thattheimagedoesnotaccuratelyshowtherelativemagnitudeof
We applied the GGM filter from SCIPY (http://www. featuresbutinsteadhighlightsthosefeatureswhicharethere.The
scipy.org/) to the exposure-corrected background-subtracted imageshowsanumberofedgesandflatstructuresaroundthenu-
image using σ =1, 2, 4, 8, 16 and 32 detector pixels, yielding cleus and inside the western cold front (see Section 2.5). Further
theimagesshowninFig.2.Regionswithlargegradients,suchas discussion of gradient filtering and its application to other X-ray
edges,arelighter,whileflatterSBareasaredarker.Pointsources clusterdatasetswillbemadeinaforthcomingpaper.
were first removed from the input image to avoid their contam-
ination of larger structures. They were initially identified using
WAVDETECTbutmanuallyeditedtoexcludefalsedetectionswhen
itbecameconfusedbythebrightcentralX-raystructures.Theval-
uesofpixelsintheremovedsourceregionswerereplacedbyran- 2.2 Temperatureandmetallicitymaps
domselectionsofthoseinthe1-pixel-wideareasimmediatelysur-
Fig.3showsmapsofthetemperatureandmetallicity,obtainedby
roundingeachpointsource.
spectralfitting(AppendixA2).Thecoolcentralplumeandcooler
Theabilityofthefiltertodetectfeaturesisafunctionofthe regions inside the western and eastern SB edges can be clearly
SBintheregionofinterestandthemagnitudeofthegradient.In seen.Fromthecoreisanotherplume-likecoolerstructureextend-
the cluster outskirts where the number of counts per pixel is low ingnorth-westwardstowardsthewesternedge.Outsidetheedges
the small length scale filters cannot detect gradients unless they the temperature is relatively flat, although there may be a cooler
areverylarge.ThisisthecasewithnormalX-rayimages,where regiontowardsthenorth-east.
the detectability of structures depends on their size, SB and con- Themetallicitymapcontainsagreatdealofstructure.There
trastwiththeirsurroundings.Thenoiseintheimages,however,is is a previously reported high metallicity region inside the west-
seenasazimuthalstructures.AsthelargestSBgradientinaclus- ern cold front edge, where the ICM is cooler (Sanders & Fabian
terisradial,PoissonfluctuationsintheX-rayimagewillamplify 2002; Fabian et al. 2005). The abundance distribution, however,
orweakenthisradialsignal.Thereforenoiseinthegradientimage isnotsteadilyrising,butisratherflatwithfluctuations.Thereare
liesperpendiculartothegradientintheoriginalimage. strongvariationsinmetallicity(seeSection2.6)withacharacteris-
To show the gradients on different scales in a single image ticlengthscaleofaround20arcsec(∼5kpc;seeSectionA4).On
(Fig.1bottompanel),weaddedtheGGM-filteredimageswithra- theeasternsideofthecluster,outsidetheeasternSBedge(Fig.1),
dialscalingfactors.Theseradialscalingswereappliedtoincrease isanotherregionofhighermetallicitygaswithatail-likeappear-
the contribution of small scales to the centre (measured from the ance.
MNRAS000,000–000(0000)
AverydeepviewoftheCentauruscluster 5
2 arcmin
26 kpc
Temperature (keV)
0.87 1.2 1.6 1.9 2.3 2.6 3 3.4 3.7 4.1 4.4
Metallicity (Solar)
0 0.17 0.34 0.51 0.68 0.85 1 1.2 1.4 1.5 1.7
Figure3.TemperatureandmetallicitymapsoftheclusterusingregionswithS/N=100.Thetemperaturemapshowsthesingle-componenttemperature.
Themetallicitymapshowstheresultsfromsingle-ortwo-componentfits,dependingonwhichispreferred.Greyregionsindicateexcludedpointsources.
Uncertaintiesinthetemperaturesvaryfrom0.6percentinthecentreto2percenttotheeastofthewesternedgeand3.5percentatlargeradius.Metallicity
uncertaintiesaretypicallyfrom0.05−0.10Z(cid:12),excludingtheverycentre.
MNRAS000,000–000(0000)
6 J.S.Sandersetal.
2.3 Sloshing
X-ray image
WecomparetheSBdeviationsintheclustertothemetallicityand
3 arcmin
temperatureinFig.4.Thereisalow-density,high-temperaturema- 38 kpc
terial on the opposite side of the core to the high-density, low-
temperaturematerialbehindthewesterncoldfront.Thismaterial
appearstosweepfromthesouththroughtheeastandtowardsthe
north.TheplotshowsstrikingcorrelationsbetweenSB,tempera-
tureanddensitydeviations.ThewesterncoldfrontislabelledE2.
TherearealsotwoedgesinmetallicityandSBintheoppositedi-
rectiontowardstheeasternsideofthecluster(markedE1andE3),
closertotheCen45subcluster.Inadditionthereisafurtheredge To Cen 45
outsidethewesterncoldfront,goingfromthewesternmostpoint
ofthecoldfrontandextendingtowardsthenorth,withmoremetal
richgasinsideit(E4).Furthertothenorthisthehighmetallicity
regionalreadynotedbyWalkeretal.(2013).Therearehintsthat
thereisaSBedgetothenorthofthisfeature,withlowermetallic-
ityoutsideit(E5).TheSBenhancementtowardsthenorth-westof
Fig. 4 may be the structure found by Churazov et al. (1999) (see -0.6 -0.4 -0.2 0 0.2 0.4 0.6
theirfigure3),suggestedtobestrippedgasfromNGC4696B.
Temperature
InFig.5areshownthreerectangularprofilesacrosstheclus-
terinaneast-westdirection.Onepassesthroughthenucleus,while
oneisnorthofitandtheothersouth.Insomeregionsthereisex-
cellentcorrelation(betweenSBandmetallicity)oranti-correlation
(ofSBormetallicitywithtemperature)betweenthevariables,but
thisisnotalwaysthecase.Forexample,inthecentralregion,the
bright,cool,plumeisrelativelylowinmetallicity.Thereisalsoa
fairlystrongeast-westSBgradient,whichmakesthefluctuations
difficulttosee.
Fig.6showsradialprofilesinmetallicityinfourquadrantsto
the north, south, east and west. Despite small-scale variation, the
overallmetallicityprofileinsidethewesternedge(roughly40kpc
radius)isremarkablyflat.Inadditionthereremainsacentraldrop
inmetallicity,whichwediscussfurtherinSection5.Intheeastern
sector,thereisamuchsmallerregionofflatmetallicitytowardsthe
cluster core. However, the tail-like structure has a flat metallicity
profilewhichdropsoutwardof60–70kpc.
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
ThespatialcorrelationbetweenSBandmetallicityenhance-
E5
ment, and anti-correlation with temperature is a typical signature Metallicity
ofsloshingandhasbeenfoundinsimulationsaswellasotherob-
servations(e.g.Virgo,Roedigeretal.2011;A496,Roedigeretal.
2012; Perseus, Simionescu et al. 2012; A2029 Paterno-Mahler
etal.2013).Similarly,sloshingintheclustercandistortandinduce
asymmetriesinthedistributionofmetalsaboutthecentralgalaxy.
The gravitational perturbation by the merger of the Cen45 E4
subcluster may have started these sloshing motions. This is sug-
gestedbythealignmentofthelargerscaleSBdeviationsalongthe
E3
axis of the merger (Fig. 4). After the interaction by the perturb- E1 E2
ingsystem,coldfrontsmoveoutwardswithtime(e.g.Ascasibar&
Markevitch2006).TheA496system,whichhasasimilartemper-
aturetoCentaurus,wassimulatedbyRoedigeretal.(2012),who
found that the radial evolution of the cold front mainly depends
ontheclusterpotential.Theradiioftheeasternandwesterninner
Centauruscoldfronts(19and42kpc,respectively)aresimilarto
the fronts in their ‘distant’ run after 1 Gyr (see their fig. 7). The
circularsymmetryofthefeaturessuggestthatthesloshinggasmo- -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
tionsareclosetobeingperpendiculartothelineofsight,aspointed
Figure4.FractionaldifferencefromtheaverageateachradiusforX-ray0.5
outbyAscasibar&Markevitch(2006).Inthiscase,theperturbing
to7keVSB,temperatureandmetallicity.TheX-rayimagewassmoothed
systemisnotmovingintheplaneofthesky.Itisnotpossibleto
bya2arcsecGaussian.Thetemperatureandmetallicitymapsusedthesame
tellwhetherCen45istheperturberbasedonitsposition,velocity
dataaspresentedinFig.3.Arrowsmarkedges(discussedintext).Also
andthetimesincetheinteraction.Indeed,Cen45maynotbeonits shownisthedirectionofthemergingCen45subcluster.
firstpassthroughthesystemandcouldbegravitationallybound.
MNRAS000,000–000(0000)
AverydeepviewoftheCentauruscluster 7
0.6 West North
+10.5 kpc
e 0.4 1.5
onal differenc 0.20 allicity (solar) 1
Fracti−−00..42 Met0.5 1 component
2 components
−0.6
0
0.6 East South
0 kpc
e 0.4 1.5
c
actional differen−00..202 Metallicity (solar) 1
Fr−0.4 0.5
−0.6
0
0.6 1 10 100 1 10 100
−10.5 kpc Radius (kpc) Radius (kpc)
e 0.4
c
n
e
al differ 0.20 Ftoirgsu.rTeh6e.nRuamdibaelrmoeftaclolimciptyonpernotfisleusoefdthinemthaepfiitnsFisigi.n3discpaltietdinbtoy9o0p◦enseoc-r
on closedsymbols.Horizontalerrorbarsshowtheradialrangeofeachbin.
cti−0.2
a
Fr−0.4 X-ray image
Temperature × 2
Metallicity at RA 192.18325 and Dec −41.31048 (marked by the sectors in
−0.6
Fig.7),11kpctothewestofthenucleus.
−128 −72 −32 −8 0 8 32 72 128 To quantify the variation and width of the edge around the
Offset (kpc) frontwefitmodelstoSBprofilesextractedin27independent10◦
sectorsspanning270◦,coveringthe180◦ towardsthewestwhere
the front is visible and 45◦ beyond this in either direction. This
Figure5.SB,temperatureandmetallicityfractionalresidualprofilesinan
analysisissimilartothatpresentedinWerneretal.(2015).Wefita
east-westdirection(westispositive)acrossthecentreoftheclusterwith
threenortherlyoffsets(−10.5,0and10.5kpc),computedfromFig.4.The brokenpowerlawwithabroadenededgemodeltotheprofiles(see
widthofeachstripis8.4kpc.Thehorizontalaxishasasquare-rootscaling. AppendixA3).Fig.9showsforeachsectortheradiusofthefront,
Notethatthetemperatureprofileshavebeenscaledbyafactorof2. thejumpmagnitude,thewidthandthepowerlawindicestoeither
sideoftheedge.
Betweenthenorthandsouth,wheretheedgeiswelldefined,
the jump radius is fairly constant at 31 kpc, with sector-to-sector
variationsofabout1kpc,likelyduetothenotchesaroundtheedge.
2.4 Westerncoldfront
Immediatelywest,thefrontisatitsbroadestat4kpc,butitismuch
ThewesterncoldfrontinCentaurusisoneoftheclearestandnear- narrowertothenorth-westandsouth-west.Insomelocationsthe
estexamplesknowninacluster,spanningnearly180◦.Thepres- edgeappearsnarrowerthan1kpc.Wherethefrontisnarrowestto-
sure across the edge is continuous, confirming it is a cold front wardsthesouth-west,forexampleinthesectorsbetween60−80◦,
(AppendixA3).Fig.7(toppanel)showsanunsharp-maskedimage the1σupperlimitsontheedgewidthare∼0.3kpc(accountingfor
of the centre and cold front region, highlighting the SB fluctua- thePSFattheedgeinthesesectors).Iftherearenomagneticfields,
tions around the front and within the central region. The edge is inthenorth-westdirectionthedensityandtemperatureimplyval-
notperfectlysmooth;thesouth-westernsideappearstobewhere uesforthecollisionalelectronmeanfreepathof0.18and0.36kpc
itissharpestanddirectlywestitappearsbroader.Tothenorthand (0.8and1.7arcsec),interiorandexteriortothefront,respectively.
southofthecoldfrontthereisarapidtransitionfromasharpedge Inthesouth-westtheseincreaseto0.30and0.48kpc(1.4and2.3
toaweakornon-existentedge.However,somecircularstructure arcsec),respectively.Wherethefrontisnarrowesttothesouth-east,
maycontinueoninthenorth-eastandsouth-eastdirections.There ourupperlimitsaresimilartotheelectronmeanfreepath.
appeartobeanumberof‘notches’intotheedgeonlengthscales The emissivity of the ICM decreases outwards over most of
of∼7kpc.Thenotchesalongtheedgecanalsobeseeninagradi- thefronttoaround60percentoftheinteriorvalue,implyingthat
entfilteredX-rayimage(Fig.7bottompanel).WediscusstheSB thedensitydecreasesbyaround20percent,whichisinagreement
featuresinsidethefrontinSection2.5. withspectralmeasurements(AppendixA3).However,immediately
Thedeviationsfromacircularcoldfrontcanbeclearlyseenif tothewestthefitspreferan80percentSBdecrease.Inthesame
theimageoftheclusterisremappedtoradialandazimuthalcoordi- region the powerlaw index of the model outside the jump is flat-
nates(Fig.8).Thenotchesappearaswave-likeirregularitiesonthe terandtheedgeiswider.Itisclearfromtheunsharp-maskedand
straightedge.Theedgeisbestrepresentedbyacirclewithacentre filteredimagesinFig.7thattheedgeismuchlessdistinctimme-
MNRAS000,000–000(0000)
8 J.S.Sandersetal.
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
Figure8.Unsharp-maskedimageoftheclusterremappedtohaveradius
andangleaxes,withacentrechosentomakethewesterncoldfrontlieat
constantradius.Horizontallyitspans360◦fromeast(left),throughnorth,
thenwest(centre),thenbacktoeast(right).Radiusrunsverticallyfrom
2 (bottom) to 68 kpc (top). The 0.5 to 7 keV input image was rebinned
into0.1kpcby0.25◦pixels.Unsharpmaskingwasthenapplied,usingthe
1 arcmin fractionaldifferencebetweenmapssmoothedby4and32pixels.
12.8 kpc
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 ever,andareunlikelytocorrespondtothesamephysicalfeatures
intheprofiles.
X1 Thenotchesalongtheedgeofthewesterncoldfront(Figs.7
and8)areconsistentwithKelvin-Helmholtzinstabilities(KHIs)of
L1 lengthscalesof∼7kpcalongtheedge.KHIsinclusterswerefirst
L2 seeninsimulations(Roedigeretal.2011;ZuHoneetal.2011).A
kink,whichmaybealargeKHI,hasbeenobservedinAbell496
L3
(Roedigeretal.2012).Similarnotchesoflengthofafewkpchave
alsobeenseeninthecoldfrontintheVirgocluster(Werneretal.
2015).Thesenotcheslikelymaketheradiusofthecoldfrontap-
peartovaryby∼1kpc(Fig.9)betweenthenorthandsouth.Com-
L4 parisonwithsimulationsofcoldfrontsincludingvaryinglevelsof
viscosityormagneticfieldssuggestslevelsofviscosityofaround
F1 10percentoftheSpitzervalueareconsistentwiththedata(Roedi-
geretal.2013;ZuHoneetal.2015).Largeramountsofviscosity
Bay (F2-F3) smoothoutallinstabilities,whileinstabilitiesgrowverylargewith
noviscosityormagneticfields.
Notches
2.5 Linearstructuresinsidethefront
ESO 322-93 L5 F4-F6
Using shorter observations we previously detected ripple-like SB
fluctuations using Fourier high pass filtering (Sanders & Fabian
2008).Thesequasi-periodicfluctuationswerealsoseeninSBpro-
Figure 7. (Top panel) Unsharp-masked 0.5 to 7 keV image. This is the files in three sectors (along the north-east, south-west and north-
fractionaldifferencebetweenGaussian-smoothedimageswithσ=2.0and westdirections)Weinterpretedthesefeaturesassoundwavesgen-
15.7arcsec,aftermaskingoutpointsources.‘×’marksthecentralnucleus
eratedbyAGNactivity.
(seeSection3).(Bottompanel)GGM-filteredimageofthesameregion,
Examiningtheunsharp-maskedimage(Fig.7toppanel)there
usingσ=7.9arcsec,takenfromFig.2.Thelabelledfeaturesaredescribed
areanumberofSBfluctuationswithasimilar∼5−10kpclength
inSection2.5.
scale to the previously found fluctuations. They are also similar
insizetothenotchesalongthecoldfront(Section2.4).However,
theydonotappeartohaveaperiodicstructure.CreatingSBpro-
diatelytothewestatthemostextremepartofthefront.Theremay filesusingthesamesectorsasinSanders&Fabian(2008),obtains
alsobeaSBedgewhichextendsfromthewesternmostpointofthe consistentSBprofilesandresidualstoβ modelfits,showingthat
fronttothenorth-west.ThiscanbeseenwhereSBedgeE4appar- thefluctuationsarerobust.
entlyconnectstothecoldfrontE2(Fig.4).Thisfeaturecanalso Using a GGM filter on the data instead of unsharp-masking
beseenastheenhancementintheremappedclusterimage(Fig.8) (Fig. 7 bottom panel) finds a number of quasi-linear features (la-
from the centre of the front extending upwards (to larger radius) belled L1–L5). These straight edges have corresponding features
andleftwards(north-west). intheunsharp-maskedimage,buttheGGMfilterappearstobea
Outsidethe180◦sectorwheretheedgeisobvious,theSBfits muchmorepowerfultoolfordetectingandconnectingtheseedges
implythatthereissomesortofjumporbreakinSBintheradial in SB. All of the labelled structures in Fig. 7 (bottom panel) can
rangeexamined.Theallowedjumpradiivaryconsiderably,how- beseenasSBedgesintherawdatawhenexaminedclosely.The
MNRAS000,000–000(0000)
AverydeepviewoftheCentauruscluster 9
6
kpc) 3346 N W S X Y
us ( 32 5
di
Jump ra 223680 2574 sseeccttoorrss e (Solar)4 30 arcsec Cbalucsktgerround
nc3
1.5 da
n
u
b
A
atio 1 2
p r
m
Ju0.5 1
abcd
0 0
O Ne Mg Si S Ar Ca Fe Ni
10
Element
8
c) 6 Figure10.Comparisonofthemetallicityinadjacenthigh(X)andlow(Y)
p
k metallicityregions,fitting(a)1and(b)2component(s),blank-skyback-
σ ( 4 ground, and (c) 1 and (d) 2 component(s), cluster background (see Ap-
pendixA4).TheinsetmetallicitymapwascreatedfromS/N=40regions.
2
0
1 aresoundwaves,thentheylieabovethiscriticalangle,andsoitis
0 possiblethatthebendsatthecold-frontedgearecausedbytotalin-
ternalreflectionofsoundwaves.Alternatively,variationsinsound
x −1
e speedcouldcausethefrontstobendifthewavepassesthrougha
d
In −2 sharptemperaturegradientatanangle.
The5kpcspacingcorrespondsto6Myrtimescaleatthesound
−3 Inside
speedat2.5keVtemperature.Thistimescaleissimilartotheage
Outside
−4 of the inner shock and cavities. It is therefore plausible that the
−100 −50 0 50 100 structurescouldbegeneratedbyAGNactivity.Thequasi-periodic
Angle (°) natureofthefeaturesarguesagainstthefeaturesbeingturbulence
generatedbytheAGN.Weanalysethesefluctuationsquantitatively
inWalkeretal.(2015)andexaminewhethertheycouldcontribute
Figure9.Fitparametersfortheprofilesofthe27sectorsaroundthewest-
erncoldfrontfromthenorth-east(−130◦)tosouth-east(130◦).Theseare significantlytoAGNfeedbackheating.
themedianand1σ percentilesobtainedfromtheanalysis(AppendixA3). Alternatively,thestructuresmaybeunrelatedtotheAGNbut
Shownaretheradiusofthejump,theratioinemissivityoutsidetoinside insteadcausedbysloshing.ThesloshingsimulationsofRoediger
(thejumpratio),itswidth(σ)andthepowerlawindicesoftheemissivity etal.(2013),inparticulartheirfig.8,showsfeaturessimilarinap-
insideandoutside.Fortheradiuswealsoshowtheresultsusingtwicethe pearance.KHIsonthecurved3Dsurfaceofthecoldfrontcanbe
numberofsectors. seenasprojectedfeaturesinsidethecoldfrontedge.Theinterac-
tion of KHIs at the cold front, combined with the slow outwards
structuresarealsoapparentinthemultiple-scaleGGM-filteredim- motionofthecoldfrontcanalsogiverisetoturbulenceinsideor
age(Fig.1bottompanel). underneath the cold front, which also adds fine structure. Linear
We tested that these features are significant by similarly fil- featureswerealsofoundbyWerneretal.(2015)insidetheM87
tering a Poisson realisation of a heavily-smoothed cluster image, coldfront.Theysuggestedthatthesecouldinsteadbegeneratedby
finding that the noise was at lower levels. In addition, all the la- theamplificationofmagneticfieldlayersbysloshing,asfoundin
belledfeaturescanbeseenbycloselyinspectingtherawimages. thesimulationsofZuHoneetal.(2011)andZuHoneetal.(2015).
L1–L4 are approximately parallel to each other. L1 and L4 (and
perhapsL2andL3)alsoappeartobendwithasharpangleclose
2.6 Metallicityblobs
totheedgeofthecoldfront.Theimagealsohighlightssomeflat
SBregions(F1–F6).Thebay(discussedinSection3)appearsas There are fluctuations in metallicity interior to the western cold
twoflatregionssurroundedbysteepgradients.Therearealsoflat front on ∼5−10 kpc scales (Fig. 3). These features are robust
regionsclosertotheedgeofthefront(F1andF4–F6).Tothenorth against changes to the binning scheme. We compared two neigh-
isanextensionoftheclusteremissionextendsnorthwards(X1). bouringregionsoffsetfromthenucleuswithhigh(X)andlow(Y)
SeveraloftheidentifiedlinearfeaturesL1-L6appeartohave metallicity (Fig. 10) to confirm that these structures are signifi-
an approximately regular separation of ∼5 kpc. L1–L3 seem to cant. Fitting the spectra separately, we obtained the metallicities
haveasharpbendnearthecoldfront,suggestiveofwavesreflect- usingoneandtwotemperaturecomponentfits,andfortwodiffer-
ingoffaboundary.Thetemperatureontheoutsideofthecoldfront entbackgroundspectra.
isafactorof∼1.4greaterthaninside.Thereforethecriticalangle The Si and Fe abundances are greater in region 1 for every
fortotalinternalreflectionatthecoldfrontis58◦.Ifthefeatures model.TheFemetallicityis30percentgreaterinregion1usinga
MNRAS000,000–000(0000)
10 J.S.Sandersetal.
Plume edge
Inner edge Depression
Central cavities
Plume
Nucleus
Bright ridge
Bay
Hook
30 arcsec
6.4 kpc
Figure11.(Toppanel)RGBimageoftheclustercentre.Thebands0.5–1,1–1.5and1.5–7keVareshownasred,greenandblue,respectively.Imageswere
extractedusingbinsof0.5detectorpixels(0.246arcsec)andsmoothedbyaGaussianofwidth0.492arcsec.Theimagemeasures2.8by2.3arcmin(35by
29kpc).(Bottompanel)Partiallyunsharp-masked0.5-7keVgreyscaleimageshowinglabelledfeatures,constructedbytakingtheX-raydatain0.246arcsec
pixelsandsmoothingbyaGaussianofwidth1pixelandsubtractingthe0.5timestheimagesmoothedby64pixels.
MNRAS000,000–000(0000)