Chandra Temperature Maps for Galaxy Clusters with Radio Halos

1.1 MB·English

by  F. Govoni

APJ,INPRESS
PreprinttypesetusingLATEXstyleemulateapjv.11/26/03
CHANDRATEMPERATUREMAPSFORGALAXYCLUSTERSWITHRADIOHALOS
F.GOVONI1,2,M.MARKEVITCH3,A.VIKHLININ3,L.VANSPEYBROECK3,4,L.FERETTI1,ANDG.GIOVANNINI1,2
Full-resolutionversionathttp://hea-www.harvard.edu/ maxim/papers/rhalos_tmaps
∼
ApJ,inpress
ABSTRACT
WeanalyzeChandratemperaturemapsforasampleofclusterswithhigh-qualityradiohalodata,tostudythe
originoftheradiohalos. ThesampleincludesA520,A665,A754,A773,A1914,A2163,A2218,A2319,and
1E0657–56. We presentnewtemperaturemapsforallbuttwo ofthem(A520andA754). Alltheseclusters
exhibitdistortedX-raymorphologyandstronggastemperaturevariationsindicatingongoingmergers. Some
4
clusters, e.g., A520, A665, 1E0657–56, exhibitthe previouslyreportedspatialcorrelationbetweenthe radio
0
halo brightness and the hot gas regions. However, it is not a general feature. While most mergers are too
0
messytoallowustodisentangletheprojectioneffects,wefindclearcounterexamples(e.g.,A754andA773)
2
where the hottest gas regions do not exhibit radio emission at the present sensitivity level. This cannot be

n explainedby projectioneffects, and thereforearguesagainstmergershocks— at least those relatively weak
a
onesresponsiblefortheobservedtemperaturestructureinmostclusters—asthemainmechanismforthehalo
J
generation. Thisleavesmerger-generatedturbulenceas a more likelymechanism. The two clusterswith the
1 clearestradiobrightness–temperaturecorrelation,A520and1E0657–56,arebothmergersinwhichasmall
2
densesubclusterhasjustpassedthroughthemain cluster, verylikely generatingturbulencein itswake. The

maximumradiobrightnessandthehotgasarebothseeninthesewakeregions. Ontheotherhand,thehalos
1
in1E0657–56andA665(bothhigh-velocitymergers)extendintotheshockregionsinfrontofthesubclusters,
v
wherenostrongturbulenceisexpected.Thus,inhigh-velocity(M≃2- 3)mergers,bothshockandturbulence
1
accelerationmechanismsmaybesignificant.
2
4 Subject headings: galaxies: clusters: general — galaxies: clusters: individual (A520, A665, A754, A773,
1 A1914,A2163,A2218,A2319,1E0657–56)—intergalacticmedium—X-rays: galaxies:
0 clusters—Radiocontinuum
4
0
/ 1. INTRODUCTION proposed (see, e.g., recent reviews by Enßlin 1999; Sarazin
h 2001;Brunetti2002;Petrosian2002andreferencestherein).
The baryonic content of the galaxy clusters is dominated
p
- by the hot(T ∼2- 10 keV) intergalacticgaswhose thermal Wewillbrieflyreviewtheoreticalargumentsin§4.
o emission is observed in X-rays. Many clusters also exhibit On the observationalside, the numberof knownradio ha-
r diffuseradiosourcesthathavenoapparentconnectiontoany los is already sufficientto start lookingfor correlationswith
t other cluster properties in order to elucidate their accelera-
s individualcluster galaxy. In this paper, we are interested in
a tion mechanism. Halos are typically found in clusters with
theextremelylow-brightness,large-scalesourcesclassifiedas
: significant substructure in the X-ray brightness which indi-
v radio halos. They often span the whole cluster and are un-
catesmergeractivity(e.g.,Feretti1999;Buote2001). Halos
i polarized. Other diffuse radio sources, not considered here,
X are present in rich clusters, characterized by high X-ray lu-
includestronglyelongatedrelics, whicharehighlypolarized
r andoftenfoundattheclusterperiphery,andmini-haloswhich minositiesandtemperatures. Thepercentageofclusterswith
a halosin a completeX-ray flux-limitedsample (thatincludes
arefoundaroundthepowerfulcentralradiogalaxy;forrecent
systemswithL >5×1044ergs- 1inthe0.1- 2.4keVband)
reviewsseee.g. Kempneretal. (2003),Feretti(2003). X
is≃5%. ThehalofractionincreaseswiththeX-rayluminos-
While the cluster X-ray emission is due to thermal elec-
ity,to≃33%forclusterswithL >1045ergs- 1(Giovannini,
tronswithenergiesofseveralkeV,theradiohaloemissionat X
∼1GHzisproducedbysynchrotronradiationofrelativistic Feretti, & Govoni1999a). Theradiopowerofa halo, if one
electronswith energiesof∼10GeVin magneticfieldswith ispresent,stronglycorrelateswiththeclusterluminosity,gas
B≃0.5- 1µG.Theseelectronsshouldcoexistwiththether- temperature(e.g.,Liangetal.2000;Colafrancesco1999;Fer-
etti1999),ortotalmass(Govonietal.2001a).Inanumberof
mal population. Their origin is still uncertain; the difficulty
well-resolvedclusters,aspatialcorrelationbetweentheradio
in explaining their presence arises from the combination of
thelargesizesofhalos(r∼1Mpc)andtheshortsynchrotron haloandX-raybrightnessisobserved(Govonietal.2001b).
lifetimeoftheseelectrons(107- 108yrs).Oneneedsamecha- These observations indicate that radio halos are closely re-
latedtotheintra-clusterthermalgas, itshistoryandenerget-
nismbywhichtheseelectronsarelocallyandsimultaneously
ics. However,detailsofthisconnectionneedfurtherinvesti-
(re-)accelerated over the halo volume. Several such mecha-
gations.
nismsoffeedingenergytotherelativisticelectronshavebeen
Because the halos appear to be related to cluster merg-
1IstitutodiRadioastronomiadelCNR,ViaGobetti101,I-40129Bologna, ers, cluster gas temperature maps, which contain informa-
Italy tion on the merger geometry, stage, and velocity, can pro-
2 Dipartimento diAstronomia, Università diBologna, ViaRanzani1,I- vide further information on their origin. Indeed, Chandra
40127Bologna,Italy studiesofA2163,A665(Markevitch&Vikhlinin2001,here-
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cam-
after MV01), 1E0657–56 (Markevitch et al. 2002a, here-
bridge,MA02138
after M02a) and A520 (Markevitch et al. 2002b, hereafter
4Wehonorthememoryofourlatecolleague
2 GOVONIETAL.
warepackagefordataprocessing,XSPECandA.Vikhlinin’s
TABLE1
ZHTOOLStofittheoverallspectraandanalyzeimages,and
X-RAYDATA
ourowncodetoderivethetemperaturemaps.
Name z ObsID exposure, Observation TheX-rayobservationsareidentifiedbytheirIDnumbers
ksec date (ObsID)inTable1,wherethecleanexposuretimeandtheob-
servingdatearelisted. Allobservationswereperformedwith
A520........... 0.1990 528 9.3 2000Oct10
theACIS-Idetectorwhichcoversafieldofviewof16′×16′.
A665........... 0.1819 531 8.5 1999Dec29
ExceptforA1914,A2163,and 1E0657–56, forwhichwe
3586 23.8 2002Dec28
usethelongerofthetwoavailableexposures(ofverydifferent
A754........... 0.0542 577 39.1 1999Oct30
length),whenmorethanoneobservationexistedforthesame
A773........... 0.2170 533 10.4 2000Sep5
object,theobservationswerecombined.
3588 8.3 2003Jan25
A1914......... 0.1712 3593 17.6 2003Sep3 2.1. X-raydatapreparation
A2163......... 0.2030 1653 69.6 2001Jun16
For each observation, standard screening was applied to
A2218......... 0.1756 553 5.2 1999Oct19
thephotonlist. We removedbadpixelsandcolumns,events
1454 10.9 1999Oct19
withASCAgrades1,5and7,andcosmicrayafterglows. The
1666 22.8 2001Aug30
backgroundwasmodeledusingacompositeblank-fieldback-
A2319......... 0.0557 3231 14.0 2002Mar15
ground dataset corresponding to the period of the observa-
1E0657–56..... 0.296a 3184 69.6 2002Jul12
tions,cleanedandnormalizedasdescribedbyMarkevitchet
Col.1:Clustername; al.(2003b). Inparticular,wefirstexcludedflareperiods,ex-
Col.2:Redshift(Struble&Rood1999;aTuckeretal.1998) tracting the X-ray light curves from the source-free parts of
Col.3:ChandraobservationIDnumber; the field and requiring that the 0.3–12 keV backgroundrate
Col.4:Cleanexposuretime; be within a factor of 1.2 of the nominal background. After
Col.5:Observingdate. theflareexclusion,wenormalizedtheblank-skybackground
datasettotheclusterobservationbytheratioofcountsathigh
energies (in the PHA interval of 2500–3000ADU; approxi-
mately10–12keV)whichisfreefromtheskyemission.This
M02b) revealed a possible spatial correlation between the
correctionwasalwayssmall, within10%oftheexposurera-
high-temperaturecluster regions and the diffuse radio emis-
tio. IfanobservationwasperformedinVFmode,andifthe
sion. A665 showed a spatial correlation between the radio
correspondingVF-modebackgrounddatasetexisted(ObsIDs
halobrightnessandthelocationofapossiblebowshock,ap-
1666,1653,3184,3231,3586,3588,3593),additionalparti-
pearing to support a merger shock origin for the relativistic
clebackgroundreduction(Vikhlinin2001)wasappliedtothe
haloelectrons.
clusterandbackgrounddata.
Clearly,anX-rayandradiostudyofagreaterclustersam-
The soft Galactic background in the direction of the ana-
pleisneededtoseewhetheraspatialcorrelationbetweenthe
lyzedclusters(Snowdenetal.1997)isnotmuchdifferentthan
radiohaloandthegastemperatureisacommonfeature. We
intheregionsincludedintheblank-skybackground—sono
presentChandraX-rayimagesandtemperaturemapsforsuch
special treatmentwas required— for all clusters but A2163
asample.
andA2319. ForA2163,treatmentofthesoftGalacticexcess
We make the radio – X-ray comparison for the clusters
is described in MV01; the central region of A2319 that we
A520, A665, A754, A773, A1914, A2163, A2218, A2319,
analyzehereissobrightthatsuchanexcessdoesnotmatter.
and 1E0657–56. Chandramapsfor A754 and A520are re-
Ingeneral,ourtemperaturemapsare limitedto thebrightest
producedfromMarkevitchet al. (2003a)and M02b, respec-
regionsoftheclusterswherethebackgroundaccuracyisnot
tively. For A2163 and 1E0657–56, we present temperature
critical.
mapsfromnew observations,deeperthan those analyzedby
MV01 and M02a; the full derivation of these maps will be
2.2. X-rayspectra
presented by Markevitch et al. (in prep.) A higher-accuracy
temperaturemap forA665is derivedin thisworkbyadding For consistency checks, we first derived the average tem-
anewlongobservationtotheoneusedinMV01. ForA773, peraturesof ourclusters, extractingspectra fromcircularre-
A1914,A2218,andA2319,wederivethefirsthigh-resolution gions of r=0.8 Mpc (or smaller if such a region did not fit
temperaturemapshere. within the field of view). This radius encompasses most of
Theradiodataweretakenfrompreviouslypublishedwork thecluster emission. Pointsourceswere excluded. The cor-
(A2319: Feretti, Giovannini, & Böhringer 1997; A665, responding spectral Redistribution Matrix Files (RMF) and
A2218: Giovannini&Feretti2000;1E0657–56: Liangetal. Auxiliary Responce Files (ARF) were generated using the
2000;A520,A773:Govonietal.2001a;A2163:Ferettietal. A. Vikhlinin’s software and were weighted by the cluster
2001;A754,A1914:Bacchietal.2003). brightness. The large quantum efficiency (QE) spatial non-
WeuseH0=70kms- 1Mpc- 1,Ω0=0.3andΩΛ=0.7. uthneifAorCmISityfoccaaulspeldanbey-C1T1I0iCn)twheasdamtaodperiloedrtuosiFnegbth2e00f0or(mwuitlha
2. X-RAYANALYSIS from Vikhlinin (2000); for other data, the standard QE uni-
formitymaps were used. ARFs for the ACIS-I observations
WeusedarchivalandourproprietaryChandraACISdatato
deriveX-rayimagesandgastemperaturemaps.ACIS5hasan alsoincludedthetime-andposition-independentfudgefactor
energybandof0.3–9keVandcombines1′′angularresolution of0.93atE<1.8keVtoaccountforthebackside/frontside
andamoderateenergyresolution. We usedthe CIAO6 soft- illuminatedCCD flux discrepancyin the currentcalibration.
The time-dependent,position-independentcorrectionfor the
5http://asc.harvard.edu/cal soft absorption caused by contamination buildup (Plucinsky
6http://cxc.harvard.edu/ciao etal.2003)wasincludedintheARFs.
TEMPERATUREMAPSFORRADIOHALOCLUSTERS 3
3.1. Abell520
TABLE2
FITSTOAVERAGECLUSTERSPECTRA Opticalspectroscopicdata onA520suggestthatthisclus-
ter is undergoing strong dynamical evolution (Proust et al.
Name T, abund. radius, NH, 2000). A short Chandra observation (Table 1) placed this
keV ′′ 1020cm- 2 X-ray-luminouscluster in the ACIS chip I3. The X-ray ob-
A520........... 7.1 0.7 0.24 0.12 245 7.80 servationconfirmsthatA520isinthemiddleofamerger. In
± ±
A665........... 8.2 0.5 0.27 0.07 260 4.24 Fig. 1, we compare an X-ray image and a temperature map
± ±
A754........... 10.0 0.3 0.30 0.05 540 4.36 (fromM02b)derivedfrom this Chandraobservation,a VLA
± ±
A773........... 7.5 0.8 0.34 0.11 230 1.44 radiohaloimageat1.4GHzfromGovonietal.(2001a),and
± ±
A1914......... 10.9 0.7 0.24 0.08 275 0.95 anopticalDigitizedSkySurvey(DSS)7plate.
± ±
A2163......... 12.4 0.7 0.20 0.03 240 18.7 3.5 The most prominent feature in the X-ray data is a dense,
± ± ±
A2218......... 6.7 0.5 0.10 0.07 270 3.24 compact—butclearlyextended—coolgasclumpsouthwest
± ±
A2319......... 10.1 0.4 0.30 0.05 540 7.93 of the center. Apparently, it has just passed straight through
± ±
1E0657–56..... 13.9 0.7 0.25 0.05 180 4.60 themainclusterfromthenortheast,accompanyingapossible
± ± groupofgalaxies(althoughnotcenteredonanyonegalaxy).
Col.1:Clustername;
Thetemperaturemapshowsthatthebrightgastrailithasleft
Col.2:Clustertemperature;
behind is cool, and reveals a hot strip of apparently shock-
Col.3:Abundancerelativetosolar;
heated gas along this trail. Most of the radio halo emission
Col.4:Radiusofthespectrumextractionregion
appearsto follow this hotstrip, and notthe coolbrighttrail.
Col.5:Galacticabsorptioncolumn(Dickey&Lockman1990),
The X-ray image hints at a possible bow shock in front of
exceptforA2163and1E0657–56,seetext.
the dense clump, coincident with the southwest edge of the
radio halo, but a longer observation (planned for late 2003)
isneededtodeterminethenatureofthatfeature. Thiscluster
Spectra were fit in the 0.8- 10 keV energy band with the is one of the best examples of a spatial correlation between
the radio halo brightnessand the gas temperature. It is also
MEKALmodel(Kaastra1992)byfixingtheabsorptioncol-
oneofonlytwoclustersinoursamplewithasimpleandclear
umn to the Galactic value (Dickey & Lockman 1990). Two
mergergeometry(theotherbeing1E0657–56).
exceptionsareA2163and1E0657–56whereitisknowntobe
significantlydifferent,higherforA2163(e.g.,Elbaz,Arnaud,
&Böhringer,1995)andlowerfor1E0657–56(e.g.,Lianget 3.2. Abell665
al.2000). ForA2163,itwasfitasafreeparameter,whilefor
TheX-rayimageofA665stronglysuggeststhatthecluster
1E0657–56,theLiangetal.’sbest-fitROSATvaluewasused,
isundergoingamerger(e.g.,Jones&Saunders1996;Gómez,
whichisingoodagreementwithourdata.
Hughes,&Birkinshaw2000). Themergerapparentlyoccurs
Theresultingaveragetemperaturesandabundances,atthe
in the direction of the elongation of the galaxy distribution
90%confidence,aregiveninTable2togetherwiththeextrac-
(Geller&Beers1982;Beers&Tonry1986).Indeed,aChan-
tion radii and absorptioncolumns. All the obtained temper-
dratemperaturemaprevealedlargevariationsandapossible
ature are in good agreement with previously derived values
shockwithM≈2intheexpectedlocation(MV01). Alumi-
foundintheliterature.
nousradiohaloinA665wasdiscoveredbyMoffet&Birkin-
shaw(1989),confirmedbyJones&Saunders(1996)andfur-
2.3. Temperaturemaps
therstudiedbyGiovannini&Feretti(2000).
Temperatureimagesoftheclustercentralregionswerede-
TheclusterwasobservedbyChandratwice (seeTable1).
rived as described in Markevitch et al. (2000a) and MV01.
Here we derive a more accurate temperature map than that
Specifically, we excludedpoint sources, extracted imagesin
presentedinMV01,byincludingtherecentlongerexposure.
severalenergybands,subtractedthebackground,dividedthe
InFig.2,theresultingmapandanACISimagearecompared
resulting images by the exposure maps, and smoothed them
to the 1.4 GHz VLA image (Giovannini& Feretti 2000)and
byaGaussianfilterwithvariablewidth(sameinallbands)to
anopticalDSSplate. Thenewmapconfirmsthepresenceof
get usefulstatistical accuracyover the interesting brightness
hot—mostprobablyshock-heated—gassouthorinfrontof
range. A temperatureineach pixelofthe mapwasobtained
the coolcore. The new image, however, still does not show
byfittingvaluesineachpixeloftheseimageswithathermal
anycorrespondingdensityfeatureassharpasthebowshock
plasma,fixingNH totheGalactic (orother,seeabove)value observed in 1E0657–56. The edge of the cool core is also
andtheelementabundanceto0.3solar. Toverifytheresult-
notassharpasinsomeotherclusters,sothemergerprobably
ingtemperaturemaps,wefitspectrafromseveralinteresting
proceedsatanangletotheskyplane.
regionsofeachcluster using XSPEC. The outlyingareasof
TheX-raybrightnesspeakisoffsetfromthecentralgalaxy,
thetemperaturemaps,wherenoisestartstodominate(usually
indicating gas motion in the core. Indeed, the new temper-
wherethe1σerrorsaregreaterthan∼2keV),werecutoff.
ature map reveals complex temperature structure inside the
core as well as in its wake, includingtwo streams (or trails)
3. RESULTS
ofcoolgasintheNWandNEdirectionsfromthecore,anda
BelowwepresentourX-rayimagesandtemperaturemaps, hotstreamnorthofthecore.
discusshowtheyelucidatethemergerstateandgeometry,and The radio halo is very extended and elongated in the SE-
comparethemtotheradiohaloimages. Foreachcluster,we NWdirection,whichistheapparentmergerdirectionandthe
present a temperature map with an X-ray and a radio con- X-rayelongation. AsnotedbyMV01,the“leadingedge”of
tour plot overlaid, an optical image with an X-ray contour thehaloextendsbeyondthecoolcoreandcoincideswiththe
plot overlaid, and an X-ray image with a radio contour plot
overlaid. 7http://archive.stsci.edu/dss
4 GOVONIETAL.
shock region. MV01 presented an image of the ratio of the collisionofthegascloudshasgeneratedashock-heatedarea
radio brightness to the square root of the X-ray brightness, there,seeninthetemperaturemap.
whichhastheapproximatemeaningofthedensityofrelativis- The radio halo does not follow the X-ray brightness, nor
tic electrons to thermal electrons (assuming a uniformmag- temperaturedistribution–moreover,thehottestclusterregion
neticfield). Itsmaximumcoincidedwiththesouthernshock is not at all bright in the radio (although a more sensitive,
region. lower spatial resolution radio image shows a faint extension
mostly southward; Govoni et al. 2001a). Instead, the radio
3.3. Abell754 haloiscenteredintherelativelycoolregionbetweenthetwo
OpticalandX-raystudies(e.g.,Fabricantetal.1986;Bird galaxysubclusters.
1994; Slezak, Durret, & Gerbal 1994; Zabludoff & Zarit-
3.5. Abell1914
sky 1995; Henry & Briel 1995; Henriksen & Markevitch
1996;Blitonetal.1998;DeGrandi&Molendi2001)showed The presence of a radio halo in A1914 was suggested by
that A754 is undergoinga violent merger. Markevitch et al. Giovanniniet al. (1999b)from an NVSS search. It was de-
(2003a)notedthatitisdifficulttoexplainthecomplexdetails tectedbyKempner&Sarazin(2001)intheWesterborkNorth-
revealed by the Chandra X-ray image and temperature map ernSkySurvey(WENSS)at0.3GHz,andconfirmedbyBac-
(whichwereproducehereinFig.3)withasimpletwo-cluster chietal.(2003)indeepVLAobservationsat1.4GHz.
mergermodel. Krivonosetal. (2003)noteda possibleweak A1914wasobservedbyChandratwice,butbecauseofthe
bowshockeastofthebrightcoreintheROSATimage;itislo- relatively short useful exposure of the first observation(Ob-
catedapproximatelyattheleftmostX-raybrightnesscontour sID 542), only the second one is used here. In Fig. 5, we
inFig.3. comparethe radioimage fromBacchi et al. (2003)with our
AradiohaloinA754wasdiscoveredbyHarris,Kapahi,& ChandraX-rayimageandtemperaturemap,andaDSSplate.
Ekers(1980)andrecentlyconfirmedbyKassimetal.(2001) Thistemperaturemap, thefirstforA1914,immediatelypro-
throughradio observation at 0.3 GHz. A hard X-ray excess vides a likely merger scenario, not at all obvious from the
atE &45keVwithrespecttothermalemissionwasalsore- X-ray and optical images alone. In this scenario, the NE-
ported (Fusco-Femianoet al. 2003). The most likely expla- SWarch-likehotregionthroughtheclustercenter,whichco-
nationisinverseComptonemissionfromthesamerelativistic incides with a distinct component in the X-ray image, is a
electronsresponsibleforthediffuseradioemission, shock between the two infalling subclusters. One of them
InFig. 3, we comparetheChandraX-rayimageandtem- has arrived from the southeast, where the map shows cool
perature map with a DSS optical plate and a 1.4 GHz VLA gas probablystripped from that subcluster. Its gas core was
imagefromBacchietal. (2003). Theradiodiffuseemission partlyshockedandstoppedbythe collisionaroundthe posi-
iscomplexandveryextended.Bacchietal.(2003)notedthat tionofthepresentX-raybrightnesspeak.Thisgasiscurrently
itconsistsoftwolargecomponents,roughlycoincidentwith squirtingsidewaysalongthehotarch,creatingtheprominent
the two main galaxy concentrations. The temperature map coolerelongationtotheeast. Itsgalaxiesappeartohavepen-
shows strong spatial variations, such as a large hot area in etratedfurther,insidetheshock-heatedgas,andarenowseen
the south and southwestand coolgasat the tip of the bright asa northeasterngalaxyconcentration. Thebrightestcluster
tongue-likestructure.Thisstructureappearstobetheremains galaxywasprobablyatthecenteroftheothercollidingsub-
ofthecoreofoneofthesubclusters,presentlyflowinginthe cluster,whicharrivedfromthewest. Itscoolercorewasleft
northeastdirection. Theeasterndiffuseradioemissionislo- behindandiscurrentlyseenasawesterncoolextension,some
catedaroundtheeasterninterfacebetweenthetongueandthe ofitpossiblysquirtingalongthewesternsideoftheshocked
surroundinggas. Thewesternradioemissionextendsbeyond region, similarly to the situation on the southeastern side of
the boundaryofour temperaturemap. Thatregionwas cov- thecluster. Ifthismergerhadasmallerimpactparameterand
eredbytheASCAmap(Henriksen&Markevitch1996)which occurred closer to the sky plane, we would now see a clas-
showedhotgas;moredetailswillsoonbeprovidedbyXMM sical picture from the simulations with two subclusters just
(P.Henry&A.Finoguenov,privatecommunication).Signifi- beforecorepassageandashockedregionbetweenthem(e.g.,
cantly,thelarge,hotsouthernregioninthepresentmapdoes Schindler&Müller1993andmanylaterworks).
notexhibitradioemissionatthepresentsensitivitylevel. ThediffuseradioemissioninA1914isunusualinthatithas
a distinct, bright, elongatedregion, approximatelyalong the
3.4. Abell773 presumedpathtraveledbytheeasternsubcluster.Itisaccom-
An irregular X-ray shape of A773 was noticed in the paniedbya moretypical, diffuse,low-brightnesshalointhe
ROSAT data (e.g., Pierre & Starck 1998; Rizza et al. 1998; clustercenter. Thebrightfeatureisclearlyextended(Bacchi
Govoni et al. 2001a). A radio halo in this cluster was sug- etal.2003).Itwouldbeinterestingtodetermineifthisregion
gested by Giovannini, Tordi, & Feretti (1999b) from the isphysicallydistinctfromtherestofthehalo.Anupperlimit
NRAOVLASkySurvey(NVSS,Condonetal.1998)andcon- onitspolarizationis3%(Bacchietal.2003),whichexcludes
firmedbyGovonietal.(2001a)throughadedicatedVLAob- thepossibilitythatitisarelicprojectedontotheclustercen-
servation at 1.4 GHz. In Fig. 4, we compare this radio im- ter. The remaining low-brightness region of the radio halo
agewithanopticalDSSplate, andourtemperaturemapand approximatelycoincideswiththehotcentralregionoftheX-
Chandraimage.ThisisthefirsttemperaturemapforA773.It rayclusterandmayevenfollowthepresumedstreamsofthe
revealsstrongtemperaturevariationsinthe6- 12keVrange. gas of the two subclusters. However, a detailed comparison
The optical image clearly shows two galaxy subclusters, requiresbetteraccuracyandaremovaloftheradiosourcesin
one in the center of the X-ray emission and another at the thesouthandtheresolutionofthenatureofthebrightfeature.
easternoutskirt. Takingintoaccountthetemperaturemap,it
3.6. Abell2163
appearsasthoughttheeasterngalaxygroupiscurrentlyexit-
ing the merger site, having shed its gas due to ram pressure A2163isamongthehottestandmostX-rayluminousclus-
at the entry into the main cluster at its southwest side. The ters known (e.g., Arnaud et al. 1992). The presence of an
TEMPERATUREMAPSFORRADIOHALOCLUSTERS 5
extended,powerfulradiohaloinA2163wasfirstreportedby Trèvese,Cirimele,&DeSimone2000)suggestedthatitcon-
Herbig & Birkinshaw (1994). The ROSAT image (e.g., El- sistsoftwocomponentssuperimposedalongthelineofsight,
bazetal.1995),crudeASCAtemperaturemap(Markevitchet with a subcluster around the second-brightest galaxy pro-
al. 1994)and the previouslypublishedChandratemperature jected about 10′ northwest of the cD galaxy (seen in Fig.
map(MV01)clearlyshowthattheclusterisamerger. How- 8a). Thatsubclusterisseen asa coolX-rayextensioninthe
ever,evenwiththetemperaturemap,itsgeometryisdifficult ROSAT imageandthecrudeASCAtemperaturemap(Marke-
todetermine. vitch1996). A2319exhibitsanextendedandpowerfulradio
InFig.6, wepresentanewtemperaturemapderivedfrom halo(Harris&Miley1978)withanirregularmorphologywell
a deeper Chandra exposure by Markevitch et al. (in prepa- correlatedwiththeX-raybrightness(Ferettietal.1997).
ration). It is in agreement with but more accurate than the WeuseaChandraobservationofA2319toderivethefirst
MV01map.TheX-rayimageandthenewmapshowstreams detailedtemperaturemapofthiscluster,showninFig.8along
ofhotandcoldgasflowingindifferentdirections,aswellas withourX-rayimageanda1.4GHzVLAimagefromFeretti
apossibleremnantofacoolgascore. Itstilldoesnotclarify et al. (1997). The most prominentfeature seen in the X-ray
the geometry of the merger, however. This, the absence of data is a sharp cold front southeast of the cD galaxy, such
anysharpfeaturesinthehigh-statisticsX-rayimage,andthe asthosediscoveredbyChandrainmanyothermergingclus-
optical spectroscopic data (Soucail et al., in preparation) all ters (e.g., Markevitchet al. 2000a; Vikhlinin, Markevitch &
pointtothepossibilitythatthemergerisoccurringatalarge Murray2001). The centralcoolgascloud is clearlymoving
angletotheskyplane. southeast with respect to the ambient hotter gas (or, equiva-
ThetemperaturemapiscomparedwithanopticalDSSplate lently, the hotter gas is flowing around it to the northwest).
andthe radiohalo imagefromFeretti etal. (2001). Thelat- ThecDgalaxyisneitheratthecentroidnoratthecoolestspot
ter combines two images with different FWHM resolutions, ofthecluster,suggestingthatthecoolgascoreismovingin-
30′′ (innerblackcontours)andthemoresensitive 45′′×60′′ dependentlyofthisgalaxy.ThecDgalaxyappearstohaveits
(outerbluecontours),respectively,toshowthelargeextentof owngasdensitypeakonasmallerlinearscale,itselfdisplaced
theradiohalo. Thehaloextension2- 3′ eastofcentercoin- eastwardfromthegalaxy. (Wenotethatthisbrightnesspeak
cideswiththehottestregionofthecluster;however,asnoted fallsexactlyontheACIS-Icentrallow-exposurespot,sothe
inMV01,thisregionalsocoincideswiththeX-raybrightness apparent temperature dip there may be an artifact.) We can
extension. In general, the halo brightnessfollows the distri- alsoseeacoolarmextendingaroundtheclustercenterfrom
butionoftheX-raybrightnessquitewell(Ferrettietal.2001). thetip ofthecoldfrontinthegeneraldirectionofthenorth-
western subcluster. It may either be a tail of that subcluster,
3.7. Abell2218 orgasstrippedfromthecoldfrontcloud,unrelatedtothesub-
InA2218,strongandweaklensingmassreconstructionre- cluster. Overall,thepicturesuggestsalaterstageofamerger,
vealedtwodistinctmasspeaksinsidetheclustercore,approx- wellpasttheinitialencounter.
imately around the two brightest galaxies along the NW-SE Theradiohalo followsremarkablycloselythe distribution
directionseen in the opticalplate Fig. 7a (e.g., Abdelsalam, ofthecoolgasinthecore,exceptfortwolow-brightnessex-
Saha,&Williams1998;Squiresetal.1996;Kneibetal.1995, tensionsintothehottergasnortheastandsouthwestofthecold
1996).ROSATimagessuggestedthattheclusterisnotrelaxed front.TheradiohaloismoreextendedsouthwestoftheX-ray
(e.g.,Kneibetal.1995;Squiresetal.1996;Markevitch1997), brightnesscentroid—towardthecoolergasthatweobserve
althoughatlargescales,itappearedrelativelysymmetric. there.
TheclusterwasobservedwithChandrathreetimes. Anal-
ysis of the first two exposureswas presented by Markevitch 3.9. 1E0657–56
etal.(2000b)whofoundacentralpeakintheradialtemper- 1E0657–56 is one of the hottest, most luminous clusters
atureprofile,andMachaceketal.(2002)whoreportedanaz- known(Tuckeretal.1998)whichalsocontainsthemostlumi-
imuthallyasymmetrictemperaturestructureinthecore. Here nousradiohalo(Liangetal.2000). AnearlyChandraobser-
weaddamorerecent,longerexposureandderivethefirstde- vationrevealedaspectacularbowshockpropagatinginfront
tailedtemperaturemapofA2218. ItisshowninFig.7along of a dense, cool bullet-like subcluster exiting the site of the
withtheX-rayimage,a1.4GHzVLAimagefromGiovannini collisionwithabiggersubcluster(M02a). Thoseauthorsde-
&Feretti(2000),andanopticalDSSplate. rivedanapproximatetemperaturemapofthismergerandno-
ThemapconfirmstheresultsfromtheearlierChandraanal- ticedthattheradiohalobrightnesspeaksatthehottestcluster
ysesandrevealsstrongasymmetrictemperaturevariationsin region.
the5- 10keVrange. Togetherwiththerelativelysymmetric In Fig. 9, we compare the Liang et al. (2000) 1.3 GHz
X-rayimage,suchanirregularbutcentrallypeakedtempera- radioimagewith a new, moreaccuratetemperaturemapob-
turemapsuggeststhatA2218isatalatermergerstage,when tainedfromadeeperChandraexposurebyMarkevitchetal.
the violentgasmotionsare starting to subside, as seen, e.g., (inpreparation). The newmap andthe X-rayimagesuggest
inthesimulationsbyRoettiger,Stone,&Mushotzky(1998). thatthesmallersubclusterhasarrivedatthecollisionsitefrom
The relatively small radio halo in A2218 (Giovannini & thesoutheast. Mostofitsoutergaswasshockedandstripped
Feretti2000)isslightly(byabout40′′)offsetfromtheX-ray during the collision with a bigger subcluster (whose largest
brightnesspeakand doesnotshowa particularlystrongcor- galaxies are now seen in the east). This stripped gas, to-
relationwiththetemperaturemap—infact,thehottestspot gether with the shocked gas from the other subcluster, form
in the cluster is notseen in the radio, as in A754and A773. the north-south bar-like structure seen in the X-ray image,
Theradioimagehasarelativelylowsensitivity,however. whichisprobablyapancakeinprojection. Shockscouldnot
penetrateand stop the dense core of the subcluster, and it is
3.8. Abell2319
now continuing to the west, preceded by a bow shock with
Optical analyses of the bright nearby cluster A2319 (e.g., M≈3.5. Thissubclustershouldbegeneratingvigoroustur-
Faber & Dressler 1977, Oegerle, Hill, & Fitchett 1995, bulenceinitswake.
6 GOVONIETAL.
TheradiohalopeakisclearlyoffsetfromtheX-raybright- strongprojectioneffects,ofcourse).
nesspeak(intheregionthatexcludesthebullet),andinstead On the other hand, the gas turbulence, although not di-
is centered in the hottest cluster region. The halo’s eastern rectly observable at present, is expected to exist throughout
part is elongated along the presumed infall trajectory of the the merging clusters, including shock-heated and cooler gas
subcluster. Interestingly,thewesternpartofthehaloextends regions(e.g.,Sunyaev,Norman&Bryan2003). Thereforein
allthewaytothebowshock. theturbulencescenario,thereshouldbenostrongcorrelation
betweentheradiobrightnessandthetemperature.
4. DISCUSSIONANDCONCLUSIONS Theanalysispresentedhereisa qualitativeattempttodis-
SubstructureintheX-rayimagesandthegalaxyspatialand tinguishbetweentheshockandturbulenceaccelerationmech-
velocity distributions, as well as complex gas temperature anisms by means of a systematic comparison of radio halo
structure, are signatures of cluster mergers. All these prop- mapswiththetemperaturemapsforasampleofhaloclusters
ertiesarecommoninclusterscontainingradiohalos. Onthe withgoodradioandX-raydata.
otherhand,wedonotknowofextendedradiohalosinrelaxed Allclustersstudiedhererevealclearsignsofongoingmerg-
clusters.Therefore,theavailabledatasuggestthatradiohalos ers and the accompanying strong spatial temperature varia-
arerelatedtomergers. tions. Althoughinmostoftheseclusters,themergergeome-
From the energetic grounds, mergerscan indeed dissipate tryisambiguousandthelikelyprojectioneffectscomplicate
enoughkineticenergy—simultaneouslyoveramegaparsec- comparisonwiththeradiodata,wecandrawseveralconclu-
scale volume — for the maintenance of a radio halo. How- sions.
ever, it is not clear how exactly the relativistic particles are We confirmthe previouslyreportedspatial coincidenceof
accelerated. In-situ acceleration (or re-acceleration) of rela- bright radio features with the high temperature regions in
tivistic electrons during a merger can occur in shocks (e.g. A665, A2163, and 1E0657–56, which was based on the
Sarazin1999,Fujita&Sarazin2001)orinthegasturbulence shorter X-ray observations. 1E0657–56 and A520 are the
(e.g. Schlickeiser,Sievers,&Thiemann1987;Brunettietal. bestexamplesofthistemperature-radioconnection.Thisspa-
2001;Ohno,Takizawa,&ShibataS.2002,Fujita,Takizawa, tial“coincidence”isnotquitea“correlation’—theseclusters
&Sarazin2003). alsoexhibitbrightradioemissionfromsomeofthe coolgas
Therearetheoreticalargumentsagainsttheshockhypothe- regions, but in the presense of projection effects, exact cor-
sis. Mostimportantly,arelativelystrongshockwithM>4- 5 relationisnotexpected. AsdiscussedbyMV01,theleading
is believed to be necessary for generation of an observable edgeofthe“limb-brightened”(Jones&Saunders1996)halo
halo (e.g., Brunetti 2002). Such high Mach numbersshould inA665coincideswiththeapparentlocationofapossiblerel-
be very rare, as most merger shocks should have M ∼1 at ativelystrongshockaheadofthefast-movingcoolcore. This
the clustercenter. Gabici& Blasi (2003)arguedthatshocks observationappeartoarguefortheshockaccelerationmech-
expected in mergersof clusters with comparablemasses are anism.
too weak to result in significant non-thermal emission (the On the other hand, in A2319, which is at a very similar
expected Mach number increases with the mass ratio of the merger stage to A665, we do not detect a particularly hot,
merging subclusters). Moreover, according to Miniati et al. shock-heatedregion in front of the moving cool core, so its
(2001),suchradioemissionwouldlookmorelikeradiorelics Machnumbershouldbelow. Yet,theclusterexhibitsaradio
at the cluster peripheryratherthan as radiohalos. However, halowhose brightestpartfollowsratherclosely the distribu-
physics of collisionless shocks in clusters is not well under- tionofthecoolergasinthecore(althoughatalowerbright-
stood, and so at present even relatively weak mergershocks ness level, the halo’s NE-SW extension appears to coincide
cannotbecompletelyruledoutasanaccelerationmechanism. withtheoutlyinghotterregions).
Indeed, comparisonof the radio halo and gas temperature Moreover, in A754 and A773, the hottest cluster regions
mapsforafewmergingclusters(MV01,M02a,M02b)hinted donotshowradioemission,atleastatthepresentsensitivity,
attheirspatialcorrelation,whichcouldbeeasilyexplainedif whilethereisradioemissionelsewhereintheseclusters. The
electrons were accelerated in shocks. A currently propagat- same appearstrue for the smallhalo in A2218, althoughthe
ingorjust-passedmergershockwouldleaveclearimprintin sensitivity of the radio data here is poor. While radio emis-
the cluster gastemperaturemaps in the form of hotregions. sionfromcoolerareascanbeexplainedbyprojectioneffects,
Aftertheshockpassage,regionsofshock-heatedgasquickly given that the X-ray-derivedprojected temperatureis biased
expandadiabaticallyandcomeintopressureequilibriumwith towardthe denser, coolergasonthe line of sight, projection
thesurroundings. Large-scalegasmotionsduringthemerger cannot explain the absence of the radio emission from the
subsequently mix gases of different temperature, resulting hottest cluster regions. We believe that this is a strong ob-
in patchytemperaturestructure (as seen in simulations, e.g., servationalargumentagainst merger shocks, at least the rel-
Roettiger, Stone, & Burns(1999),Takizawa 2000,Ricker & ativelyweakonesexpectedinmostmergers,asthemainac-
Sarazin2001)whichpersistsforaconsiderabletimeintheab- celerationmechanism.
sence ofthermalconduction(Markevitchet al. 2003a). Any This leaves turbulence as a more viable mechanism. In-
relativistic electronsacceleratedas the shockpasses through deed,allclustersinoursamplewherethemergergeometryis
thegas,willbepreventedfromdiffusingfarfromtheirorigin tractable—1E0657–56,A520,A2319,andA665—exhibit
bythemagneticfields(thesamefieldsthatsuppressthermal relativelysmall, dense, coolmovingcloudswhich are likely
conductionanddiffusionofthermalelectrons,e.g.,Vikhlinin togenerateturbulenceintheirwake. Theradiohalosareob-
etal.2001;Markevitchetal.2003a),andwillfollowthebulk served along the path of these moving clouds. In A754, the
motionoftheirhostgas. Thus,ifrelativisticelectronsareac- easternhaloliesalongthebrightcoolX-raytongue,whichis
celeratedbymergershocks,andassumingauniformmagnetic likely to be the flow of dense gaspossibly generatingturbu-
field (or, at least, a field uncorrelated with the gas tempera- lenceatitsinterface.InA773,thepassageoftheeasternmass
ture),oneexpectsstrongspatialcorrelationbetweentheradio peakthroughthe gasofthemainsubclustermayalsogener-
halobrightnessandthehottestgasregions(intheabsenceof ateturbulenceinthebrightradioarea.Theremainingclusters
TEMPERATUREMAPSFORRADIOHALOCLUSTERS 7
havesufficientlyuncertainmergergeometriestobeconsistent radio halo emission would be invaluable for pinpointingthe
withthepossibilityofstrongturbulenceintherightregions. electronaccelerationsites (because the halo spectrum steep-
However,withturbulencealone,itisdifficulttoexplainthe ensastherelativisticelectronsloseenergy).Suchdatawould
observedextensionoftheradiohaloin1E0657–56aheadof be especially illuminating in clusters such as 1E0657–56,
thegasbulletallthewaytothebowshock,asimilarextension A665 (Feretti et al., in preparation), and A520, where both
intotheshockregionaheadofthecoreinA665,andpossibly turbulenceandshocksarelikelytobepresentandcanbesepa-
inA520.Turbulencecausedbythesecorescannotprecedethe ratedspatially.Also,fortheseclusters,higher-resolutionhalo
fast-moving core, especially in such a high-velocity merger imagesarerequiredtodetermineifthefrontedgesofthehalos
as 1E0657–56. Thus, in systems like these, at least some indeedcoincidewiththeshockfronts.
of the relativistic electrons should be accelerated in shocks.
Incidentally,1E0657–56hasashockwiththehighest-known
Machnumber(M≈3.5,M02a)andA665appearstocontain FGthanksthehospitalityoftheHarvard-SmithsonianCen-
arelativelyhigh-Mshockaswell(M≈2,MV01). terforAstrophysicswheremostofthisworkwasdone. Sup-
Weconcludethatinmostclusters,theradiohaloelectrons portwasprovidedbyNASAcontractNAS8-39073,Chandra
areprobablyacceleratedbyturbulence,butinthoserarecases grantsGO2-3164XandGO2-3165X,andtheSmithsonianIn-
when shocks with M ≃2- 3 are present, these shocks also stitution. We thank M. Murgia for the use of the Synage++
appear to contribute in the acceleration. In this regard, we program.
notethatKempner&David(2003)performedasimilarradio–
X-ray comparisonfor anotherhalo cluster, A2744, and con- NOTE ADDED IN PROOF: After this paper was submit-
cluded that both turbulence and shock acceleration may be ted, an independentlyderivedChandra temperaturemap for
presentinthatcluster. A2319 has been published by O’Hara, Mohr, & Guerrero
Inconclusion,wenotethatmapsofthespectralindexofthe (2004).Itisingoodagreementwithourmap.
REFERENCES
Abdelsalam,H.M.,Saha,P.,&Williams,L.L.R.,1998,AJ,116,1541 Henriksen,M.J.,&MarkevitchM.L.,1996,ApJ,466,L79
ArnaudM.,HughesJ.P.,FormanW.,JonesC.,Lachieze-ReyM.,Yamashita HenryJ.P.,&BrielU.G.,1995,ApJ,443,L9
K.,&HatsukadeI.,1992,ApJ390,345 HerbigT.,&BirkinshawM.,1994,BAAS,26,1403
Bacchi,M.,Feretti,L.,Giovannini,G.,&Govoni,F.,2003,A&A,400,465 Kaastra,J.S.,1992,“AnX-RaySpectralCodeforOpticallyThinPlasmas”
Beers,T.C.,&Tonry,J.L.,1986,ApJ,300,557 (InternalSRON-LeidenReport,updatedversion2.0)
BirdC.M.,1994,AJ107,1637 Kassim,N.E.,Clarke,T.E.,Enßlin,T.A.,Cohen,A.S.,&Neumann,D.M.,
BlitonM.,RizzaE.,BurnsJ.O.,OwenF.N.,&LedlowM.J.,1998,MNRAS 2001,ApJ,559,785
301,609 Kempner, J.C.,Blanton, E.L.,Clarke, T.E.,Enßlin, T.,Johnston-Hollitt,
Brunetti,G.,Setti,G.,Feretti,L.,&Giovannini,G.,2001,MNRAS,320,365 M.,&Rudnick,L.,2003,Proc.“TheRiddleofCoolingFlowsinGalaxies
Brunetti G., 2002, Proc. “Matter and Energy in Clusters of Galaxies”, andClustersofGalaxies”,eds.T.H.Reiprich,J.C.Kempner,&N.Soker
Taiwan, ASP Conf. Series, eds. S. Bowyer & C.-Y. Hwang, in press (astro-ph/0310263)
(astro-ph/0208074) Kempner,J.C.,&David,L.P.,2003,MNRAS,submitted(astro-ph/0310185)
Buote,D.A.,2001,ApJ,553,L15 KempnerJ.C.,SarazinC.L.,2001,ApJ,548,639
Colafrancesco S.,1999,Proc.RingbergworkshoponDiffuseThermaland Krivonos,R.A.,Vikhlinin, A.A.,Markevitch, M.L.,&Pavlinsky,M.N.,
Relativistic PlasmainGalaxyClusters,ed.H.Böhringer,L.Feretti,&P. 2003,AstronomyLetters,29,425
Schuecker,MPEreport271,269 KneibJ.P.,MellierY.,PelloR.,Miralda-EscudeJ.,LeBorgneJ.-F.,Böhringer
Condon,J.J.,etal.1998,AJ115,1693 H.,&PicatJ.-P.,1995,A&A,303,27
DeGrandiS.,&MolendiS.,2001,ApJ551,153 Kneib,J.P.,Ellis,R.S.,Smail,I.,Couch,W.J.,&Sharples,R.M.1996,ApJ,
DickeyJ.M.,&LockmanF.J.,1990,ARA&A,28,215 471,643
ElbazD.,ArnaudM.,&BöhringerH.,1995,A&A293,337 Liang,H.,Hunstead,R.W.,Birkinshaw,M.,&Andreani,P.,2000,ApJ544,
EnßlinT.A.,1999,inTheUniverseatlowradiofrequencies,Symp.IAU199, 686
Pune(India),inpress,(astro-ph/0001433) Machacek, M.E.,Bautz,M.W.,Canizares, C.,&Garmire,G.P.,2002,ApJ
Faber,S.M. &Dressler,A.,1977,AJ,82,187 567,188
Fabricant D.,Beers T.C.,Geller M.J.,Gorenstein P.,HuchraJ.P.,&Kurtz Markevitch,M.,Yamashita,K.,Furuzawa,A.,&Tawara,Y.,1994,ApJ,436,
M.J.,1986,ApJ308,530 L71
FerettiL.,GiovanniniG.,&BöhringerH.,1997,NewAstronomy,2,501 Markevitch,M.,1996,ApJ,465,L1
FerettiL.,1999,inTheUniverseatlowradiofrequencies,Symp.IAU199, Markevitch,M.,1997,ApJ,483,L17
Pune(India),inpress(astro-ph/0006379) Markevitch,M.,etal.,2000a,ApJ,541,542
FerettiL.,Fusco-FemianoR.,GiovanniniG.,&GovoniF.,2001,A&A,373, Markevitch, M., Vikhlinin, A., Mazzotta, P., & VanSpeybroeck, L.,
106 2000b, Proc. “X-ray astronomy 2000”, Palermo, September 2000, eds.
Feretti L., 2003, XXI Texas Symposium on Relativistic Astrophysics , R. Giacconi, L. Stella, S. Serio, ASP Conf. Ser., 234, CDROM
Florence,Italy(astro-ph/0309221) (astro-ph/0012215)
Fujita,Y.&Sarazin,C.L.,2001,ApJ,563,660 Markevitch,M.,&Vikhlinin,A.,2001,ApJ563,95(MV01)
Fujita,Y.,Takizawa,M.,&Sarazin,C.L.,2003,ApJ,584,190 Markevitch, M., Gonzalez, A. H., David, L., Vikhlinin, A., Murray, S.,
Fusco-Femiano, R.,Orlandini, M.,DeGrandi, S.,Molendi, S.,Feretti, L., Forman,W.,Jones,C.,&Tucker,W.,2002a,ApJ567,L27(M02a)
Giovannini,G.,Bacchi,M.,&Govoni,F.,2003,A&A398,441 Markevitch,M.,Vikhlinin,A.,&Forman,W.R.,2002b,Proc.“Matterand
Gabici,S.&Blasi,P.,2003,ApJ,583,695 EnergyinClustersofGalaxies”,Taiwan,ASPConf.Series,eds.S.Bowyer
Geller,M.J.,&Beers,T.C.,1982,PASP,94,421 &C.-Y.Hwang,301,37(M02b),(astro-ph/0208208)
Giovannini,G.,Feretti,L.,&Govoni,F.,1999a,inTheUniverseatlowradio Markevitch,M.,etal.,2003a,ApJ,586,L19
frequencies,Symp.IAU199,Pune,inpress,(astro-ph/0006380) Markevitch,M.,etal.,2003b,ApJ583,70
Giovannini,G.,Tordi,M.,&Feretti,L.,1999b,NewAst.4,141 Miniati,F.,Jones,T.W.,Kang,H.,&Ryu,D.2001,ApJ,562,233
Giovannini,G.,&Feretti,L.,2000,NewAstronomy,5,335 Moffet,A.T.&Birkinshaw,M.,1989,AJ,98,1148
Gómez,P.L.,Hughes,J.P.,&Birkinshaw,M.,2000,ApJ,540,726 Oegerle,W.R.,Hill,J.M.,&Fitchett,M.J.,1995,AJ,110,32
Govoni, F., Feretti, L., Giovannini, G., Böhringer, H., Reiprich, T.H., & O’Hara, T. B., Mohr, J. J., & Guerrero, M. A. 2004, ApJ, submitted
Murgia,M.,2001a,A&A,376,803 (astro-ph/0311402)
Govoni,F.,Enßlin,T.A.,Feretti,L.,&Giovannini,G.,2001b,A&A,369, Ohno,H.,Takizawa,M.,&Shibata,S.2002,ApJ,577,658
441 Petrosian,V.,2002,Proc.“MatterandEnergyinClustersofGalaxies”,ASP
Harris,D.E.&Miley,G.K.,1978,A&AS,34,117 Conf.Series,eds.S.Bowyer&C.-Y.Hwang,inpress,(astro-ph/0207481)
Harris,D.E.,Kapahi,V.K.,&Ekers,R.D.,1980,A&AS,39,215 Pierre,M.,&Starck,J.L.,1998,A&A330,801
8 GOVONIETAL.
Plucinsky,P.P.,etal.,2003,Proc.SPIE,4851,89 SquiresG.,etal.,1996,ApJ,461,572
Proust,D.,etal.,2000,A&A355,443 Struble,M.F.,&Rood,H.,1999,ApJ,125,35
Jones,M.,&Saunders,R.,1996,inRöntgenstrahlungfromtheUniverse,ed. Sunyaev,R.A.,Norman,M.L.,&Bryan,G.L.,2003,AstronomyLetters,in
H.U.Zimmermannetal.(MPEReport263),553 press,(astro-ph/0310041)
Ricker,P.M.&Sarazin,C.L.,2001,ApJ,561,621 Takizawa,M.,2000,ApJ,532,183
Rizza,E.,Burns,J.O.,Ledlow,M.J.,Owen,F.N.,Voges,W.,&Bliton,M., Trèvese,D.,Cirimele,G.,&DeSimone,M.,2000,ApJ,530,680
1998,MNRAS,301,328 Tucker,W.,etal.,1998,ApJ,496,L5
Roettiger,K.,Stone,J.M.,&Mushotzky,R.F.,1998,ApJ493,62 Vikhlinin, A., 2000, Chandra calibration memo,
Roettiger,K.,Stone,J.M.,&Burns,J.O.1999,ApJ,518,594 http://asc.harvard.edu/cal/Links/Acis/acis/Cal- prods/qe/12- 01- 00
Sarazin,C.L.,1999,ApJ,520,529 Vikhlinin, A., 2001, Chandra calibration memo,
Sarazin,C.L.,2001,in“MergingProcessesinClustersofGalaxies”,edited http://asc.harvard.edu/cal/,”ACIS”,“ACISBackground”,“VFmode”
byL.Feretti,I.M.Gioia,andG.Giovannini,inpress,(astro-ph/0105418) Vikhlinin,A.,Markevitch,M.,&Murray,S.S.,2001,ApJ,551,160
Schlickeiser,R.,Sievers,A.,&Thiemann,H.,1987,A&A,182,21 Zabludoff,A.I.,&Zaritsky,D.,1995,ApJ,447,L21
Schindler,S.,&Müller,E.,1993,A&A,272,137
Slezak,E.,Durret,F.,&Gerbal,D.,1994,AJ,108,1996
Snowden,S.L.,etal.,1997,ApJ,485,125
TEMPERATUREMAPSFORRADIOHALOCLUSTERS 9
FIG.1.—A520.(a):X-raycontoursoverlaidontheopticalDSSimage.The0.8- 4keVX-rayimageisadaptivelysmoothed;contoursarespacedbyafactor
of√2.(b):X-raycontourplotoverlaidonthetemperaturemap(colors).(c):Theisocontourmapat1.4GHzofthecentralregionofA520overlaidontheX-ray
image(colors).TheradioimagehasaFWHMof15′′ 15′′.Thecontourlevelsare:0.06,0.12,0.24,0.48,0.96,1.92,3.84,7.68mJy/beam.(d):Radiocontours
overlaidonthetemperaturemap(colors);crossesmar×ksomeradiosourcesunrelatedtothehaloemission.
10 GOVONIETAL.
FIG.2.—A665.(a):X-raycontoursoverlaidontheopticalDSSimage.The0.8- 4keVX-rayimageisadaptivelysmoothed;contoursarespacedbyafactor
of√2.(b):X-raycontourplotoverlaidonthetemperaturemap(colors).(c):Theisocontourmapat1.4GHzofthecentralregionofA665overlaidontheX-ray
image(colors). TheradioimagehasaFWHMof52′′ 42′′. Thecontourlevelsare: 0.20.40.81.5361225mJy/beam. (d):Radiocontoursoverlaidonthe
temperaturemap(colors);crossesmarksomeradiosou×rcesunrelatedtothehaloemission.