Table Of ContentACCEPTEDFORPUBLICATIONAPJL
PreprinttypesetusingLATEXstyleemulateapjv.03/07/07
DORADIOCORE-HALOSANDCOLDFRONTSINNONMAJORMERGINGCLUSTERSORIGINATEFROMTHE
SAMEGASSLOSHING?
PASQUALEMAZZOTTA1,2ANDSIMONAGIACINTUCCI2,3
AcceptedforpublicationApJL
ABSTRACT
Weshowaninterestingcorrelationbetweenthesurfacebrightnessandtemperaturestructureoftherelaxed
clustersRXJ1720.1+2638 andMS1455.0+2232,hostingapairofcoldfronts,andtheircentralcore–haloradio
source. Wediscussthepossibilitythattheoriginofthisdiffuseradioemissionmaybestrictlyconnectedwith
8
thegassloshingmechanismsuggestedtoexplaintheformationofcoldfrontsinnonmajormergingclusters.
0
0 We show that the radiative lifetime of the relativistic electrons is much shorter than the timescale on which
2 they can be transported from the central galaxy up to the radius of the outermost cold front. This strongly
indicatesthattheobserveddiffuseradioemissionislikelyproducedbyelectronsre–acceleratedviasomekind
n
ofturbulencegeneratedwithintheclustervolumelimitedbythecoldfrontsduringthegassloshing.
a
J Subject headings: galaxies: clusters: general — galaxies: clusters: individual (RXJ1720.1+2638,
MS1455.0+2232)—X-rays:galaxies—coolingflows
2
1
] 1. INTRODUCTION metallicityappropriateforeachcluster(NH=3.6×1020cm- 2;
h A number of cool core clusters of galaxies host very pe- Z = 0.39 Z⊙, and NH = 3.0×1020 cm- 2; Z = 0.5 Z⊙, for
p RXJ1720.1+2638 and MS1455.0+2232, respectively). The
culiar diffuse radio sources at their center characterized by
- temperature maps were produced using a radially increas-
o a core–halo (CH herafter) structure (e.g. PKS0745–191;
ing Gaussian smoothing of σ = 1 pixel in the center up to
r Baum&O’Dea 1991). These objects show an amorphous
t σ=10pixelsinthemoreexternalregions(seeVikhlininetal.
s steepspectrumhalowhichsurroundsabrightcorecoincident
2001fordetails.)
a withthedominantgalaxy,withnojetsonthekpcscale. Even
[ thoughtheconnectionbetweenCHsandcoolcoreclustersis
3. X-RAYIMAGEANDTEMPERATUREMAP
well established, the origin of these sources remains uncer-
1
v tain. Sarazinetal.(1995)arguedthattheymightbeproduced BothRXJ1720.1+2638 andMS1455.0+2232 wereknown
5 by the interaction between the radio source and the thermal to host two sharp variations of the surface brightness
0 gas cooling at the cluster center, i.e. in the form of disrup- (edges) on opposite side with respect to the cluster center
9 tion of the jets by the high–pressure ambient medium and (Mazzottaetal.2001bandMazzottaetal.2001a).Itwassug-
1 buoyant effects on the radio plasma from the disrupted jets. gested that such features were CFs but no final conclusion
. In this Letter we show an interesting spatial correlation be- was possibledue to the limited statistics available. The new
1
tweentheCHemissionandX-rayfeaturesintwogalaxyclus- deeperobservationsallowustodisentanglethenatureofthe
0
8 tersRXJ1720.1+2638 (z=0.159)andMS1455.0+2232 (z= observededges.Tobetterhighlightthespatialstructureofthe
0 0.258), both showing a pair of cold fronts (CF hereafter) in edges,wecreatedaratioimageforbothclustersbydividing
: theircore. Wesuggestaconnectionbetweentheoriginofthe thephotonimageinthe[0.5,2.5]keVenergybandbyitsra-
v CHstructureandthesamegassloshingmechanismresponsi- dialmeanvaluessimplyobtainedfromtheradialprofilecen-
Xi blefortheformationoftheCFs. WeuseH =70kms- 1kpc- 1, tered in the X-ray peak. The resulting image was smoothed
0
Ω=0.27,andΛ=0.73. usingatop–hatfunctionwithr=3pixels.Theratioimageand
r
projectedtemperaturemapofRXJ1720.1+2638 areshownin
a
2. X-RAYDATAPREPARATIONANDANALYSIS theleftand rightpanelsof Fig. 1, respectively. Overlaidare
ForthepurposeofthisLetterweprovideashortsummary the1.5GHzradiocontours(fromVLA–Barchivedata)ofthe
of the X-ray data preparationand analysis. We refer to two centralCH.IntheleftandrightpanelsofFig.2weshowthe
companionpapersfordetails(Mazzottaetal. inpreparation). same images for MS1455.0+2232, with overlaid the GMRT
TheanalysisofRXJ1720.1+2638 wasperformedcombining 610MHzcontours. Figs.1and2clearlyshowthattheedges
3 Chandra observations (OBS_ID=1453, 3224, and 4631). appearaspartofamoreextendedspiral–likeexcessstructure
A single Chandra observation (OBS_ID=4192) was used at the center of both clusters. They seem to confine the CH
for MS1455.0+2232. After flare cleaning, the useful expo- radioemission. Furthermore,theradiostructureappearsspa-
sure time is 42508s and 88556s for RXJ1720.1+2638 and tially correlated with the X–ray excess, following the spiral
MS1455.0+2232, respectively. All images are background patterninitsmoreexternalregions.
subtractedandvignettingcorrected.Thespectralanalysiswas
carriedoutinthe0.7–9keVenergybandinPIchannels. The 4. COLDFRONTS
spectralfittingwasperformedassumingfixedabsorptionand InthisSectionwedemonstratethatalltheedgesindicated
by blue lines in Figs. 1 and 2 are CFs. We provide a brief
1Dipartimento di Fisica Universitá di Roma “Tor Vergata”, Via della descriptionofthesefeatures,andrefertotheforthcomingpa-
ricercascientifica1,I-00133Roma,Italy;[email protected] pers for a detailed analysis and discussion. For each edge
2Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cam-
we extracted the surface brightness profile within the rela-
bridge,MA02138
3INAF-IstitutodiRadioastronomia,viaGobetti101,I-40129,Bologna, tive sector. The centers and position angles for each sector
Italy;[email protected] arereportedin Tab.1. In Figs. 3 and4 we show the surface
2 MAZZOTTA&GIACINTUCCI
FIG.1.—Leftpanel–RatioChandra X–rayimageofRXJ1720.1+2638 inthe[0.5,2.5]keVbandoverlaidtothe1.5GHzVLA–Bcontours.TheX-rayratio
imageisobtainedbydividingtheactualclusterimagebyitsradialmeanvaluesandbysmoothingtheresultusingatophatfunctionwithr=3pixels.Eachpixel
correspondsto4′′. BluelinesindicatethepositionoftheCFs. Radiocontoursarelogarithmicallyspacedbyafactor2from0.10mJyb- 1. Theradiobeamis
5.7′′×4.8′′.Rightpanel–X-rayprojectedtemperaturemapofRXJ1720.1+2638 inKeV.Contoursandlinesasintheleftpanel.
FIG. 2.—Leftpanel–RatioChandra X-rayimageofMS1455.0+2232 inthe[0.5,2.5]keVbandoverlaidtotheGMRT610MHzcontours. TheX-rayratio
imageisobtainedbydividingtheactualclusterimagebyitsradialmeanvaluesandbysmoothingtheresultusingatophatfunctionwithr=3pixels.Eachpixel
correspondsto1′′. BluelinesindicatethepositionoftheCFs. Radiocontoursarelogarithmicallyspacedbyafactor2from0.15mJyb- 1. Theradiobeamis
5.9′′×4.6′′.Rightpanel–X-rayprojectedtemperaturemapofMS1455.0+2232 inKeV.Contoursandbluelinesasintheleftpanel.
brightnessandprojectedtemperatureprofilesofthetwosec- and
torsofRXJ1720.1+2638 andMS1455.0+2232,respectively.
1 r<r
Wemodeledtheedgeusingasimpleanalyticfunctionforthe jump
ceolencttirnounitdye(nasijtuym(pn)e)aatnthde3fDrotnetm: perature(T),bothwithadis- T =T0(DT 1+1(+rj(urm/pr/Tr)T2)2 c r>rjump. (2)
h i
Assuming spherical symmetry we projected both density
and temperature models, and fitted them simultaneously to
theobservedsurfacebrightnessandtemperatureprofiles. For
ne=n0 Dn((rr//rrjjuummpp))αα21 rr><rrjjuummpp, (1) tihneititoenmTpselriantturroedpurcoejdecbtyioMnwazezuostteadetthaels.p(2ec0t0r4o)s.copiclikedef-
(cid:26)
Coldfrontsinclustersandradiocore-halos 3
highlysubsonic.
FIG. 5.—Electrondensity(ne),temperature(T)andthermalpressure(P)
profilesofthesectorscontainingtheSouth-East(left)andNorth-West(right)
FIG.3.—Projectedsurfacebrightnessandtemperatureprofilesoftheclus-
CFsofRXJ1720.1+2638.Theshadowregionsindicatetherelative68%con-
ter sectors containing the South-East (left) and North-West (right) CFs of
fidencelevelerrors.
RXJ1720.1+2638. Verticaldottedlinesindicatethepositionsoftheedges.
Upperpanels) Surface brightness profiles. Thecontinuous histograms are
theprojected bestfitmodelsshownintheupperpanels ofFig.5. Middle
panels)Residualsofthesurfacebrightnessprofilewithrespecttothebestfit
modelinsigmas.Lowerpanels)ProjectedTemperatureprofiles.Thecontin-
ueshistogramsaretheprojectedbestfitmodelsshowninthemiddlepanels
ofFig5
FIG. 6.—SameasinFig.5butfortheNorth-East(left)andSouth-West
(right)CFsofMS1455.0+2232.
5. DISCUSSIONANDCONCLUSIONS
Using high resolution simulations differetauthorsshowed
thatCFsinthecoreofrelaxedgalaxyclustersmaybethere-
FIG. 4.—SameasinFig.3butfortheNorth-East(left)andSouth-West sultof gassloshinginducedby a minormergerwith a small
(right)CFsofMS1455.0+2232. dark matter halo occurred within the last few Gyrs (see e.g
Ascasibar&Markevitch2006–AMhereafter–andreferences
therein). The only condition for the CF to form is that the
If Figs. 5 and 6 we show the best fit within the 68% con- cluster has a steep entropy profile (as observed in cool core
fidencelevelerrorcurvesrelativeto thedensity,temperature clusters) and the dark matter sub–halo has a non–zero im-
andpressureprofilesofeachedge.Wealsoreportassolidline pact parameter. When the sub–halo flies through the main
thebestfitmodelsofthe surfacebrightnessandtemperature cluster,itproducesagravitationaldisturbancethatpushesthe
profiles to highlightthe good agreementbetween the fit and cool gas away from its initial position. This makes the gas
thedata. Furthermore,themiddlepanelsofthesamefigures sloshandtwoormoreCFsformonoppositesidewithrespect
show the departuresin sigmasof the surface brightnessdata to the cluster center. AM noticed that, when the gas is dis-
with respect to the best fit. All the observededges are CFs. placedfromthecenterforthefirsttime,itdoesnotfallback
We notice that the pressure profile is continuous, within the radially, and the subsequent CF(s) is (are) not exactly con-
errors, acrossthe front(Tab.1), so the speedof the frontsis centric but combines into a spiral pattern (see 1.9–2.1 Gyrs
4 MAZZOTTA&GIACINTUCCI
TABLE1
PARAMETERSOFTHECOLDFRONTS
Sector x,y Positionangle rjump Dn DT rT c
(J2000) (deg)† (Mpc) (Mpc)
RXJ1720.1+2638 N (17:20:10,+26:37:25) 2–80 0.0994±0.0005 1.68±0.03 1.68±0.12 0.1 0
RXJ1720.1+2638 S (17:20:10,+26:37:25) 180–280 0.1551±0.0007 1.66±0.05 1.49±0.16 0.1 0
MS1455.0+2232 N (14:57:15,+22:20:35) 46–163 0.1367±0.002 1.67±0.06 1.72±0.24 1.9±0.3 2.6±0.27
MS1455.0+2232 S (14:57:15,+22:20:30) 270–330 0.0397±0.003 1.50±0.08 2.50±0.30 0.23±0.04 0.5±0.10
†PositionanglesaremeasuredanticlockwisefromWest
panelsinFig.7 ofAM).Asshownin§ 3, boththeCF pairs time is much larger than the radiative lifetime of relativis-
inRXJ1720.1+2638 andMS1455.0+2232,whichareclearly tic electrons which suffer energy losses mainly due to In-
twodistinctnonconcentricstructures,seemtocombineintoa verseComptonwiththecosmicmicrowavebackgroundpho-
spiralpatternverysimilarlytowhatpredictedbyAM.These tons(t≈107- 8/(1+z)4yr;seee.g. Brunettietal.2001). This
structuresareremarkablysimilartothesimulatedonesinthe simple calculationsuggeststhatthe observedradio emission
surface brightness map (compare left panels of Figs. 1 and mightbeproducedbyre–accelerationofapopulationofrelic
2 with upper–left panel of Fig. 19 of AM) as well as in the electrons,injectedintotheICMbyapreviousactivityofthe
temperaturemap(comparerightpanelsofFigs.1and2with centralAGN.Duetothesubsonicnatureofthegassloshing,
lower–leftpanelofFig.19ofAM).ThustheX-rayobserva- wecanexcludethatthere–accelerationmaybedrivenbyther-
tionsofthesetwoclustersseemstoconfirmthepredictionof malshock. Analternativeandplausiblemechanismsmaybe
AMforthegassloshing. the re–accelerationby MHD turbulence, as suggested to ex-
Whatappeartobeveryinterestingisthatbothclustershost plaintheclustermini–halos(Gittietal.2002)andgiantradio
a CH radio source. These sources seem to be well confined halos(e.g. Brunettietal.2004). Theseauthorsclaimthat,to
(in projection) within the CF pairs of the respective cluster. be effective, this re–acceleration mechanism requires only a
Furthermore,wenoticedaspatialcorrelationbetweenthera- modestlevel(few percent) of turbulence. In fact, 2D simu-
dioemissionandtheX-rayspiralstructurewhichmightsug- lationswithmuchhigherresolutionshaveshownthatthegas
gestthattheradioemittingelectronsaretrappedintothesame sloshingcouldautomaticallycreateturbulenceinacore(e.g.
ICM flows that produced the CFs. Given the lack of jets Fujitaetal.2004).
on the kpc scale, the relativistic electrons need to be trans- It is worth noting that the CH/CFs correlation described
ported by some other mechanism from the central galaxy in this Letter is not limited to these two clusters but
up to the radii of the outermost CFs. We might speculate seems to be present also in other clusters hosting a CH,
that the relativistic electrons injected by the central AGN, as e.g. 2A0335+096 (Mazzottaetal. 2003), PKS0745–191
and trapped into the ICM magnetic field, are transported at (Baum&O’Dea1991),andA2052(Zhaoetal.1993). These
the CFs radii by the motion of the low entropy thermal gas resultscallsforfurtheranddeeperinvestigationoftheradio–
induced by the gas sloshing. In fact, the AM simulations X-rayconnectioninrelaxedclusterswithCFs,whichwillbe
show that the CFs are formed by the continuous circulation addressedinforthcomingpapers.
of lower entropy gas that comes from the inner core region
and falls back soon after (see Fig 7 of AM). However, AM
show that the speed of this gas circulation is relatively low
with an upperlimitof v =500km/s. In bothclusters the We thank M. Markevitch, G. Brunetti, T. Venturi, and A.
max
outermost CF is at a radial distance of ∆r ≈150 kpc. As- Vikhlininforconstructivecriticism. Thisworkwassupported
suming radial transport, we estimate that the minimumtime by contract ASI-INAF I/023/05/0,CXC grants AR6-7015X,
neededfortherelativisticelectronstobe displacedupto the GO5-6124X,NASAgrantNNG04GK72GandbytheSmith-
position of the outermost front is t ≥ 3×108yr. This sonianInstitution.
min
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