Table Of ContentAstronomy&Astrophysicsmanuscriptno.ms4 (cid:13)c ESO2010
January27,2010
LettertotheEditor
The diffuse radio filament
in the merging system ZwCl 2341.1+0000
G.Giovannini1,2,A.Bonafede1,2,L.Feretti2,F.Govoni3,M.Murgia3
(1)DipartimentodiAstronomia,viaRanzani1,40127Bologna,I
(2)IstitutodiRadioastronomia-INAF,viaP.Gobetti101,40129Bologna,I
(3)OsservatorioAstronomicodiCagliari-INAF,Strada54,Loc.PoggiodeiPini,09012Capoterra(Ca),I
0
1 October14,2009
0
2 ABSTRACT
n Context.Insomeclustersofgalaxies,adiffusenon-thermalemissionispresent,notobviouslyassociatedwithanyindividualgalaxy.
a Thesesourceshavebeenidentifiedasrelics,mini-halos,andhalosaccordingtotheirpropertiesandpositionwithrespecttothecluster
J center.Moreoverinafewcaseshavebeenreportedtheexistenceofadiffuseradioemissionnotidentifiedwithacluster,butwitha
7 largescalefilamentaryregion.
2 Aims.Theaimofthisworkistoobserveanddiscussthediffuseradioemissionpresentinthecomplexmergingstructureofgalaxies
ZwCl2341.1+0000.
] Methods.WehaveobtainedVLAobservationsat1.4GHztoderiveadeepradioimageofthediffuseemission.
O
Results.LowresolutionVLAimagesshowadiffuseradioemissionassociatedtothecomplexmergingregionwithalargestsize=
C 2.2Mpc. Inaddition tothepreviously reportedperipheral radioemission, classifiedasadouble relic,diffuseemissionisdetected
alongtheopticalfilamentofgalaxies.
.
h Conclusions.Thegiantradiosourcediscussedhereshowsthatmagneticfieldsandrelativisticparticlesarepresentalsoinfilamentary
p structures.Possiblealternatescenariosare:agiantradiohaloinbetweentwosymmetricrelics,orthemergingoftwoclustersboth
- hostingacentralradiohalo.
o
r Key words. galaxies:cluster:non-thermal – Clusters: individual: ZwCl 2341.1+0000 – Cosmology: large-scale structure of the
t
s Universe
a
[
1 1. Introduction presenceoflargescaleshocksoriginatingintheaccretionflows
v of intergalactic gas, inferred from the Mpc scale diffuse ra-
Diffusenon-thermalradiosourceswithsteepspectrahavebeen
0 dio emission. vanWeerenetal.2009b presented GMRT obser-
8 foundinarelativelylargenumberofclustersofgalaxies(seee.g. vations at 610 MHz, 241 MHz and 157 MHz of this region,
9 Giovanninietal.2009 and references there in). These sources combinedwith X-ray and optical data. The radio images show
4 arenotdirectlyassociatedwiththeactivityofindividualgalax- twodiffusesourcestothenorthandsouthoftheclusterposition
. ies and are related to physical properties of the whole cluster.
1 whichtheyclassifiedasdoubleradiorelics.Theserelicsareper-
According to their properties and location they are commonly
0 pendiculartotheX-rayaxiswhichcanbeconsideredthemerger
classifiedasRelicsandHalos.
0 axis.Theyaresuggestedtobeduetooutwardtravellingshocks
1 Relics have been found at the cluster peripheries and the causedbya majormergerevent.The distancebetweenthe two
: mostcommoninterpretationisthattheyarerelatedtothepres- relicsis∼2.2Mpc.
v
enceofshocksoriginatedbyclustermergers.Halosarelocated
i WepresentherenewlowresolutionVLAimagesofthedif-
X inthecentralclusterregionsandhavealwaysbeenfoundinclus-
fuseradioemissionat1.4GHz.Despiteofthehigherfrequency,
r terswithevidenceofmergeractivity,whichplaysanimportant the better sensitivity to surface brightness allowed us to detect
a roleinparticlere-accelerationprovidingtheenergythatpowers veryextendeddiffuseradioemission,whichspansalongtheop-
thesesources(Brunettietal.2009,Feretti&Giovannini2008).
ticalfilament.Wediscussitsnatureandproperties.
In addition some evidence has been found of the existence The intrinsic parametersquotedin this paperare computed
of non thermal emission on even larger scales. Bridges of ra- foraΛCDMcosmologywithH =71kms−1Mpc−1,Ω =0.27,
0 m
dio emission have been observed in the regionsbetween relics andΩ =0.73.Atz=0.27theangularconversionfactoris4.1
Λ
and halos in a few clusters, including Coma (Kimetal.1989, kpc/”.
Giovanninietal.1990), A2255(Ferettietal.1997), andA2744
(Govonietal.2001). Diffuse emission have been found at
large distance from A2255 (Pizzoetal.2008), and A2256
2. Radioimages
(vanWeerenetal.2009a).
A convincing evidence of radio emission from a filament The ZwCl2341.1+0000 region was observed for 6 hrs with
structure was reported by Bagchietal.2002, identified with the VLA at 1.4 GHz in the D configuration on July 24 2008,
the multi-Mpc scale filamentary network of galaxies in the at two differentpointings:RA=23h43m40sDEC=+00◦20’00”,
ZwCl2341.1+0000region at z = 0.27. The authors discuss the andRA=23h43m50sDEC=+00◦14’00”,to avoidhigh primary
2 Giovanninietal.:Thediffuseradiofilament
1.5,expectedfromextra-galacticradiosources(forthestrongest
subtractedsource(A)α1.4 =1.2).Cleancomponentsweresub-
0.6
tractedfromtheuvdata.Withthenewdatasetsweproducedlow
resolutionimagestoenhancethelowbrightnessextendedemis-
25:00.0 sion.Finalimageswerecombinedandcorrectedfortheprimary
beam attenuation. The final combined, and primary beam cor-
rectedimageisshownintherightpanelofFig.2.Inthisimage,
theradioemissioniscontinuous,andthegapbetweenthenorth-
0:20:00.0 ern and southern regionsvisible in Fig. 1 is no longer present,
on clearlybecauseofapositivenoiseinthisarea,enhancedbythe
nati slightlylargerbeam.
cli
e
D
15:00.0 2.2.Polarizedemission
Weinvestigatedthepresenceofpolarizedemissionfromthera-
diodiffuseemission.We producedStokesQ andU lowresolu-
tion images. The discrete sourceswere subtracted as explained
10:00.0
aboveforthetotalintensityimage.We derivedthepolarization
angleimageandthepolarizationintensityimagewithoutimpos-
10.0 23:44:00.0 50.0 40.0 30.0 20.0 43:10.0 inganycut.Fromthepolarizationintensityimagewederivedthe
Right ascension
fractionalpolarizationimage,andweconsideredasvalidpixels
those whose signal-to-noise ratio was >3. This cut is done in
Fig.1.RadioemissionfromZwCl2341.1+0000at1.4GHzob-
orderto getrid of possiblespuriouspolarization.The resulting
servedatfullresolution.TheHPBWis50′′×43′′.Thefirstcon-
imageisshowninFig.3.Wecangatherthatthereisadetection
tour is drawn at 3σ level (0.12 mJy/beam), other contours are
ofpolarizationinthenorthernandsoutherncomponents,aswell
spacedby a factor2. The first negativecontour(at3σ level)is
asinthecentralregion.
incyan.
3. Results
beam attenuation and bandwidth smearing. Each field was ob-
served for 3 hrs, and we moved the pointing center every half In the final image, obtained after the subtraction of discrete
anhourtoobtainabetteruv-coverage.Calibrationandimaging sources, an extended emission is detected, which is consistent
wereperformedwiththeNRAOAstronomicalImageProcessing with the result of Bagchietal.2002. Owing to the better sen-
System(AIPS).Thesources3C48,3C138and2316+040were sitivity of our image, the diffuse source is well defined. The
used as primary flux-density calibrator, absolute reference for radio morphology is elongated, clearly following the distribu-
the electric vector polarization angle and phase calibrator re- tion of the optical galaxies and of the X-ray emission, shown
spectively.Severalcyclesofself-calibrationwereappliedinor- by vanWeerenetal.2009b. We note that the regions of high-
dertoremoveresidualphaseandgainvariations.Imagesofthe est brightness are coincident with the two relics presented by
total intensity (Stokes I) as well as of the Stokes parameters vanWeerenetal.2009b. In the central region, we detect a low
U and Q were produced for the two pointings separately fol- surfacebrightnessemission,atthelevelof0.4– 0.6mJy/beam
lowingthecommonprocedures:Fourier-Transform,Clean,and (corresponding to ∼ 7 10−5 mJy/arcsec2), which could be de-
Restore. The images resulting from the two pointings were fi- tected at 610 MHz with a 15” beam only if the spectral index
nally combined together using the AIPS task LTESS. We then α1.4issteeperthan2.5-3.
0.6
derived images of the polarized intensity P = p(U2+Q2), The total size of the diffuse emission is ∼ 2.2 Mpc. The
corrected for the positive bias, and of the polarization angle measured total flux at 1.4 Ghz is 28.5 mJy, correspondingto a
Ψ=0.5arctan(U/Q).Calibrationerrorsareoftheorderof5%. radio power log P = 23.66 W/Hz. The radio emission is ir-
1.4
regularanddecreasesfromthe two brightouterregionstoward
the cluster center. A plateau of radio emission, at the level of
2.1.Totalintensityradioemission
about 0.6 mJy/beam is present at the location of the southern-
InFig.1weshowthefinalimageobtainedwithnaturalweights. mostX-raypeak,detectedbyChandraandpublishedinFig1of
Thediffuseemissionis easily visibleevenif a largenumberof vanWeerenetal.2009b.
discrete sourcesare present.To obtain the image of the diffuse Wedetectpolarizedemissionfromlargeareasofthediffuse
source, and measure its flux, we subtracted unrelated discrete radio source, both from the outer and the central regions. The
sourcesintheuvplane.Tothisaim,weproducedhighresolution polarized flux is more prominentin the eastern side of the ex-
imageswithuniformweight(ROBUST=-5),aftercuttingbase- tendedsource. Once the discrete sourceshave beensubtracted,
linesshorterthan2Kλ.Intheseimagesonlydiscretesourcesare thetotalpolarizedfluxis∼2.4mJy.Thepolarizedpercentagein
present, whereas the extended diffuse emission is resolved out thenorthernandinthesouthernbrightregionsis∼15%and8%
(seeFig.2,leftpanel). respectively, while the mean fractional polarization in the cen-
Discretesourcespresentinourimagesareinagreementwith tral region is ∼11%. The polarization vectors are very regular
the list of unrelated sources found by vanWeerenetal.2009b andorientedtowardtheNE-SWdirectioninthenorthernsource
using GMRT data at lower frequencies, but at higher resolu- region.In the otherregionsthey follow the eastern edge of the
tion. A comparisonbetween GMRT flux densities at 610 MHz totalintensityemissionstillshowingsomelevelofordering.
and at 1.4 GHz, taking into accountthe differentangular reso- From present images and following the detailed discussion
lutionanduv-coverage,givesspectralindexesintherange0.8- byBagchietal.2002,andvanWeerenetal.2009b,weruledout
Giovanninietal.:Thediffuseradiofilament 3
D 25:00.0
25:00.0
B+C
E+F
0:20:00.0
Declination 0:20:00.0 M G - K A Declination
15:00.0
15:00.0
L
10:00.0
10:00.0
20.0 10.0 23:44:00.0 50.0 40.0 30.0 20.0 43:10.0 30.0 20.0 10.0 23:44:00.0 50.0 40.0 30.0 20.0 43:10.0
Right ascension Right ascension
Fig.2. Left: The large scale galaxydistribution aroundZwCl 2341.1+0000is shown in colors. The opticalimage is from GSCII
(Laskeretal.2008).Contoursoftheradioemissionfromdiscretesources(seetext)areoverlaid.TheHPBWis39′′×35′′.Thefirst
contour is drawn at 3σ level (0.15 mJy/beam). Following contoursare spaced by a factor of 2. The first negative contour at 3σ
level is displayed in cyan. Labels refer to the sources found by vanWeerenetal.2009b. Right: Colors and contoursrefer to the
radioemission after the subtractionof the discrete sources. The HPBW is 83′′×75′′. The first contouris drawnat 3σlevel(0.15
mJy/beam).Followingcontoursarespacedbyafactorof2.
00 28 4. Discussion
26 Radio emission of very low brighthness is detected in the
ZwCl2341.1+0000 complex, owing to the high sensitivity to
24
surface brightness of the present observations. The data re-
22 ported in this paper indicate the existence of diffuse radio
DECLINATION (J2000) 211086 eBasumtaggtighscseehisoitoeneudtataetllhor.en2sg0oe0xuth2irsce,teeinwnbcahoedoudloneiftdiooatpwrnytoit,coarwtelhlhfiieeclbrsae.rmiTgvehhantent,rWalaodsweioefirarreesnntggieruoetlnpaasrol.rlrot2eec0sdao0tlb9euydb-
tionofthepresentdatadoesnotallowustoseparatethetheradio
14 emissionofthesefeaturesfromtherestofthediffuseemission.
12 The most obvious interpretation is, as suggested by
Bagchietal.2002,thatthisregioniswitnessingtheprocessofa
10
largescalestructureformation,wherecosmicshocksoriginated
08 by a complex multiple merger are able to accelerate particles
23 44 15 00 43 45 30 15 and amplify seed magnetic fields. The large-scale radio emis-
RIGHT ASCENSION (J2000)
sionwouldoriginatefromlarge-scaleshocksandpossiblyturbu-
Fig.3.PolarizedemissionfromZwCl2341.1+0000at1.4GHz.
lence,andtheidentificationofsingleradiosourcecomponentsas
Contoursshowthetotal-intensityemissionatlowresolution(see
relics,wouldbeproblematic.Theexistenceofsignificantpolar-
Fig.2.LinesrefertotheEvectorsTheirorientationrepresentthe
izedemissionimagecanbeeasilyexplainedinthisscenario.The
projected E-field not corrected for the galactic rotation. Their
shocks connected to large scale filaments are indeed expected
length is proportional to the fractional polarization: 1′′ corre-
tocompressthemagneticfield,allowingthedetectionofpolar-
spondsto0.2%.
izedemissionalsoatsuchlowresolution.Furthermore,wenote
that the lower gas density of filaments with respect to clusters
would cause less depolarization, and internal Faraday rotation,
thatcouldexplaintheorderedfeaturesrevealedbythepolariza-
tion image. The high fractionalpolarization in the eastern side
the identification of this extended emission as a giant radio could be related to the direction and dynamics of the merging
galaxy,and we comparedits propertieswith those of radio ha- process.
lospresentedin Giovanninietal.2009.The2.2Mpcsize isre- Analternatepossibilitycouldbethattheextendedemission
markablyhigh,butitiscomparabletothatoftheradiohalosin inthisclusteractuallyconsistsoftwoperipheralrelicsandacen-
A2163and1E0657-56.Onthecontrary,thetotalradiopowerat tralradiohalo,asintheclustersA1758(Giovanninietal.2009)
1.4GHzisconsiderablylowerthantheexpectedvalueforagi- and RXCJ1314.4-2515(Ferettietal.2005, Venturietal.2007).
antradiohalofromthecorrelationbetweenhalo’ssizeandradio In this scenario, shocks originated in the merging processes
powershowninGiovanninietal.2009(andreferencestherein). of this complex structure could give origin to the peripheral
4 Giovanninietal.:Thediffuseradiofilament
relics, while a large scale turbulence at the cluster center as does notallow us to separate the radio emission of the various
enhanced by X-ray data (vanWeerenetal.2009b) could give features.Wesuggestthreepossiblescenarios:
origin to the central halo. Although it is not obvious to sepa-
rate in ZwCl12341.1+0000the relic and halo emission, we es- 1. The cluster is the site of cosmic shocks originated by the
timate that the radio halo size is ∼ 4’ (∼ 1 Mpc) with a flux multiplemergersduringthelargescalestructureformation,
density ∼ 10 mJy (log P 23.2 W/Hz), and the Northern and as proposed by Bagchietal.2002. In this case, the radio
1.4
Southernrelicshaveafluxdensity∼5mJyand13mJy,respec- emission would be related with the galaxy filament as a
tively. The spectral index of the two relics between 1.4 GHz whole. The polarized emission image could be easily rec-
and 610 MHz, estimated from the present data and those by onciledwith thisscenario,asthe shocksconnectedto large
vanWeerenetal.2009b,is∼1.2(forbothrelics).Thisvalueis scale filamentsare expectedto compressthemagneticfield
higherwithrespecttoα610=0.49and0.76fortheNandSrelics toahighdegreeofordering.
241
respectivelyreportedbyvanWeerenetal.2009b.Althoughthis 2. The diffuse radio emission could consist of a central radio
may be due to a genuine flattening at low frequency, we note halo and two opposite radio relics, as found in A1758 or
thatbecauseofdifferentUVcoveragesandsensitivities,itisnot RXCJ1314.4-2515.Despite of the irregularstructure of the
possibletoderivefirmconclusionsaboutthespectralindexprop- central halo, its size and power would be consistent with
erties. the correlation between radio power and size reported by
From the image presented in this paper, we derive that the Giovanninietal.2009.Thestructureofthetworelicsisdif-
central radio halo is likely to show a quite irregular struc- ferent from that of typical elongated relics, however some
ture, however its radio size and power are in good agreement other cases of irregular relics are found in the literature
with the correlation between radio power and size reported by (A1664,A548b).Thefractionalpolarizationdetectedinthe
Giovanninietal.2009.Alsothetwopossiblerelicsarepeculiar, diffuse source is consistent with the polarization of relics,
indeedtheirshapeisnotregularandelongatedasintypicalrelics whereas the radio halos are generally unpolarized (but see
like 1253+275 in the Coma cluster (Giovanninietal.1991) or A2255andMACSJ0717+3745).
the double relics in A3376 (Bagchietal.2005), and A3667 3. The complex structure could derive from a double clus-
(Ro¨ttgeringetal.1997). However,exceptionsare known in the ter, hosting two radio halos, similar to the cluster system
literatureastherelicsinA1664(Govonietal.2001),andA548b A399/A401(Murgiaetal.2009),butatalaterstageofmerg-
(Ferettietal.2006). The presence of polarized emission in the ing, therefore the system has developed a radio bridge be-
two relics is in favour of this scenario, whereas the detection tweenthetwointeractingclusters.Thepresenceofpolarized
of polarization in the central halo would be against it. Indeed, fluxcannotbe easilyreconciledwith thisscenario,asradio
radio halos are typically unpolarized, with upper limits of the halosaregenerallyunpolarized.Moreover,fromtheoptical
order of few percent. The only two cases known so far where isodensitycontourspresentedbyvanWeerenetal.2009b,in
polarized emission has been detected from radio halos are the the brightest radio emitting regions there is no evidence of
clusterAbell2255(Govonietal.2005)andMACSJ0717+3745 a galaxyoverdensitywhichis the expectedsignatureof the
(Bonafedeetal.2009). twomergingclusters.
As a third possiblity, we wish to remember the case of
Acknowledgements. TheNationalRadioAstronomyObservatoryisoperatedby
two halos found for the first time in the double cluster system
Associated Universities, Inc., under cooperative agreement with the National
A399/A401 (Murgiaetal.2009). These two clusters are prob-
ScienceFoundation.
ably in a pre-merging phase: the X-ray excess, the slight tem-
perature increase in the region between the two clusters, and
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