The two-component giant radio halo in the galaxy cluster Abell 2142

17 Pages·2017·14.19 MB·English

A&A603,A125(2017) Astronomy
DOI:10.1051/0004-6361/201630014
&
(cid:13)c ESO2017 Astrophysics
The two-component giant radio halo in the galaxy
cluster Abell2142
T.Venturi1,M.Rossetti2,G.Brunetti1,D.Farnsworth3,4,F.Gastaldello2,S.Giacintucci5,6,D.V.Lal7,L.Rudnick3,
T.W.Shimwell8,D.Eckert9,S.Molendi2,andM.Owers10,11
1 INAF–IstitutodiRadioastronomia,viaGobetti101,40129Bologna,Italy
e-mail:[email protected]
2 INAF–IASF-Milano,viaBassini15,20133Milano,Italy
3 MinnesotaInstituteforAstrophysics,SchoolofPhysicsandAstronomy,UniversityofMinnesota,116ChurchStreetSE,
Minneapolis,MN,55455,USA
4 Cray,Inc.,380JacksonStreet,Suite210,St.Paul,MN55101,USA
5 NavalResearchLaboratory,Washington,DC20375,USA
6 DepartmentofAstronomy,UniversityofMaryland,CollegePark,MD,20742-2421,USA
7 NationalCentreforRadioAstrophysics,TIFR,PostBag3,Ganeshkhind,411007Pune,India
8 LeidenObservatory,LeidenUniversity,POBox9513,2300RALeiden,TheNetherlands
9 DepartmentofAstronomy,UniversityofGeneva,ch.d’Écogia16,1290Versoix,Switzerland
10 AustralianAstronomicalObservatory,POBox915,NorthRyde,NSW1670,Australia
11 DepartmentofPhysicsandAstronomy,MaquarieUniversity,NSW2109,Australia
Received4November2016/Accepted10March2017
ABSTRACT
Aims.WereportonaspectralstudyatradiofrequenciesofthegiantradiohaloinA2142(z=0.0909),whichweperformedtoexplore
itsnatureandorigin.TheopticalandX-raypropertiesoftheclustersuggestthatA2142isnotamajormergerandthepresenceofa
giantradiohaloissomewhatsurprising.
Methods. We performed deep radio observations of A2142 with the Giant Metrewave Radio Telescope (GMRT) at 608 MHz,
322MHz,and234MHzandwiththeVeryLargeArray(VLA)inthe1–2GHzband.Weobtainedhigh-qualityimagesatallfrequen-
ciesinawiderangeofresolutions,fromthegalaxyscale,i.e.∼5(cid:48)(cid:48),upto∼60(cid:48)(cid:48)toimagethediffusecluster–scaleemission.Theradio
haloiswelldetectedatallfrequenciesandextendsouttothemostdistantcoldfrontinA2142,about1Mpcawayfromthecluster
centre.Westudiedthespectralindexintworegions:thecentralpartofthehalo,wheretheX–rayemissionpeaksandthetwobrightest
dominantgalaxiesarelocated;andasecondregion,knownastheridge(inthedirectionofthemostdistantsouth–easterncoldfront),
selectedtofollowthebrightpartofthehaloandX-rayemission.WecomplementedourdeepobservationswithapreliminaryLOw
FrequencyARray(LOFAR)imageat118MHzandwiththere-analysisofarchivalVLAdataat1.4GHz.
Results.Thetwocomponentsoftheradiohaloshowdifferentobservationalproperties.Thecentralbrightestparthashighersurface
brightessandaspectrumwhosesteepnessissimilartothoseoftheknownradiohalos,i.e.α1.78GHz =1.33±0.08.Theridge,which
118MHz
fadesintothelargerscaleemission,isbroaderinsizeandhasconsiderablylowersurfacebrightessandamoderatelysteeperspectrum,
i.e.α1.78GHz ∼ 1.5.WeproposethatthebrightestpartoftheradiohaloispoweredbythecentralsloshinginA2142,inaprocess
118MHz
similartowhathasbeensuggestedformini-halos,orbysecondaryelectronsgeneratedbyhadroniccollisionsintheICM.Onthe
otherhand,thesteeperridgemayprobeparticlere-accelerationbyturbulencegeneratedeitherbystirringthegasandmagneticfields
onalargerscaleorbylessenergeticmechanisms,suchascontinuousinfallofgalaxygroupsoranoff-axis(minor)merger.
Keywords. galaxies:clusters:general–galaxies:clusters:individual:A2142–radiocontinuum:general
1. Introduction historyandprovideinvaluableinformationconcerningtheirdy-
namical state (see Markevitch & Vikhlinin 2007). At the same
time,thedeepradioimagingachievedbelow1GHzbytheGi-
Cluster mergers are the most energetic events in the Universe.
Withtotalenergyoutputsoforder1063−1064 erg,mergersarea antMetrewaveRadioTelescope(GMRT),andmorerecentlyby
LOFAR,issheddinganewlightonthepropertiesofnon-thermal
naturalwaytoaccountformassassembly:galaxyclustersform
componentsingalaxyclusters.
asaconsequenceofmergertreestoreachandexceedmassesof
order1015 M .Thegravitationalenergyreleasedintothecluster Radio halos represent the most spectacular non-thermal ef-
(cid:12)
volumeduringmergersdeeplyaffectsthedynamicsofthegalax- fectsofclustermergers.Radiohalosarediffusesynchrotronra-
iesandthepropertiesofthethermalintraclustermedium(ICM) diosourcesthatcoverthewholeclustervolume,i.e.upandbe-
andnon-thermalrelativisticparticleandmagneticfieldemission yondMpcsize.Thesehaloshavesteepspectra,forwhichtypical
(Brunetti & Jones 2014). The impressive quality of the X–ray valuesareα ∼ 1.2–1.3,forS ∝ ν−α,whereS isthefluxdensity
ChandraandXMM–Newtonimages(e.g.Markevitchetal.2000; andαisthespectralindexofthesynchrotronradiospectrum,and
Rossettietal.2013)showsavarietyoffeaturesinthehotICM, extremely low surface brightness of a fraction of µJy arcsec−2
suchascoldfrontsandshocks,whichtracetheclusterformation (see the Feretti et al. 2012, for a recent observational review).
ArticlepublishedbyEDPSciences A125,page1of17
A&A603,A125(2017)
Radiohalosarenotubiquitousingalaxyclusters;only30–40% attheunprecedenteddistanceof∼1Mpcfromtheclustercentre
ofmassiveclustersintheUniverse(M ≥ 1015 M )hostaradio was detected with XMM–Newton and studied by Rossetti et al.
(cid:12)
halo.Inthosecases,the1.4GHzradiopoweroftheMpc-scale (2013),whoproposedthatlarge-scalesloshingwastherespon-
halocorrelateswithboththeclusterX-rayluminosityandmass siblemechanismforitsorigin;thiseitherresultedfromthelong-
(e.g.Liangetal.2000;Brunettietal.2007;Basu2012;Cassano termevolutionofcentralsloshingcommoninmanyrelaxedclus-
etal.2013;Cucitietal.2015).Forthoseclusterswithoutaradio tersorfromamergerofintermediatestrength.
halo, upper limits to the radio power derived from the GMRT Diffuse radio emission on the scale of few hundred kpc
radiohalosurveyareordersofmagnitudebelowthecorrelation around the two brightest cluster galaxies (BCGs) was detected
(Venturi et al. 2007; and 2008; Cassano et al. 2013; Kale et al. byGiovannini&Feretti(2000).Amorerecentstudyperformed
2013and2015a). withtheGreenBankTelescope(GBT)at1410MHzshowsthat
Theoriginofgiantradiohalosisstilladebatedissue,how- thesizeofthisdiffuseradioemissionis∼2Mpc,extendingeven
ever the connection between radio halos and cluster mergers is beyondthemostdistantcoldfront.Themajoraxisofthisgiant
quantitativelysupportedbythedistributionofclusterswithand radiohaloisalignedinthesamesouth-eastdirectionwherethe
without radio halos as a function of a number of indicators of large-scale cold front is located (Farnsworth et al. 2013). The
the X-ray substructure: giant radio halos are always found in GBT image shows that its surface brightness is very low, i.e.
unrelaxed clusters (Buote 2001; Cassano et al. 2010). On the ∼0.2 µJyarcsec−2, but the poor angular resolution does not al-
other hand, a strong correlation is found between the cool-core lowadetailedcomparisonwiththeX-rayimages.Toovercome
strengthinrelaxedclustersandthepresenceofradiomini-halos; the resolution limitations of the GBT image and to study the
thatis,diffusecluster-scalesourcesthataresmallerinsizethan spectralpropertiesofthisexceptionalradiohaloforcomparison
giant radio halos (of the order of up to few hundreds of kpc), withtheX-rayemissionandopticalinformation,weundertooka
which always encompass the radio emission from the cluster studywiththeGMRTat608MHz,322MHzand234MHz,and
dominant galaxy, but whose origin is not closely related to the with the Karl Jansky Very Large Array (VLA) in the 1–2 GHz
currentcycleofactivity(e.g.Bravietal.2016;Giacintuccietal. band.WecomplementedouranalysiswithLOwFrequencyAR-
2014;Kaleetal.2015b;Mittaletal.2009). ray(LOFAR)dataat118MHzandwitharchivalVLAobserva-
A possible explanation for the origin of giant radio halos tionsat1.4GHz.
is the re-acceleration of in situ relativistic particles by turbu- In this paper we present the results of our work. The pa-
lence injected into the ICM during cluster merger events (the perisorganizedasfollows:inSect.2wedescribetheobserva-
so-called re-acceleration model; see Brunetti et al. 2001). Sec- tionsanddataanalysis;theimagesarepresentedinSect.3;the
ondary(hadronic)modelsfortheoriginoftherelativisticparti- spectral study is presented in Sect. 4; the results are discussed
cles(e.g.Dennison1980;Blasi&Colafrancesco1999)arecur- in Sect. 5; and the summary and future prospects are given in
rentlylessfavouredbecauseofthelackofdetectionofpredicted Sect.6.InAppendixAwecomplementtheinformationandre-
γ-ray emission from galaxy clusters by the Fermi satellite, and port on the radio emission associated with the cluster galaxies.
becauseofthediscoveryofradiohaloswithultra-steepspectra Throughout the paper we use the convention S ∝ ν−α. We as-
(α ≥ 1.5), whose relativistic energy would exceed the thermal sumeastandardΛCDMcosmologywithH =70kms−1Mpc−1,
o
energy under the secondary model assumptions (see the proto- Ω =0.3,Ω =0.7.Attheclusterredshift(z=0.0909)thiscor-
M v
type case of A521; Brunetti et al. 2008). Mixed hadronic and respondstoascaleof1.705kpc/(cid:48)(cid:48) andtoaluminositydistance
re-accelerationmodelshavebeenproposedandsomelevelofra- D =418.6Mpc.
L
dioemissionisexpectedin“radiooff-stateclusters”(e.g.Brown
etal.2011;Brunetti&Lazarian2011;Zandaneletal.2014).
Despite the statistical connection between radio halos and 2. Observationsanddatareduction
mergers, a few radio halos have been found in less disturbed
2.1. ObservationswiththeGMRT
systems. An example of these outliers is the giant radio halo
in the strong cool-core cluster CL1821+643 (Bonafede et al. We observed A2142 with the GMRT (Pune, India) in March
2014). A study of the X-ray morphological parameters of this 2013 at 608, 322, and 234 MHz. The logs of the observations
cluster shows that it shares the same properties of galaxy clus- aregiveninTable1.
ters hosting a radio halo, suggesting that it may be a case of a Thedatawerecollectedinspectral-linemodeatallfrequen-
clustermergerinwhichthecoolcorehasbeenpreserved(Kale cies, i.e. 256 channels at 322 and 608 MHz, and 128 channels
& Parekh 2016). More recently, a giant radio halo has been re- at 234 MHz, with a spectral resolution of 125 kHz/channel at
portedinA2261andA2390,neitherofwhichisamajormerger 322MHzand608MHz,and65kHz/channelat234MHz.The
(Sommer et al. 2016). Current models predict that radio halos raw data were first processed with the software flagcal (Prasad
canalsobegeneratedinlessdisturbedsystems,althoughwitha & Chengalur 2013; Chengalur 2013) to remove RFI and ap-
probabilitythatissignificantlylowerthaninthecaseofmassive ply bandpass calibration, then further editing, self-calibration,
majormergers(Cassanoetal.2006;andBrunetti&Jones2014, andimagingwereperformedusingtheNRAOAstronomicalIm-
forareview).Suchoutliersarethusveryimportant,astheymay ageProcessingSystem(AIPS)package.Thesources3C286and
provide important constraints on the origin of radio halos, and 3C48wereusedasprimary(amplitude)calibrators.Inorderto
probe a piece of the theoretical framework that is still poorly findacompromisebetweenthesizeofthedatasetandtheneedto
explored. minimizebandwidthsmearingeffectswithintheprimarybeam,
ThegalaxyclusterA2142(z=0.0909)isanotherchallenge after bandpass calibration the central channels in each individ-
toourunderstandingoftheformationofgiantradiohalos. ualdatasetwereaveragedto30,39,and26channelsof∼1MHz
A2142 is massive (M ∼ 8.8×1014 M , Cuciti et al. 2015) eachat608MHz,322MHz,and234MHz,respectively.
(cid:12)
and is located at the centre of a supercluster (Einasto et al. Ateachfrequencyweperformedmulti-facetimaging,cover-
2015; Gramann et al. 2015) with ongoing accretion groups. inganareaof∼2.5◦×2.5◦at234MHz,∼1.8◦×1.8◦at322MHz
It is the first object where cold fronts were discovered by and∼1.4◦×1.5◦ at608MHz,respectively.ThefieldofA2142
Chandra (Markevitch et al. 2000). Recently, another cold front isextremelycrowded,asisclearfromFig.1,andincludesmany
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T.Venturietal.:Thetwo–componentgiantradiohaloinA2142
Table1.Observations.
Array ProjectID Obs.Date ν ∆ν Time FWHM rmsa
MHz MHz h (cid:48)(cid:48)×(cid:48)(cid:48), ◦ mJyb−1
GMRT 23_017 23-03-13 234 16 10 12.7×11.0,68 0.18
23_017 28-03-13 322 32 10 9.8×7.5,58 0.13
23_017 22-03-13 608 32 10 5.4×4.5,59 0.03
VLAb VLA11B-156 09-10-11 1500 1000 1.5 c c
Notes. (a) Valuemeasuredfarfromthefieldcentre;(b) theVLAobservationsconsistofthreepointingswiththesameset-upandexposuretime
(seeSect.2.2).Herewegivethetotaltimeoverthethreepointings;(c)seeSect.2.2.andTable2.
strong sources, which had to be properly self-calibrated and Table2.ParametersofthefullresolutionVLAimages.
cleanedtoreachthetargetedrms(seeTable1).
At each frequency we produced final images, over a wide ν ∆ν FWHM rms
range of resolutions and with different tapering and weight- MHz MHz (cid:48)(cid:48)×(cid:48)(cid:48) mJyb−1
ing schemes, to account for the complexity of the radio emis- 1380 250 40(cid:48)(cid:48)×37(cid:48)(cid:48) 0.15
sion in the field. In particular, full resolution images were used 1780 200 32(cid:48)(cid:48)×29(cid:48)(cid:48) 0.07
to subtract the strongest radio sources at distances larger than
∼0.8◦−1.5◦ (depending on the frequency) from the field cen-
tre,andthentaperingandrobustweightingwereusedtoimage
the diffuse radio sources and the radio halo. Images with mul- noise and less complete u−v coverage. Standard data flagging
andreductiontechniqueswereperformedwithCASA,usingthe
tiple resolutions were produced with resolutions ranging from
5.5(cid:48)(cid:48) × 4.5(cid:48)(cid:48) to 39.1(cid:48)(cid:48) × 36.6(cid:48)(cid:48) at 608 MHz, 9.8(cid:48)(cid:48) × 7.5(cid:48)(cid:48) to VLAcalibratorsourcesJ1331+3030(3C286)andJ1609+2641
53.4(cid:48)(cid:48)×43.7(cid:48)(cid:48)at322MHz,and12.7(cid:48)(cid:48)×11.0(cid:48)(cid:48)to45.19(cid:48)(cid:48)×44.80(cid:48)(cid:48) for flux and phase calibration, respectively. After editing for
RFI, roughly 45% of the total bandwidth remained, yielding
at234MHz.
∼450MHzoversevencleanspectralwindows.Wecreatedtwo
Finally,toimagetheradiohalowefirstproducedhighreso-
images, one using 250 MHz bandwidth around 1.38 GHz and
lutionimagesusingtheu−vspacings>2kλ.Thenweusedthese
one using 200 MHz around 1.78 GHz. The standard phase
tosubtractthecleancomponentsofthediscretesourcesfromall
and amplitude calibration were successful enough that self-
of the u−v data, and imaged the residual emission with a taper
calibrationdidnotproduceasignificantimprovement,soitwas
and natural weighting (ROBUST=+2 in AIPS) to a resolution
not used in the image presented here. Residual amplitude cali-
of∼50(cid:48)(cid:48)−60(cid:48)(cid:48) atallfrequencies.Allimageswereprimarybeam
brationerrorsareestimatedtobewithin3%.
correctedusingthetaskPBCORinAIPS.Theshortestbaselines
Weusedthemulti-frequencymulti-scalecleantaskinCASA
inourdatasetsare∼0.2kλ,∼0.1kλ,and∼0.07kλ,respectively
(Rau&Cornwell2011)bothtodeconvolveandcreateamosaic
at 608 MHz, 322 MHz, and 234 MHz. The largest detectable
featuresarehence17(cid:48),32(cid:48)and44(cid:48),respectively. fromthethreepointingsforeachofthe1.38and1.78GHzmaps.
Correction for primary beam attenuation was performed using
Thefinalrmsvaluesforthefullresolutionimagesaregiven
theCASAtaskimpbcor.
inTable1.Atlowerresolutionweobtainedrms∼0.05mJyb−1
Toisolatethediffuseclusteremissionwesubtractedthecon-
at 608 MHz (13.18(cid:48)(cid:48) × 11.06(cid:48)(cid:48)), ∼0.25 mJy b−1 at 322 MHz
tribution from radio galaxies, as follows. We used the C con-
(14.80(cid:48)(cid:48) × 13.06(cid:48)(cid:48)), and ∼0.3 mJy b−1 at 234 MHz (19.46(cid:48)(cid:48) ×
figuration 1.6 GHz maps (resolution of 11(cid:48)(cid:48), rms sensitivity of
17.92(cid:48)(cid:48)).Weestimatethatresidualcalibrationerrorsateachfre-
90 µJy b−1) to create masks of the location and extents of ra-
quency are within 4–5% at 608 MHz and of the order of 10%
dio galaxies; these data were not calibrated accurately enough
at 234 MHz and 322 MHz. We point out that the difference in
to directly subtract from the D configuration data. With these
the flux density between the Baars et al. (1977) scale adopted
masks, we then performed an interactive single-scale clean of
here and the Scaife & Heald (2012) scale, suggested for obser-
the1.38GHzand1.78GHzfullresolution∼40(cid:48)(cid:48)Dconfiguration
vations at ν ≤ 500 MHz, is of the order of few percent, hence
images until the radio galaxies were no longer visible. Model
withinourfinalestimatederrors.Figure1showsthefullfieldof
u−vdatasetsrepresentingtheradiogalaxyemissionwerecreated
viewat234MHzandhighlightsthesizeofthefieldsimagedat
fromthesecleancomponentsandsubtractedfromtheoriginalD
322MHzand608MHz.
configurationu−vdata.Theresidualu−vdatasetswerethenim-
agedwithmulti-scalecleanandcorrectedfortheprimarybeam,
2.2. JanskyVLAobservations convolving the final maps to 60(cid:48)(cid:48) with an rms of ∼180 µJy b−1
(∼140 µJy b−1) at field centre for 1.38 (1.78) GHz, to increase
A2142 was observed with the Karl G. Jansky VLA in D and thesignal-to-noiseratioofthediffuseemission.Thelargestde-
C configurations at 1–2 GHz as part of NRAO observing pro- tectableangularsizeoftheseobservationsis<∼1000(cid:48)(cid:48)and<∼820(cid:48)(cid:48),
gramme VLA11B-156. Three pointings were acquired to re- at1.38GHzand1.78GHz,respectively.
cover the full extent of the radio halo detected with the GBT
(Farnsworth et al. 2013) with 28 min of integration time per
pointing. Observations were made in spectral line mode with 3. Radioemissionfromclustergalaxies
16 spectral windows, each 64 MHz wide, spread across the
full 1–2 GHz band. For technical reasons due to the recent TheinnerportionofthefieldofviewoftheGMRTobservations,
VLA upgrade, only two seconds of every five second integra- i.e.A2142itself,isshowninFigs.2and3,wherethecontours
tion on source were recorded, which resulted in higher thermal of the 234 MHz and 608 MHz radio emission are overlaid on
A125,page3of17
A&A603,A125(2017)
0
.
0
0
:
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3
608 MHz field
0
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0
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0
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8
2
0
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0
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o 3
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n 0
li .
c 0
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0
0
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3
322 MHz field
04:00.0 02:00.0 16:00:00.0 58:00.0 56:00.0 15:54:00.0
Right ascension
Fig.1.FieldofA2142atGMRT234MHz.Thelowresolutionimageisrestoredwithabeamof45.2(cid:48)(cid:48)×40.8(cid:48)(cid:48),PA–37.9◦.Thecontinuousand
dashedblackboxesshowthesizeofthe322MHzand608MHzfields,respectively.
theredplateoftheDigitizedSkySurveyDSS–2andtheXMM– CFHT (Canadian French Hawaii Telescope) MegaCam g-band
Newtonimage,respectively. image.
ThecentralregionofA2142isdominatedbythepresenceof
two extended FRI radio galaxies (Fanaroff & Riley 1974) with
3.1. Radiogalaxiesattheclustercentre
head-tailmorphology(Sect.3.1)andbydiffuseemissioncoinci-
dentwiththebrightestpartoftheX-rayemissionfromtheintra- ThemoststrikingradiogalaxiesatthecentreofA2142aretwo
clustermedium,whichweclassifyasagiantradiohalo(Sect.4). long tailed sources labelled T1 and T2 in Fig. 2. A zoom on
Beyond these striking features, many radio sources in the field eachofthemisshownintheupperleftandupperrightpanelsof
areassociatedwithclustergalaxies. Fig.A.1.
TableA.1reportsthefullsampleofradiosourceswithopti- ThesourceT1istheradiogalaxyB21556+27(Collaetal.
calcounterpartattheredshiftofA2142inthe608MHzfieldof 1972, Owen et al. 1993), associated with a m = 17.5 clus-
g
view (Fig. 1). The list is based on the full resolution 608 MHz ter galaxy (z = 0.0955). A compact counterpart is visible in
imagewithadetectionlimitS =0.25mJy(i.e.5σ)prior the XMM-Newton image. The head of this radio galaxy is co-
608MHz
totheprimarybeamcorrection.Forthisreason,theradiosource incident with the north-western cold front in the cluster. The
catalogueisnotcompleteinradiopower,whosedetectionlimit length of the tail is ∼610 kpc. Figure A.1 clearly shows that
increases away from the cluster centre. Flux density values at the long and straight tail has small amplitude wiggles. The
234MHzhavealsobeenreportedinthetable.Theradiogalax- imaging process at all frequencies reveals that it is embedded
ies presented in Sect. 3.1 are shown in Fig. A.1, where the full in the diffuse emission of the radio halo. Its radio power is
resolution 234 MHz and 608 MHz contours are overlaid on a logP = 24.80 W Hz−1, which is high for this class of
608MHz
A125,page4of17
T.Venturietal.:Thetwo–componentgiantradiohaloinA2142
0
.
0 W1
0
: T2
5
2 G
0
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0
0
:
0
2
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7
2 W2 T1
0
n .
0
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at 5:
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0
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1 C4
0 C1 C2 C3
.
0
0
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5
0
59:00.0 40.0 20.0 15:58:00.0 57:40.0
Right ascension
Fig.2.RadioemissionofA2142overlaidontheredplateofDSS–2.BlackcontoursshowalowresolutionGMRT234MHzimagerestoredwith
abeamof45.2(cid:48)(cid:48)×40.8(cid:48)(cid:48),PA–37.9◦(sameasFig.1);contourlevelsare±3,6,12mJy/b;andthermsintheimageis∼0.9mJyb−1farfromthefield
centre(negativecontoursarewhitedashed).Redcontoursshowthe608MHzfullresolutionimagerestoredwithabeamof5.2(cid:48)(cid:48)×4.5(cid:48)(cid:48),PA52.6◦;
contourlevelsare±0.1,0.2,0.4,0.8,1.6,6.4,25.6,102.4mJyb−1;andthermsintheimageis∼35µJyb−1 farfromthefieldcentre.Negative
contoursareshowninwhite.
objects.Wemeasureα608MHz = 0.73±0.15,whichsteepensto Two wide-angle tail (WAT) sources are also present, and
234MHz
α1400MHz =1.04±0.111. labelled W1 and W2 in Fig. 2. A zoom on each of them is
608MHz
shown in the left and right central panels of Fig. A.1, respec-
The source T2 is associated with the galaxy 2MASX
tively. Interestingly, neither of these is located at the cluster
J15582091+2720010 (m = 16.6, z = 0.0873 from the
g redshift. The source W1 has a very faint optical counterpart
NASA/IPACExtragalaxticDatabase;NED),locatednorthofthe
(m = 23 from NED) with z = 0.574; its flux density is
cluster centre and outside the brightest part of the X–ray emis- g phot
S = 37.99 mJy. The radio peak of W2 coincides with
sion,asisclearfromFig.3.Itslengthis∼370kpcanditsradio 608MHz
anX-raysourceandhasaveryfaintopticalcounterpart.Despite
power is logP = 24.51 W Hz−1. From Table A.1 it is
608MHz thepresenceofthreeverynearbyclustergalaxies,anassociation
clear that T2 has a very steep integrated spectrum. If we com-
with any of these seems unlikely. Considering that wide-angle
plementourGMRTfluxdensitymeasurementswiththearchival
tails are tracers of galaxy clusters (e.g. Giacintucci & Venturi
1.4GHzdata,weobtainα608MHz =1.21±0.14,whichsteepens
234MHz 2009;Maoetal.2009)andthatbothW1andW2areassociated
toα1400MHz =1.92±0.11.
608MHz withveryfaintobjects,atleastanotherclusteralongthelineof
sightandbehindA2142mustbepresent.
TheradioemissionlabelledG(bottomleftpanelofFig.A.1)
1 TheVLAC+Dconfiguration1.4GHzobservationsareare-analysis
waspresentedinEckertetal.(2014)at608MHz.Itisaremark-
ofprojectAG344.Weproducedimageswithangularresolution24.1(cid:48)(cid:48)×
21.9(cid:48)(cid:48) and ∼38.6(cid:48)(cid:48) ×34.9(cid:48)(cid:48) with rms of the order of ∼15 µJy b−1 and able blend of discrete radio sources associated with a group at
∼20µJyb−1,respectively.Individualsourcesubtractionwasperformed z ∼ 0.094 located north-east of the cluster centre. The south-
followingthesameproceduredescribedinSect.2.1toobtainimagesof erntipoftheradioemissioniscoincidentwiththebrighttipof
thediffuseemission. thelong(∼800kpc)X-raytailvisibleinFig.3,whichhasbeen
A125,page5of17
A&A603,A125(2017)
Southern X-ray tip
0
.
0
0
:
5
2
0
.
0
0
:
0
2
:
7
2
n
o
ati 0.0 1 Mpc
n 0
cli 5:
e 1
D
0
.
0
0
:
0
1
0
.
0
0
:
5
0
59:00.0 30.0 15:58:00.0 57:30.0
Right ascension
Fig.3.RadioemissionofA2142overlaidontheX-rayimagefrom XMM-Newton.Magentacontoursshowa234MHzlowresolutionimage
restoredwithabeamof60(cid:48)(cid:48)×60(cid:48)(cid:48).Contoursaredrawnat1.5,3,6,12,24,48mJyb−1.Negativescontoursareshowninblue(–3mJyb−1).The
whitecontoursshowthe608MHzimage(samecontoursandresolutionasinFig.2).
interpretedasthesignatureoftheinfallofthegroupintoA2142 and T2 (GMRT J155820+272000), the two tailed radio galax-
(Eckertetal.2014and2017).Weassociatethesouthernpeakof iesattheclustercentre,allradiosourcesareeitherpoint–likeor
thisemissionwiththebrightest(m =16.1)galaxyinthegroup barelyextendedatthefullresolutionofthe608MHzimagewith
g
(TableA.1),eventhoughtheoverlayshowninFig.A.1suggests radio powers in the range 21.85 ≤ logP608MHz <∼ 23 W Hz−1.
thatitcouldbeablendofemissionfrommoregalaxies. ThesevaluessuggestthattheradiogalaxypopulationofA2142
ThethreecompactradiogalaxieslabelledC1,C2,andC3in mayincludebothstarburstgalaxiesandfaintradioactivenuclei.
Fig.2formanotherinterestinggroup.Theyarealignedandasso- Forthelatter,theradiopowersarewellbelowtheFRI/FRIIdivi-
ciatedwithclustergalaxiesofsimilaropticalmagnitude(inthe sionandaretypicalofthefaintFR0radiogalaxies,whoseradio
rangem =17.4–17.7fromNED).Afourthclusterradiogalaxy, morphologylacksextendedemissionintheformofradiolobes
g
C4,islocatedjustwestofthistriplet,asseeninthebottomright (Baldietal.2015,andreferencestherein).
panelofFig.A.1.
The radio emitting galaxies in A2142 span a range of red-
The high resolution images at 608 MHz reveal that some shifts from ∼0.079 to >∼0.12 (see Table A.1); this is consistent
clustergalaxiesembeddedinthegianthalohaveassociatedradio withthepresenceofmultiplegroups,ashighlightedinthespec-
emission.ThemostluminousBCG(m = 16.2andz = 0.0904) troscopic analysis performed by Owers et al. (2011) and in the
g
hosts a faint radio source (see upper left panel of Fig. A.1). structureanalysisshowninEinastoetal.(2015).InFig.A.2we
Moreover, a fainter cluster galaxy (m = 18.8, z = 0.0806) showthelocationoftheclusterradiogalaxieslistedinTableA.1
g
justnorth-westofthemostluminousBCGandtwomorecluster withcolourcodestohighlightthedifferentredshifts.
galaxies located along the X-ray elongation from the BCGs to
The wider field of view of the 234 MHz (Fig. 1) includes
theC1–C2–C3groupshowradioemission.
manymoreradiosourcesassociatedwithclustergalaxies.Start-
ingfromthevisualinspectiononDSS–2andafterconsultation
3.2. OverallradiopropertiesofthegalaxiesinA2142 of the NED database we found 71 counterparts in the redishift
rangez∼0.07−0.13.Thesearemostlylocatedsouthandeastof
A total of 42 radio sources have an optical counterpart. These theclustercentre,followingtheoverallelongationofthecluster
numbers refer to the region covered by the 608 MHz observa- subgroups (Owers et al. 2011) and of the supercluster (Einasto
tions.WithexceptionsmadeforT1(GMRTJ155814+271619) et al. 2015). A full characterization of the population of radio
A125,page6of17
T.Venturietal.:Thetwo–componentgiantradiohaloinA2142
00.0 0.0
18: 8:0
1
Declination 14:00.0 Declination 14:00.0
0
27:10:00. 7:10:00.0
2
59:00.0 40.0 20.0 15:58:00.0 50.0 40.0 30.0 20.0 10.0 15:58:00.0
Right ascension Right ascension
Fig.4.Leftpanel:radiohaloinA2142overlaidontheXMM–Newtonemission.The234MHzimageisshowninblack(negativecontoursin
yellow).Contourlevelsaredrawnat±2.5,4,8mJyb−1,θ = 44.86(cid:48)(cid:48) ×40.96(cid:48)(cid:48),PA−39◦.The608MHzemissionisshowninwhite(negative
contoursshowningreen).Contourlevelsaredrawnat±0.6,1.2,2.4mJyb−1,θ=50(cid:48)(cid:48)×50(cid:48)(cid:48).Bothimageswereobtainedaftersubtractionofthe
discretesourcesfromtheu−vdata.Rightpanel:VLAcontoursat1377MHz(θ =60(cid:48)(cid:48)×60(cid:48)(cid:48),contoursaredrawnat±0.2,0.4,0.8,1.6mJyb−1)
overlaidonthe322MHzGMRTimage.Negativecontoursaredrawnasdashedlines.Thegreenarrowhighlightsthenorth-westernextension(see
Sect.4.1).
sourcesinA2142isbeyondthescopeofthispaperandwillbe
presentedinafuturework. 0
0
7:
1
4. Radiohalo 00
6:
1
4.1. Morphology
The diffuse extended emission in A2142 is well visible in ation 5:00
n 1
Figs. 1–3, and this emission is best highlighted in the low res- cli
e 0
olutionimagesshownintheleftandrightpanelsofFig.4(ob- D 4:0
tained after subtraction of the individual radio galaxies at each 7:1
2
frequency;seeSects.2.1and2.2),andinFig.5.Ourimagesare
0
suggestive of a multi-component cluster-scale emission, which 0
3:
alltogetherwerefertoastheradiohalo.Forourstudyweiden- 1
tifytworegions,whichareshownintheleftpanelofFig.6and 100 kpc
0
0
are named H1 and H2. The operational definition of these two 2:
1
regionsisgiveninSect.4.2. 30 25 15:58:20 15 10 05
The region H1 is the brightest part of the halo, and this re-
Right ascension
gion is best highlighted in Fig. 5, which shows an intermediate
Fig.5.Zoomonthecentralpartoftheradiohalo.TheGMRT608MHz
resolution608MHzimageoverlaidontheChandraX-rayemis-
image(discreteradiosourcesnotsubtracted)attheresolutionof16.3(cid:48)(cid:48)×
sion.TheradioandX-raypeaksarecoincident.Thisregionwas
14.2(cid:48)(cid:48),PA−84.6◦,isoverlaidonChandra.Contoursaredrawnat±0.18,
formerly classified as mini–halo (Giovannini & Feretti 2000).
0.36, 0.72, 1.44, 2.88 mJy b−1; positive contours are shown in black,
Figure5clearlyshowsthatitisconfinedbytheinnermostcold
negative contours white). The green arrows show the location of the
front, whose position is indicated by the green arrows. Even innerSEcoldfront.
though the north-western boundary is more difficult to define,
owing to the presence of the radio emission from T1, H1 does
not seem to extend beyond the north-western cold front. This brightness distribution of this component is very different from
regionhasthesameextentandboundariesatallfrequencies. H1.ItisconsiderablymoreextendedthanH1andlacksacentral
The left panel of Fig. 4 shows the entire extended radio peak. The difference between H1 and H2 is confirmed by our
emission in A2142 at 234 MHz and 608 MHz overlaid on the analysis of the radio brightness profile across the two regions
X-ray emission detected by XMM-Newton, while the emission shownintheleftpanelofFig.6(seeSect.4.2fordetails).The
at 1.38 GHz is given as contours in the right panel, overlaid regionH1showsaregularprofilewithaprominentpeak,follow-
with the 322 MHz image. At all frequencies, the radio halo is ingcloselythebrightnessdistributionofthehotgas.Theprofile
elongated in the same north-west to south-east direction of the of H2 is less regular, with a flat shoulder and a steeper profile
X-rayemissionfromtheintraclustergas,andcoversthebright- farfromthecore,andshowsnoclearcorrelationwiththeX-ray
est X-ray ridge of emission out to the most distant cold front. profile. The north-western extension visible at 1.38 GHz (right
Its largest angular size is ∼10(cid:48), i.e. ∼1 Mpc at the cluster red- panelofFig.4)isnotdetectedatlowerfrequenciesanditismost
shift. We define as H2 the ridge-like emission extending from likelyduetoincompletesubtractionofT1.
H1 towards the most distant old front in the south-east direc- Finally, our VLA datasets detect further extended emission
tion.InspectionofbothpanelsofFig.4suggeststhatthesurface surroundingH1andH2(seetheleftpanelofFig.6),whichwe
A125,page7of17
A&A603,A125(2017)
Fig.6.Leftpanel:tworegionsH1andH2usedfortheevaluationoftheintegratedspectrum(seeSect.4.2)shownonthe1.38GHzVLAimageat
theresolutionof60(cid:48)(cid:48).Rightpanel:sliceofthesurfacebrightnessofthe1.38GHzimagealongthedirection−33◦(seeSect.4.2),sothatlengths
alongtheslicecanbecalculatedas(Dec2-Dec1)/cos(33d).Theverticalgreydottedlineshowsthelocationofthesouth-eastinnermostcoldfront,
whiletheXMMX-raybrightnessprofileconvolvedwiththesameresolution(60(cid:48)(cid:48))isshownasablackdottedline.
interpretastheremainsofthe2Mpcscaleemissionimagedwith Finally,weinspectedthe74MHzimageintheVLSS–Redux
theGBT(Farnsworthetal.2013),whosefullextentisunrecov- (VLA Low-Frequency Sky Survey) to check for the presence
eredinallourimages.Thisclearlyshowsthelimitationsofinter- of diffuse radio emission at this frequency and found no clear
ferometricobservations,whoselackofzerospacingsisasevere emissionabovethenoiselevel.
limit in imaging very low brightness extended and complex ra- TodeterminethespectralindicesforH1andH2,wecarried
dioemission(asisthecaseoftheradiohaloinA2142)atleast out the following steps: (a) we defined the H1 and H2 bound-
outtoz∼0.1. aries; (b) we adopted a procedure that was insensitive to varia-
tions in the detailed structures at different frequencies and that
In the next sections we refer to H1 as the central emission,
H2astheridge,andthelargerscaleemissionforthemorediffuse removed the varying amounts of flux from the 2 Mpc compo-
nentstomeasurethefluxdensityofeachcomponent;and(c)we
emissiondetectedwiththeVLAandGBT.
evaluatedtheu−vcoveragetoensurereliablefluxestimates.We
describeeachofthesebelow.
4.2. Radiospectrum
a) H1andH2definition–TheextentsofH1andH2wereini-
tiallyestimatedbyeyefromgreyscaleimagesofthebestim-
ThecomplexityoftheradioemissioninA2142atallfrequencies
ages. To look at this more quantitatively, we calculated the
doesnotallowreliableimagingofthespectralindexdistribution
averagefluxateachpositionalongthemajoraxisofthera-
throughouttheradiohalo.Toovercomethisdifficultyandobtain dioemissioninastrip160(cid:48)(cid:48) widealongtheminoraxis.The
someinformationaboutpossiblechangesofthespectralproper-
sliceofthesurfacebrightness,obtainedusingtheVLA1.38
ties,wederivedtheintegratedspectrumoftheradiohaloinH1 GHz image convolved to 60(cid:48)(cid:48), is shown in the right panel
and H2 (see the left panel of Fig. 6, where the two regions are
ofFig.6.BecauseH1andH2overlap,thedivisionbetween
overlaidontheVLAimage).
themissomewhatarbitrary.ItwasfirstchosenaswhereH1
To complement and further extend the frequency cov- beginstosignificantlyaffecttheshapeoftheH2profileand
erage of the GMRT and VLA observations we re-analysed thencomparedtothepositionofthecoldfront.TheH1,H2
1.4 GHz archival data in the C and D configuration (see division is at the southern base of the cold front (see Fig. 5
Sect. 3.1) and used a preliminary image obtained with LOFAR and right panel of Fig. 6). The north-west boundary of H1
at118MHz.TheLOFARhba_dual_inner118-190MHzdata isapproximatelyatitshalf-powerlevel,asisthesouth-east
were recorded on April 19 2014 (project ID LC1_017). A sub- boundary of H2. Because of the variations in the detailed
setofthetargetdatasetintheband118–124MHzwascalibrated structure at the different frequencies, the widths of the H1
andimagedusingthestandarddirectionindependentcalibration and H2 regions were chosen to be the FWHM of the emis-
procedure(seee.g.Shimwelletal.2017).Theresultingimage, sionaftersmoothingby160(cid:48)(cid:48) alongthemajoraxis.ForH1,
made from the visibilities from baselines shorter than 7kλ, has we used the width of the bright core, rather than the lower
a sensitivity of 3 mJy b−1, an angular resolution of ∼50(cid:48)(cid:48) and brightness emission. The H2 profile is somewhat asymmet-
the peak flux density measurements of compact sources are in ric,sothelarger(SW)widthwasused.Theseproceduresled
agreement with the 150MHz TGSS ADR (Intema et al. 2016) tothelengthandwidthoftheboxesas160(cid:48)(cid:48)×136(cid:48)(cid:48)(6square
surveytowithin30%.Afulldirectiondependentcalibrationof arcmin)forH1and216(cid:48)(cid:48)×160(cid:48)(cid:48)(9.6squarearcmin)forH2,
the dataset (see e.g. van Weeren et al. 2016a,b; Williams et al. orientedat−33deg.
2016;Hardcastleetal.2016;andShimwelletal.2016)willbe b) Flux density measurements – All images were first con-
appliedinthefuturetocorrectforionosphericdisturbancesand volved to a resolution of 60(cid:48)(cid:48), except for the VLSSr, which
improvetheimagefidelity,sensitivityandresolution,butthisis has a beam size of 75(cid:48)(cid:48). The total flux densities in the H1
beyondthescopeofthispaper. and H2 boxes were then measured as follows. For each
A125,page8of17
T.Venturietal.:Thetwo–componentgiantradiohaloinA2142
Fig.7.Integratedspectrumoftheradiohalo.Leftpanel:plotofH1.Theupperlimitat74MHzisshownasaredtriangle.Rightpanel:plotofH2.
Filledbluecircleshighlightthemeasurementswithgoodu−vcoverageatshortspacings;theremainingmeasurementsareshownasfilledgreen
dotsseeSect.4.2).Inbothpanelstheweightedlinearfitsareshownwiththesamecolourcodeandtheupperlimitat74MHzisshownasared
triangle.
component,wecalculatedtherunningsumofthefluxwithin frequency,canleadtoelevatedbackgroundlevelsaroundH1
aboxfixedtothesamelengthandwidthasH1andH2,re- andH2.Thedeterminationofabackgroundlevelinthe1D
spectively.Theboxwasslidalongthelineperpendicularto cuts,asdiscussedabove,removesanysucheffects.Thetotal
themajoraxisoveralengthof1120(cid:48)(cid:48)crossingthemajoraxis spectrum of H1 and H2 between 118 MHz and 1.778 GHz
(andH1andH2,respectively)nearthemiddle.Wethenmea- is shown in the left and right panel of Fig. 7, respectively.
sured both the off-source background level in each running We point out that H2, whose largest extent along its ma-
sum,alongwiththepeakfluxabovethebackground.These joraxis(216(cid:48)(cid:48))iswellwithinthenominallargestdetectable
peak fluxes are reported in Table 3. The background level structure, is not sufficiently well sampled in our interfero-
includesanysmallcontributionsfromthemuchlargerscale metric images owing to to its surface brightness, which is
(20(cid:48) component detected in our single dish measurements), considerably lower than H1. In particular, the inner portion
aswellasanyinstrumentaleffectsintheinterferometerim- of the u−v coverage (within 1 kλ) is much better sampled
ages. We then calculated the residual rms scatter in the off- at 118 MHz (LOFAR), 322 MHz (GMRT), 1.38 GH, and
sourcebackgroundofeachrunningsumandreporttheseas 1.78 GHz (VLA-D). For this reason we refer to those im-
the errors for the peak fluxes. Such errors therefore reflect agesastothewell-sampled(weareusingthisorgoodcov-
random noise and instrumental artifacts but do not reflect erage)images.Bycontrast,theu−vcoverageofthe1.4GHz
any systematic effects such as those due to u−v coverage, archival VLA and the 608 MHz and 234 MHz GMRT data
which we discuss below. It is important to note that the H1 ispoorerontheshortestspacingsandsomeofthefluxden-
andH2fluxesarenotthe“totalflux”fromeachcomponent, sityfromH2ismissing.Thisproblemiswellknownandits
since they each have broad wings beyond their box widths. effecthasbeenquantifiedinearlierworksongiantradioha-
ItisnotclearwhetherthesewingsareextensionsofH1and los (see Brunetti et al. 2008; and Venturi et al. 2008). The
H2 or part of the larger scale emission. However, since the flux density measurements for H2 are plotted separately in
boxes were kept the same at all frequencies, they provide a therightpanelofFig.7.
well-defined sample of the H1 and H2 emission. The right
panel of Fig. 6 shows the surface brightness profile along A weighted mean square fit provides α1.78GHz = 1.33 ± 0.08
theridgeorientedat−33deg.Thecleardifferenceinsurface for H1 and α1.78GHz = 1.42 ± 0.08 for11H82M.HTzhe difference is
brightness and extent between H1 and H2 separately moti- 118MHz
onlymarginal.However,itbecomesmoresignificantifwetake
vatedourstudyofthetotalspectrumforthesetworegions. thedifferentu−vcoverageintoaccount.Afitofthetwosetsof
c) Evaluationoftheu−vcoverage–Despitethenominallargest
measurementsforH2separately(filledgreenandfilledbluedots
detectableangularsizeofourobservationsateachfrequency in the right panel of Fig. 7), provides α1.78GHz = 1.55±0.08.
(seeSects.2.1and2.2,forcompletenessnotethatthelargest 118MHz
Thefluxdensityupperlimitsat74MHzareconsistentwiththe
angularsizedetectablebyLOFARat118MHzis∼1◦),be-
spectral index both for H1 and H2. The future analysis of the
causeoftheirverylowsurfacebrightnessnoneofourinter-
LOFARdatawillallowustoobtainbetterconstraints.
ferometer data sample well the largest scale (20(cid:48)) emission
seenwiththesingledish.Wenotethatthetotalfluxdensity
detectedwiththeVLAat1377MHzis23±2mJy,whichin-
5. Originoftheradiohalo
cludes H1, H2, and the emission beyond these two regions
until they fade into the background, while the 1410 MHz The observations presented in this work confirm that in many
flux density measured in the GBT image is ∼55 mJy (see respectsA2142isacasestudyofgalaxyclusters.Ourmostim-
Farnsworth et al. 2013). However, the partial sampling of portant results is the finding that the radio halo consists of two
thisverydiffusecomponent,whichvariesfromfrequencyto regionswithdifferentmorphologicalandspectralproperties.
A125,page9of17
A&A603,A125(2017)
Table3.Radiohalofluxdensity. theturbulenteddiesof∼100kms−1 onascaleofabout50kpc.
Theserequirementsareindeedconsistentwiththosemeasuredin
Region Frequency Fluxdensity simulationsofgassloshingandthosenecessaryforturbulentre-
MHz mJy accelerationtomaintain(ortore-accelerate)radioemittingelec-
trons in these regions (ZuHone et al. 2013); in addition, values
H1 1778 4.4±0.5
ofthesamemagnituderangehavebeenderivedforthePerseus
1465 7.0±0.5
cluster(Hitomicollaboration2016).
1377 7.9±0.4
608 16.5±1.4
322 56.2±4.1
5.2. Theridge
234 69.3±4.9
118 275.0±94.1 TherightpanelofFig.6clearlyshowsthatthesurfacebrightness
74 <700 and extent of the radio emission in the ridge (H2) are very dif-
H2 1778 3.3±0.5 ferent from those in the core region (H1). The clear separation
1465 2.2±0.4 between these two components suggests that they might origi-
1377 4.9±0.4 natefromdifferentmechanismsorthattheytracedifferentevo-
608 6.8±1.4 lutionary stages of the same phenomenon. On the scale of the
322 40.5±4.1 ridgetheclusterappearsunrelaxedwithaclearelongationinthe
234 34.5±4.9 south-eastern direction. The radio emission follows the spatial
118 260.0±94.1 distributionoftheX-rayemittinggasandextendsuptothecold
74 <700 frontlocatedat1Mpcsouth-eastofthecore.Onthesescalesthe
clustermayhavebeenperturbedbyanumberofprocessesthat
inducegassloshingandgeneratetheexternalcoldfront.Thede-
tectionofabridgeoflow-entropygasbetweenthecentralregion
Ouranalysis,whichspreadsoverafrequencyrangeofmore
oftheclusterandthemostdistantcoldfrontisinsupportofthis
thanoneorderofmagnitude(118–1.78GHz),suggeststhatthe
picture(Rossettietal.2013).Itislikelythatthegasandmagnetic
regionofemissionextendingsouth-eastoftheclustercore(H2;
fieldsinthisregionhavebeenmoderatelyperturbedandstirred.
seeleftpanelofFig.6)hasalowersurfacebrightness,isbroader
We can speculate that this induces turbulence that cascades on
insize,andhasamoderatelysteeperspectrumthanthebrighter,
smaller scales and damps into particle re-acceleration and fast
morecompactregioninthecore(H1).Inthissectionwediscuss
the possible origin of such differences in the framework of the magetic reconnection, which are two interconnected processes
ifincompressibleturbulenceisconsidered(Brunetti&Lazarian
cluster dynamics, as inferred from its broadband properties. In
2016).
Fig.8weshowalltheobservationalinformationthatisrelevant
tothediscussion. Thelowluminosityoftheridgeandofthe2Mpcscaleemis-
sion,andthemoderatelysteeperspectrumoftheridge,suggest
thattheenergybudgetthatbecomesavailabletothenon-thermal
5.1. Thecentralemission components in H2 is smaller than that of classical giant radio
halos,thatindeedaregeneratedduringmajormergerevents.An
The radio emission observed in the core of A2142 (H1) is
alternative possibility is that the radio emission is very old and
bounded by the two inner cold fronts that are detected on the
marks the switched off phase of the radio halo, when merger
100–200kpcscale,asclearfromFigs.5and8.Thismaysuggest
turbulence is dissipated at later times. Both cases are possible
that this emission traces the dissipation of the energy produced
for massive clusters that show intermediate properties between
by the sloshing of the low-entropy gas oscillating between the
mergingandrelaxedsystems(e.g.Cassanoetal.2006;Brunetti
twoinnercoldfronts.Thisscenarioissimilartowhathasbeen
et al. 2009; Donnert et al. 2013). This is indeed the case of
suggestedfortheoriginofmini–halosincool-coreclusters(e.g.
A2142. As a matter of fact, this radio halo is considerably un-
Mazzotta&Giacintucci2008;ZuHoneetal.2013).TwoBCGs
derluminous compared to the correlation between radio power
are present in A2142 (see Figs. 8 and A.1) and the sloshing in
andclustermass(Cassanoetal.2013;Cucitietal.2017),even
thecoremaybeduetotheperturbationinducedbyagaslessmi-
considering the GBT measurement in Farnsworth et al. (2013,
nor merger with the group associated with the secondary BCG
seeSects.4.2and6).
(Owersetal.2011).ThesetwoBCGsmayalsoplayaroleinthe
originoftheradioemission.Atpresentonlythemostluminous Differentscenarioscanexplaintheoriginoftheperturbations
BCG hosts a radio source (Sect. 3.1, Table A.1 and Fig. A.1), inducedintheICM.Apossibilityisthatthethreecoldfrontsde-
but it is likely that over the last Gyr or so both BCGs have re- tected in X-rays (see their location in Fig. 8) are generated by
leasedrelativisticparticlesandmagneticfieldsinthecoreregion. a single event (Rossetti et al. 2013). In this case, H1 and H2
Duringthistimescalerelativisticelectronscouldbeadvectedby mayprobetheevolutionofthisphenomenonondifferentscales
turbulentmotionsinthesloshinggas.Theveryweakcorrelation and/ortimesandmaytracedifferentlevelsofperturbationsand
between the radio power of the BCG and that of the mini-halo magneticfieldstrengththatarepresentintheICM.Thesymme-
foundforasizeablesampleofmini-haloclusters(Govonietal. tryofthethreecoldfrontsandthewaytheyencompassH1and
2009;Giacintuccietal.2014)isindeedsuggestiveofthefactthat H2isinsupportofthisscenario.
thecentralAGNactivitymaynotbepoweringtheradioemission Anotherpossibilityisthattheridgetracesaturbulentregion
directly,butitisthemostlikelysourceofseedelectronsforre- that is generated by the continuous accretion of subhalos along
accelerationintheICM. theS–Edirection.Thishypothesismaybesupportedbythefact
Inordertocoveradistanceoftheorderof100kpcin1Gyr, that optical data show the presence of several groups of galax-
thespatialdiffusioncoefficientduetoturbulenttransportshould iesthattracealarge-scalefilamentintheS–Edirection(Owers
be of the order of D ∼ δV L ∼ 2 × 1030 cm2s−1 (Brunetti etal.2011;Einastoetal.2015),andgroupaccretioninA2142
L
& Jones 2014, and references therein), implying a velocity of iscaughtinaction(Eckertetal.2014).
A125,page10of17