Sardinia Radio Telescope observations of Abell 194

26 Pages·2017·18.87 MB·Italian

A&A603,A122(2017) Astronomy
DOI:10.1051/0004-6361/201630349
&
(cid:13)c ESO2017 Astrophysics
Sardinia Radio Telescope observations of Abell 194
The intra-cluster magnetic field power spectrum
F.Govoni1,M.Murgia1,V.Vacca1,F.Loi1,2,M.Girardi3,4,F.Gastaldello5,6,G.Giovannini7,8,L.Feretti7,
R.Paladino7,E.Carretti1,R.Concu1,A.Melis1,S.Poppi1,G.Valente9,1,G.Bernardi10,11,A.Bonafede7,12,
W.Boschin13,14,15,M.Brienza16,17,T.E.Clarke18,S.Colafrancesco19,F.deGasperin20,D.Eckert21,T.A.Enßlin22,
C.Ferrari23,L.Gregorini7,M.Johnston-Hollitt24,H.Junklewitz25,E.Orrù16,P.Parma7,R.Perley26,M.Rossetti5,
G.BTaylor27,andF.Vazza7,12
(Affiliationscanbefoundafterthereferences)
Received23December2016/Accepted23March2017
ABSTRACT
Aims.Westudytheintra-clustermagneticfieldinthepoorgalaxyclusterAbell194bycomplementingradiodata,atdifferentfrequencies,with
dataintheopticalandX-raybands.
Methods.WeanalyzednewtotalintensityandpolarizationobservationsofAbell194obtainedwiththeSardiniaRadioTelescope(SRT).Weused
theSRTdataincombinationwitharchivalVeryLargeArrayobservationstoderiveboththespectralagingandrotationmeasure(RM)imagesof
theradiogalaxies3C40Aand3C40BembeddedinAbell194.Toobtainnewadditionalinsightsintotheclusterstructure,weinvestigatedthe
redshiftsof1893galaxies,resultinginasampleof143fiducialclustermembers.WeanalyzedtheavailableROSATandChandraobservationsto
measuretheelectrondensityprofileofthegalaxycluster.
Results. The optical analysis indicates that Abell 194 does not show a major and recent cluster merger, but rather agrees with a scenario of
accretionofsmallgroups,mainlyalongtheNE−SWdirection.Undertheminimumenergyassumption,thelifetimesofsynchrotronelectrons
in3C40Bmeasuredfromthespectralbreakarefoundtobe157±11Myr.Thebreakfrequencyimageandtheelectrondensityprofileinferred
fromtheX-rayemissionareusedincombinationwiththeRMdatatoconstraintheintra-clustermagneticfieldpowerspectrum.Byassuming
a Kolmogorov power-law power spectrum with a minimum scale of fluctuations of Λ =1kpc, we find that the RM data in Abell 194 are
min
well described by a magnetic field with a maximum scale of fluctuations of Λ =(64±24)kpc. We find a central magnetic field strength
max
of(cid:104)B (cid:105)=(1.5±0.2)µG,which isthelowest evermeasuredso faringalaxy clustersbasedon Faradayrotationanalysis. Further out,thefield
0
decreaseswiththeradiusfollowingthegasdensitytothepowerofη = 1.1±0.2.ComparingAbell194withasmallsampleofgalaxyclusters,
thereisahintofatrendbetweencentralelectrondensitiesandmagneticfieldstrengths.
Keywords. galaxies:clusters:general–galaxies:clusters:individual:Abell194–magneticfields–large-scalestructureofUniverse
1. Introduction Highly polarized elongated radio sources named relics are
alsoobservedattheperipheryofmergingsystems(e.g.,Clarke
&Ensslin2006;Bonafedeetal.2009b;vanWeerenetal.2010).
Galaxy clusters are unique laboratories for the investigation of
These radio sources trace the regions where the propagation of
turbulent fluid motions and large-scale magnetic fields (e.g.,
mildly supersonic shock waves compresses the turbulent intra-
Carilli & Taylor 2002; Govoni & Feretti 2004; Murgia 2011).
Inthelastfewyears,severaleffortshavebeenfocusedondeter- clustermagneticfield,therebyenhancingthepolarizedemission
miningtheeffectivestrengthandstructureofmagneticfieldsin andacceleratingtherelativisticelectronsresponsibleforthesyn-
chrotronemission.
galaxyclustersandthistopicrepresentsakeyprojectinviewof
theSquareKilometreArray(e.g.,Johnston-Hollittetal.2015).
Acomplementarysetofinformationongalaxyclustermag-
Synchrotronradiohalosatthecenterofgalaxyclusters(e.g., netic fields can be obtained from high quality rotation measure
Feretti et al. 2012; Ferrari et al. 2008) provide direct evidence (RM)imagesofpowerfulandextendedradiogalaxies.Thepres-
ofthepresenceofrelativisticparticlesandmagneticfieldsasso- ence of a magnetized plasma between an observer and a radio
ciated with intra-cluster medium. In particular, the detection of source changes the properties of the incoming polarized emis-
polarized emission from radio halos is key to investigating the sion. In particular, the position angle of the linearly polarized
magneticfieldpowerspectrumingalaxyclusters(Murgiaetal. radiationrotatesbyanamountthatisproportionaltothelinein-
2004;Govonietal.2006,2013,2015;Vaccaetal.2010).How- tegralofthemagneticfieldalongtheline-of-sighttimestheelec-
ever,detectingthispolarizedsignalisaveryhardtaskwithcur- trondensityoftheinterveningmedium,i.e.,theso-calledFara-
rentradiofacilitiesandsofaronlythreeexamplesoflarge-scale day rotation effect. Therefore, information on the intra-cluster
filamentarypolarizedstructureshavebeendetectedthatarepos- magnetic fields can be obtained, in conjunction with X-ray ob-
siblyassociatedwithhaloemission(A2255;Govonietal.2005; servationsofthehotgas,throughtheanalysisoftheRMofradio
Pizzo et al. 2011, MACS J0717.5+3745; Bonafede et al. 2009; galaxiesinthebackgroundorinthegalaxyclustersthemselves.
A523;Girardietal.2016). Rotation measure studies have been performed on statistical
ArticlepublishedbyEDPSciences A122,page1of26
A&A603,A122(2017)
samples(e.g.,Clarkeetal.2001;Johnston-Hollitt&Ekers2004; (18−26GHz),amono-feedC-bandreceiver(5700−7700GHz),
Govonietal.2010)aswellasindividualclusters(e.g.,Perley& and a coaxial dual-feed L/P-band receiver (305−410 MHz,
Taylor 1991; Taylor & Perley 1993; Feretti et al. 1995, 1999; 1300−1800MHz).
Taylor et al. 2001; Eilek & Owen 2002; Pratley et al. 2013). The antenna was officially opened on 30 September 2013,
These studies reveal that magnetic fields are widespread in the upon completion of the technical commissioning phase (Bolli
intra-clustermedium,regardlessofthepresenceofadiffusera- et al. 2015). The scientific commissioning of the SRT was car-
diohaloemission. riedoutintheperiod2012–2015(Prandonietal.2017).Atthe
The RM distribution seen over extended radio galaxies is beginningof2016thefirstcallforsingledishearlysciencepro-
generallypatchy,indicatingthatmagneticfieldsarenotregularly gramswasissued,andtheobservationsstartedonFebruary1st,
ordered on cluster scales, but instead they have turbulent struc- 2016.
turesdowntolinearscalesassmallasafewkpcorless.There- The SMOG project is an SRT early science program (PI
fore,RMmeasurementsprobethecomplextopologyoftheclus- M. Murgia) focused on a wide-band and wide-field single
termagneticfieldandindeedstate-of-the-artsoftwaretoolsand dish spectral-polarimetric study of a sample of galaxy clusters.
approaches based on a Fourier domain formulation have been By comparing and complementing the SRT observations with
developed to constrain the magnetic field power spectrum pa- archivalradiodataathigherresolutionandatdifferentfrequen-
rameters on the basis of the RM images (Enßlin & Vogt 2003; cies, but also with data in optical and X-ray bands, we want to
Murgia et al. 2004; Laing et al. 2008; Kuchar & Enßlin 2011; improve our knowledge of the non-thermal components of the
Bonafede et al. 2013). The magnetic field power spectrum has intra-clustermediumonalargescale.Ouraimisalsotounder-
beenestimatedinsomegalaxyclustersandgalaxygroupscon- stand the interplay between these components (relativistic par-
taining radio sources with very detailed RM images (e.g., Vogt ticles and magnetic fields) and the life cycles of cluster radio
&Enßlin2003;Murgiaetal.2004;Vogt&Enßlin2005;Govoni galaxiesbystudyingboththespectralandpolarizationproperties
et al. 2006; Guidetti et al. 2008; Laing et al. 2008; Guidetti oftheradiosourceswiththeSRT(see,e.g.,thecaseof3C129;
et al. 2010; Bonafede et al. 2010; Vacca et al. 2012). RM data Murgiaetal.2016).Forthispurpose,weselectedasuitablesam-
are usually consistent with volume averaged magnetic fields of pleofnearbygalaxyclustersknownfromtheliteraturetoharbor
(cid:39)0.1–1 µG over 1 Mpc3. The central magnetic field strengths diffuseradiohalos,relics,orextendedradiogalaxies.
are typically a few µG, but stronger fields, with values exceed- We included Abell 194 in the SMOG sample because it is
ing (cid:39)10µG, are measured in the inner regions of relaxed cool- one of the rare clusters hosting more than one luminous and
ingcoreclusters.Thereareseveralindicationsthatthemagnetic extended radio galaxy close to the cluster center. In particular,
field intensity decreases going from the center to the periphery it hosts the radio source 3C40 (PKS 0123-016), which is in-
followingtheclustergasdensityprofile.Thishasbeenillustrated deedconstitutedbytworadiogalaxieswithdistortedmorpholo-
bymagneto-hydrodynamicalsimulations(see,e.g.,Dolagetal. gies (3C40A and 3C40B). This galaxy cluster has been exten-
2002;Brüggenetal.2005;Xuetal.2012;Vazzaetal.2014)and sivelyanalyzedintheliteraturewithradiointerferometers(e.g.,
confirmedinRMdata. O’Dea & Owen 1985; Jetha et al. 2006; Sakelliou et al. 2008;
Inthispaperweaimatimprovingourknowledgeoftheintra- Bogdánetal.2011).Herewepresent,forthefirsttime,totalin-
cluster magnetic field in Abell 194. This nearby (z = 0.018; tensity and polarization single-dish observations obtained with
Struble&Rood1999)andpoor(richnessclassR=0;Abelletal. the SRT at 6600 MHz. The importance of mapping the radio
1989) galaxy cluster belongs to the Sardinia Radio Telescope galaxies in Abell 194 with a single dish is that interferometers
(SRT)MultifrequencyObservationsofGalaxyclusters(SMOG) suffer the technical problem of not measuring the total power;
sample, an early science program of the new SRT radio tele- the so-called missing zero spacing problem. Indeed, they filter
scope. For our purpose, we investigated the total intensity and outstructureslargerthantheangularsizecorrespondingtotheir
polarization properties of two extended radio galaxies embed- shortestspacing,thuslimitingthesynthesisimagingofextended
dedinAbell194incombinationwithdatainopticalandX-ray structures. Single dish telescopes are optimal for recovering all
bands.Thepaperisorganizedasfollows.InSect.2,wedescribe of the emission on large angular scales, especially at high fre-
theSMOGproject.InSect.3,wepresenttheradioobservations quencies(>1GHz).Althoughsingledishestypicallyhavealow
anddatareduction.InSect.4,wepresenttheradio,optical,and resolution,theradiogalaxiesatthecenterofAbell194aresuffi-
X-ray properties of Abell 194. In Sect. 5, we complement the cientlyextendedtobewellresolvedwiththeSRTat6600MHz
SRT total intensity data with archival VLA observations to de- attheresolutionof2.9(cid:48).
rivespectralaginginformationoftheradiogalaxies.InSect.6, The SRT data at 6600 MHz are used in combination with
we complement the SRT polarization data with archival VLA archival Very Large Array observations at lower frequencies to
observations to derive detailed multiresolution RM images. In derive the trend of the synchrotron spectra along 3C40A and
Sect. 7, we use numerical simulations to investigate the cluster 3C40B. In addition, linearly polarized emission is clearly de-
magnetic field by analyzing the RM and polarization data. Fi- tected for both sources and the resulting polarization angle im-
nally,inSect.8wesummarizeourconclusions. ages are used to produce detailed RM images at different an-
ThroughoutthispaperweassumeaΛCDMcosmologywith gular resolutions. 3C40B and 3C40A are very extended both
H0 =71kms−1Mpc−1, Ωm =0.27, and ΩΛ =0.73. At the dis- in angular and linear size, therefore they represent ideal cases
tanceofAbell194,1(cid:48)(cid:48) correspondsto0.36kpc. forstudyingtheRMoftheclusteralongdifferentlinesofsight.
In addition, the close distance of Abell 194 permits a detailed
investigation of the cluster magnetic field structure. Following
2. SRTMultifrequencyobservationsofGalaxy Murgia et al. (2004), we simulated Gaussian random magnetic
field models and we compared the observed data and the syn-
clusters
thetic images with a Bayesian approach (Vogt & Enßlin 2005)
The SRT is a new 64 m single dish radio telescope located to constrain the strength and structure of the magnetic field as-
north of Cagliari, Italy. In its first light configuration, the SRT sociated with the intra-cluster medium. Until recently, most of
is equipped with three receivers: a 7-beam K-band receiver the work on cluster magnetic fields has been devoted to rich
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Table1.DetailsoftheSRTobservationscenteredonAbell194.
Frequency Resolution Timeonsource SRTproj. Obs.date OTFmapping Calibrators
(MHz) ((cid:48)) (min)
6000−7200 2.9 32 S0001 1-Feb-2016 1RA 3C138,3C84
6000−7200 2.9 384 S0001 3-Feb-2016 6RA×6Dec 3C138,3C286,3C84
6000−7200 2.9 448 S0001 6-Feb-2016 7RA×7Dec 3C286,3C84
Notes.RA =01h25m59.9s;Dec =−01◦20(cid:48)33(cid:48)(cid:48).Column1:SRTfrequencyrange;Col.2:SRTresolution;Col.3:timeonsource;Col.4:
J2000 J2000
SRTprojectname;Col.5:dateofobservation;Col.6:numberofimagesonthesource;Col.7:calibrators.
galaxy clusters. Little attention has been given in the literature positionofthepolarizationangleusingasreferencetheprimary
to magnetic fields associated with poor galaxy clusters such as polarization calibrator 3C286 and 3C138. The difference be-
Abell 194 and galaxy groups (see, e.g., Laing 2008; Guidetti tween the observed and predicted position angle according to
et al. 2008, 2010). Magnetic fields in these systems deserve to Perley&Butler(2013)wasdeterminedandcorrectedchannelby
be investigated in detail since, being more numerous, are more channel. The calibrated position angle was within the expected
representativethanthoseofrichclusters. value of 33◦ and −11.9◦ for 3C286 and 3C138, respectively,
witharmsscatterof±1◦.
In the following we describe the total intensity and polar-
3. Radioobservationsanddatareduction ization imaging. For further details on the calibration and data
handlingoftheSRTobservations,seeMurgiaetal.(2016).
3.1. SRTobservations
We observed with the SRT, for a total exposure time of about
3.1.1. Totalintensityimaging
14.4 h, an area of 1 deg × 1 deg centered on the galaxy clus-
terAbell194usingtheC-bandreceiver.Full-Stokesparameters TheimagingwasperformedinSCUBEbysubtractingthebase-
wereacquiredwiththeSARDARAbackend(SArdiniaRoach2- line from the calibrated telescope scans and by projecting the
based Digital Architecture for Radio Astronomy; Melis et al. datainaregularthree-dimensionalgrid.At6600MHzweused
2017),whichisoneofthebackendsavailableattheSRT(Melis aspatialresolutionof0.7(cid:48)/pixel(correspondingtotheseparation
etal.2014).Weusedthecorrelatorconfigurationwith1500MHz ofthetelescopescans),whichisenoughinourcasetosamplethe
and 1024 frequency channels of approximately 1.46MHz in beamFWHMwithfourpixels.
width. We observed in the frequency range 6000−7200 MHz Asafirststep,thebaselinewassubtractedbyalinearfitin-
at a central frequency of 6600 MHz. We performed several volving only the 10% of data at the beginning and end of each
on-the-fly (OTF) mappings in the equatorial frame alternating scan. The baseline removal was performed channel by chan-
the right ascension (RA) and declination (Dec). The telescope nelforeachscan.Allfrequencycubesobtainedbygriddingthe
scanning speed was set to 6 arcmin/s and the scans were sep- scans along the two orthogonal axes (RA and Dec) were then
arated by 0.7(cid:48) to properly sample the SRT beam whose full stackedtogethertoproducefull-StokesI,Q,Uimagesofanarea
widthathalfmaximum(FWHM)is2.9(cid:48)inthisfrequencyrange. of1squaredegreecenteredonthegalaxyclusterAbell194.In
We recorded the data stream sampling at 33 spectra per sec- thecombination,theindividualimagecubeswereaveragedand
ond,thereforeindividualsampleswereseparatedontheskyby destrippedbymixingtheirStationaryWaveletTransform(SWT)
10.9(cid:48)(cid:48) alongthescanningdirection.AsummaryoftheSRTob- coefficients (see Murgia et al. 2016, for details). We then used
servations is listed in Table1. Data reduction was performed thehighersignal-to-noise(S/N)imagecubesobtainedfromthe
with the proprietary Single-dish Spectral-polarimetry Software SWT stacking as a prior model to refine the baseline fit. The
(SCUBE;Murgiaetal.2016). baseline subtraction procedure was then repeated including not
Bandpass,fluxdensity,andpolarizationcalibrationwerecar- just the 10% from the begin and the end of each scan. In the
riedoutwithatleastfourcrossscansonsourcecalibratorsper- refinementstage,thebaselinewasrepresentedwitha2ndorder
formedatthebeginningandattheendofeachobservingsection. polynomialusingtheregionsofthescansfreeofradiosources.
Bandpassandfluxdensitycalibrationwereperformedbyob- A new SWT stacking was then performed and the process was
serving 3C286 and 3C138, assuming the flux density scale of iteratedafewmoretimesuntiltheconvergencewasreached.
Perley & Butler (2013). After a first bandpass and flux density Close to the cluster center, the dynamical range of the
calibration cycle, persistent radio frequency interference (RFI) C-bandtotalintensityimagewaslimitedbythetelescopebeam
were flagged using observations of a cold part of the sky. The patternratherthanbytheimagesensitivity.InSCUBE,weused
flaggeddatawerethenusedtorepeatthebandpassandfluxden- abeammodelpattern(Murgiaetal.2016)foraproperdeconvo-
sitycalibrationforafinerRFIflagging.Theprocedurewasiter- lutionoftheskyimagefromtheantennapattern.Thedeconvolu-
atedafewtimes,eliminatingallofthemostobviousRFI.Weap- tionalgorithminteractivelyfindsthepeakintheimageobtained
pliedthegain-elevationcurvecorrectiontoaccountforthegain fromtheSWTstackingofallimagesandsubtractsafixedgain
variation with elevation due to the telescope structure gravita- fraction(typically 0.1)of thispointsource fluxconvolved with
tionalstresschange. the reprojected telescope “dirty beam model” from the individ-
Weperformedthepolarizationcalibrationbycorrectingthe ualimages.Inthereprojection,theexactelevationandparallac-
instrumental polarization and the absolute polarization angle. tic angle for each pixel in the unstacked images are used. The
The on-axis instrumental polarization was determined through residual images were stacked again and the CLEAN continued
observations of the bright unpolarized source 3C84. The leak- until a threshold condition was reached. Given the low level of
age of Stokes I into Q and U is in general less than 2% across thebeamsidelobescomparedtointerferometricimages,ashal-
the band with a rms scatter of 0.7–0.8%. We fixed the absolute lowdeconvolutionwassufficientinourcase,andwedecidedto
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Table2.DetailsofVLAarchivalobservationsofAbell194analyzedinthispaper.
Frequency VLAConfig. Bandwidth Timeonsource VLAProj. Obs.date Calibrators
(MHz) (MHz) (min)
C-band
4535/4885 D 50 15 AC557 01-Oct.-2000 3C48,3C138,0122−003
L-band
1443/1630 C 12.5 142 AV102 02-Jun.-1984 3C48,3C138,0056−001
1443/1630 D 25 82 AV112A 31-Jul.-1984 3C286,0106+013
1465/1515 D 25 9 AL252 18-Sep.-1992 3C48,3C286,0056−001
P-band
327.5/333.0 B 3.125 316 AE97 16-Aug.-1994 3C48
327.5/333.0 C 3.125 40 AE97 20-Nov.-1994 3C48
Notes.Column1:observingfrequency(IF1/IF2);Col.2:VLAconfiguration;Col.3:observingbandwidth;Col.4:timeonsource;Col.5:VLA
projectname;Col.6:dateofobservation;Col.7:calibrators.
stop the CLEAN at the first negative component encountered. self-calibrationwereappliedwhenproventobeusefultoremove
As a final step, CLEAN components at the same position were residualphasevariations.ImagesoftheStokesparametersI,Q,
merged, smoothed with a circular Gaussian beam with FWHM andU wereproducedforeachfrequencyandconfigurationsep-
2.9(cid:48), and then restored back in the residuals image to obtain a aratelyprovidingdifferentangularresolutions.SincetheC-band
CLEANedimage. dataconsistofthreeseparateshortpointings,inthiscase,thefi-
nal I, Q, and U images were obtained by mosaicking the three
differentpointingswiththeAIPStaskFLATN.Finally,P,FPOL,
3.1.2. Polarizationimaging andΨimageswerethenderivedfromtheI,Q,andU images.
ThepolarizationimagingatC bandofStokesparametersQand Data in P band were obtained in spectral line mode divid-
U was performed following the same procedures described for ingthebandwidthof3.125MHzin31channels.Weeditedthe
the total intensity imaging: baseline subtraction, gridding, and data to excise RFI channel by channel. We performed the am-
SWTstacking.Therewerenocriticaldynamicrangeissueswith plitudeandbandpasscalibrationwiththesource3C48.Theflux
thepolarizationimage,andthusnodeconvolutionwasrequired. density of the calibrator was calculated accordingly to the low
However,sincethecontributionoftheoff-axisinstrumentalpo- frequencycoefficientsofScaife&Heald(2012).Intheimaging,
larization can affect the quality of polarization data if bright thedatawereaveragedintofivechannels.Thedataweremapped
sources are present in the image, we corrected for the off-axis usingawide-fieldimagingtechnique,whichcorrectsfordistor-
instrumental polarization by deconvolving the Stokes Q and U tions in the image caused by the non-coplanarity of the VLA
beam patterns. In particular, SCUBE uses the CLEAN compo- overawidefieldofview.Asetofsmalloverlappingmapswas
nents derived from the deconvolution of the beam pattern from usedtocoverthecentralareaofabout∼2.4◦ inradius(Cornwell
the total intensity image to subtract the spurious off-axis polar- & Perley 1992). However, at this frequency confusion lobes of
ization from each individual Q and U scans before their stack- sources far from the center of the field are still present. Thus,
ing.Theoff-axisinstrumentalpolarizationlevelcomparedtothe we also obtained images of strong sources in an area of about
StokesIpeakis0.3%. ∼6◦ inradius,searchedintheNRAOVLASkySurvey(NVSS;
Finally,polarizedintensityP= (cid:112)Q2+U2(correctedforthe Condon et al. 1998) catalog. All these “facets” were included
positivebias),fractionalpolarizationFPOL=P/I,andposition in CLEAN and used for several loops of phase self-calibration
angleofpolarizationΨ=0.5tan−1(U/Q)imageswerethende- (Perley 1999). To improve the u-v coverage and sensitivity we
combinedthedatasetsinBandCconfiguration.
rivedfromtheI,Q,andU images.
3.2. ArchivalVLAobservations 4. MultiwavelengthspropertiesofAbell194
WeanalyzedarchivalobservationsobtainedwiththeVLAatdif- Inthefollowingwepresenttheradio,optical,andX-rayproper-
ferent frequencies and configurations. The details of the obser- tiesofthegalaxyclusterAbell194.
vationsareprovidedinTable2.Thedatawerereducedusingthe
NRAOAstronomicalImageProcessingSystem(AIPS)package.
4.1. Radioproperties
ThedatainC bandandin Lbandwerecalibratedinampli-
tude,phase,andpolarizationangleusingthestandardcalibration In Fig.1, we show the radio, optical, and X-ray emission of
procedure. The flux density of the calibrators were calculated Abell 194. The field of view of the left panel of Fig.1 is
accordinglytothefluxdensityscaleofPerley&Butler(2013). (cid:39)1.3×1.3 Mpc2. In this panel, the contours of the CLEANed
Phasecalibrationwasderivedfromnearbysources,whichwere radio image obtained with the SRT at 6600 MHz are overlaid
periodicallyobservedoverawiderangeinparallacticangle.The on the X-ray ROSAT PSPC image in the 0.4–2keV band (see
radio sources 3C286 or 3C138 were used as reference for the Sect.4.3).TheSRTimagewasobtainedbyaveragingallthefre-
absolutepolarizationangles.Phasecalibratorswerealsousedto quency channels from 6000 MHz to 7200 MHz. We reached
separatethesourcepolarizationpropertiesfromtheantennapo- a final noise level of 1 mJy/beam and an angular resolution
larizations.Imagingwasperformedfollowingthestandardpro- of 2.9(cid:48) FWHM. The radio galaxy 3C40B, close to the clus-
cedure: Fourier transform, clean, and restore. A few cycles of ter X-ray center, extends for about 20(cid:48). The peak brightness
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G
F
Q
O
P
-01 00 00
500 kpc N
S -01 10 00
I
0) J E
0
0
2
N (J -01 15 00 D 000) -01 15 00
TIO M N (J2
A O
CLIN LINATI -01 20 00 3C40B
DE -01 30 00 L C K DEC 3C40A
-01 25 00
H
A B
-01 45 00 -01 30 00
01 26 20 01 26 00 01 25 40 01 25 20
RIGHT ASCENSION (J2000)
01 28 00 01 27 00 01 26 00 01 25 00 01 24 00
RIGHT ASCENSION (J2000)
Fig.1. Left:SRTradioimageat6600MHz(contours)overlaidontheROSATPSPCX-rayimage(colors)inthe0.4–2keVband.TheSRTimage
resultsfromthespectralaverageofthebandwidthbetween6000and7200MHz.Ithasasensitivityof1mJy/beamandanangularresolutionof
√
2.9(cid:48).Thefirstradiocontourisdrawnat3mJy/beam(3σ)andtherestarespacedbyafactorof 2.SRTsourceswithanNVSScounterpartare
I
labeledwiththelettersAtoS.TheX-rayimage(ID=800316p)isexposurecorrected(T (cid:39)24ks)andadaptivelysmoothed(seeSect.4.3).
exp
ThegreencircleiscenteredontheclusterX-raycentroidandtheradiusindicatestheclustercoreradiusr .Right:VLAradioimageat1443MHz
c
(contours)isoverlaidontheopticalemissioninthegMega band(grayscale).TheVLAimagehasasensitivityof0.34mJy/beamandanangular
resolutionof19(cid:48)(cid:48).Thefirstradiocontourisdrawnat1mJy/beam(3σ)andtherestarespacedbyafactorof2.
I
of 3C40B ((cid:39)600 mJy/beam) is located in the southern lobe. blendsofmultipleNVSSsources.Therearealsoafewsources
The narrow angle-tail radio galaxy 3C40A (peak brightness visibleintheNVSSbutnotdetectedatthesensitivitylevelofthe
(cid:39)150mJy/beam)isonlyslightlyresolvedattheSRTresolution SRTimage.Thesearelikelysteepspectralindexradiosources.
anditisnotclearlyseparatedfrom3C40B. The SRT contours show an elongation in the northern lobe of
Thedetailsofthemorphologyofthetworadiogalaxiescan 3C40Btowardthewest.Thiselongationislikelyduetothepoint
beappreciatedintherightpanelofFig.1,whereweshowafield sourcelabeledwithS,whichisdetectedbothinthe1443MHz
ofviewof(cid:39)0.5×0.5Mpc2.Inthispanel,thecontoursofthera- VLA image at 19(cid:48)(cid:48) resolution in the right panel of Fig.1, and
dioimageobtainedwiththeVLAat1443MHzareoverlaidon intheNVSS.The1443MHzimagealsoshowsthepresenceof
the optical emission of the cluster. We retrieved the optical im- anotherpointsourcelocatedontheeastofthenorthernlobeof
ageinthegMegabandfromtheCADCMegapipe1archive(Gwyn 3C40B. This point source is blended with 3C40B both at the
2008).TheVLAradioimagewasobtainedfromtheLbanddata SRTandattheNVSSresolution.
setinCconfiguration.Ithasasensitivityof0.34mJy/beamand Given that 3C40A and 3C40B are not clearly separated
an angular resolution of 19(cid:48)(cid:48). The distorted morphology of the at the SRT resolution, we calculated the flux density for the
tworadiogalaxies3C40Band3C40Aiswellvisible.Bothra- two sources together by integrating the total intensity image at
dio galaxies show an optical counterpart and their host galax- 6600MHzdowntothe3σ isophote.Itresults(cid:39)1.72±0.05Jy.
I
ies are separated by 4.6(cid:48) ((cid:39)100 kpc). The core of the extended This flux also contains the flux of the two discrete sources lo-
source 3C40B is associated with NGC547, which is known to catedinthenorthernlobeof3C40Bmentionedabove.
formadumbbellsystemwiththegalaxyNGC545(e.g.Fasano In Table3, we list the basic properties of the faint radio
et al. 1996). 3C40A is a narrow angle-tail radio galaxy associ- sources detected in the field of the SRT image. For the unre-
ated with the galaxy NGC541 (O’Dea & Owen 1985). The jet solved sources, we calculated the flux density by means of a
emanating from 3C40A is believed to be responsible for trig- two-dimensional Gaussian fit. Along with the SRT coordinates
gering star formation in Minkowski’s object (e.g., van Breugel andtheSRTfluxdensities,wealsoreporttheirNVSSname,the
et al. 1985; Brodie et al. 1985; Croft et al. 2006), which is a NVSSfluxdensityat1400MHz,andtheglobalspectralindices
star-formingpeculiargalaxynearNGC541. (S ∝ν−α)between1400and6600MHz.
ν
In addition to 3C40A and 3C40B, some other faint radio In Fig.2, we show the total intensity SRT contours lev-
sourcesweredetectedintheAbell194field.Intheleftpanelof els overlaid on the linearly polarized intensity image P at
Fig.1,SRTsourceswithanNVSScounterpartarelabeledwith 6600 MHz. The polarized intensity was corrected for both
thelettersAtoS.SourceslabeledwithE, J,andQareactually the on-axis and off-axis instrumental polarization. The noise
level, after the correction for the polarization bias, is σ =
P
1 http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/ 0.5 mJy/beam. Polarization is detected for both 3C40B and
megapipe/ 3C40A with a global fractional polarization of (cid:39)9%. The peak
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A&A603,A122(2017)
Table3.FluxdensitymeasurementsoffaintradiosourcesdetectedintheAbell194field.
Source Label RAJ2000 DecJ2000 S S α1400MHz
6600MHz 1400MHz 6600MHz
(hms) (◦(cid:48)(cid:48)(cid:48)) (mJy) (mJy)
NVSSJ012748-014134 A 012748 −014105 11.5±0.7 24.6±2.3 0.49±0.13
NVSSJ012622-013756 B 012621 −013800 5.3±0.7 11.0±0.6 0.47±0.29
NVSSJ012513-012800 C 012513 −012758 13.3±0.8 32.9±1.1 0.58±0.10
NVSSJ012438-011232 D 012438 −011225 11.0±0.8 35.7±1.5 0.76±0.10
BlendsofNVSSsources E 012403 −010411 13.9±1.2 − −
NVSSJ012542-005302 F 012541 −005306 7.7±0.7 8.4±1.4 0.06±0.36
NVSSJ012624-005101 G 012624 −005126 5.8±0.8 21.0±1.1 0.83±0.18
NVSSJ012401-013706 H 012403 −013718 9.7±2.1 19.3±0.8 0.44±0.17
NVSSJ012628-010418 I 012626 −010419 4.1±0.8 19.4±1.4 1.00±0.22
BlendsofNVSSsources J 012651 −010735 4.8±0.8 − −
NVSSJ012410-013357 K 012410 −013351 5.0±0.8 7.3±0.6 0.24±0.42
NVSSJ012746-013446 L 012746 −013410 2.7±0.7 3.9±0.5 0.24±0.78
NVSSJ012450-012208 M 012447 −012200 3.7±0.8 5.1±0.7 0.21±0.60
NVSSJ012431-010459 N 012436 −010446 2.9±0.7 7.1±1.5 0.58±0.47
NVSSJ012426-005827 O 012425 −005825 4.0±1.6 7.9±0.6 0.44±0.40
NVSSJ012557-005442 P 012557 −005519 2.9±0.6 4.6±0.7 0.30±0.67
BlendsofNVSSsources Q 012658 −005646 4.6±0.7 − −
NVSSJ012406-005638 R 012403 −005705 3.6±0.7 4.0±0.5 0.07±0.75
NVSSJ012538-011139 S − − − 16.1±1.5 −
Notes.Column1:NVSScross-identification;Col.2:sourcelabel(seeFig.1);Cols.3and4:coordinatesofthepeakintensityintheSRTimage;
Col. 5: flux density at 6600MHz, taken from the SRT image; Col. 6: flux density at 1400MHz, taken from the NVSS; Col. 7: spectral index
between1400and6600MHz.
polarizedintensityisof(cid:39)19mJy/beam,locatedinthesouthlobe Previous studies of Abell 194, based on redshift data, have
of 3C40B, not matching the total intensity peak. The origin of found a low value of the velocity dispersion of galaxies within
thismismatchcouldbeattributedtodifferenteffects.Apossibil- the cluster (∼350−400 km s−1; Chapman et al. 1988; Girardi
ity is that the magnetic field inside the lobe of the radio source etal.1998)andalowvalueofthemass.Twoindependentanaly-
isnotcompletelyordered.Indeed,alongthelineofsightatthe sesagreeindeterminingamassofM ∼1×1014h−1M within
200 (cid:12)
position of the peak intensity, we may have by chance two (or theradius2R ∼1Mpc(Girardietal.1998;Rinesetal.2003).
200
more)magneticfieldstructuresthatarenotperfectlyalignedin Early analyses of redshift data samples of ∼100–150 galaxies
the plane of the sky so that the polarized intensity is reduced, within3–6Mpc,mostlyderivedbyChapmanetal.(1988),con-
while the total intensity is unaffected. This depolarization is a firmed that Abell 194 is formed of a main system elongated
pure geometrical effect related to the intrinsic ordering of the alongtheNE−SWdirectionanddetectedminorsubstructurein
magnetic field of the source and may be present even if there boththecentralandexternalclusterregions(Girardietal.1997;
is no Faraday rotation inside and/or outside the radio emitting Bartonetal.1998;Nikogossyanetal.1999).
plasma.AnotherpossibilityisthatFaradayrotationisoccurring To obtain new additional insights into the cluster struc-
inanexternalscreenandthepeakintensityislocatedinprojec- ture, we considered more recent redshift data. Rines et al.
tioninaregionofahighRMgradient(seeRMimageinFig.8). (2003)compiledredshiftsfromtheliteratureascollectedbythe
In this case, the beam depolarization is expected to reduce the NASA/IPAC Extragalactic Database (NED), including the first
polarized signal but not the total intensity. Finally, there could SloanDigitalSkySurvey(SDSS)data.Here,wealsoaddedfur-
be also internal Faraday rotation, but the presence of an X-ray ther data extracted from the last SDSS release (DR12). In par-
cavity(seeSect.4.3)suggeststhatthesouthernlobeisdevoidof ticular,toanalyzethe2R clusterregion,weconsideredgalax-
200
thermal gas and, furthermore, the observed trend of the polar- ies within 2 Mpc (∼93(cid:48)) from the cluster center (here taken as
izedanglesareconsistentwiththeλ2 lawthatpointsinfavorof the galaxy 3C40B/NGC547). Our galaxy sample consists of
anexternalFaradayscreen(seeSect.6). 1893galaxies.AftertheapplicationoftheP+Gmembershipse-
In 3C40B the fractional polarization increases along the lection (Fadda et al. 1996; Girardi et al. 2015) we obtained a
source from 1–3% in the central brighter part up to 15–18% in sample of 143 fiducial members. The P+G membership selec-
thelowsurfacebrightnessassociatedwiththenorthernlobe.The tion is a two-step method. First, we used the one-dimensional
otherfaintsourcesdetectedintheAbell194fieldarenotsignif- adaptive-kernelmethodDEDICA(Pisani1993)todetectthesig-
icantlypolarizedintheSRTimage. nificant cluster peak in the velocity distribution. All the galax-
iesassignedtotheclusterpeakareanalyzedinthesecondstep,
whichusesacombinationofpositionandvelocityinformation,
4.2. Opticalproperties
i.e., the “shifting gapper” method by Fadda et al. (1996). This
procedure rejects galaxies that are too far in velocity from the
OneoftheintriguingpropertiesofAbell194isthatitappearsas
mainbodyofgalaxiesandwithinafixedbinthatshiftsalongthe
a “linear cluster”, as its galaxy distribution and X-ray emission
are both linearly elongated along the NE−SW direction (Rood
& Sastry 1971; Struble & Rood 1987; Chapman et al. 1988; 2 TheradiusR istheradiusofaspherewithmassoverdensityδtimes
δ
Nikogossyanetal.1999;Jones&Forman1999). thecriticaldensityattheredshiftofthegalaxysystem.
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F.Govonietal.:SardiniaRadioTelescopeobservationsofAbell194
0 0.005 0.01 0.015
JY/BEAM
100%
-01 00 00
)
0
0
0
2
J -01 15 00
(

N
O
I
T
A
N
I
L
C
E -01 30 00
D
-01 45 00
300 kpc
01 27 00 01 26 00 01 25 00 01 24 00
RIGHT ASCENSION (J2000)
Fig.2.SRTlinearlypolarizedintensityimageat6600MHzofthegalaxyclusterAbell194,resultingfromthespectralaverageofthebandwidth
between6000and7200MHz.Thenoiselevel,afterthecorrectionforthepolarizationbias,isσ =0.5mJy/beam.TheFWHMbeamis2.9(cid:48),as
P
indicatedinthebottomleftcorner.Contoursrefertothetotalintensityimage.Levelsstartat3mJy/beam(3σ)andincreasebyafactorof2.The
I
electricfield(E-field)polarizationvectorsareonlytracedforthosepixelswherethetotalintensitysignalisabove5σ,theerroronthepolarization
I
angleislessthan10◦,andthefractionalpolarizationisabove3σ .Thelengthofthevectorsisproportionaltothepolarizationpercentage(with
FPOL
100%representedbythebarinthetopleftcorner).
distance from the cluster center. The procedure is iterated until Byapplyingthebiweightestimatortothe143clustermem-
thenumberofclustermembersconvergestoastablevalue. bers (Beers et al. 1990, ROSTAT software), we computed
a mean cluster line-of-sight (LOS) velocity (cid:104)V(cid:105) = (cid:104)cz(cid:105) =
The comparison with the corresponding galaxy samples of
(5375 ± 143) km s−1, corresponding to a mean cluster red-
Chapmanetal.(1988),whichisformedof84galaxies(67mem-
shift (cid:104)z(cid:105) = 0.017929 ± 0.0004 and a LOS velocity dispersion
bers),showsthatwehavedoubledthedatasampleandstresses
the difficulty of improving the Abell 194 sample when going σV = 425+−3340 km s−1, in good agreement with the estimates by
Rines et al. (2003, see their Table2). There is no evidence of
down to fainter luminosities. Unlike the Chapman et al. (1988)
non-Gaussianityinthegalaxyvelocitydistributionaccordingto
sample,thesamplingandcompletenessofbothoursampleand
tworobustshapeestimators,theasymmetryindexandtailindex,
thatofRinesetal.(2003)arenotuniformand,inparticular,since
andthescaledtailindex(Bird&Beers1993).Wealsoverified
thecenterofAbell194isattheborderofaSDSSstrip,thesouth-
that the three luminous galaxies in the cluster core (NGC547,
ernclusterregionsareundersampledwithrespecttothenorthern
NGC545,andNGC541)havenoevidenceofpeculiarvelocity
regions.Asaconsequence,welimitedouranalysisofsubstruc-
accordingtotheindicatortestbyGebhardt&Beers(1991).
turetothevelocitydistributionandtotheposition-velocitycom-
bineddata.
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A&A603,A122(2017)
Fig. 4. Spatial distribution of the 143 cluster members (small black
Fig. 3. Spatial distribution of the 143 cluster member galaxies. The
points). The galaxies of the two main subgroups detected in the 3D-
largerthecircle,thelargeristhedeviationofthelocalmeanvelocity,
DEDICA analysis are indicated by large symbols. The red squares
ascomputedonthegalaxyandits10neighbors,fromtheglobalmean
and blue rotated squares indicate galaxies of the main and secondary
velocity.Thebluethin-lineandredthick-linecirclesshowwherethelo-
subgroups, respectively. The secondary subgroup is characterized by
calvalueofmeanvelocityislowerorhigherthantheglobalvalue.The
a lower velocity. The larger the symbol size, the larger is the devia-
greendashedellipsesindicatetheregionsofthesubgroupsdetectedby
tion of the galaxy from the mean velocity. The center is fixed on the
Nikogossyanetal.(1999;seetheirFig.13,thesubgroupNo.2isoutside
NGC547galaxy(largeblackcircle).NGC545andNGC541areindi-
ofourfield).ThecenterisfixedontheNGC547galaxy.
catedbysmallcircles.
Weappliedthe∆-statisticsdevisedbyDressler&Schectman
(1988; hereafter DS-test), which is a powerful test for level of 31 and 15 galaxies. Minor subgroups have very low
three-dimensional substructure. The significance is based on density and/or richness and are not discussed. Figure4 shows
1000 Monte Carlo simulated clusters obtained shuffling galax- that both the two DEDICA subgroups are strongly elongated
ies velocities with respect to their positions. We detected a and trace the NE−SW direction but have different velocities.
marginal evidence of substructure (at the 91.3% confidence The main subgroup has a velocity peak of 5463 km s−1, close
level). Figure3 shows the comparison of the DS bubble plot, to the mean cluster velocity, and contains NGC547, NGC545,
here obtained considering only the local velocity kinematical and NGC541. The secondary subgroup has a lower velocity
DSindicator,withthesubgroupsdetectedbyNikogossyanetal. (4897kms−1).
(1999).ThemostimportantsubgroupdetectedbyNikogossyan The picture resulting from new and previous optical results
etal.(1999),No.3,tracestheNE−SWelongatedstructure.In- agreeinthatAbell194doesnotshowtraceofamajorandrecent
sidethis,theregionscharacterizedbyloworhighlocalvelocity clustermerger(e.g.,asinthecaseofabimodalhead-onmerger),
correspond to their No. 1 and No. 5 subgroups. A SE region but rather agrees with a scenario of accretion of small groups,
characterizedbyahighlocalvelocitycorrespondstotheirNo.4 mainlyalongtheNE−SWaxis.
subgroup,theonlyoneoutsidethemainNE−SWclusterstruc-
ture.Theaboveagreementisparticularlymeaningfulwhencon-
sidering that the Nikogossyan et al. (1999) and our results are 4.3. X-rayproperties
basedonquitedifferentsamplesandanalyses.Inparticular,their
hierarchical-treeanalysisweightsgalaxieswiththeirluminosity, The cluster Abell 194 has been observed in X-rays with the
while no weight is applied in our DS test and plot. The galaxy ASCA,ROSAT,Chandra,andXMMsatellites.Sakelliouetal.
withaverylargebluethin-linedcirclehasadifferencefromthe (2008) investigated the cluster relying on XMM and radio ob-
meanczof488kms−1andliesat0.42Mpcfromtheclustercen- servations. The X-ray data do not show any signs of features
ter,whichiswellinsidethecausticlinesreportedbyRinesetal. expected from recent cluster merger activity. They concluded
(2003,seetheirFig.2)andthusitisdefinitelyaclustermember. that the central region of Abell 194 is relatively quiescent and
However, this galaxy lies at the center of a region inhabited by doesnotsufferamajormergerevent,whichisinagreementwith
severalgalaxiesatlowvelocity,resultinginthelargesizeofthe the optical analysis described in Sect.4.2. Bogdán et al. (2011)
circle.Infact,asmentionedinthecaption,thecirclesizerefers analyzed X-ray observations with Chandra and ROSAT satel-
tothelocalmeanvelocityascomputedwithrespecttothegalaxy lites. These authors mapped the dynamics of the galaxy cluster
andits10neighbors. and also detected a large X-ray cavity formed by the southern
We also performed the three-dimensional optimized radio lobe arising from 3C40B. Therefore, this target is partic-
adaptive-kernel method of Pisani (1993, 1996; hereafter 3D- ularlyinterestingforFaradayrotationstudiesbecausethepres-
DEDICA;seealsoGirardietal.2016).Themethoddetectstwo enceofanX-raycavityindicatesthattherotationofthepolariza-
important density peaks, significant at the >99.99% confidence tionplaneislikelytooccurentirelyintheintra-clustermedium,
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F.Govonietal.:SardiniaRadioTelescopeobservationsofAbell194
and flare cleaning. We used blank-sky observations to subtract
thebackgroundcomponentsandtoaccountforvariationsinthe
normalizationoftheparticlebackgroundwerescaledtheblank-
sky background template by the ratio of the count rates in the
10–12keVenergyband.Weextractedaspectrumfromacircular
regionofradius1arcminaroundthecentroidposition.Wefitted
apec
thedatawithan (Smithetal.2001)modelwithATOMDB
xspec
code v2.0.2. in v.12.8.2 (Arnaud 1996). We fixed the
Galactic column density at N = 4.11 × 1020 cm−2 (Kalberla
H
etal.2005);theabundanceisquotedinthesolarunitsofAnders
& Grevesse (1989) and we used the Cash statistic. The best fit
modelgivesatemperatureofkT =2.4±0.3keV,anabundanceof
0.27+0.15Z ,andaxspecnormalizationof1.0±0.1×10−4cm−5
−0.11 (cid:12)
foracstat/d.o.f.=82/83.
Using the ROSAT best fit β-model and the spectral param-
etersobtainedwithChandrathecentralelectrondensitycanbe
expressedbyasimpleanalyticalformula(Eq.(2)ofEttorietal.
2004). In order to assess the error we repeated the measure-
ments after 1000 random realizations of the normalization and
theβ-modelparametersdrawnfromGaussiandistributionswith
mean and standard deviation set by the best fit results. We ob-
tainavalueforthecentralelectrondensityofn =(6.9±0.6)×
0
Fig.5. SurfacebrightnessprofileoftheX-rayemissionofAbell194. 10−4cm−3.Thedistributionofthethermalelectronsdensitywith
TheprofileiscenteredatthepositionoftheX-raycentroid.Thebestfit the distance from the cluster X-ray center r was thus modeled
β-modelisshowninblue. with
(cid:32) r2(cid:33)−23β
n (r)=n 1+ · (1)
since comparatively little thermal gas should be present inside e 0 r2
theradio-emittingplasma. c
Thetemperatureprofileandcoolingtimeof21.6±4.41Gyr
5. Spectralaginganalysis
determinedbyLovisarietal.(2015)andthetemperaturemapby
Bogdán et al. (2011) indicate that Abell 194 does not harbor a We investigated 3C40A and 3C40B at different frequencies to
coolcore.
studytheirspectralindexbehaviorindetail.
We analyzed the ROSAT PSPC pointed observation of We analyzed the images obtained from the VLA Low-
Abell194followingthesameprocedureofEckertetal.(2012) Frequency Sky Survey redux (VLSSr at 74MHz; Lane et al.
to which we refer for a more detailed description. Here we 2014), the VLA archive data in P band (330MHz), the VLA
briefly mention the main steps of the reduction and analy- archive data in L band (1443 and 1630 MHz), and the SRT
sis.TheROSATExtendedSourceAnalysisSoftware(Snowden (6600MHz)data.Wesmoothedalltheimagestothesameres-
et al. 1994) was used for the data reduction. The contribution olutionasthatoftheSRTimage(seetoppanelsofFig.6).The
of the various background components, such as scattered solar relevant parameters of the images smoothed to a resolution of
X-rays, long-term enhancements, and particle background, has 2.9(cid:48) arereportedinTable4.
been taken into account and combined to get a map of all the Wecalculatedthefluxdensitiesof3C40Aand3C40Bto-
non-cosmic background components to be subtracted. We then gether.Thetotalspectrumofthesourcesisshowninthebottom
extractedtheimageintheR37ROSATenergyband(0.4–2keV) panelofFig.6.AlthoughtheradiosourcesinAbell194havea
corrected for vignetting effect with the corresponding exposure large angular extension, the interferometric VLA data at 1443
map. We detected and excluded point sources up to a constant and 1630 MHz do not seem to suffer from missing flux prob-
fluxthresholdtoensureaconstantresolvedfractionofthecos- lem.Tocalculatetheerrorassociatedwiththefluxdensities,we
micX-raybackground(CXB)overthefieldofview.Thesurface addedinquadraturethestatisticalnoiseandanadditionaluncer-
brightnessprofilewascomputedwith30arcsecbinscenteredon tainty of 10% to take into account a different uv coverage and
thecentroidoftheimage(RA=012554;Dec=–012105)out apossiblebiasduetothedifferentfluxdensityscaleofthedata
to50arcmin.Thesurfacebrightnessprofilewasfittedwithasin- sets.
gleβ-model(Cavaliere&Fusco-Femiano1976)plusaconstant Wefittedtheintegratedspectrumwiththecontinuousinjec-
(to take into account the sky background composed by Galac- tion model (CI; Pacholczyk 1970) with the software SYNAGE
proffit
tic foregrounds and residual CXB) with the software (Murgia et al. 1999). The CI model is characterized by three
v1.3(Eckertetal.2011).Thebestfittingmodelhasacoreradius freeparameters:theinjectionspectralindex(α ),thebreakfre-
inj
rc = 11.5±1.2 arcmin (248.4 ± 26 kpc) and β = 0.67±0.06 quency (νb), and the flux normalization. In the context of the
(1σ)foraχ2/d.o.f.=1.77.InFig.5weshowthesurfacebright- CI model, it is assumed that the spectral break is due to the
nessprofileoftheX-rayemissionofAbell194withthebestfit energylossesoftherelativisticelectrons.Forhigh-energyelec-
β-modelshowninblue. trons, energy losses are primarily due to the synchrotron radi-
For the determination of the spectral parameters represen- ation itself and to inverse Compton scattering of cosmic mi-
tative of the core properties, we analyzed the Chandra ACIS- crowave background (CMB) photons. During the active phase,
S observation of Abell 194 (ObsID: 7823) with CIAO 4.7 and the evolution of the integrated spectrum is determined by the
CALDB 4.6.8. All data were reprocessed from the level = shiftwithtimeofν tolowerandlowerfrequencies.Indeed,the
b
1 event file following the standard Chandra reduction threads spectralbreakcanbeconsideredtobeaclockindicatingthetime
A122,page9of26
A&A603,A122(2017)
Table4.Relevantparametersoftheimagessmoothedtoaresolutionof174(cid:48)(cid:48)(2.9(cid:48))and60(cid:48)(cid:48).
Frequency Orig.beam Conv.beam σ σ σ Conv.beam σ σ σ
I Q U I Q U
(MHz) ((cid:48)(cid:48)×(cid:48)(cid:48)) ((cid:48)(cid:48)) (mJy/beam) ((cid:48)(cid:48)) (mJy/beam)
74(VLSSr) 75×75 174 260 − − − − − −
330 21.8×18.9 174 30 − − − − − −
1443 59.3×46.1 174 2.0 0.2 0.2 60 0.50 0.09 0.07
1465 52.8×46.1 174 3.2 0.7 1.1 60 1.17 0.19 0.28
1515 49.8×43.8 174 7.7 1.1 1.4 60 1.80 0.34 0.35
1630 52.8×40.5 174 3.0 0.2 0.1 60 0.59 0.09 0.08
6600 174×174 174 1.0 0.4 0.5 − − −
Notes.Column1:observingfrequency;Col.2:originalBeam;Col.3:smoothedBeam(174(cid:48)(cid:48));Col.4–6:RMSnoiseoftheI,Q,andU images
smoothedtoaresolutionof174(cid:48)(cid:48);Col.7:smoothedBeam(60(cid:48)(cid:48));Cols.8–10:RMSnoiseoftheI,Q,andU imagessmoothedtoaresolutionof
60(cid:48)(cid:48).
0 2 4 6 8 10 0 1 2 3 4 5 0 0.5 1 1.5 2 0 0.5 1 1.5 0 0.1 0.2 0.3 0.4 0.5 0.6
JY/BEAM JY/BEAM JY/BEAM JY/BEAM JY/BEAM
DR~45 DR~170 DR~1040 DR~645 DR~620
74 MHz 330 MHz 1443 MHz 1630 MHz 6600 MHz
Fig.6. Top:imagesof3C40Aand3C40Batdifferentfrequencies,smoothedtoaresolutionof2.9(cid:48).TheimagesareVLSSr(74MHz;Laneetal.
2014),VLAPband(330MHz),VLALband(1443and1630MHz),andSRT(6600MHz).Thedynamicrange(DR)ofeachimageisshownon
eachofthetoppanels.Bottom:totalspectrumofsources3C40Aand3C40Btogether.ThebluelineisthebestfitoftheCImodel.
elapsedsincetheinjectionofthefirstelectronpopulation.Below of 3C40B, at a relatively high resolution ((cid:39)20(cid:48)(cid:48)), between the
andaboveν ,thespectralindicesareα andα +0.5,respec- L-band(1443MHz)andtheP-band(330MHz)images.
b inj inj
tively. We also studied the variation pixel by pixel of the syn-
For 3C40A and 3C40B the best fit of the CI model to the chrotron spectrum along 3C40A and 3C40B using the images
observed radio spectrum yields a break frequency ν (cid:39) 700± inthetoppanelsofFig.6.3C40Bisextendedenoughtoinvesti-
b
280MHz.Tolimitthenumberoffreeparameters,wefixedα = gatethespectraltrendalongitslength.Afewplotsshowingthe
inj
0.5,whichisthevalueofthespectralindexcalculatedinthejets radiosurfacebrightnessasafunctionoftheobservingfrequency
A122,page10of26