Diffuse steep-spectrum sources from the 74 MHz VLSS survey

1.2 MB·English

by  R. J. van Weeren

Astronomy&Astrophysicsmanuscriptno.15991 c ESO2011
(cid:13)
January21,2011
Diffuse steep-spectrum sources from the 74 MHz VLSS survey
R.J.vanWeeren1,H.J.A.Ro¨ttgering1,andM.Bru¨ggen2
1 LeidenObservatory,LeidenUniversity,P.O.Box9513,NL-2300RALeiden,TheNetherlands
e-mail:[email protected]
1
2 JacobsUniversityBremen,P.O.Box750561,28725Bremen,Germany
1
0
Preprintonlineversion:January21,2011
2

n ABSTRACT
a
J Context.Galaxyclustersgrowbyasequenceofmergerswithotherclustersandgalaxygroups.Duringthesemergers,shocksand/or
0 turbulencearecreatedwithintheintraclustermedium(ICM).Inthisprocess,particlescouldbeacceleratedtohighlyrelativisticener-
2 gies.Thesynchrotronradiationfromtheseparticlesisobservedintheformofradiorelicsandhalosthataregenerallycharacterized
byasteepradiospectralindex.ShockscanalsorevivefossilradioplasmafromapreviousepisodeofAGNactivity,creatingaso-
] calledradio “phoenix”. Herewepresent multi-frequency radioobservations of diffusesteep-spectrum radio sources selectedfrom
O the74MHzVLSSsurvey.PreviousGiantMetrewaveRadioTelescope(GMRT)observationsshowedthatsomeofthesesourceshad
filamentaryandelongatedmorphologies,whichareexpectedforradiorelics.
C
Aims.Weattempttounderstandthenatureofdiffusesteep-spectrumradiosourcesandcharacterizetheirspectralindexandpolariza-
.
h tionproperties.
p Methods.Wecarriedoutradiocontinuumobservationsat325MHzwiththeGMRT.ObservationswiththeVeryLargeArray(VLA)
- andWesterborkSynthesisRadioTelescope(WSRT)weretakenat1.4GHzinfullpolarizationmode.Opticalimagesaroundtheradio
o sourcesweretakenwiththeWilliamHerschelandIsaacNewtonTelescope(WHT,INT).
r Results.MostofthesourcesinoursampleconsistofoldradioplasmafromAGNslocatedinsmallgalaxyclusters.Thesourcescan
t
s beclassifiedasAGNrelicsorradiophoenices.Thespectralindicesacrossmostoftheradiosourcesdisplaylargevariations.
a Conclusions.Weconcludethatdiffusesteep-spectrumradiosourcesarenotonlyfoundinmassiveX-rayluminousgalaxyclustersbut
[ alsoinsmallersystems.Futurelow-frequencysurveyswilluncoverlargenumbersofsteep-spectrumradiorelicsrelatedtoprevious

episodesofAGNactivity.
1
v Keywords.RadioContinuum:galaxies–Galaxies:active–Clusters:general–Cosmology:large-scalestructureofUniverse
5
6
8
1. Introduction & Enßlin 2000). The turbulent re-acceleration model currently
3
. seemstoprovideabetterexplanationoftheoccurrenceofradio
1 Studiesoflarge-scalestructureformationshowthatgalaxyclus- halos(e.g.,Brunettietal.2008).
0 tersgrowthroughmergerswithotherclustersandgalaxygroups, Mini-halos(alsocalledcore-halosystems) arediffuseradio
1
aswell asthroughthe continuousaccretionof gasfromthe in- sourceswithsizes. 500kpclocatedinrelaxedgalaxyclusters,
1
: tergalactic medium (IGM). The baryonic content of clusters is in which diffuse emission surrounds the central cluster galaxy
v mostly in the form of hot thermal gas visible at X-ray wave- (Murgiaetal.2009;Govonietal.2009;Gittietal.2007,2004;
Xi lengths. Several clusters also have a non-thermal component Bacchietal.2003;Gittietal.2002;Burnsetal.1992).
withintheICM,whichisobservableatradiowavelengths(e.g.,
Radio relics are irregularlyshapedradio sourceswith sizes
r Ferrarietal.2008;Giovanninietal.2009;Giovannini&Feretti
a rangingfrom50kpcto2Mpc,whichcanbedividedintothree
2000; van Weeren et al. 2010). The idea is that shocks and/or
groups(Kempneretal.2004).Radiogischtarelargeelongated,
turbulence generated during cluster merger events can acceler-
oftenMpc-sized,radiosourceslocatedattheperipheryofmerg-
ate particles to relativistic energies, and in the presence of a
ing clusters. They probably trace shock fronts in which parti-
magnetic field, synchrotron radiation is emitted (e.g., Ensslin clesareacceleratedviathediffusiveshockaccelerationmecha-
et al. 1998; Miniati et al. 2001; Hoeft & Bru¨ggen 2007;Hoeft
nism (DSA; Krymskii 1977;Axford et al. 1977;Bell 1978a,b;
etal.2008;Pfrommer2008;Battagliaetal.2009;Skillmanetal.
Blandford & Ostriker 1978; Drury 1983; Blandford & Eichler
2010).Theseradiosources,whichtracethenon-thermalcompo-
1987; Jones & Ellison 1991; Malkov & O’C Drury 2001).
nentoftheICM,canbedividedintoseveralclasses.
Amongtheseareraredouble-relicsthathavetworelicslocated
Radiohalosarefoundatthecenterofmerginggalaxyclus- onbothsidesoftheclustercenter(e.g., Bonafedeetal.2009b;
ters and have typical sizes of about a Mpc. They follow the vanWeerenetal.2009a;Venturietal.2007;Bagchietal.2006;
X-ray emission from the thermal ICM and are mostly unpo- Ro¨ttgering et al. 1997; van Weeren et al. 2010; Brown et al.
larized, although there are some exceptions (see Govoni et al. 2010).AccordingtoDSAtheory,theintegratedradiospectrum
2005;Bonafedeetal.2009a).Radiohaloshavebeenexplained shouldbea singlepower-law.RadiophoenicesandAGN relics
byturbulenceinjectedbyrecentmergerevents.Thisinjectedtur- arebothrelatedtoradiogalaxies.AGNrelicsareassociatedwith
bulencemightbecapableofre-acceleratingrelativisticparticles extinct or dying radio galaxies. The radio plasma has a steep
(e.g., Brunettietal.2001;Petrosian2001).Alternatively,theen- curvedspectrum1duetosynchrotronandinverseCompton(IC)
ergeticelectronsaresecondaryproductsofproton-protoncolli-
sions(e.g.,Dennison1980;Blasi&Colafrancesco1999;Dolag 1 F να,whereαisthespectralindex
ν ∝
2 R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey
losses.FossilradioplasmafromapreviousepisodeofAGNac- Table1.GMRT325MHzobservations
tivity canalso be compressedbya mergershockwave produc-
ing a radio phoenix (Enßlin & Gopal-Krishna 2001; Enßlin & rmsnoise beamsize
Bru¨ggen 2002), these sources again having steeply curved ra- µJybeam−1 arcsec
diospectra.ProposedexamplesofthesearethosefoundbySlee VLSSJ1431.8+1331 178 11.8′′ 7.8′′
VLSSJ1133.7+2324 132 10.4 ×7.7
etal.(2001). ′′ ′′
×
Abell2048 248 9.7 9.6
InvanWeerenetal.(2009b),wepresentedobservationsofa ′′× ′′
24P73 162 13.6 9.0
sampleof26diffusesteep-spectrumsources,withα 1.15,se- ′′× ′′
lectedfromthe1.4GHzNVSS(Condonetal.1998)≤an−d74MHz VVLLSSSSJJ00090145..97−[phone] 1134..57′′×18.11.0′′
′′ ′′
VLSS(Cohenetal.2007)surveys.Thesesourceswereeitherre- ×
solved out in the VLA B-array 1.4 GHz snapshot observations
or1.4GHzFIRSTsurvey(Beckeretal.1995).GMRT610MHz
observationsof these 26 sources detected one distant powerful
radiohalowitharadiorelic(vanWeerenetal.2009c),fiveradio The data was visually inspected for the presence of RFI,
relicsincludingtworadiophoenices,andonepossiblemini-halo. which was subsequently removed (i.e., “flagged”). We carried
Theremainingsourceswereclassifiedasradiosourcesdirectly out an amplitude and phase calibration on the flux and band-
relatedto AGNactivity.The spectralindicesofthe radiorelics pass calibrators3C147and3C286on a timescale of 8 sec. For
inthesamplearegenerallysteeperthanmostpreviouslyknown this,wechosethreeneighboringfrequencychannelsfreeofRFI.
relics.Bycomplementingourobservationswithresultsforother Thesegainsolutionswereappliedbeforedeterminingtheband-
relics fromthe literature, we foundthat the largerrelics gener- pass response of the antennas. This assures that any amplitude
ally have flatter spectra and are located farther away from the and/or phase variations during the scans on the calibrators are
clustercenter.Thisisinlinewithpredictionsfromshockstatis- correctedbeforedeterminingthe bandpasssolutions. Athigher
tics derived from cosmological simulations (Hoeft et al. 2008; frequencies(e.g., 1.4 GHz), both amplitude and phases are as-
Skillmanetal.2008;Battagliaetal.2009).Inthesesimulations, sumedtobeconstantduringbandpasscalibration.However,for
itisfoundthatlargershockwavesoccurmainlyinlower-density theGMRTobservingatlowfrequencies,thisassumptionisnot
regionsandhavelargerMachnumbers,andconsequentlyshal- alwaysvalidandcanaffectthequalityofthebandpasssolutions
lowerspectra.Ontheotherhand,smallershockwavesaremore aswellasthedeterminationofthefluxscale.
likelytobefoundinclustercentersandhavelowerMachnum- After correctingfor the bandpassresponse,boththe ampli-
bers,thussteeperspectra. tude and phase solutions for both primary and secondary cali-
Inthispaper,wepresentfollow-upradioobservationsofsix brators were determined but in this case using the full channel
sources(i.e.,thosemostlikelyrelatedtoradiorelicsandhalos) range.The fluxesofthe primarycalibratorswere setaccording
found in van Weeren et al. (2009b). GMRT 325 MHz as well tothePerley&Taylor(1999)extensiontotheBaarsetal.(1977)
asVLAandWSRT1.4GHzobservationswereobtainedtocre- scale.Thefluxdensitiesforthesecondarycalibratorswereboot-
ate spectral index and polarization maps. Optical images were strappedfromtheprimarycalibrators.Theamplitudeandphase
acquired with the 4.2m WHT and 2.5m INT telescopes at the solutions were interpolated and applied to the target sources.
position of the radio sources to search for optical counterparts Sometargetswereobservedovermultipledays(observingruns),
andidentifygalaxyclustersassociatedwiththeradiosources. the resulting different data sets were combined with the AIPS
Thelayoutofthispaperisasfollows.InSect.2,wepresent task‘DBCON’.
an overviewof the observationsand data reduction.In Sect. 3, Foreachofthetargetsources,wecreatedamodelofthesur-
we present the radio and spectral maps as well as optical im- roundingfieldusingtheNVSSsurveywithaspectralindexscal-
ages around the radio sources. In Sect. 4, we show additional ingof 0.7.Wecarriedoutaphase-onlyself-calibrationagainst
−
opticalimagesaroundfiveslightlymorecompactradiosources this model to improve the astrometric accuracy. This was fol-
(alsofromthesampleof26sources)tosearchforopticalcoun- lowedby severalroundsofphase self-calibrationand two final
terparts.Thesesourcesarenotlocatedinnearbygalaxyclusters roundsof amplitude and phase self-calibration.To producethe
andtheirnatureremainsunclear.Weendwithadiscussionand images,weusedthepolyhedronmethod(Perley1989;Cornwell
conclusionsinSects.5and6. & Perley 1992) to minimize the effects of non-coplanar base-
Throughoutthispaper,weassumeaΛCDMcosmologywith lines. The modelwas then subtractedfromthe data,a step that
H =71kms 1Mpc 1,Ω =0.3,andΩ =0.7.Allimagesare facilitatedtheremovalofadditionalRFIorbaselineswithprob-
0 − − m Λ
intheJ2000coordinatesystem. lems. Final images were made using robust weighting (robust
= 0.5, Briggs 1995). Images were cleaned using the automatic
clean-boxwindowingalgorithminAIPSandcleaneddownto2
2. Observations&datareduction timesthermsnoiselevel(2σ )withinthecleanboxes.Thefi-
rms
nalimageswerecorrectedfortheprimarybeamresponse2.The
2.1.GMRT325MHzobservations
uncertaintyinthecalibrationoftheabsoluteflux-scaleisinthe
RadiocontinuumobservationswiththeGMRTat325MHzwere range 5 10%, see Chandra et al. (2004). The resulting noise
−
carried out on 14, 15, and 17 May, 2009. Both upper (USB) levelsandbeamsizesareshowninTable1.
andlower(LSB)sidebands(IFs,whichincludedRRandLLpo- Radioobservationsat610MHzweretakenwiththeGMRT
larizations) were recorded with a total bandwidth of 32 MHz. inFebruaryandNovember2008ofthesourcesinTable1.The
The observations were carried out in spectral line mode with reductionoftheseobservationsissimilartotheGMRT325MHz
128channelsperIFtofacilitatetheremovalofradiofrequency dataandisdescribedinmoredetailinvanWeerenetal.(2009b).
interference(RFI)andreducetheeffectofbandwidthsmearing. Weusedtheseimagestocreatethespectralindexmaps.
The integration time per visibility was 8 sec. Each source was
observedforabout4hrsintotal.Thedatawerereducedwiththe 2 http://gmrt.ncra.tifr.res.in/gmrt hpage/Users/doc/manual/
NRAOAstronomicalImageProcessingSystem(AIPS)package. UsersManual/node27.html
R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey 3
Table2.VLA1.4GHzobservations
VLSSJ1133.7+2324 MaxBCGJ217.95869+13.53470 VLSSJ0004.9 3457 Abell2048
−
Frequencybands(IFs) 1385,1465MHz 1385,1465MHz 1385,1465MHz 1385,1465MHz
Bandwidth 2 50MHz 2 50MHz 2 50MHz 2 50MHz
× × × ×
Polarization RR,LL,RL,LR RR,LL,RL,LR RR,LL,RL,LR RR,LL,RL,LR
Observationdates 4Nov2008,17Apr2009, 2Nov2008,18Apr2009, 11Jun2009 15Jul2009
10Aug2009 31Jul2009
Projectcode AV305,AV312 AV305,AV312 AV312 AV312
Integrationtime 3.3s 3.3s 3.3s 3.3s
Totalon-sourcetime 5.0hr,6.8hr,3.9hr 4.9hr,6.8hr,3.9hr 4.0hr 4.0hr
VLAconfiguration A+B+C A+B+C CnB C
Beamsize 1.3 1.4 a,6.7 6.7 b 1.6 1.5 a,6.4 5.3 b 19.8 10.3 13.0 12.4
′′ ′′ ′′ ′′ ′′ ′′ ′′ ′′ ′′ ′′ ′′ ′′
Rmsnoise(σ ) 14a,1×9bµJybeam×1 15a,1×7bµJybeam×1 49µJy×beam 1 88µJy×beam 1
rms − − − −
aBriggsweighting(robust= 1.0)
bnaturalweighting −
2.2.VLA1.4GHzobservations gainsolutionsweredeterminedusingobservationsoftwostan-
dard calibrators, both at the start and end of an observing run.
We carried out L-band observations of four sources with the
The fluxes for the calibrators were set according to the Perley
VLA(seeTable2).Theobservationsweretakeninstandardcon-
& Taylor (1999)extensionto the Baars et al. (1977)scale. We
tinuummodewithtwoIFs,eachhavingabandwidthof50MHz
usedbothpolarized(3C138or3C286)andunpolarized(CTD93,
recordingallpolarizationproducts(RR,LL,RL,andLR).Gain
3C48)calibratorsources.Thebandpassandgainsolutionswere
solutions were determinedfor the calibrator sources and trans-
applied and the data were calibrated for the leakage terms us-
ferredtothetargetsources.Thefluxesfortheprimarycalibrators
ing the unpolarized calibrator source. The polarization angles
were set according to the Perley & Taylor (1999) extension to
were set using the polarized calibratorssources, the anglesbe-
theBaarsetal.(1977)scale.Theeffectivefeedpolarizationpa-
ing the same as in Sect. 2.2. The data was then exported into
rameters(theleakagetermsorD-terms)werefoundbyobserv-
AIPSfortworoundsofphaseonlyandtworoundsofamplitude
ingthe phase calibratorovera wide rangeof parallacticangles
andphaseself-calibration(separatelyforeachIF).Thesolutions
andsimultaneouslysolvingfortheunknownpolarizationprop-
for the amplitude and phase self-calibrations were determined
erties of the source. The polarizationangles were set using the
by combing both XX and YY polarizationsas Stokes Q is not
polarizedsources 3C286 and 3C138. For the R-L phase differ-
necessarilyzero.
ence,weassumedvaluesof 66.0and15.0degfor3C286and
−
3C138,respectively.StokesQandUimageswerecompiledfor
The images for each IF were cleaned to about 2σ using
eachsource.FromtheStokesQandUimages,thepolarization rms
cleanboxes.TheimagesforeachIFwerecombinedintoadeep
angles(Ψ)weredetermined(Ψ = 1arctan(U/Q)).Totalpolar-
2 imagebyconvolvingtheimagesoftheindividualsIFstoacom-
izedintensity(P)imageswerealsomade(P= Q2+U2).The monresolutionandusingaspectralindexscalingof 1.
polarizationfractionwerefoundbydividingtheptotalpolarized −
intensitybythetotalintensity(StokesI)image( Q2+U2/I).
p
2.4.OpticalWHT&INTimaging
2.3.WSRT1.3 1.7GHzobservationsof24P73
−
WSRT observationswere taken of a single source (24P73)not Optical images around the radio sources were made using the
included in the VLA observations.Every 5 min, the frequency PFIPcameraonthe4.2mWHTtelescopeandtheWFCcamera
setup was changed within the L-band, alternating between the ontheINT.Theobservationswerecarriedoutbetween15and19
21cm and 18cm setups. Both of these frequency setups have April,2009(WHT)and1 8October,2009(INT).Thefieldof
−
160 MHz bandwidth divided over 8 IFs, each having 20 MHz viewwas16 16 forthePFIPand34 34 fortheWFCcam-
′ ′ ′ ′
× ×
bandwidth. The data were recorded in spectral line mode with era. The seeing varied between 0.6 and 2.0 , but was mostly
′′ ′′
64spectralchannelsperIFin4polarizations.Theobservations between1.0 and1.5 .Mostnightswerephotometric.Thetotal
′′ ′′
were carried out in three runs on 11, 17, and 18 March, 2009 integrationtimepertargetwasabout1500sforbothV,R,andI
resultingin a more or less complete12-hoursynthesisrun,see bandsfortheWHTobservationsand4000sfortheINTobser-
Table3. vations. The data were reduced with IRAF (Tody 1986, 1993)
The data were partly calibrated using the CASA (formerly and the mscred package (Valdes 1998). All images were flat-
AIPS++)3 package. The L-band receivers of the WSRT tele- fieldedandbias-corrected.TheIandRbandimageswerefringe
scopes have linearly polarized feeds4. The leakage terms (D- corrected.Theindividualexposureswereaveraged,withpixels
terms) for the WSRT are frequency dependent(e.g., Brentjens being rejected above 3.0σrms to remove cosmic rays and other
2008). As a first step, we flagged the autocorrelations, and re- artifacts.
movedanyobviousRFIandcorrupteddata.Timerangesofan-
tennasaffectedbyshadowingwerealsotakenout.Bandpassand Zero-pointswere determined using various observationsof
standard stars taken during the nights. Images taken on non-
3 http://casa.nrao.edu/ photometricnightswerescaledsuchthatthefluxofafewtargets
4 TheWSRTrecordsXX=I Q,YY=I Q,XY= U+iV,and withinthefieldofviewagreedwiththatoftheimagestakenon
XY= U iV,whereI,Q,U,a−ndVaretheS−tokesparam−eters. photometricnights.
− −
4 R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey
Table3.WSRTobservations
Frequencybands21cm(IFs) 1311,1330,1350,1370,1392,1410,1432,1450MHz
Frequencybands18cm(IFs) 1650,1668,1686,1704,1722,1740,1758,1776MHz
BandwidthperIF 20MHz
NumberofchannelsperIF 64
Channelwidth 312.5kHz
Polarization XX,YY,XY,XY
Observationdates 11,17,and18March,2009
Integrationtime 30s
Totalon-sourcetime 6.5hr21cm+6.5hr18cm
Beamsize 19.0 16.5
′′ ′′
Rmsnoise(σ ) 37µJy×beam 1
rms −
3. Results The spectral index map, between 325 and 610 MHz, is
shown in Fig. 2 (left panel). The spectral index of the south-
A list of the observed sources with their integrated fluxes and
ern partof the filamentary source is 2.0, while for the rest of
redshiftsisgiveninTable4.Forsourceswithoutaspectroscopic the diffuse emission it is α 1.7.−Towards UGC 6544, the
redshift, we used the Hubble-K or Hubble-R relations to esti- ∼ −
spectralindexflattens.Thespectralindexmapbetween610and
mate the redshift (Willott et al. 2003; Snellen et al. 1996; de
1425MHzisshowninFig.2(rightpanel).Herethespectralin-
Vries etal. 2007).Spectralindexmaps were also made for the
dexsteepenstoα 2.5forthesouthernpartofthefilamentary
radiosources.OnlycommonUV-rangeswereusedtominimize ≤−
source.ThespectralindexoftheforegroundgalaxyUGC6544
errors due to differences in the UV-coverage and the individ-
isrelativelyflatwithα 0.5.
ual maps were convolved to the same resolution. For sources ∼−
VLSSJ1133.7+2324wasalsoobservedbyDwarakanath&
withhigh-resolution(.10 )1.4GHzobservations,wecreateda
′′ Kale(2009)at1287MHzwiththeGMRTandat330MHzwith
high-resolutionspectralindexmapbetween610and1425MHz.
the VLA. The reportedintegratedspectralindiceswere 1.6
Alow-frequencyspectralindexmapbetween325and610MHz − ±
0.03 between 74 and 328 MHz and 1.9 0.08 between 328
wasalsomadeformostsources.Pixelsbelow4σ ateitherone − ±
rms and1278MHz.Theintegratedfluxdensitywas151 12mJyat
ofthetwofrequencymapswereblanked.Low-resolutionspec- ±
328MHz.Wemeasureafluxof273 28mJyat325MHz,which
tralindexmapsbetween325,610,and1425MHzwerecreated ±
issignificantlyhigherthanthereportedvaluefromDwarakanath
tomapthespectralindexinlowsignal-to-noiseratio(SNR)re-
& Kale. This may be partly caused by the higher SNR of our
gions.Wefittedasingle(power-law)spectralindexthroughthe
image as we may pick up additional emission beyond what is
threefrequencies,pixelsbelow2.5σ beingneglected.
rms visibleintheDwarakanath&Kaleimage.Although,thiscannot
Fortwosources,theSNRwashighenoughtocreatespectral completelyexplainthedifferenceinfluxes.
curvaturemaps.Thespectralcurvaturewedefinedasα
325 610 The source could be old radio plasma from a previous
α .Pixelswithaspectralindexerrorlargerthan0.06−wer−e
610 1425 episode of AGN activity from UGC 6544, but UGC 6544 is
blan−kedinthespectralcurvaturemaps.Theerrorsinthespectral
a spiral galaxy, which normally do not host AGN. If the radio
index map are based on the noise levels (rms) of the individ-
emissionwereexplainedbyreliclobes,theywouldbeexpected
ualimages.Inthefollowingsubsections,theradioandspectral
to be located symmetrically with respect to the nucleus of the
indexmapsarepresentedtogetherwithopticaloverlays.
galaxy,which isnotthecase. Theflat spectrumradioemission
wedetectfromthegalaxyisfullyconsistentwiththatpredicted
bythefar-infraredradiocorrelation(vanWeerenetal.2009b).
3.1.VLSSJ1133.7+2324,7C1131+2341
Atthelocationofthefilamentaryradiosource,wedetected
VLSSJ1133.7+2324isafilamentaryradiosource,possiblyas- anoverdensityoffaintgalaxies(seeFig.3).Thesegalaxiesare
sociated with a galaxy cluster at z = 0.61 (van Weeren et al. partlyhiddenbehindUGC6544.ThemedianSDSSphotometric
2009b). To the west of the filamentary source, our 610 MHz redshift is 0.61 for these galaxies. To confirm the presence of
imagedetecteddiffuseemissionassociatedwiththeforeground a cluster, X-ray observations and/or spectroscopic redshifts of
galaxyUGC6544locatedatz=0.02385(Haynesetal.1997). severalgalaxiesareneeded.
The325MHzimage(seeFig.1leftpanel),issimilartoour If this is indeeda distant galaxycluster, the radio emission
previous 610 MHz image. In the 325 MHz image, the south- isverylikely tobeassociated withthecluster. Thesourcemay
ern part of the filamentary source is significantly brighter than thentraceashockwaveintheclusterwhereparticlesareaccel-
the northern part, while little emission from UGC 6544 is de- erated by the DSA mechanism. In that case the integrated ra-
tectedat325MHz.TheVLA1.4GHzimagemadewithnatural dio spectrum should be a single power-law. Our flux measure-
weightingisshowninFig.1(rightpanel)andrevealsmuchmore ments however indicate a slightly curved spectrum. A redshift
emissionfromUGC6544.Thesouthernpartofthefilamentary of 0.61 would correspond to a physical size of about 500 kpc.
source is quite faint at 1.4 GHz, confirming that it has a steep The very steep and somewhatcurvedradio spectrum then sug-
spectrum. A high-resolution VLA image, also at 1.4 GHz, is gestthesourcetobeanAGNrelic,ratherthana radiophoenix
shownasanoverlayinFig.3.Thisimagerevealsonlythreefaint becausethesourceisquitelargeandthetimetocompresssucha
compact radio sources. This shows that the emission from the largeradio“ghost”wouldremovemostoftheelectronsrespon-
steep-spectrumsource is truly diffuse, and cannotbe attributed sible for the radio emission by radiative energy losses (Clarke
tothecombinedemissionfromcompactsources.Wedonotde- &Ensslin2006).Additionalfluxmeasurementsatlowerand/or
tect any polarized emission from the source at 1425 MHz. We higherfrequencieswillbe neededtoconfirmwhetherthe radio
placea5σupperlimitof5%onthepolarizationfraction. spectrumisindeedcurved.
R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey 5
Table4.Sourcelist&properties
source/cluster z S S α curvature LLS classificationh
325 1425 74 1400
Jy mJy − α α kpc
VLSSJ1431.8+1331 0.1599 0.373 0.040 14.6 1.1 2.03 0.05 1.7043−6f10− 610−1425 125 AGN+AGNRorPHNX
VLSSJ1133.7+2324 0.61 0.16g 0.273±0.028 12.3±1.1 −1.69±0.06 0.96 570g AGNRorDSAR
± ± ± − ±
Abell2048 0.0972 0.559 0.061 18.9 4.3 1.50 0.05 1.6 310 PHNX
24P73 0.15 0.1b,g 0.307±0.033 12.0±3.0 −2.20±0.06 2.0 270 PHNX
VLSSJ0004.9 3457 0.3 ±0.1e 0.417±0.046 32.2±1.9 −1.40±0.04 0.30f 200 AGN(orMH+DSAR?)
VLSSJ0915.7+−2511 0.32±4 0.417±0.046 24.7±1.5a −1.52±0.04 1.02 190 AGNRorPHNX
VLSSJ1117.1+7003 0.8 0.4d 0.030±0.006c 2.9 ±0.5a −1.87±0.07 0.0 130 AGN?
VLSSJ2043.9 1118 0.5±0.3d ... ± 7.7±0.6a −1.74±0.05 0.84 250 AGN(MH?+DSAR?)
VLSSJ0516.2+−0103 1.2±0.7d ... 4.3±0.4a −1.73±0.06 0.0 290 AGN(orMH?)
VLSSJ2209.5+1546 1.1±0.7d ... 7.0±0.9a −1.56±0.07 0.59 500 AGN?
VLSSJ2241.3 1626 0.5±0.3d ... 14.6± 1.1a −1.44±0.06 0.0 290 AGN?
− ± ± − ±
afluxfromNVSS(Condonetal.1998)
bidentificationofthecDgalaxyuncertain
cfluxfromWENSS(Rengelinketal.1997)
d redshift estimated using the fitted Hubble-R relation from de Vries et al. (2007), since it is unclear whether there is a common underlying
populationofmassiveellipticalgalaxiesforextendedsteep-spectrumradiosourceswehavetakenthe3CHubble-RrelationfromSnellenetal.
(1996)asanupperlimitfortheredshift(i.e.,the3Cgalaxiesareabout1magbrighteratthesameredshift)
eredshiftestimatedusingthefittedHubble-KrelationfromWillottetal.(2003)
f variesacrossthesource
gassociationwithclusteruncertain
hPHNX=radiophoenix,AGNR=AGNrelic,MH=radiomini-halo,DSAR=relictracingshockwavewithDSA
Fig.1.Left:GMRT325MHzmap.Contourlevelsaredrawnat √[1,2,4,8,...] 4σ .ThepositionofisUGC6544isindicatedbythedashed
× rms
ellipse.Right:VLA1425MHzmap.Contourlevelsaredrawnasintheleftpanel.
3.2.VLSSJ1431.8+1331,MaxBCGJ217.95869+13.53470 first two components are connected by a faint radio “bridge”.
Thisbridgewasnotseen inourprevious610MHz image.The
VLSS J1431.8+1331 is located in the galaxy cluster bright source is a currently active radio galaxy with the radio
MaxBCG J217.95869+13.53470 (z = 0.1599, Koester core clearly being visible in our VLA 1.4 GHz images (see
et al. 2007) and associated with the central cD galaxy of Fig. 4 top middle and right panels). Probably, radio plasma
the cluster. The cluster has a moderate X-ray luminosity of from the core flows westwards and then forms the north-south
L 1 1044 erg s 1 based on the ROSAT count elongatedstructure.
X,0.1 2.4keV −
rate (V−oges et∼al.×1999). The GMRT 325 MHz image (see Thespectralindexmaps(see Fig. 4 bottompanels),are in-
top left panel Fig. 4) shows a bright elongated source. To the dicative of a relatively flat spectral index of 0.5 for the ra-
−
west a somewhat fainter diffuse component can be seen. This diocorebetween325and610MHz.Spectralsteepeningisob-
componentisnotassociatedwithanyopticalgalaxy(seeFig.5). served to the north and south of the elongated structure. The
A third fainter source is located further to the southwest. The spectral index for the southern part of the elongated structure
6 R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey
Fig.2.Left:Spectralindexmapbetween325and610MHzataresolutionof11.75 7.65 .Contourlevelsarefromthe325MHzGMRTimage
′′ ′′
×
anddrawnatlevelsof[1,2,4,8,...] 6σ .Right:Spectralindexbetween610and1425MHz.Contourlevelsarefromthe1425MHzVLA
× rms
imageanddrawnatlevelsof[1,2,4,8,...] 5σ .Thebeamsizeis6.7 6.7 .
× rms ′′× ′′
Fig.3. Optical WHT color image for VLSS J1133.7+2324. GMRT Fig.5.OpticalWHTcolorimageforMaxBCGJ217.95869+13.53470.
610MHzcontoursareoverlaidinyellow.Thebeamsizeis6.6 3.9 . GMRT 610 MHz contours are overlaid in yellow. The beam size is
′′ ′′
×
The VLA 1425 MHz high-resolution map is overlaid with red con- 5.3 4.8 .Contourlevelsaredrawnat[1,2,4,8,...] 4σ .
tours. The beam size is 1.36 1.32 and the image has a noise of ′′× ′′ × rms
′′ ′′
14µJybeam 1.Contourlevelsa×redrawnat[1,2,4,8,...] 4σ .
− × rms
ably old “bubbles” of radio plasma linked to this AGN, which
is consistent with the curved radio spectrum. The presence of
steepensto 3.5between610and1425MHz.Thespectralindex afaintradiobridgealsosuggestsarelationbetweenthissouth-
ofthe south−westerncomponentis about 1.5between325and westerncomponentandtheradiogalaxy.Thesouthwesterncom-
610 MHz, there being smaller spectral i−ndex variations across ponentcanthereforebeclassifiedasaradiophoenix(iftheradio
it than in the brighter western component. Between 610 and plasma has been compressed) or an AGN relic. XMM-Newton
1425MHz,thespectralindexsteepenstoabout 2.5.Thespec- observations of the cluster will be presented by Ogrean et al.
tralcurvaturemap(α α )(seeFig−.4bottomright (submitted).
325 610 610 1425
panel),showsthattheso−uthw−estern−sourcehasaverycurvedra-
dio spectrum. The southernend of the radiostructure from the
3.3.VLSSJ2217.5+5943,24P73
activeAGNis alsoquitecurved.Thehighspectralcurvatureis
likelytobetheresultofspectralageing,thegradientinthespec- This source was discovered during the Synthesis Telescope of
tral index away from the core providing evidence of this. The the Dominion Radio Observatory (DRAO) Galactic plane sur-
two diffuse sources the southwest of the active AGN are prob- vey at 408 MHz and 1.42 GHz (Higgs 1989; Joncas & Higgs
R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey 7
Fig.4.Topleft:GMRT325 MHzmap. Contour levelsaredrawnasinFig.1.Topmiddle: VLA1425MHzmap. Contour levelsaredrawnas
inFig.1.Topright:VLA1425MHzhigh-resolutionimage.TheimagewasmadeusingBriggsweightingwithrobustsetto 1.Contourlevels
−
aredrawnat √[1,2,4,8,...] 4σ .Bottomleft:Spectralindexmapbetween325and610MHzataresolutionof10.4 7.7 .Contourlevels
× rms ′′× ′′
arefromthe325MHzGMRTimageanddrawnatlevelsof[1,2,4,8,...] 6σ .Bottommiddle:Spectralindexbetween610and1425MHz.
× rms
Contourlevelsarefromthe610MHzGMRTimageanddrawnatlevelsof[1,2,4,8,...] 5σ .Thebeamsizeis6.41 5.26 .Bottomright:
× rms ′′× ′′
Spectralcurvaturemap.Contourlevelsaredrawnasintheleftpanelandtheresolutionis10.4 7.7 .
′′ ′′
×
1990). Green & Joncas (1994) found the source to be diffuse andextremespectralindex.Infact,therelicisverysimilartothe
andhaveanultra-steepspectrum(α= 2.58 0.14).OurGMRT proposedphoenixinAbell13(Sleeetal.2001).Toconfirmthe
− ±
610MHzobservations(seeFig.6bottomrightpanel)detecteda presenceofacluster,deepnear-infrared(NIR)imagingwillbe
verycomplexfilamentarysource,resemblingtherelicsfoundin necessary.
Abell13andAbell85(Sleeetal.2001).OurGMRT325MHz
image, Fig. 6 top left panel, is similar to the 610 MHz image.
InourcombinedWSRT 1.3 1.8GHzimage(toprightpanel), 3.4.VLSSJ0004.9 3457
− −
thefainterwesternpartoftherelicisonlymarginallydetected,
The radio source is located in a small galaxy cluster or group,
whichisindicativeofasteepspectralindexinthisregion.Weset
B02291(Zanichellietal.2001).Thecluster/groupislocatedat
an upperlimit to the polarizationfractionof 5% forthe source
aredshiftof0.3 0.1(vanWeerenetal.2009b).NoX-rayemis-
at1.4GHz. ±
sionfromthesystemisdetectedintheROSATAll-SkySurvey
Thespectralindexmapbetween325and610MHzisshown
(Vogeset al. 1999,2000),which implies that the system is not
inthebottomleftpanelofFig.6.Thespectralindexvariesbe-
very massive. An opticalPOSS-II colorcomposite is shown in
tween 1.4 and 2.9 over the source. The western part of the
− − Fig. 7 (bottom right panel). The various radio components are
source has the steepest spectrum. Towards the southwest, the
labeledalphabetically(seeFig.7toprightpanel).
spectral index flattens to about 1.0. This part may be associ-
− Our GMRT 325 MHz image of VLSS J0004.9 3457 (top
atedwithaseparatecompactradioAGN.
−
left panel) displays a diffuse source (A) centered on a K-
OuropticalWHTimage(bottomrightpanelFig.6)isdomi-
mag=14.86galaxy.Thesourceextendssomewhatfurthernorth-
natedbyforegroundstarsasthesourceislocatedintheGalactic
wardsthaninthe610MHzimage(bottomrightpanel).
plane, at Galactic latitude of 2.44 degrees. However, there are
also several faint red galaxies seen in the image that may be- SourceBisassociatedwithanothergalaxy,Cdoesnothave
long to a galaxy cluster, and these are marked by red circles. an optical counterpart and seems to be related to source A.
Thebrightestgalaxy(locatedsouthwest)hasanR-bandmagni- Source D is a fainter source (resolved in the 610 MHz image)
tudeof20.69.UsingtheHubble-Rrelation(deVriesetal.2007), located just east of VLSS J0004.9 3457 at RA 00h 04m 50s,
−
we estimate a redshiftof 0.15 0.1,includingan extinctionin Dec 34◦56′38′′.IntheCnB-arrayVLA1.4GHzimage,shown
the R-band of 4.173 mag (Sch±legel et al. 1998). We note that inFi−g.7(topright),componentCislessprominentthaninthe
thisredshiftestimate is basedon thecorrectedidentificationof 325MHzimage,whilesourceBisclearlyvisible.
the cD galaxy in the cluster. If the source is indeed located at Thespectralindexmapbetween325and610MHzisshown
z = 0.15,thenitslargestphysicalextentis270kpc.Weclassify in Fig. 8 (right panel). Source B has a flat spectral index of
∼
thesourceasaradiophoenixgiventhefilamentarymorphology 0.5.SourceDhasasteeperspectralindexof 1.1.Thespectral
− −
8 R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey
Fig.6.Topleft:GMRT325MHzmap.ContourlevelsaredrawnasinFig.1.Topright:WSRT1.3 1.8GHzmap.Contourlevelsaredrawnasin
−
Fig.1.Bottomleft:Spectralindexmapbetween325and610MHzataresolutionof13.44 8.96 .Contourlevelsarefromthe325MHzimage
′′ ′′
×
anddrawnasinFig.2.Bottomright:OpticalWHTcolorimagefor24P73.GMRT610MHzcontoursareoverlaidinyellow.Thebeamsizeis
6.4 4.3 .ContourlevelsaredrawnasinFig.5.Severalfaintgalaxiesintheimagearemarkedbyredcircles.
′′ ′′
×
index of A steepens away from the center (defined as the peak spectralsteepeningawayfromthecoreisthentheresultofsyn-
flux and located at the position of the K-mag=14.86 galaxy). chrotronandIClosses. Theeasternarc-likeextensioncouldbe
The central region has a spectral index of 1.2. Outwards, the aradiorelicbasedonitselongatednatureandhighpolarization
−
spectral index steepens to < 2. Component C has a spectral fraction.Thehighpolarizationfractionisindicativeofthepres-
−
indexof about 1.5.A spectralindex map, for the frequencies ence of ordered magnetic fields. Therefore it is likely that this
−
325,610,and1425MHz,isshowninFig.8(middlepanel).The componenttracesaregionintheICMthathasbeencompressed
spectral curvature map, in the right panel of Fig. 8, shows the byashockwave.Radiomini-halosusuallyoccurinmassivere-
least curvature(i.e., less than 0.5 units) for the western part of laxedgalaxyclusters.Thepossiblepresenceofaradiorelicin-
AandsourceB.EmissiontotheeastofAshowsmorespectral dicatesmergeractivitysoitispossiblethatthe“mini-halo”can
steepening,withacurvatureofabout1.0units. alsobelinkedtothemergeractivityofthesystem.Radioplasma
Polarized emission from source A, B, and componentC is from the central AGN might have been re-accelerated or com-
observedintheVLAimages,Fig.7(bottomleftpanel).Source pressed by this merger event. The source is somewhat similar
B is polarized at the 8% level. The polarization fraction at the to MRC 0116+111studied by Gopal-Krishnaet al. (2002)and
centerofAisroughly6%.Thearc-likesoutheastextension(C) Bagchietal.(2009).WefindthatMRC0116+111exhibitstwo
ishighlypolarized,withapolarizationfractionbetween25and bubble-likeradiolobes,whereasVLSSJ0004.9 3457seemsto
−
35%. consistofasinglecomponent.Themorphologyofthesourceis
moresimilartothecore-halosysteminZwCl1454.8+2223and
We tentativelyclassify thesourceasa200kpc“mini-halo”
candidate core-halosystem in Abell 3444 (Venturiet al. 2008,
(or core-halo), because the radio emission of diffuse source A
2007). The cluster/group would make an interesting target for
surrounds a central galaxy in a cluster or galaxy group. The
R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey 9
Fig.7.Topleft:GMRT325MHzmap.ContourlevelsaredrawnasinFig.1.Topright:VLA1425MHzmap.ContourlevelsaredrawnasinFig.1.
Bottomleft:VLA1425MHzpolarizationmap.Totalpolarizedintensityisshownasgrayscaleimage.VectorsrefertothepolarizationE-vectors,
withtheirlengthrepresentingthepolarizationfraction.Areferencevectorforapolarizationfractionof100%isshowninthebottomleftcorner.
Thepolarization fractions werecorrected for Ricean bias(Wardle& Kronberg 1974). No polarization E-vectorsweredrawn for pixelswitha
SNRlessthan3inthetotalpolarizedintensitymap.Contoursshowthetotalintensityimage(StokesI)at1425MHz.Contourlevelsaredrawnat
[1,16,256,4096,...] 0.147mJybeam 1.Bottomright:OpticalPOSS-IIcolorimageforVLSSJ0004.9 3457.GMRT610MHzcontoursare
−
× −
overlaidinyellow.Thebeamsizeis6.4 4.3 .ContourlevelsaredrawnasinFig.5.
′′ ′′
×
Fig.8.Left:Spectralindexmapbetween325and610MHzataresolutionof13.5 11.0 .Contour levelsarefromthe325MHzimageand
′′ ′′
×
drawnasinFig.2.Middle:Power-lawspectralindexfitbetween325,610,and1425MHz.Contoursarefromthe325MHzimageanddrawnat
levelsof[1,2,4,8,...] 4σ .Thebeamsizeis19.83 12.0 .Right:Spectralcurvaturemap.Contouraredrawnasinthemiddlepanel.
× rms ′′× ′′
10 R.J.vanWeerenetal.:Diffusesteep-spectrumsourcesfromthe74MHzVLSSsurvey
futureX-rayobservationstostudytherelationbetweentheradio
sourcesandthesurroundingICM.
3.5.VLSSJ0915.7+2511,MaxBCGJ138.91895+25.19876
The radio source is located in the cluster
MaxBCG J138.91895+25.19876. The source consists of a
northern component(Fig. 9 left panel) and a slightly more ex-
tendedfaintercomponenttothesouth.Acompactsourcetothe
westisassociatedwiththegalaxySDSSJ091539.68+251136.9.
Thissourcehasaspectroscopicredshift(SDSSDR7, Abazajian
etal.2009)of0.324.Thisclusterhasa photometricredshiftof
0.289 (Koester et al. 2007),but the galaxy seems to be part of
the cluster, hence we adopt a redshift of 0.324 for the cluster.
Thecompactsourceisresolvedin our610MHz GMRT image
anddisplaysadoublelobestructure(seeFig.9rightpanel).
Thespectralindexmapbetween325and610MHzisshown
inFig.9(middlepanel).Thespectralindexmapisnoisybecause
ofdynamicrangelimitationsfromthesource4C+25.24(1.35Jy
at325MHz)locatedabout3 tothesoutheast.Theeasternpartof
′
thenortherncomponenthasthesteepestspectrumwithanindex
ofα 2,althoughtheSNRislowinthisregion.Thecompact
∼− Fig.11. Optical WHT color image for VLSS J2043.9 1118. GMRT
sourcetothewesthasaflatspectralindexofabout 0.2. −
− 610MHzcontoursareoverlaidinyellow.Thebeamsizeis5.8′′ 4.2′′.
Theclassificationofthesourceisunclear.Thesourcemight ×
ContourlevelsaredrawnasinFig.5.Aredcircleindicatestheproposed
containoldradioplasmathatoriginatedintheAGNtothewest. opticalcounterpart.
Inthiscase,thesourcecouldbeclassifiedasaradiophoenixor
AGNrelic.
galaxiesintheclusteris857kms 1(Shenetal.2008).Thebolo-
−
metricX-rayluminosityis1.914 1044ergs 1.Onthebasisof
−
3.6.VLSSJ1515.1+0424,Abell2048 ×
theL σrelation(X-rayluminosityversusvelocitydispersion)
X
−
fromShenetal.
VLSS J1515.1+0424is located in the cluster Abell 2048 (z =
0.0972; Struble&Rood1999)totheeastoftheclustercenter.
L σ
Thesourcehas a largestextentof 310kpc(see Fig. 10top left log X =4.39log 0.530, (1)
1044ergs 1! 500kms 1!−
panel),andhasacomplexmorphology.Onlythebrighterpartsof − −
thesourceareseenintheVLA1.4GHzC-arrayimage(Fig.10
we predict a velocity dispersion of 765 40 km s 1 given the
top rightpanel).No polarizedflux is detected fromthe source. ± −
X-rayluminosity.Thisislowerthanthe observedvalue,which
We setan an upperlimiton the polarizationfractionof 8%for
is not inconsistent with the cluster having undergone a recent
thesource,againrequiringaSNRof5foradetection.Anoptical
mergerevent.A shockwavegeneratedbytheproposedmerger
V, R, and I color image of the cluster with 610 MHz contours
eventmighthavecompressedfossilradioplasmaandproduced
overlaid does not reveal an obviousoptical counterpartfor the
the radiophoenix.FutureX-rayobservationswill beneededto
source.
studythedynamicalstateoftheclusterandtherelationbetween
The spectral index map between 325, 610, and 1425 MHz,
theICMandtheradiophoenix.
is shownin Fig. 10 (bottomleft panel). No systematic spectral
indexgradientsare seen acrossthe source.A regionwith a flat
spectral index (α > 0.5) is located under the southern “arm” 4. Opticalimagingaroundfivecompact
ofthesourceatRA1−5h15m08.6s,Dec+04 23 08 .Thispart
◦ ′ ′′ steep-spectrumsources
is associated with the galaxy 2MASX J15150860+0423085in
frontofthecluster(z=0.047856fromSDSSDR7)(seeFig.10 We present optical images around five slightly more compact
bottomrightpanel).Thespectralindexoftherelicissteepwith steep-spectrumradiosources,whichnaturewasfoundtobeun-
anaveragevalueofabout 1.7between1425and325MHz. clearinvanWeerenetal.(2009b).
−
The complex morphologyof the radio source suggests that
thesourcecanbeclassifiedasaradiophoenix.Thesteepcurved
4.1.VLSSJ2043.9 1118
radiospectrumisconsistentwiththisinterpretation.Ifthesource −
is indeed a radio phoenix, the radio plasma should have orig- The radio source has a largest angular size of 41 . An optical
′′
inated in a galaxy that has gone through phases of AGN ac- counterpart(R band magnitudeof 20.1)is visible in our WHT
tivity. A candidate is the elliptical galaxy MCG +01 39 011 image (see Fig. 11). The radio emission surrounds the galaxy
− −
(z = 0.095032; Slinglendetal.1998).Thisgalaxyiscurrently andthereisahintofafaintextensiontotheeast.Weestimatea
activeandlocatedclosetotheeasternendofthesouthern“arm”. redshiftof0.5 0.3fortheopticalcounterpart,whichimpliesa
±
However, there are several other elliptical galaxies around, al- physicalextentof250kpcfortheradiosource.Thesourcecould
thoughatthemomenttheyarenotradio-loud.AROSATimage beamini-haloorcore-halosystemgivenitssteepspectralindex
(seevanWeerenetal.2009b)oftheclustershowsasubstructure of 1.74 0.05(between74and1400MHz).Alternatively,we
− ±
to theeast ofthe maincluster,whichimpliesthatthecluster is are detecting radio plasma from an AGN that has undergonea
presentlyundergoingamerger.Thevelocitydispersion,σ,ofthe significantamountofspectralageing.