Multifrequency study of a new Hybrid Morphology Radio Source

2.2 MB·

by  F. de Gasperin

Mon.Not.R.Astron.Soc.000,1–8(2013) PrintedJanuary25,2017 (MNLATEXstylefilev2.2)
Multifrequency study of a new Hybrid Morphology Radio Source
F. de Gasperin1,2
1LeidenObservatory,LeidenUniversity,P.O.Box9513,NL-2300RA,Leiden,TheNetherlands
2HamburgObservatory,UniversityofHamburg,Gojenbergsweg112,D-21029,Hamburg,Germany
7
1
0
2

n
a ABSTRACT
J HybridMorphologyRadioSources(HyMoRS)areaclassofradiogalaxieshavingthelobe
4 morphology of a Fanaroff-Riley (FR) type I on one side of the active nucleus and of a FR
2 typeIIontheother.TheoriginofthedifferentmorphologiesbetweenFRIandFRIIsources
has been widely discussed in the past 40 years, and HyMoRS may be the best way to un-

] derstand whether this dichotomy is related to the intrinsic nature of the source and/or to its
A environment. However, these sources are extremely rare ((cid:46) 1% of radio galaxies) and only
G for a few of them a detailed radio study, that goes beyond the morphological classification,
. hasbeenconducted.InthispaperwereportthediscoveryofonenewHyMoRS;wepresent
h
X-rayandmulti-frequencyradioobservations.Wediscussthesourcemorphological,spectral
p
andpolarisationpropertiesandconfirmthatHyMoRSareintrinsicallybimodalwithrespect
-
o to these observational characteristics. We notice that HyMoRS classification based just on
r morphologicalpropertiesofthesourceishazardous.
t
s
a Key words: galaxies: active - galaxies: nuclei - radio continuum: galaxies - galaxies: indi-
[ vidual:NVSSJ232149+484951-galaxies:individual:NVSSJ232147+482956

1
v
5
1 1 INTRODUCTION whileathigh-zFRIIsarealsofoundinclustercentres(Hill&Lilly
9 1991).
6
0 Fanaroff&Riley(1974)dividedradiogalaxiesintotwocategories Apeculiarsub-classofradiogalaxieshaveahybridFRI/FRII
. basedontheirmorphology.TypeIhaveslower,moreturbulent,and morphology. Hybrid morphology radio sources (HyMoRS) were
1
less collimated jets. Conversely, typeII have powerful collimated firstdefinedbyGopal-Krishna&Wiita(2000)asa“classofdou-
0
jets (often only one is visible) which terminate into bright, com- ble radio sources in which the two lobes exhibit clearly different
7
1 pact“hotspots”.Intheseregionsmuchofthebulkenergyofthejet Fanaroff-Riley morphologies”. Gawronski et al. (2006) inspected
: isconvertedintoacceleratingrelativisticparticlesandamplifying 1700 sources from the FIRST survey finding three certain and
v magneticfieldsthroughshocksproducedbythebeamagainstthe two possible HyMoRS showing that these type of sources are
Xi intergalacticmedium(Blandford&Rees1974;Carillietal.1991). rare ((cid:46) 1% of radio galaxies belong to this category). Currently,
WhiletheFRIclassismadeupofsourceswithdifferentmorpholo- around10HyMoRSareconsideredgenuinebutonlyasmallfrac-
r
a gies,themajorityofthemshowjetsthatappearlaminarintheinner- tionofthemhavebeenstudiedindetail.Piryaetal.(2011)made
mostregioninflatingturbulentlobesafterpassingthroughaflare adetailedmulti-wavelengthradiostudyoftwopossibleHyMoRS
point.Thischaracteristiccanbewellexplainedbytheinteraction (J1211+743andJ1918+742).While,inotherwavelengths,Miller
with the ambient medium (Laing & Bridle 2014). The magnetic & Brandt (2009) made Chandra observations of the two hybrids
field orientation for FRIs is often perpendicular to the jet, apart 3C433 and 4C65.15. They found that both sources have unab-
fromtheinnermostregion.Conversely,FRIIjetshaveamagnetic sorbed X-ray luminosity, radio luminosity, and optical spectra of
fieldparalleltothejetorientation(Bridle1984). a typical FRII, while the FRI side is likely a consequence of
jet-mediuminteraction.Finally,Ceglowski,Gawronski&Kunert-
Another distinction between FRI and FRII was based on
Bajraszewska (2013) examined the central 10 kpc of five known
jet/lobeluminosity,withFRIhavinglowerluminosity(< 1025 W
HyMoRS using VLBI observations. Their work underlines that
Hz−1at1.4GHz)thanFRII.Thevalueoftheradioluminositytran-
genuineHyMoRSarenotaconsequenceofthejetorientationand
sitiondependsonthepropertiesofthehostgalaxiesasittendsto
thaton1–10kpcscaletheirtargetshaveweakjets,compatiblewith
increasewiththehostopticalluminosity(Ledlow&Owen1996),
FRII-like structures. The study of hybrid radio sources might be
butthisresulthasnotbeenconfirmedbysubsequentanalysis(Best
offundamentalimportancetounderstandtheoriginoftheFRdi-
2009;Wing&Blanton2010).Thishasbeeninterpretedasalink
chotomy and assess whether the large scale morphology of radio
between the morphological dichotomy of radio galaxies and the
galaxiesislinkedtothecentralengineproperty,theenvironmentor
propertiesofeitherthenucleus(e.g.Ghisellini&Celotti2001)or
toacombinationofthetwo.
theenvironment(e.g.Bicknell1995).FRIsarefoundinricherclus-
ters(Prestage&Peacock1988)andthisrelationevolvesintime, Even though recent efforts have been undertaken to find hy-
(cid:13)c 2013RAS
2 F.deGasperin
bridsources,theknownpopulationofHyMoRSisstillsmalland, 4.5
to our knowledge, no study has so far combined high-resolution,
4.0
polarisation and spectral index radio maps to characterise a can-
didate HyMoRS. In this paper we report the discovery of a new 3.5
HyMoRS:NVSSJ232149+484951(hereafterRG1).InSec.2we
3.0
presentmulti-frequencyobservationsofthesourceandweanalyse
high-resolutionandpolarisationradiomaps.InSec.3wepresent z) 2.5
thenewlydiscoveredHyMoRSandwemakespectralindexchar- P( 2.0
acterisationinSec.3.1.InSec.4wediscussanotherradiogalaxy
(NVSSJ232147+482956,hereafterRG2)asanexampleofthedif- 1.5
ficulties in the HyMoRS classification when using only morpho- 1.0
logicaldata.DiscussionandconclusionsareinSec.5.
0.5
ThroughoutthepaperweadoptafiducialΛCDMcosmology
withH0 = 70kms−1 Mpc−1,Ωm = 0.3andΩΛ = 0.7.Atthered- 0.00.0 0.2 0.4 0.6 0.8 1.0
shiftofthetarget(z≈0.16)1(cid:48)(cid:48)=2.8kpc.Allregressionsaremade Redshift, z
withabootstrapleastsquaresalgorithmthattakesintoaccounter-
rorsonthedependantvariables.Unlessotherwisespecifiederrors Figure2.Normalisedredshiftprobabilitydensityfunctionofsourceswith
areat1σ.Thespectralindexisdefinedas:Fν ∝να,whereFνisthe W1andW2magnitudeswithin2-sigmaofRG1forasetofsemi-analytic
fluxdensity. modelsfromSAGE(Crotonetal.2016).Thepeakofthedistributionisat
z=0.16.
2 OBSERVATIONSANDDATAREDUCTION
fluxesofRG1withtheredshiftdistributionofsourcesintheSAGE
2.1 Radio (Semi-AnalyticGalaxyEvolution;Crotonetal.2016)modelswith
comparable WISE fluxes and colours (see Fig. 2). The predicted
WeobtainedGMRTdataat323and607MHz,WSRTdataat1380
redshiftprobabilitydensityfunction,P(z),wasgeneratedbyaver-
MHz and VLA data at 1–2 GHz (see Table 1). The observations
agingtheredshiftdistributionsofphotometricallysimilarsources
arepointedtowardsthenearbygalaxyclusterPSZ1G108.18-11.53 in5modellightconeswithdifferentsightlines.Themodelphotom-
(z=0.335),butbothradiogalaxiesdescribedinthispaperfallwell
etrywasgeneratedusingtheFlexibleStellarPopulationSynthesis
withintheprimarybeamfullwidthhalfmaximumoftheseobser-
stellarpopulationmodels(Conroy&Gunn2009)andassuminga
vations.ThedatareductionproceduresfortheGMRTandWSRT
Chabrierinitialmassfunction(Chabrier2003).Additionally,dust
observations are described in de Gasperin et al. (2015) and take
attenuationisincorporatedintothemodelphotometryfollowingthe
advantagefromtheSPAM(SourcePeelingandAtmosphericMod-
method outlined in Tonini et al. (2012). However, given the long
eling)packageforionosphericcalibrationtoachievehigh-fidelity, wavelength of the filters being used, the effect of dust on the ap-
thermalnoiselimitedimages(Intemaetal.2009).
parentmagnitudesofmodelsourcesislikelyverysmall.Themost
VLAdatahasbeentakenintheL-band(1–2GHz)inBand likelyredshiftisaroundz=0.16,butthisestimationmustbetaken
C configuration. The data were reduced using the CASA1 pack-
justasaqualitativeindicationanditwillnotbeusedtoderiveany
age.ThevisibilitieswereHanning-smoothed,bandpass-calibrated
importantconclusionintherestofthepaper.
andflaggedusingtheautomatictoolAOflagger(Offringa,vande
Gronde&Roerdink2012).Weused3C147asfluxcalibratorand
3C138tocalibratethepolarisationangle.Thefluxscalehasbeen 2.3 X-ray
set to Perley & Butler (2013) which above 1 GHz is in-line with
Scaife & Heald (2012) used for the low frequency observations. AChandraACIS-Iobservationof27ks(obs.801493,performedon
Bandpass,scalardelays,cross-handdelays,andpolarisationangle 09/23/2014), pointed on the nearby cluster PSZ1 G108.18-11.53,
correctionsweretransferredtothetargetandthephasecalibrator. hasbeenreducedfollowingthestrategydescribedinvanWeeren
Then,phaseandrescaledamplitudefromthephasecalibratorwere etal.(2016).Theonlysignificantemissionthatisassociatedwith
transferredtothetargetfield.Finally,asinglecycleofphase-only the candidate HyMoRS RG1 is co-located with the radio galaxy
self-calibrationwasappliedonthetargetfield. core(seeFig.3).Giventhenumberofphotonsdetectedwewere
Wenotethatallflu√xdensityerrorsforextendedemissionare able to fit only a simple power-law. Galactic NH absorption was
computedasS =σ· N ,whereσisthelocalimagermsand fixed at 1.25×1021 cm−2 (Kalberla et al. 2005). We obtained a
err beam
N isthenumberofbeamscoveringthesourceextension. slopeof theelectrondistribution equalsto1.67±0.21.The fitis
beam
showninFig.4and,althoughthenoiseislarge,itdoesnotshow
evidenceforstronginternalabsorption.
2.2 Optical
AlthoughpointedopticalobservationsofthenearbyclustersPSZ1
G108.18-11.53areavailable,noneofthemcovertheregionofthe 3 ATRUEHYMORS:NVSSJ232149+484951(RG1)
hostinggalaxyofRG1orRG2.Theredshiftofbothgalaxiesiscur-
RG1 is likely not part of the cluster PSZ1 G108.18-11.53 but
rentlyunknown,butaphotometricredshiftshouldbeincludedin
rather a foreground object. This conclusion is based on its dis-
theforthcomingreleaseofPanSTARSS.Tohavearoughestimation
tancefromtheclustercentreandangularsize.Theroughredshift
oftheredshiftofRG1wecomparedtheWISE(W1andW2band)
estimationmadeinSec.2.2(z = 0.16),alsoplacesthesourcein
the foreground with respect to the cluster (z = 0.335). Assum-
1 https://casa.nrao.edu/ ingthatredshift,theradiogalaxyhasaluminosityat1.4GHzof
(cid:13)c 2013RAS,MNRAS000,1–8
MultifrequencystudyofanewHyMoRS 3
Telescope Frequency Obs Totalobservingtime Bandwidth Maxresolution Rmsnoise Figure
(MHz) Date (hours) (MHz) (arcsec) (µJybeam−1)
GMRT 323 02Jun14 9 32 10.5(cid:48)(cid:48)×8.2(cid:48)(cid:48) 92 1Top-left
GMRT 607 06Jun14 8 32 5.4(cid:48)(cid:48)×5.3(cid:48)(cid:48) 47 1Top-right
WSRT 1381 02Jan14 8 160 17.3(cid:48)(cid:48)×13.3(cid:48)(cid:48) 32 1Bottom-left
VLA 1519 02/05Apr15;30Jan16 10a 1000 4.8(cid:48)(cid:48)×4.3(cid:48)(cid:48) 10 1Bottom-right
a6hinB-configurationand4hinC-configuration
Table1.Radioobservations
Figure1.BlackcontoursarefromGMRT/WSRT/VLAdata(323,607,1381,and1519MHz,aslabelledintheimages)at3...30×σwithsigmaandbeam
sizeasdescribedinTable1.Bluecontoursare(3,6)×σfromalow-resolutionimages(σ = 170µJybeam−1,beam=18(cid:48)(cid:48)×18(cid:48)(cid:48)forGMRT607MHzand
σ = 15µJybeam−1,beam=11(cid:48)(cid:48)×10(cid:48)(cid:48) forVLA1519MHz).PolarisationvectorsshowtheB-mode(paralleltomagneticfieldorientation)withalength
proportional to the fractional polarisation which, for the VLA image, is on average 18% in the northern lobe and 35% at the end of the southern lobe.
PolarisationanglehasbeencorrectedforGalacticrotationwhichisthisregionoftheskyisexpectedtobe−14radm−2Oppermannetal.(2012).
(cid:13)c 2013RAS,MNRAS000,1–8
4 F.deGasperin
RA DEC Flux323MHza Flux607MHza Flux1380MHza Flux1519MHza Integrated
hh:mm:ss dd:mm:ss (mJy) (mJy) (mJy) (mJy) spectralindexb
RG1(HyMoRS) 23:21:51.1 48:50:20 78±1 41±1 15.0±0.2 15.1±0.3 −1.10±0.01
RG2 23:21:48.1 48:30:04 41±1 27±1 12.1±0.2 14.9±0.3 −0.75±0.02
aFluxdensitycalculatedwithinthe3σcontourofthe323MHzmap.
bUsingallavailablefrequencies.
Table2.Radiogalaxiesfluxdensities
(178MHz,log(νL /ergs−1)=40.3)pointstowardsaclassification
ν
aslow-excitationradiogalaxy(LERG,Mingoetal.2014).
TheextensionofRG1isapproximately5(cid:48),whichcorresponds
toaprojectedlinearsizeof840kpcatz=0.16.Thenucleusofthe
sourcehasafaintinfraredWISEcounterpart,anopticalDSScoun-
terpart(Fig.5)andanX-rayChandracounterpart(Fig.3)atRA:
23h21m51.1s,Dec:48◦50(cid:48)20(cid:48)(cid:48).Theactivegalacticnucleus(AGN)
is also identifiable in the spectral index map as a region with an
invertedspectrum(α=+0.14±0.03,Fig.6).
ThesouthernlobehasaclearFRImorphology.Itisbentand
doesnothaveaclearterminationpoint,insteaditfadesintonoise.
Thesurfacebrightnessisratheruniformalongthelobe(seeFig.7).
The emission shows signs of polarisation to an average level of
20%.However,themagneticfieldshowsaquitedisorderedpattern,
similartothetotalintensity,whichindicatesaturbulentmedium.
Onthecontrary,thenorthernlobehastypicalcharacteristics
ofFRIIsources.Thelobebrightnessisenhancedtowardsitster-
mination and a discontinuity in the flux density, which increases
by a factor of 3, is detected in the same region where the spec-
tralindexflattens(Fig.7).Inthelocationofenhancedbrightness,
thelobeemits16%polarisedradiation.Themagneticfieldorienta-
Figure 3. Chandra X-ray image of the field surrounding NVSS
tionisparalleltothejetdirection,asseeninFRIIsources(Bridle
J232149+484951(RG1).Afewpointsourcesarevisible,oneisco-located
1984).However,noprominentsinglehotspotisdetectedinthatre-
with the HyMoRS core, no X-ray excess is present in the location of
gion.Wereportinsteadthreesurfacebrightnesspeaksalignedwith
theexpectedFRIIhotspot.RadiocontoursarefromVLAimageat1519
MHz:(3..100)×σ,logarithmically-spaced,withσ=15µJybeam−1(beam: thejetwhichmightindicatethepresenceofmultiplehotspots(as
11(cid:48)(cid:48)×10(cid:48)(cid:48)). seeninotherradiosourceslike3C20,3C405and3C351;Hard-
castle&Looney2008).NoX-rayemissionisdetectedintheloca-
tionofthehotspot,whichisnotuncommonandhasbeenreported
foranumberofbonafideFRIIradiogalaxies(seee.g.Hardcastle
et al. 2004). RM-synthesis can be a useful tool to study inhomo-
geneousmediumaroundHyMoRScores.However,inthiscaseit
does not show relevant features and the measured Faraday depth
in both lobes is compatible with being generated by our Galaxy
(−14radm−2,Oppermannetal.2012).
3.1 Spectralanalysis
Integrated flux density values and spectral index are listed in Ta-
ble2.Moreinterestingistostudythespectralindexprofilesacross
thesourceextensionsastheyaredistinctmarkersofFRIorFRII
sources. In radio galaxies the spectral energy distribution of the
synchrotronemissionisratherflatclosetothelocationwhereelec-
tronsareaccelerated.SynchrotronandinverseComptonlossesde-
plete the more energetic electrons faster then the less energetic
Figure4.X-rayspectraofthepointsourceco-locatedwiththecoreofRG1. ones,steepeningthespectrumwithtime.InthecaseofFRIsources
thejetisdisruptedclosetothecentralengineandasteepeningof
thespectrumshouldbevisiblemovingawayfromthecore.Onthe
(1.16±0.02)×1024 W/Hz.AccordingtothehistoricalFRI/FRII contrary,inFRIIsourcesthejetpropagatesrelativelyunperturbed
luminositydivision(Owen&Ledlow1994)thesourcewouldfall until the “hotspots” where it forms shocks that re-accelerate the
intheFRIregime,howeveritisnowknownthatboththemorpho- electrons. These electrons then flow back towards the core. As a
logical modes populate these luminosity ranges (Best 2009). The consequence,theradiospectraisexpectedtobeflatterclosetothe
luminosityininfrared(WISEat12µm,log(νL /ergs−1) = 42.7), hotspotandsteeperclosetothecore,wheretheagedplasmaaccu-
ν
X-ray(Chandra2−10kev,log(L /ergs−1) = 43.0),andradio mulates.
unabs
(cid:13)c 2013RAS,MNRAS000,1–8
MultifrequencystudyofanewHyMoRS 5
(a)RG1(DSS) (b)RG2(DSS)
Figure5.DSSimages(IRfilter)ofthecoresoftheHYMORSRG1(left)andthenon-HYMORSRG2(right).ContoursinthetwopanelsarefromtheVLA
imageinFig.1andFig.8respectively.Theopticalcounterpartsareindicatedbythearrow.Thehostinggalaxymightbeinamergingstateinbothcases.
Totestthesescenarios,weusedtheobservationsat323and
1519 MHz, where the most detailed radio images were obtained.
Thedatasetswerere-imagedtaperingthedatasotoobtainthesame
resolution(∼ 11(cid:48)(cid:48))inbothmaps.GMRTat325MHzandVLAat
L-bandinC-configurationcoverthesameminimumuv-distancein
wavelength,thismeansthatthetwointerferometersaresensitiveto
thesamemaximumangularscale.Thisminimisesthebiasdueto
thedifferentuv-coveragesthatisunavoidablewhendoingaperture
synthesisofextendedsources.Ofthetwoimages,weretainedonly
pixelsthatwereabove3σineachmap.Wedividedthesourcelobes
into circular regions with a diameter twice the synthesised beam
FWHM(seeFig.6).Foreachregionwecomputedtheintegrated
spectralindexusingabootstrapleastsquaresmethodtoincludeflux
densityerrors.TheresultsofthisanalysisareshowninFig.7.RG1
shows opposite trends in the two lobes. The northern lobe has a
steepspectralindex(α=−1.2±0.1)closetothecorewhichflattens
intheouterregion(α=−0.88±0.03)duetoparticlereacceleration
inthehot-spotandback-flowofagedelectronstowardsthenucleus.
Thesouthernlobehasanoppositetrend:thejetisdisruptedcloseto
thecoreandtheelectrons’spectrasteepensfromα=−0.86±0.06
toα=−1.3±0.1duetosynchrotronandinverseComptonlooses
asthedistancefromthecoreincreases.
Morphology,polarisationandspectralindexcharacteristicsall Figure6.Spectralindexmapsobtainedfromthe1519and323MHzmaps.
pointtotheclassificationofRG1asagenuineHyMoRS. Over-plottedthecontoursofthe323MHzmapat3...50×σlogarithmically
spaced,withσ = 170µJybeam−1andbeam=18(cid:48)(cid:48)×18(cid:48)(cid:48) forbothimages.
Dashed lines represent the regions used to extract the flux densities and
spectralindexvaluesplottedinFig.7.Thespectralindexofthecoresis
4 CLASSIFICATIONRISKS:NVSSJ232147+482956(RG2) ratherflat,whilegradientsareseeninthelobesofRG1.Thegradientsare
in-linewiththedefinitionofFRIandFRIIassociatedwiththesouthernand
In this section we present the analysis of another candidate Hy-
thenorthernlobe,respectively.Onthecontrary,RG2doesnotshowclear
MoRSthatcannotbeclassifiedassuch.Thiscaseexemplifiesthe gradient,butthespectralindexallowstoidentifythecoreposition.
dangerofmorphology-basedclassificationofhybridradiogalaxies.
ThecoreofRG2isidentifiablebyaflatspectralindex(α =
−0.01±0.06)andbyacounterpartvisibleintheWISEandDSS thesource.Ithasaratherflatspectralindexofα=−0.62±0.05and
imagesatRA:23h21m48.1s,Dec:48◦30(cid:48)04(cid:48)(cid:48) (Fig.5b).Thenorth- itispolarisedtothelevelof∼16%.Thepolarisationanglesuggests
ernlobeofthesourcehasanFRImorphologyanditbendsintwo amagneticfieldorientedperpendiculartothejetdirectionasinthe
locations. Its spectral index is quite uniform along its extension caseoftheinitialpartofthejetinFRIsources.Weinterpretthis
(α(cid:39)−1). emissionasbeamedradiationfromanincomingjet.
Acompactemissionclosetothecoreisthebrightestregionof Low surface brightness emission is present 2(cid:48) south-west of
(cid:13)c 2013RAS,MNRAS000,1–8
6 F.deGasperin
7 5 DISCUSSIONANDCONCLUSIONS
North lobe (FR II)
South lobe (FR I)
6 SomerecentworkssupporttheideathattheFRI/FRIIdichotomy
might be related to environmental effects. In fact, in their central
5 region,HyMoRSappearasFRIIsources.Thisfindingissupported
y) byVLBIobservationsofthecentral10kpc(Ceglowski,Gawron-
mJ
y (4 ski&Kunert-Bajraszewska2013)andbymeasuringX-ray–radio
sit scalingrelationsofthenuclearemission(Miller&Brandt2009).In
n
x de3 thisscenario,onesideofthesourcemaydecelerateandturnintoan
Flu FRItype.Atheoreticalframeworkthatcanexplainthisbehaviour
wasdevelopedbyMeliani,Keppens&Giacomazzo(2008),where
2
the jet deceleration is caused by a density jump in the external
medium.AccordingtoPeruchoetal.(2012),theFRIItypejetcan
1
alsobedisruptedbythegrowthofhelicalinstabilityoriginatedby
theinteractionwiththesurroundingmedia.Itisinterestingtono-
0.70 ticethatthemagneticfieldontheFRIsideofRG1seemsindeedto
traceanhelicalflowwhichmighthavedisruptedtheoriginalFRII
0.8 straightjet.Alternatively,thejetsofFRIIsourcesmightbeeroded
by the entrainment due to the interactions with their surrounding
0.9
lobes(Wangetal.2011).
x 1.0
ne Other theories link the arising of an FRI or an FRII to the
d
al i 1.1 nucleus properties (Wold,Lacy & Armus 2007). However, X-ray
ctr andopticalspectroscopicstudies(Buttiglioneetal.2010;Best&
e
p
S 1.2 Heckman2012)supportthedivisionofradiosourcesbasedonline
ratios(lowexcitationgalaxiesorLEGandhighexcitationgalaxies
1.3 orHEG)ratherthentheirobservedmorphology,andlinkthedis-
cussiontothepropertiesofthecentralengine:blackholespinor
1.4 modeofaccretion.Inthisclassification,weakFRIIs(LEGs)fueled
viaradiativelyinefficientflowsatlowaccretionratesaregrouped
1.5 togetherwithFRIs(e.g.Best&Heckman2012).
0 10 20 30 40 50 60 70
Distance (arcsec)
Thepicturethatseemstoariseisthatallradiosourcesbegin
Figure7.Top:integratedfluxdensityat323MHzoftheregionsshownin
life as FRII sources, with collimated jets, but that some sources
Fig.6.Onthexaxisisthedistanceoftheregionfromthesourcecore;error-
do not survive as such to large sizes, getting disrupted into FRIs
barsareusedtoshowtheregionextension.Bottom:integratedspectraindex
(Kaiser&Best2007;Kunert-Bajraszewskaetal.2010).Therefore,
valuesoftheregionsshowninFig.6.ThetwolobesofRG1showopposite
trendsasexpectedforHyMoRS.ThisisnotthecaseforRG2,whereboth theFRI/FRIInatureofaradiogalaxymustberelatedwithboththe
lobesarecompatiblewithaconstantspectralindexalongtheirextensions. centralenginepropertiesandaccretionmode,whichdrivesthejet
power,aswellaswiththesurroundingenvironment,whichopposes
the jet advancement. In this picture, HyMoRS are FRII sources
withajetpowernotmuchhigherthanthatofanFRI.Asaconse-
quence,avariationintheenvironmentaroundoneofthetwojets
could cause its disruption and transformation in a turbulent FRI
lobe.
the core. This emission might be part of the source lobe. There
isnoanapparentconnectionbetweenthisemissionandthecore- In this work we showed that HyMoRS have a complete bi-
jetregion,ashasbeenseeninFRIIlobes.However,theputative modalnatureunderthepointofviewofmorphology,polarisation
“lobe”lacksacompacthotspotanditsspectralindexisrathersteep andspectralproperties,showingnoapparentdifferencebetweena
(α (cid:46) −1). We note the presence of a compact, flatter spectrum “standard”FRI(FRII)lobeandanHyMoRSFRI(FRII)lobe.We
source(PS2inFig.8,α = −0.77±0.06)withnoWISEnorDSS presentedmulti-frequency,polarimetricobservationoftwocandi-
counterpart on the lobe’s west side. A point source with a WISE dateHyMoRS.NVSSJ232149+484951(RG1)appearstobearare
counterpart is also present on the northern edge of this extended exampleofanintrinsically-truehybridsourceasindicatedbymor-
emission(PS3inFig.8).Aalternativeinterpretationcouldbethat phology,spectralindexandpolarisationanalysis.Ontheotherhand
thisextendedemissionisnotpartoftheRG2sourcebutitisalobe NVSSJ232147+482956(RG2)appearsstronglybeamedanddoes
relatedwiththecompactsourcePS3. notshowanyhotspotontheputativeFRIIsideandcannotbeclas-
Although the source is difficult to interpret and probably sifiedasanHyMoRS.Ourworksolidifiestheimportancetoanal-
stronglybeamed,thedataavailabletousdonotsuggestRG2tobe ysemulti-frequency,high-resolutionandpolarisationmapstoac-
ahybridradiogalaxy.However,alowresolutionimage(seeFig.8) curatelydistinguishHyMoRS.MoreinsightontheHyMoRSphe-
withoutspectralindexorpolarisationinformation(asaradiosur- nomenonwillcomefromtheanalysisoflargersamplesthatwillbe
vey image) could have led to the HyMoRS classification of RG2 assembledsoonthankstohighresolution,highsurvey-speedtele-
basedonitsmorphology. scopessuchasLOFARandSKA.
(cid:13)c 2013RAS,MNRAS000,1–8
MultifrequencystudyofanewHyMoRS 7
Figure8.RadioimagesofRG2,abeamedFRIradiogalaxythatcannotbeclassifiedasHyMoRS.Left:blackcontoursarefromVLAdata(1519MHz)at
3...80×σlogarithmicallyspaced,withσ=20µJybeam−1,beam=4.8(cid:48)(cid:48)×4.3(cid:48)(cid:48).Bluecontoursare(3,6)×σfromalow-resolutionimages(σ=35µJybeam−1,
beam=11(cid:48)(cid:48)×11(cid:48)(cid:48)).Right:contoursfromWSRTdata(1381MHz)at3...50×σlogarithmicallyspaced,withσ=37µJybeam−1,beam=17.3(cid:48)(cid:48)×13.3(cid:48)(cid:48).In
bothimages,vectorstracetheB-modesofpolarisedemission.
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