Table Of ContentAstron. Nachr./AN0,No.0,000–000(2009)/DOIpleasesetDOI!
Transverse motions in CSOs?
C.Stanghellini1,⋆,D.Dallacasa1,2,T.Venturi1,T.An3,andX.Y.Hong3
1 IstitutodiRadioastronomia–INAF,Bologna,Italy
2 DipartimentodiAstronomia-Universita`diBologna,Italy
3 ShanghaiObservatory
9
0 Received08Dec2008,accepted18Dec2008
0 Publishedonlinelater
2
n Keywords Galaxies:active–radiocontinuum:galaxies–quasars:general
a
ThemeasurementofpropermotionsinCSOsisapowerfultooltodeterminethedynamicalevolutionofthenewlyborn
J
extragalacticradiosources.Weobserved3CSOswiththeVLBAin2004andin2006tomonitorchangesintheirstructure
0
andmeasuretheseparationvelocityofthehotspots.ItisimportanttoincreasethesizeofthesamplesofCSOswithmea-
2
suredexpansionvelocitytotesttheexistanceoffrustratedobjects,andputstringentconstraintsonthecurrentmodels.We
foundforallthethreeobjectsobservedatransversemotionofthehotspots,andwesuggestasthemorelikeyexplanation
]
O aprecessioninthejetaxis.Thisbehaviourlikelyinhibitsoratleastslowsdowntheradiosourcegrowthbecausethehead
ofthehotspotcontinuouslyhitsnewregionsoftheISM.Thereforetheseradiosourcesmayrepresentanoldpopulationof
C GPS/CSOs.
.
h
(cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
p
-
o
r 1 Introduction Table1 Informationontheobservedradiosources
t
s
a source mr z RA(J2000) DEC(J2000)
CompactSymmetricObjects(CSOs)areradiosourceswith
[
sub-kpcsizewithamorphologyandradioluminositysimi-
J0706+4647 - - 070648.147 +464756.21
1 lartotheclassicaldoubles,whicharetypically1000times J1335+5844 22.0 0.57 133525.928 +584400.29
v
larger. The integrated radio spectrum is generally convex, J1823+7938 16.7 0.224 182314.109 +793849.00
5
with the peakfrequencyarounda few GHz. The projected
6
0 linearsizegenerallyneverexceeds1kpc,andsomesources
3 areafewtensofpcinsizeonly.
. •Simpledoubleortriplestructureonmasscale;
1 A key issue in the study of these sources is their age.
0 Theycouldberatheryoung(<104yr)orsubstantiallyolder • High declination, allowing a better sampling of the UV
9 (>106yr). plane;
0
: It has also been suggested that many CSOs are short • High flux density at 8.4 GHz, where the VLBA has the
v livedobjectswhodiebeforereachinglargesizes(Readhead bestcombinationofsensitivityandresolution;
i
X etal.1994).
•Sizebetween10and∼100mas,tofullyexploitthespatial
r At present there is largesupportto the youthscenario. frequenciessampledbytheVLBA.
a
Expansionspeedsdetectedareinagreementwithestimated
radiativeages to giveto CSOs an age of a thousandto ten •Notalreadystudiedorunderstudyforthispurpose.
thousand years (Murgiaet al. 1999,Polatidis and Conway Not many sources satisfied all these requirements. A
2003,Gugliuccietal.2005).Stilltherearethesourceswith searchintheliteratureandamongtheVLBIdatabasesavail-
upperlimitsto theirexpansionspeed,anditis notdemon- able online was carried out to select suitable sources be-
strated that all the sources are growing fast. In this con- longing to the samples of GPS radio sources and CSOs (
text monitoring new CSOs increases the number of mea- Snellenetal.1998,Stanghellinietal.1998,Dallacasaetal.
surmentsofpropermotionsandimprovesthestatistics. 2000, Dallacasa et al. 2002a). From the candidate sources
wechoseasthebestsuitable,the3objectslistedintable1,
which satisfy the above requirements. Two objects do not
2 The selectionofnew targetCSOs
have a measuredredshift, buttheybelongto GPS samples
with planned/ongoingoptical studies (e.g. Dallacasa et al.
WesearchedCSOssourcesfulfillingthefollowingrequire-
2002b), and in any case a rough redshift can be estimated
ments:
for one of them (J1335+5844)by its optical magnitude as
⋆ Correspondingauthor:e-mail:[email protected] itisknownthatopticalmagnitudesofGPS/CSOscorrelate
wellwithredshift(O’Deaetal.1996,Snellenetal.1996).
(cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
Astron.Nachr./AN(2009) 1
10
5 2σ
J0706+4647
0
C
E -5
S
C
R
A
Milli-10
-15
-20
2σ
-25
50 40 30 20 10 0 -10
MilliARC SEC
Center at RA 07 06 48.147 DEC 46 47 56.21
Cont peak flux = 25.73 mJY/beam
Levs = 0.6 mJy * (-3, 3, 5, 12, 25, 50, 100, 200, 400, 800, 1500, 3000, 6000)
Fig.1 J0706+4647,thearrowsshowtheexprapolatedmotionsinabout500yr.Seetextforanexplanationofthearrows.
Hereandinthenextfiguresthefirstpositivecontouristhreetimesther.m.s.noiseontheimage
3 Observations,data reduction, and results
2
We observedthese 3radiogalaxieswiththe VLBA in two
epochs for 24 hours each. Observationsfor the first epoch
0 took place in 2 observing sessions in January and March
2004, while observations for the second epoch took place
about2.5yearslater,alwaysintwoseparateobservingses-
2σ
-2 sionsinSeptemberandNovember2006.
At each epoch each source was observed 6/8 hours at
8.4GHzinsinglerightpolarization.
-4 The calibration and mapping has been done in a stan-
J1335+5844
C dard way using the NRAO AIPS software. Once the final
E
S imageshavebeenobtained,weused theDIFMAPdatare-
C -6
R duction package to model-fit the radio sources with a few
A
Milli discretepointlikeorgaussiancomponentsonthefullycali-
bratedvisibilities.
-8
The typical resolutions of the images (FWHM) are ∼
0.8mas.
InJ0706+4647andJ1335+5844averyweakradiocore
-10
isdetectedforthefirsttimeatthecenterofthedoublestruc-
ture.Suchdetectionistrulyimportanttoconfirmtheclassi-
2σ
-12 ficationoftheradiosorceasaCompactSymmetricObject
(CSO)andpossiblytoreferthehotspotmotiontothecen-
ter ofactivity.Inthepresentobservationshowever,evenif
-14 detected,thepositionofthecoresarenotwellconstrained,
andreferringthepositionoftheHot-Spotrespecttothecore
givestoolargeerrors.
2 0 -2 -4 -6 -8
MilliARC SEC Therefore the relative position of the 2 hot-spots have
Center at RA 13 35 25.928 DEC 58 44 00.29
beenmeasuredinallthethreeobjectsmodelfittingthedata
Cont peak flux = 299.02 mJY/beam
Levs = 0.1 mJy * (-3, 3, 6, 12, 25, 50, 100, 200, 400 intheUVplanewithseveralgaussiancomponents.Wecon-
400, 800, 1500, 3000, 6000)
sidered as hot-spots the two strongest and more compact
Fig.2 J1335+5844,thearrowsshowtheextrapolatedmo-
componentsderivedfromthemodelfit.Theestimateofthe
tionsinabout100yr.
accuracy and precision of this measures is usually a diffi-
www.an-journal.org (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
2 Stanghellinietal.:TransversemotionsinCSOs?
2
J1823+7938
0
C-2
E
S
C
R 2σ
A
Milli-4 2σ
-6
-8
-10
2 0 -2 -4 -6 -8 -10 -12 -14 -16
MilliARC SEC
Center at RA 18 23 14.109 DEC 79 38 49.00
Cont peak flux = 175.35 mJY/beam
Levs = 0.8 mJy * (-3, 3, 6, 12, 25, 50, 100, 200, 400, 800, 1500, 3000, 6000)
Fig.3 J1823+7938,thearrowsshowtheextrapolatedmotionsinabout150yr.
culttask,butgiventhehighdynamicalrangeoftheimages ingthe motionofthe2 hotspotsis symmetricwith respect
itismuchbetterthanthebeamsize.Intheassumptionofa tothecenterofactivity,themorelikelyexplanationforthe
linear motion the errors can be estimated by the scatter of transverse hotspot motion is precession of the jet axis. An
thedatathemselves,whichisnotpossibleinourcasewith alternativeinterpretationofthedifferentpositionseeninthe
only two epochs. In a previous work on OQ208 we could twoepochsisachangeinthestructureoftheradiosource.
be able to estimate a 1 sigma error of 0.02 mas based on We note that the hotspots show trail of extended emission
the scatter of the data, and we use the same value for the consistent with the direction of the motion, supportingthe
presentdata,whichareofthesametype(Stanghellinietal. hypothesisofrealmotion.
inpreparation).
Tomakethemotionvisible,inFig.1,2,and3weshow
Table 2 Hot-spot relative position in the 2 epochs. The the extrapolatedmotions in 500 years for the radio source
positionofthesouthernhot-spotrelativetothenorthernone J0706+4647(v =∼0.4c,assumingz=0.5),in100yearsfor
is indicated in polar coordinates in columns 3 and 4. The J1335+5844(v =∼ 0.4c,photometricz=0.57),andin 150
differencein positionfromfirstand secondepochisgiven yearsforJ1823+7938(v =∼ 0.15c,z=0.224).Thearrows
incolumn5and6. donotshowtheexactpositionofthecomponentsafterthat
source epoch r θ r θ time,becausewedonotknowifthemotionisconstantand
mas deg mas deg which is the period of the jet precession (if this is the real
J0706+4647 2004 52.126 109.35 natureofthemotion).Itisonlyshowntomakeitpossibleto
J0706+4647 2006 52.059 109.45 0.11 -145.79 evaluatebyeyethedirectionandthevelocityofthemotion.
J1335+5844 2004 12.964 -164.84
J1335+5844 2006 12.891 -165.18 0.11 61.41
J1823+7938 2004 15.765 -115.60 Itseemssurprisingthatwefindthesefirstcasesoftrans-
J1823+7938 2006 15.745 -115.38 0.06 -7.20 versemotionsinallthethreeobjectsobserved,butweare
notawareofanybiaswecouldhaveputinourselectioncri-
teria. These objects deserve to be observed again together
We present in Tab. 2 the relative position of the two with other candidates of the original list, and it is impor-
hotspotsforeachepochandthedifferencebetweenthetwo tanttomonitorallCSOsofacompletesampletofindhow
epochs.Theerrorsareratherlarge,butthereisaclearevi- frequenttranversemotionsare,butweareconfidentwere-
denceofatransversemotionrespecttothejetaxis.Assum- vealedarealphenomenon.
(cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2009) 3
4 Conclusions R.S.BoothandY.M.Pihlstrm,publishedOnsalaSpaceObser-
vatory,p.99
Wesuggestascenariowherethejetprecessionstronglyin- Stanghellini,C.,Dallacasa,D.,O’Dea,C.P.,Baum,S.A.,Fanti,R.,
Fanti,C.:2001,A&A377,377
fluencesthe size and morphologyof a radiosource.When
the precession is negligible the radio source expands nor-
mally to become an FRI or an FRII. When the jet preces-
sion is small the jet is able to travel through the channel,
void ofISM particlesbecausethetransverseshocksit cre-
ated, and the hot spot has a slow transverse motion, as in
theobjectspresentedinthiswork.Whenthejetprecession
is large,the jethits the ISM at a shorterdistance andan S
shape is formed, as sometime seen in GPS radio galaxies
(e.g.B0500+019,Stanghellinietal.2001).
Rapisprecessionislikelytoinhibitorslowdownatleast
theradiosourcegrowthbecausetheheadofthehotspotcon-
tinuously hits new regionsof the ISM. These objects with
a precessing jet may represent population of CSOs older
thanthosewhithexpansionspeedmeasuredsofar,andmay
help to explain the higher fraction of CSOs in samples of
radiogalaxieswith respectto the predictionsofthe models
(Snellenetal2000,Alexanderetal.2000).
Acknowledgements. TheVLBAisoperatedbyU.S.NationalRa-
dioAstronomyObservatorywhichisafacilityoftheNationalSci-
enceFoundation,operatedundercooperativeagreementbyAsso-
ciatedUniversities,inc.
References
Alexander,P.2000,MNRAS319,8
Dallacasa, D., Stanghellini, C., Centonza, M., Fanti, R.: 2000,
A&A363,887
Dallacasa, D., Tinti, S., Fanti, C., Fanti, R., Gregorini, L.,
Stanghellini,C.,Vigotti,M.:2002a,A&A389,115
Dallacasa,D.,Falomo,R.,Stanghellini,C.:2002b,A&A382,53
Gugliucci,N.E.,Taylor,G.B.,Peck,A.B.,Giroletti,M.:2005,ApJ
622,136
O’Dea,C.P.,Stanghellini,C.,Baum,S.A.,CharlotS.:1996,ApJ
470,806
Murgia,M.,Fanti,C.,Fanti,R.,etal.:1999,A&A345,769
Polatidis,A.G.,Conway,J.E.:2003,PASA20,69
Readhead, A.C.S., Xu, W., Pearson, T.J., Wilkinson, P.N., Pola-
tidis, A.G.: 1994 in Compact Extragalactic Radio Sources,
Proceedings of the NRAO workshop held at Socorro, New
Mexico,February11-12,1994.EditedbyJ.AntonZensusand
KennethI.Kellermann.GreenBank,WV:NationalRadioAs-
tronomyObservatory(NRAO),1994.,p.17
Snellen, I.A.G., Bremer, M.N., Schilizzi, R.T., Miley, G.K., van
Ojik,R.:1996,MNRAS279,1294
Snellen, I.A.G., Schilizzi, R. T., de Bruyn, A. G., Miley, G. K.,
Rengelink, R.B., Ro¨ttgering, H. J. A., Bremer, M. N.:1998,
A&AS131,435
Snellen,I.A.G.,Schilizzi,R.T.,Miley,G.K.,etal.2000,MNRAS
319,445
Stanghellini,C.,O’Dea,C.P.,Dallacasa,D.,Baum,S.A.,Fanti,R.,
Fanti,C.:1998,A&AS131,303
Stanghellini,C.,Dallacasa,D.,Bondi,M.,XiangL.:2000,EVN
Proceedingsofthe5theuropean VLBINetworkSymposium
heldatChalmersUniversityofTechnology,Gothenburg,Swe-
den,June29-July1,2000,Eds.:J.E.Conway,A.G.Polatidis,
www.an-journal.org (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
