Table Of ContentMon.Not.R.Astron.Soc.000,1–5(2012) Printed22January2013 (MNLATEXstylefilev2.2)
Probing the nature of compact ultra-steep spectrum radio
sources with the e-EVN and e-MERLIN
3
M. K. Argo1, Z. Paragi2, H. Ro¨ttgering3, H.-R. Kl¨ockner4, G. Miley3, M. Mahmud5
1
0 1ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
2 2Joint Institute for VLBI inEurope (JIVE), Postbus 2, 7990 AA, Dwingeloo, The Netherlands
3Leiden Observatory,UniversiteitLeiden, NielsBohrweg 2, NL-2333 CA Leiden, The Netherlands
n
4Max-Planck-Institute fu¨r Radioastronomie, Auf dem Hu¨gel 69, 53121 Bonn, Germany
a
J 5Formerly at JIVE
1
2
] RevisionJanuary2013
O
C
ABSTRACT
.
h WepresentfirstresultsfromelectronicMulti-ElementRemotelyLinkedInterferometer
p Network (e-MERLIN) and electronic European VLBI Network (e-EVN) observations
-
of a smallsample of ultra-steep spectrum radio sources,defined as those sources with
o
r aspectralindexα<−1.4between74MHzand325MHz,whichareunresolvedonarc-
t secondscales.Such sourcesare currently poorly understood anda number of theories
s
a astotheiroriginhavebeenproposedintheliterature.Thenewobservationsdescribed
[ herehaveresultedinthefirstdetectionoftwoofthesesourcesatmilliarcsecondscales
andshowthatasignificantfractionofultra-steepspectrumsourcesmayhavecompact
1
structures which can only be studied at the high resolution available with very long
v
3 baseline interferometry (VLBI).
1
Key words: radio continuum: galaxies – galaxies: active
8
4
.
1
0
3 1 INTRODUCTION gave an understanding of their enormous importance for
1 studies of galaxy and cluster evolution.
:
v
Radio sources with an ultra steep radio spectrum (USS; A third class of USS has not yet received much atten-
Xi defined here as having α < −1.4 where Sν ∝ να) are of- tion: those which are unresolved on arcsecond scales. The
ten associated with extreme phenomena in the universe. aimoftheobservationspresentedhereistotakeafirststep
r
a USS sources with a classical FRII type radio morphology towards determining the nature of these objects. There are
(highly collimated jets and well-defined lobes with promi- a great number of possible scenarios for the origin of such
nent hotspots) are, for the most part, associated with very compact USSobjects including(i) radio galaxies located at
distant galaxies (2 < z < 5.2; DeBreuck et al. 2000; or near the epoch of re-ionisation (it has been suggested
Ro¨ttgering et al. 1997). Being very massive and often lo- that objects with the steepest spectral index correspond to
cated in proto-clusters, these sources can be used to study the highest redshift objects, e.g. Krolik & Chen 1991), (ii)
the origin and evolution of massive galaxies and clusters. youngobscuredradiogalaxies withtheearlyphaseperhaps
Recenthigh-redshiftresultsinclude(i)thefindingthatmas- accompanied by a starburst (Reuland et al. 2004 inferred
sive radio galaxies with masses of around 1012M⊙ exist at starformationratesof∼1000M⊙/year),(iii)steepspectrum
up to z ∼ 5 (Seymouret al. 2007), and (ii) the discovery corequasarswith thesteepeningperhapscausedbycooling
of proto-clusters up to z ∼ 4 with total masses as ex- of the plasma via scattering of infrared photons from the
pected for progenitors of nearby clusters (a few 1014M⊙; torusbythesynchrotron-emittingelectrons,and(iv)pulsars
Venemanset al. 2007). USS sources with mostly diffuse ra- (DeBreuck et al.2000 foundaclearoverdensityofsources
dio emission are virtually always associated with close-by withα<−1.6neartheGalacticplane).Longbaselineradio
clustersuptoz∼0.5(van Weeren et al.2009)andtheemit- interferometric observations may give very important clues
ting regions can have spectacular sizes up of to a few Mpc. aboutthenatureoftheseobjects.Starformationactivityin
The most likely explanation for this class of object is ra- galaxies is expected to produce low brightness temperature
dioemittingplasmatracingshocksinmergingclusters.The emission (∼ 104 K, see Alexandroff et al. 2012 and refer-
study of these two types of USS source started in the late encestherein)anddiffusemorphology.Synchrotronemission
seventies with studies of their morphology, and follow-up fromradiolobesonkiloparsecscalescanbeverywellimaged
work with large optical and X-ray telescopes subsequently withe-MERLIN,butwillstillbemostlyresolvedoutonmil-
2 M.K. Argo et al.
liarcsecond scales. Finally, steep spectrum quasars and pul- and J151229+470245 were detected. Note that initially
sarswillbecompactwhenprobedwithVLBI;thepotential J130612+514407 was not detected because of a 12.8-
Galactic origin of the source can then be verified by multi- arcsecond error in the pointing position; a clear detection
epoch proper motion measurements. This letter describes of the source was obtained only after this error was discov-
e-MERLINande-EVNobservationsofasampleofcompact ered and a wide-field image was made. Table 2 summarises
ultra-steepspectrumsources,aimedatattemptingtoinves- thedetections of thesesources. Since, forthesources which
tigatetheirmorphologiesandnarrowdownthepossibilities. weredetected,thee-EVNobservationsrecoveredonly∼20%
Section 2 describes the sample selection and observations, oftheVLAfluxatthesamefrequency,e-MERLINcommis-
the results are discussed in section 3 and in section 4 we sioning observations were requested in order to investigate
draw some conclusions. their structures on intermediate scales. Comments on each
of these sources are given in thenext section.
2 SAMPLE SELECTION AND OBSERVATIONS
2.2 e-MERLIN observations
The initial sample of USS sources was found by correlating
e-MERLINobservationsoftwooftheEVN-detectedsources
the74MHzVLSScatalogue(Cohen et al.2007)againstthe
wererequestedinordertoinvestigatetheirstructuresonin-
325MHz WENSS database (Rengelink et al. 1997) and se-
termediatescales.Thee-MERLINobservationswerecarried
lectingthesmallfractionofVLSSsourceswithasufficiently
outinMarch2011,duringthecommissioningperiod.Atthis
steep spectrum. A sub-sample of compact sources was then
point in thecommissioning phase, fivestations of thearray
selected bychoosing objects which were both unresolved in
were available with the new C-band (4- to 6-GHz) system:
theFIRSTsurvey(Becker et al.1995),andhadafluxratio
Mk2, Tabley, Darnhall, Knockin and Defford. Observations
between the NVSS (Condon et al. 1998) and FIRST cata-
werecarriedoutat6.6GHzusingthenewcapabilitiesofthe
loguesof0.8<R<1.2, ensuringthatthe1.4GHzemission
is indeed compact on scales of ∼1arcsecond. The FIRST upgradede-MERLINarray2andusedfouradjacentbandsof
128MHz, each split into 512 channels, starting at 6.64GHz
andNVSSmapswerealsovisually inspected toensurethat
with a central frequency of 6.89GHz. In both cases 3C286
the sources were unresolved and that there were no nearby
wasusedastheprimaryfluxcalibratorandOQ208wasused
objectswhichcouldhaveledtoambiguitiesinthemeasured
as the bandpass calibrator. Both sources were observed for
fluxes. The resulting sample contained 17 sources. A sub-
∼4.5 hours on 26th March and ∼10 hours on March 30th
sampleoffivesourceswasselected foranexploratory VLBI
2011. J151229+470245 used the source 1500+478 for phase
survey, all of these sources had NVSS flux densities larger
referencing, J072212+291042 used 0736+299 as the phase
than 10mJy (see Table 1).
calibrator. Of the total observing time, only ∼9 hours on
each source were ultimately usabledueto avariety of com-
2.1 e-EVN and WSRT observations missioningissues.Thedatawereflaggedspectrally,averaged
infrequency,fringefittedandthencalibratedusingstandard
The five sources (J072212+291042, J082916+383453, methodsfor phase referencing experiments.
J130612+514407, J131655+483200 and J151229+470245)
were observed with the EVN at 1.6 GHz in e-VLBI mode
(Szomoru 2008) over two 10-hour runs carried out on
10th and 11th June 2010 (programmes EP070A and B; PI 3 RESULTS
Ro¨ttgering).Tenstationsparticipatedintheexperiment:Ef-
3.1 J072212+291042
felsberg,Medicina,Onsala,Torun,Westerbork(12antennas
tiedarray),Lovell,Cambridge,Darnhall,KnockinandShe- With a flux density of 10mJy in FIRST and 11mJy in the
shan. The target sources were phase-referenced to nearby NVSS(Table2),J072212+291042istheweakestofthethree
calibrators; the typical on-source time was 2.5 hours. The VLBIdetections.Thee-EVNobservationsshoweda∼2mJy
totalaggregate bitratepertelescope was1024Mb/s, except source with no clear evidence of structure on VLBI scales,
for the MERLIN antennas which participated with a lower consistent with a point source model, albeit at significantly
rate of 128 Mbps. Both LCP and RCP polarizations were lowerfluxdensitythanthatdetectedbytheVLAandWest-
observed with 2-bit sampling. The data were processed us- erbork: only ∼30% of the Westerbork flux is recovered by
ing standard procedures following the EVN Data Analysis thee-EVNatthesamefrequencyindicatingthepresenceof
Guide1. significant structureon intermediate scales.
During the e-EVN observations, synthesis array data The source is not detected with e-MERLIN at 6.9GHz
were also recorded at Westerbork and were independently to a 3σ limit of 0.12mJy/beam. Comparing the 3-σ limit
processed in AIPS. The data were recorded using 8 bands at 6.9GHzwith thefluxdetectedby thee-EVNat 1.4GHz
of 20MHz, each split into 64 channels. For amplitude cal- givesalimitonthespectralindexofα<−1.4,showingthat,
ibration we used 3C286. There was significant interference in this case, the spectral index remains steep at higher fre-
during the observation resulting in the loss of the first two quencies.AsearchofthecataloguesintheNASA/IPACEx-
subbands. tragalactic Database (NED)3 shows that J072212+291042
Outofthefiveobjects from thesamplewhich wereob-
served with the e-EVN, J072212+291042, J130612+514407
2 See http://www.e-merlin.ac.uk/tech/ for the current capabili-
tiesofthee-MERLINarray.
1 http://www.evlbi.org/user guide/guide/userguide.html 3 http://ned.ipac.caltech.edu/
Probing the nature of USS sources 3
Table 1.Compactsourceswithaspectralindexα74MHz <−1.4observedwiththee-EVN.NamescorrespondtotheNVSScatalogue
325MHz
names. The coordinates are from the VLA FIRST Survey. The spectral index is calculated fromthe VLSS and WENSS flux densities,
andthefluxdensityratioiscalculatedfromtheNVSSandFIRSTcatalogues.TheVLSS,WENSS,NVSSandFIRSTfluxdensitiesare
giveninmJy.PositionsaregiveninJ2000coordinates. NotethatboththeWENSSfluxdensityandspectralindexofJ151229+470245
wererevisedaftertheobservationsduetoanerrordiscoveredinoneofthecatalogues (seetext).
Name —–RA—– —–Dec—– α74MHz S1N4V00SSMHz SVLSS SWENSS SNVSS SFIRST
325MHz SFIRST 74MHz 325MHz 1400MHz 1400MHz
1400MHz
J072212+291042 07:22:12.603 +29:10:41.51 −1.45 1.1 650 76 11 10
J082916+383453 08:29:17.351 +38:34:52.62 −1.44 0.9 940 112 11 12
J130612+514407 13:06:12.176 +51:44:06.93 −1.58 1.1 1640 159 27 25
J131655+483200 13:16:55.642 +48:32:00.13 −1.52 1.1 2390 252 67 61
J151229+470245 15:12:29.174 +47:02:45.59 −0.50 1.0 1130 543 217 221
has no known counterparts in other wavebands. The com- 3.3 J151229+470245
pact nature of this object suggests that the most likely ex-
Incontrast, J151229+470245 is clearly resolved in both the
planationiseitherasteepspectrumquasarcoreoraGalac-
e-EVN and e-MERLIN images (see Fig. 1). The WSRT-
tic pulsar. Although this source has a low galactic latitude
only data show an unresolved source, as expected from the
(see Table 2) a search of pulsar catalogues around this lo-
NVSS/FIRST images, with a peak flux of 192mJy/beam
cationshowsnocurrentlyknownpulsarorpulsarcandidate
andanintegratedfluxdensityof203mJy.Thisiswithin8%
at these coordinates (Eatough, privatecommunication).
of the flux recorded in the FIRST and NVSS catalogues.
The e-EVN observation shows a source elongated in a NE-
SW direction, with a peak brightness to total flux density
ratio of 1:4.
3.2 J130612+514407
The map obtained with e-MERLIN at 6.9GHz (Fig.
Of the detected sources, J130612+514407 has the steepest 1) shows a very different structure. The source is clearly
spectral index at low frequencies. Without an observation resolved and appears to show a core-jet morphology. The
at 6.8GHz, however, it is impossible to tell if the spectrum e-EVN component is coincident with the brightest feature
remains steep at higher frequencies - the positional offset observedwithe-MERLIN.Atwo-componentfitwascarried
in the e-EVN observations was only discovered after the e- outonthee-MERLINmapusingtheAIPStaskjmfit:arel-
MERLIN observations had taken place. The e-EVN data ativelycompact(comparedtothee-MERLINbeam),bright
showanunresolvedpointsourcewithanintegratedfluxden- Gaussian with a peak of 39.5mJy/beam, an integrated flux
sityof7.1mJy,recovering∼30%oftheemissiondetectedby of 66.1mJy and a size of 98×73 mas at a position angle
the Westerbork array during the same observation, imply- of 71 degrees, and a more extended Gaussian “jet” com-
ing structure on intermediate scales at 1.4GHz. However, ponent with a fitted peak brightness of 4.14mJy/beam, an
the primary beam of the Westerbork tied-array is signifi- integrated flux density of 28.3mJy with a size of 0.2×0.12
cantly smaller than the 12.8” offset of the source position arcseconds at a position angle of 145 degrees.
fromthepointingcentreofthee-EVNobservation;imaging The nature of this source is rather puzzling. Interest-
thesourcewithoutincludingtheWesterborkarrayresultsin ingly,whilethee-MERLINmap(takenonitsown)appears
a peak flux density of 18.5 mJy and an integrated intensity to show a classic core-jet morphology, the e-EVN (“core”)
of 21.6 mJy, recovering almost all of the WSRT-only flux, component is very resolved suggesting that it is not a sim-
although it should be noted that these measurements are ple AGN core. Unless there is very strong scatter broaden-
somewhat uncertain dueto the large positional offset. ing of the target, the observed structure is more consistent
Unlike J072212+291042, J130612+514407 has an opti- withlobeemission detectedon10-masscales. Theobserved
cal counterpart in SDSS, J130612.15+514407.0, a diffuse- steepspectrum,lowbrightnesstemperatureandnovariabil-
looking galaxy with an SDSS spectrum giving a red- ity (between VLA and WSRT epochs) all support this sce-
shift of 0.2773. The source also has a counterpart in the nario. Therefore we may have identified a “naked jet-lobe”
JVAS/CLASS sample with an integrated flux density at source without strong AGN core emission. The absence of
8.4GHz of 16.2mJy. If the source does not vary, then this a compact, self-absorbed core is a sign of a Type 2 AGN,
resultsinaspectralindexof−0.31between1.4(NVSS)and whenthejetisviewedatalargeanglecomparedtoourline
8.4GHz making it slightly steep, but certainly not ultra- ofsight.Itisintriguingthough,whysuchan unbeamedob-
steepatthesefrequencies.However,theCLASSobservations ject would have a prominent one-sided jet morphology. An
werecarriedoutwiththeVLAinitslargestAconfiguration alternative interpretation is that we are observing a two-
resulting in an angular resolution of 220 milli-arcseconds sided jet-lobe with a veryweak core within thestructure.
(mas), very different to the beam size of the NVSS survey, On inspection of the WENSS map, we found that
makingthisspectralindexmeasurementunreliable.Thisob- J151229+470245 is a complex source at the resolution of
jectalsohascounterpartsin2MASSandtheROSATX-ray Westerborkat325MHz.Anexaminationofthemapincom-
catalogue,andisclassifiedasthebrightestclustergalaxyin parison with the survey database, the corresponding fields
Koester et al. (2007), making the quasar core scenario the in both NVSS and FIRST, and the new Westerbork data
most likely. obtained during the e-EVN observations, determined that
4 M.K. Argo et al.
Blue contours: 1512+470 IPOL 6888.675 MHz e-MERLIN twoareundetectedwithVLBI,twoaredetectedbutareun-
Red contours: 1512+470 IPOL 1664.490 MHz e-EVN
resolvedonmilli-arcsecondscales.Thesenewhighresolution
data are not sufficient to uniquely determine the nature of
4477 0022 4455..7700
thesesources,howevertheydoprovidevaluableinformation
4455..6655 onthevarietyofsourcesonemay findin alarger USSsam-
4455..6600 ple.Asonemayexpect,asignificantfractionofthesesources
are completely resolved out since steep spectrum emission
0)0) 4455..5555
0000 is often related to very extended lobes in radio galaxies. A
22
N (JN (J 4455..5500 detectionrateoffiftypercent,however,indicatesthatvery
OO
ATIATI 4455..4455 highresolutionstudiesofamoderatefractionofUSSsources
LINLIN will bepossible.
ECEC 4455..4400 Both of our USS source detections, J072212+291042
DD
4455..3355 and J130612+514407, are consistent with a point source
on VLBI scales. Note that in these short, exploratory ob-
4455..3300
servations with limited uv-coverage (especially on the long
4455..2255 baselines) we cannot adequately constrain the source sizes,
buttheydo appearmore compact thanourbeam of 32×26
4455..2200
mas.Theemissioninthesetwocasesis,therefore,consistent
1155 1122 2299..2211 2299..2200 2299..1199 2299..1188 2299..1177
RRIIGGHHTT AASSCCEENNSSIIOONN ((JJ22000000)) witheitheraGalacticpulsaroraverysteepspectrumquasar
Levs = 2.320E-04 * (-1, 1, 2, 4, 8, 16, 32, 64, 128)
Levs = 3.607E-04 * (-1, 1, 2, 4, 8, 16) core. The former hypothesiswill be easily tested byfurther
VLBIobservationsthatcanpotentiallyshowthepropermo-
Figure 1. e-MERLIN map of J151229+470245 at 6.9GHz tion of thepulsar in our Galaxy.
(blue contours) together with the e-EVN map at 1.4GHz (red Sincethere aregenerally nodetections of thesesources
contours). Contours are plotted at -1,1,2,4,8,16,32,64,128 × inanyotherwavelengthregimeandnoredshift information
0.23mJy/beam for the e-MERLIN data and -1,1,2,4,8,16 × is available, they are very difficult to classify. These new
0.36mJy/beam for the e-EVN data. The boxes in the lower left
radioobservations providefresh information and vitalclues
indicatethesizeoftherestoringbeamforeachimage.
in the search for an understandingof their nature.
J072212+291042 is compact on VLBI scales but the e-
thefluxeslistedintheWENSScataloguedonotrefertothe EVN observations presented here recover less than 20% of
sourcecomponentsatthelistedcoordinates.Thecalibrated the flux seen at the same frequency on VLA scales, clearly
fits file for the WENSS field was retrieved and Gaussian indicating the existence of structure on intermediate spa-
components were fitted to the three sources in the region. tial scales to which neither of these arrays are sensitive. e-
Thisshowedthatthesourceinwhichweareinterestedactu- MERLINobservationsat6.9GHzfailtodetectthesourceto
allyhadanintegratedfluxof543mJy,not123mJyaslisted a3-σ limitof120µJy/beam,confirmingthesteepspectrum
intheWENSScatalogue.Thisgivesthesourceaspectralin- extendstohigherfrequencies.Observationswithe-MERLIN
dexof-0.50between74and325MHz,makingitsteepspec- at1.4GHzwouldprobethisintermediate-scaleemission,but
trum, but not ultra-steep. The spectrum is slightly steeper this band was not available at the time when the commis-
betweenWENSSandFIRST/NVSSwithα=-0.62between sioningobservationsdescribedherewereobtained.Thefact
325MHzand1.4GHz.Therefore,basedonitsspectrumand that only ∼20 per cent of the FIRST/NVSS flux is recov-
the observed VLBI and e-MERLIN radio structure we may eredonVLBIscalesstronglysuggeststheexistenceofstruc-
classify J151229+470245 as a peculiar compact steep spec- ture on intermediate scales for these sources. A study by
trum radio source (CSS). Klo¨ckneret al. (2009) of a sample of 11 high-redshift ob-
This source is also an example of the class of radio scured quasars also found similar evidence for structures
sourcesknownasInfraredFaintRadioSources(IFRS),those on intermediate scales, although the VLBI-recovered fluxes
with clear radio emission butno corresponding infrared de- there ranged from 30 to 100 per cent of the low-resolution
tection(e.g.Norris et al.2006).Suchsourcesaredifficultto measurements.
identify due to their lack of counterparts outside the radio J130612+514407 is also compact on VLBI scales with
regime and, as yet, no IFRS source has a measured red- significantly less flux lost when imaged with long baselines.
shift, but they are thought to be high redshift quasars due Unlike J072212+291042, however, this source does have
totheirgenerally compact nature,mostly steepspectral in- counterparts in other wavebands which suggest the source
dexesand high brightness temperatures. While some of the is associated with a galaxy at a redshift of 0.2773.
known IFRS sources are extended on arcsecond scales with While J151229+470245 appears to be a compact steep
the VLA, and a small number of such sources have previ- spectrumobject,ratherthananultrasteepspectrumsource,
ously been detected at VLBI-resolution (e.g. Norris et al. it is still compact with a steep spectrum at low frequen-
2007; Middelberg et al. 2008), this appears to be the only cies and interesting structure at higher resolutions. VLBI
example so far which is resolved on VLBI scales. observations of J151229+470245 only recover 20% of the
NVSS flux. In sharp contrast to J072212+291042 and
J130612+514407 however, the object is clearly resolved on
both e-MERLIN and VLBI scales, showing different struc-
4 DISCUSSION AND CONCLUSIONS
tures in the two maps. While the e-MERLIN map shows
Of the five sources in the sample, one, J151229+470245, is whatappearstobeaclassiccore-jetmorphology,the“core”
not an ultra steep spectrum source. Of the remaining four, is clearly resolved in the e-EVN map. Observations with e-
Probing the nature of USS sources 5
Table2.e-EVNdetectedsources.ThemeasuredcoordinatesintheVLBIexperiment(±0.1mas)andthecorrespondinggalacticcoordi-
natesarebothgiven.Fluxdensitiesaregivenforeachobservation:WSRT(1.6GHz),e-EVN(1.6GHz)ande-MERLIN(6.8GHz),and
the NVSS-WSRTspectral index,and VLBI-WSRT fluxdensityratios arecalculated. ForJ072212+291042 a3σ upper limitisgiven at
6.89GHz.NotethattheWSRT andVLBIfluxes at1.6GHzwereobtainedsimultaneously,andthatJ130612+514407 wasnotobserved
withe-MERLIN.
Name RA Dec (l,b) SWSRT Se-EVN Se-MERLIN α1400MHz S1e6-E70VMNHz
1670MHz 1670MHz 6890MHz 1670MHz SWSRT
1670MHz
J072212+291042 07:22:12.60906 +29:10:41.6648 189,19 7.6 2.1 <0.12 −1.82 0.28
J130612+514407 13:06:12.16262 +51:44:06.9627 118,65 23.1 21.6 − −1.28 0.94
J151229+470245 15:12:29.19905 +47:02:45.3837 78,56 203 42.0 94.0 −0.51 0.21
MERLINatL-band(1.4GHz)wouldagainallowustoprobe DeBreuckC., vanBreugel W., Ro¨ttgeringH.J.A., Miley
themorphology of themissing flux. G.,2000,AstronomyandAstrophysicsSupplementSeries,
While firm conclusions regarding the nature of these 143, 303
sourcesremainelusive,theobservationsandunexpectedre- Klo¨cknerH.-R.,Mart´ınez-SansigreA.,RawlingsS.,Garrett
sultspresentedheredoshowthatcompactultra-steepspec- M. A., 2009, Mon. Not.R. Astr. Soc., 398, 176
trum sources are a diverse class of object whose naturecan Koester B. P., McKay T. A., Annis J., Wechsler R. H.,
beprobed with VLBI. Evrard A., Bleem L., Becker M., Johnston D., Sheldon
E., Nichol R., Miller C., Scranton R., Bahcall N., Bar-
entine J., Brewington H., Brinkmann J., Harvanek M.,
Kleinman S., Krzesinski J., Long D., Nitta A., Schneider
D. P., Sneddin S., Voges W., York D., 2007, Astrophys.
ACKNOWLEDGMENTS
J., 660, 239
The authors thank Ralph Eatough for useful discussions. Krolik J. H., Chen W., 1991, Astron.J., 102, 1659
e-MERLIN is a National Facility operated by the Univer- MiddelbergE.,NorrisR.P.,TingayS.,MaoM.Y.,Phillips
sityofManchesteratJodrellBankObservatoryonbehalfof C. J., Hotan A. W., 2008, Astron. Astrophys., 491, 435
STFC. We thank the e-MERLIN operations team for data NorrisR.P.,AfonsoJ.,AppletonP.N.,BoyleB.J.,Ciliegi
taken during commissioning time. The EVN is a joint fa- P.,CroomS.M.,HuynhM.T.,JacksonC.A.,Koekemoer
cility of European, Chinese, South African and other radio A.M.,LonsdaleC.J.,MiddelbergE.,MobasherB.,Oliver
astronomyinstitutesfundedbytheirnationalresearchcoun- S.J.,Polletta M.,SianaB.D.,SmailI.,VoronkovM.A.,
cils. e-VLBI research infrastructure in Europe is supported 2006, Astron. J., 132, 2409
by the European Union’s Seventh Framework Programme Norris R. P., Tingay S., Phillips C., Middelberg E., Deller
(FP7/2007-2013) under grant agreement RI-261525 NEX- A., Appleton P. N., 2007, Mon. Not. R. Astr. Soc., 378,
PReS.TheWSRTisoperatedbyASTRON(NetherlandsIn- 1434
stituteforRadioAstronomy)withsupportfromtheNether- Rengelink R. B., Tang Y., de Bruyn A. G., Miley G. K.,
lands foundation for Scientific Research. This research has Bremer M. N., Ro¨ttgering H. J. A., Bremer M. A. R.,
made use of the VizieR catalogue access tool, CDS, Stras- 1997, Astronomy and Astrophysics Supplement Series,
bourg,France.TheoriginaldescriptionoftheVizieRservice 124, 259
waspublishedinA&AS143,23.Thisresearchalsomadeuse ReulandM.,Ro¨ttgeringH.,vanBreugelW.,DeBreuckC.,
oftheNASA/IPACExtragalacticDatabase(NED)whichis 2004, Mon. Not. R. Astr.Soc., 353, 377
operatedbytheJetPropulsionLaboratory,CaliforniaInsti- Ro¨ttgering H. J. A., van Ojik R., Miley G. K., Chambers
tuteof Technology, undercontract with theNational Aero- K. C., van Breugel W. J. M., de Koff S., 1997, Astron.
nautics and Space Administration. Astrophys.,326, 505
Seymour N., Stern D., De Breuck C., Vernet J., Rettura
A., Dickinson M., Dey A., Eisenhardt P., Fosbury R.,
Lacy M., McCarthy P., Miley G., Rocca-Volmerange B.,
Ro¨ttgeringH.,StanfordS.A.,TeplitzH.,vanBreugelW.,
REFERENCES
ZirmA.,2007, Astrophys.J. SupplementSeries,171, 353
Alexandroff R., Overzier R. A., Paragi Z., Basu-Zych A., Szomoru A.,2008, in Therole of VLBI in theGolden Age
HeckmanT.,KauffmannG.,BourkeS.,LobanovA.,Ptak for Radio Astronomy
A., Schiminovich D., 2012, Mon. Not. R. Astr. Soc., 423, vanWeerenR.J.,Ro¨ttgeringH.J.A.,Bru¨ggenM.,Cohen
1325 A.,2009, Astron. Astrophys.,508, 75
BeckerR.H.,WhiteR.L.,HelfandD.J.,1995,Astrophys. Venemans B. P., Ro¨ttgering H. J. A., Miley G. K., van
J., 450, 559 Breugel W. J. M., de Breuck C., Kurk J. D., Pentericci
Cohen A. S., Lane W. M., Cotton W. D., Kassim N. E., L.,StanfordS.A.,OverzierR.A.,CroftS.,FordH.,2007,
Lazio T. J. W., Perley R. A., Condon J. J., Erickson Astron.Astrophys., 461, 823
W. C., 2007, Astron. J., 134, 1245
Condon J. J., Cotton W. D., Greisen E. W., Yin Q. F.,
PerleyR.A.,TaylorG.B.,Broderick J.J.,1998, Astron.
J., 115, 1693
