High Frequency GPS sources in the AT20G Survey

0.31 MB·English

by  P. J. Hancock

Astron.Nachr./AN999,No.88,789–793(2006)/DOIpleasesetDOI!
High Frequency GPS sources in the AT20G Survey
P. J. Hancock1,⋆
SydneyInstituteforAstronomy(SIFA),SchoolofPhysics,UniversityofSydney2006NSWAustralia
Received30May2005,accepted11Nov2005
9 Publishedonlinelater
0
0
Keywords Galaxies-GPS,Surveys-AT20G
2
TheAustraliaTelescope20GHz(AT20G)surveywasusedtoselectacompletesampleof656GigahertzPeakedSpectrum
n
(GPS)sourceswithspectralturnoversabove5GHz.TheAT20Ghasnearsimultaneousobservationsat4.8,8.6and20GHz,
a
whichmakesitpossibletoexcludeflatspectrumvariabilityasacauseofasource’speakedspectrum.Opticalidentification
J
ofthesampleresultsin361QSOsand104galaxiesand191blankfields.Redshiftsareknownfor104oftheGPSsources.
9
TheGPSsourcesfromtheAT20Garediscussed andcomparedtopreviouslyknown samples.Thenewsampleofhigh
2
frequencypeakingGPSsourcesisfoundatalowerredshiftthanprevioussamplesandtoalsohavealower5GHzradio

power.EvidenceisfoundtosupporttheideathattheoriginoftheGPSspectralshapeareintrinsicallydifferentforgalaxies
A] andQSOs.Thispaperisanelaborationandextensionofthetalkgivenatthe4thCSS/GPSconferenceinRiccioneinMay
thisyear.
G
. (cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
h
p
-
o 1 Introduction were created from the scanning observationswere used to
r create a list of candidate sources that were then followed
t
s TheAustraliaTelescope20GHzsurvey(AT20G)isthefirst upinthesecondstageofobserving.Sourcesthatwerecon-
a
large area sensitive survey at frequenciesabove 5GHz. A- firmed at 20GHz were re-observed at 4.8 and 8.6GHz to
[
mong the new and interesting sources detected within the providea near simultaneousthree pointspectrum for each

1
AT20G are the High Frequency Peaking (HFP) subset of ofthe sources.Notallsourceswereableto beobservedat
v
Gigahertz Peaked Spectrum (GPS) sources. HFP sources thelowerfrequenciesdueagaintotimeconstraintsandalso
2
9 aretypifiedashavingaspectralturnoveraboveafewGHz thepoor(u,v)coverageavailablefromtheATCAfor4.8and
5 (Dallacasa et al. 2002) and thus far have eluded detailed 8.6GHzatdeclinationsabove-15◦.
4 study,duetotheverynatureoftheirspectrumandthelack Further details of the AT20G survey and its pilot can
.
1 of sensitive large area surveys above 5GHz. In this paper befoundin Riccietal.(2004),Massardietal. (2008),and
0 wediscussagroupofHFPsourcesthatwereselectedfrom Hancocketal.(2008inprep).
9 theAT20Gsurveywhicharegoodcandidatesforsuchade-
0
tailed study. All the GPS sources detected in the AT20G
:
v surveyhaveturnoverfrequenciesabove5GHzandarethus 3 SampleSelection
Xi HFPs.
r TheAT20Gcatalogcontainsatotalof5450radiosources.
a
2 AT20G Crossmatchingwithopticaldatagivesa90%identification
rate which breaks down into 65% QSOs, 25% Galaxies,
and 10% faint or blank fields. Almost all the extragalac-
TheAT20Gsurveyusedawidebandanalogcorrelatorwith
tic sources are identified with AGN and are unresolved at
threetelescopesfromtheAustraliaTelescopeCompactAr-
20GHz(largestangularsize<5asec).Figure1showsatwo
ray(ATCA)tosurveythesouthernskyat20GHz.Thesur-
colour plot of the 4404 sources in the AT20G which have
veywasbegunin2004andobservationswerecompletedin
threefrequencyobservations.Sourcesthatare invertedbe-
2007. The survey covers the entire sky south of the equa-
tweenthelowerfrequencies(α8.6 > +0.2whereS = να)
tor.Thesurveywasconductedintwostages,afastscanning 4.8
are considered to be HFP candidates and make up around
stage that imaged the entire sky, and a followupstage that
20% of the sample. Sources that are inverted between 8.6
observed candidate sources in order to refine flux and po-
and20GHzhavebeenfoundtoturnoverbefore95GHzal-
sitionmeasurements.Thescanningobservationsweredone
most 100% of the time (Sadler et al. 2008), and are thus
usingthewidebandcorrelatorsetup,andthedatawaspro-
included in the GPS sample as sources that have spectral
cessed using custom software. The images of the sky that
peaksabove20GHz.Sourcesthatarewithin2.5◦oftheGa-
⋆ Correspondingauthor:e-mail:[email protected] lactic plane are excluded. The resulting list contains 656
GPSsourceswithpeaksabove5GHz.
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
790 Hancock:HFPsourcesintheAT20G
Galaxies QSOs
3
Number 104 362
Knownz 25 75
Upturn 11% Inverted 13% Averagez 0.2 1.2
2
log (PW/Hz) 24.4 26.6
5
GHz) 1 <<SB2m0a>g >(Jy) 01.92.04±±00..024 01.93.86±±00..014
0
2 Table 1 The properties of the GPS galaxies and QSOs
o
z t selectedfromtheAT20Gsurvey.Thelistederrorsareerrors
H
8.6G 0 inthemean.
x (
de G
al in 1.4 QSO
ctr -1
e
p
S 1.2
Steep 54% Peaked 22%
-2
1
-3 hift 0.8
-3 -2 -1 0 1 2 3 ds
e
Spectral index (4.8GHz to 8.6GHz) R
Fig.1 Near simultaneous two colour spectral index plot 0.6
for the 4404 sources that have three frequency measure-
mentsintheAT20G.Thefourregionsdelineatedbythex-y 0.4
axisareidentifiedbyspectralbehavior.
0.2
4 Additional Information
0
8-15 15-30 30+
AdditionalinformationabouteachGPSsourcewasalsocol- Spectral Peak (GHz)
lected from SUMSS (280 fluxes at 843MHz, Mauch et al. Fig.2 Redshiftasafunctionofspectralturnover.Galax-
2003),NVSS (365fluxes at 1.4GHz, Condonet al. 1998), iesareshownbysquaresandQSOsbytriangles.
NEDand6dF(104redshifts,Jonesetal.2004),SuperCOS-
MOS(466opticalidentificationsandB magnitudes,Ham-
blyetal.2001). Table 1 shows the various properties of galaxies and
QSOs within the sample. The percentage of galaxies and
OpticalidentificationsweretakenfromtheSuperCOS-
QSOsthathaveknownredshiftsaresimilarat20%and24%
MOSSkySurvey(SSS)classID.Sourcesthatdonothavea
respectively.AsexpectedtheQSOsarefoundata system-
SuperCOSMOSclassIDwereremovedfromtheGPSsam-
atically higherredshiftthan the galaxiesand have a 5GHz
ple. Redshiftinformationwas taken from the NASA/IPAC
radio power that is 2 orders of magnitude stronger. There
Extragalactic Database (NED) and are almost exclusively
isacleardifferencebetweenthe20GHzfluxdensityofthe
fromthe6dFredshiftsurvey.
galaxiesandQSOswithQSOsbeingfoundatmuchhigher
flux levels on average. If this trend is borne out in future
5 Sample Properties surveysthen we can expectto find manymore galaxiesas
thefainterradiosourcesareprobedat20GHz.
The 104 sources for which there is existing redshift infor- Figure2showstheredshiftdistributionofGPSsources
mation, were separated into galaxies and QSOs using the asafunctionofspectralbehavior.Inanysampleofsources
class ID flag from the SuperCOSMOS database. As noted thatisdefinedbyaselectionfrequency,theGPSsourcesthat
insection6theoriginoftheGPSspectrumingalaxiesand are found within the sample will have the observed peak
QSOs is intrinsically different and thus the galaxies and closer to the selection frequency. This is because sources
QSOs were treatedseparately.Due to thesmallnumberof that peak further from the selection frequency will appear
data points available for the radio spectra of each object fainterandarethuslesslikelytobedetected.Thelowerred-
the spectralturnoverwasestimatedandbinnedintooneof shift of the higherfrequencyturnoversourcesthatis com-
threecategories:8-15GHzforsourcesthathaveaclearpeak montoboththeQSOsandgalaxiesinfigure2isconsistent
around the 8.6GHz data point, 15-30GHz for sources that with that which is expected from the shape of their spec-
are inverted but show a flattening at 20GHz, and 30GHz+ trum.TheseparationinredshiftbetweengalaxiesandQSOs
forsourcesthatareinvertedat20GHzbutshownosignof isduetotheirdifferingvolumedensityandluminosity.We
flattening. donotexpecttoseeasmanyGPSgalaxiesathighredshift
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2006) 791
Masterlist Thispaper
30
Q TypicalPeak 1-5GHz 8-15GHz
G
Numberofsources1 172(136) 656(466)
25
Galaxy/QSOpercent 58/42 23/77
Averagez 0.5/1.9 0.2/1.2
20
TypicallogP5(W/Hz) 26.3/27.72 24.4/26.6
mber 15 Table 2 AcomparisonofthemasterlistofGPSsources
u presentedbyLabianoetal.2007,andthesourcespresented
N
in this paper. 1Onlysourceswithopticalidentifications (numberedin
10 parenthesis)areusedinthecalculationsofthistable.2Onlysourceswitha
listedredshiftand5GHzfluxdensitywereincludedinthiscalculation(89
5 intotal).
0
0.035 0.1 0.35 1.0 3.5 10
listofLabianoetal.(2007)1(hereaftermasterlist)contains
Estimated S (Jy)
p
themostcompletelistingofGPSandHFPsourcescurrently
Fig.3 Distributionofthefluxdensityatthespectralpeak,
known. The master list consists of 172 sources with 79 of
S ,forgalaxiesandQSOs.
p
these identifiedas galaxiesand57asQSO. A summaryof
themasterlistandthesourcesinthispaperisshownintable
2.
becausetheyarelessluminousandthusnotaslikelytobe
Themasterlistcontainsmostlysourcesfromthenorth-
seeninafluxlimitedsampleofsources.Wedonotexpectto
ernhemisphereduetotheincreasedavailabilityoflargearea
seeasmanyQSOsatlowredshiftbecauseoftheirrelatively
surveys in the northern hemisphere, however there are 57
lowvolumedensity.
southernhemispheresourcespresentinthemasterlist.Only
Figure3showstheestimatedpeakfluxdensitydistribu-
threeofthemasterlists’southernhemispheresourceshave
tion forgalaxiesandQSOs. As with thespectralpeak,not
alistedpeakabove5GHz(0745-004at5.8GHz,1522-2730
enoughspectralinformationwasavailabletoreliablyfitthe
at5.8GHz,and1837-7108at8.2GHz).TheAT20Gsurvey
spectrumsothepeakfluxdensitywasestimatedbyeye.The
doesnothave4.8or8.6GHzobservationsforsourcesabove
distributionoftheGPSgalaxiesandQSOsarequitediffer- -15◦declinationso 0745-004is notincludedin the AT20G
ent, with the galaxies being more sharply peaked around
sample.1522-2730ispresentinbothsamplesandpeaksin
0.1 to 0.35 Jy estimated peak flux and QSOs being more
the 8-15GHz bin. 1837-7108is seen in the AT20G survey
broadly peaked around 0.35 to 1 Jy estimated peak flux. buthasasteepspectrumwithα = −0.27,andisidentified
EachoftheGPSsourcesconsideredinthispaperwereclas-
withaQSOatredshift1.35.
sifiedaseithergalaxyorQSObyadoptingtheSSSclassID
WhileGPSsourcesaregenerallytheleastvariableclass
as stated in section 4. This classification was done largely
of radio sources (O’Dea 1998), GPS QSOs are known to
automaticallyandwasbasedonwhetherornotthesources
have a large amount of variability which can often cause
wereresolvedonvariousopticalplatesandisnotinfluenced
falseGPSidentificationwhenthespectralobservationsare
byopticalmagnitude,redshiftoranyspectralbehavior.This
notsimultaneous(Torniainenetal.2005).Thechangeinthe
clear separation of the two groups of sources in figure 3
spectrumof1837-7108maybeduetothenonsimultaneous
showsthattherearetwopopulationsofsources,andnotjust
observationsusedintheinitialclassification.
thesamepopulationviewedatdifferentdistances.
Takingthemasterlistasarepresentativesampleofpre-
Itshouldbenotedthatfigure3showsonlythe sources
viously known GPS sources we can see that the sources
for which redshifts are known. Whilst the total fraction of
detected in the AT20G survey differ in a number of ways.
sources with known redshifts is the same for galaxies and
ThetypicalpeakfrequencyoftheAT20Gselectedsources
QSOs, the distribution of known redshifts is not the same
ismuchhigherthanthemasterlist,whichisexpectedsince
within each of the samples. The percentage of QSOs with
the AT20G will not detect any sources with peaks below
known redshift increases with S and may be biasing the
p 5GHz.ThenumberofsourcesintheAT20Gselectedsam-
peakoffigure3inthesamedirection.Thefractionofgalax-
ple is about four times larger than the master list, but the
ieswithknownredshiftdoesnotshowanysuchcorrelation.
amount of information known about each object is much
Thewidthofthetwodistributionswouldnotbeeffectedby
less than the more well studied sources of the master list.
suchabias,onlythelocationofthepeak.
Thegalaxyto QSO ratio ofthetwo samplesarealso quite
different with the master list having slightly more galax-
ies than QSOs and the AT20G sample having about three
6 Comparisonto known GPSsamples times as many QSOs as galaxies. This change in galaxy
to quasar ratio has been noted previously by Stanghellini
Previous GPS studies have contained mainly sources with 1 Onlineathttp://damir.iem.csic.es/∼labiano/GPSlist/GPS List.txt,ac-
spectral turnovers below 5GHz. The updated GPS master cessed2008August11
www.an-journal.org (cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
792 Hancock:HFPsourcesintheAT20G
(2003), among others. The fraction of galaxies in a sam- radiosource.Figure4showsonesuchsource.Theresultsof
ple of GPS sourcesis seen to increase as the turnoverfre- theseobservationswillbediscussedinmoredetailinHan-
quency decreases even into the Compact Steep Spectrum cocketal.(2009inprep).
radio sources that can be thought of as GPS sources with
a peak frequency below our detection limit. This supports
8 Conclusions andFuture Work
the idea that the GPS emission from galaxies and quasars
are intrinsically different, with the quasar emission origi-
natingfromthecoreandthatofgalaxiesfrommicrolobes TheAT20Gsurveyhasbeenusedtoselectacompletesam-
and hot spots. The AT20G selected sample of sources are ple of 656 GPS sources using near simultaneous observa-
at lower redshift and 5GHz radio power than those in the tionsat4.8,8.6and20GHz.The361QSOsand104galax-
masterlist.Galaxiesatlowradiopowerandredshiftareun- ies within this sample are found to be at lower redshifts
der representedin currentGPS samples and correspondto and lower radio power than those listed in the master list
stages of radio source evolution that precede the currently of Labiano et al. (2007),and supportthe idea that the ori-
studiedsampleofsources. ginsoftheGPSspectraareintrinsicallydifferentforgalax-
ies and QSOs. Contamination of the GPS sample by vari-
ablesourceswaslimitedbythenearsimultaneousnatureof
7 Observations at40and 95GHz
theobservations,resultinginthelargestlistofgenuineHFP
GPS sourcesso far discovered.Future workto bedoneon
InNovember2007asampleofsourcesfromtheAT20Gcat-
thesampleincludes:eVLBIobservationsofasampleoflow
alogwereobservedat40and95GHzattheATCA.Theob-
redshift, low radio power galaxies to probe their milliarc-
servationswereacontinuationofthe95GHzstudyofSadler
second structure; 40 and 95GHz observations of inverted
et al. (2008),and contained a subset of GPS sources from
andHFPgalaxies;andaprogramtoexpandthenumberof
thispaper.TheGPSsourceswereselectedtobeinvertedor
knownredshifts.
tobeturningoveraround20GHz,andtohavea20GHzflux
or spectral index that put their predicted higher frequency
fluxeswithindetectionlimits. In total21sourceswere ob- Acknowledgments
served.Withanincreasedknowledgeofthesourcespectra,
the estimated spectral turnover frequency and flux density
I wish to thank the AT20Gteam for the use of pre-release
wereabletobefittedusingtheGPSspectrumofSnellenet
data in the publicationof thispaper,aswell as the partici-
al.(1998):
pants of the 4th CSS/GPS conferencewho gave their time
Sν = 1−Spe−1 (cid:18)ννp(cid:19)k(1−e−(cid:0)ννp(cid:1)l−k) (1) taollodwisicnugssmaentdhecoomppmoretnutnoitnytmoyspweaokrk. and to the SOC for
Where:ν andS arethepeakfrequencyandfluxdensity, This research has made use of the NASA/IPAC Extra-
p p
andkandlaretheopticallythickandthinspectralindicies. galacticDatabase(NED)whichisoperatedbytheJetPro-
pulsionLaboratory,CaliforniaInstituteofTechnology,un-
der contractwith the National Aeronauticsand Space Ad-
Data
Fit ministration.
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