Deep 610-MHz GMRT observations of the Spitzer extragalactic First Look Survey field - I. Observations, data analysis and source catalogue

1.1 MB·English

by  Timothy Garn

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed5February2008 (MNLATEXstylefilev2.2)
Deep 610-MHz GMRT observations of the Spitzer extragalactic
First Look Survey field – I. Observations, data analysis and source
catalogue
⋆
Timothy Garn , David A. Green, Sally E. G. Hales, Julia M. Riley, Paul Alexander
7
0 AstrophysicsGroup,CavendishLaboratory,19J.J.ThomsonAve.,CambridgeCB30HE
0
2

n 5February2008
a
J

8 ABSTRACT
1 ObservationsoftheSpitzerextragalacticFirstLookSurveyfieldtakenat610MHzwiththe
GiantMetrewaveRadioTelescopearepresented.Sevenindividualpointingswereobserved,

1 coveringa totalareaof 4deg2 witha resolutionof5.8 4.7,PA 60 .Ther.m.s.noiseat
′′ ′′ ◦
v thecentreofthepointing∼sisbetween27and30m Jybefore×correctionfortheGMRTprimary
4
beam. The techniques used for data reduction and productionof a mosaicked image of the
3
regionare described,and the final mosaic, along with a catalogueof 3944sourcesdetected
5
above 5s ,arepresented.Thesurveycomplementsexistingradioandinfrareddataavailable
1
∼
0 forthisregion.
7
Keywords: surveys–catalogues–radiocontinuum:galaxies
0
/
h
p
-
o 1 INTRODUCTION gion,madewiththeVeryLargeArray(VLA)inB-arrayconfigura-
r
tion,andtheWesterborkSynthesisRadioTelescope(WSRT).The
st The Spitzer First Look Survey was the first major scientific pro- VLAsurveycovered 4deg2witharesolutionof5 ,toa1s depth
a gram carried out by the Spitzer Space Telescope (Werneretal. of 23m Jy(Condon∼etal.2003)andcontains3565′′sources,while
v: 2004),observingfor∼110hoursofDirector’sDiscretionaryTime the∼WSRTsurvey(Morgantietal.2004)concentratedon 1deg2
i inDecember2003.Theaimoftheextragalacticcomponentofthe ofthexFLSregiontoadepthof8.5m Jybeam 1withare∼solution
X FirstLookSurvey(xFLS)wastostudyaregionwithlowGalactic −
of14 11 ,PA0 ,andcontains1048sources.
r backgroundtoasignificantlydeeperlevelthananypreviouslarge- ′′× ′′ ◦
a There is a tight correlation between the far-infrared and ra-
area infrared survey, in order to accurately characterise the dom-
dio luminosities of galaxies, which has been known for many
inant infrared source populations. Observations were made over
four square degrees centred on 17h18m00s, +59 3000 (J2000 years (see e.g. Helouetal. 1985; Condon 1992). The correla-
◦ ′ ′′
tion applies to both the local (Hughesetal. 2006) and global
coordinates, which are used throughout this paper) with two in-
(Murphyetal. 2006) properties of galaxies in the local and dis-
struments–theInfraredArrayCamera(IRAC,Fazioetal.2004),
usingthe3.6,4.5,5.8and8 m mbands, andtheMultibandImag- tantuniverse(Garrett2002;Gruppionietal.2003;Chapmanetal.
2005;Luo&Wu2005).Therehavebeen twoprevious studiesof
ingPhotometerforSpitzer(MIPS,Riekeetal.2004)at24,70and
160m m.Thedatahavebeenprocessedandmapsandsourcecata- theIR/radiocorrelationinthexFLSregion(Appletonetal.2004;
loguesarecurrentlyavailable(IRAC–Lacyetal.2005;24 m m– Frayeretal.2006).Appletonfoundnoevidenceforavariationof
Faddaetal.2006;70and160m m–Frayeretal.2006). the correlation with redshift, while Frayer, using more sensitive
infrared data, found a decrease in the infrared/radio flux density
Complementary observations have been taken at a range of
ratio with z. To study the luminosity variation it is necessary to
wavelengths to fully exploit the new deep infrared data. Deep
accuratelyk-correcttheobservedradiofluxdensitiestotheirrest-
optical surveys of the region have been completed in the R-
framevaluesusingthespectralindexa (wherea isheredefinedso
band (KPNO 4 m, Faddaetal. 2004), and in the u - and g-
∗ thatfluxdensityscaleswithfrequencyasS(cid:181) n a ).Bothprevious
bands(CFHT3.6m,Shimetal.2006).ThexFLSregionwascov- −
worksinthisregionassumedthatallsourceshavethesameradio
ered by the early data release of the Sloan Digital Sky Survey
spectralindex(takentobea =0.7and0.8respectively),butinor-
(Stoughtonetal.2002).Afurtherredshiftsurveytargetingselected
24-m msourceswasmadewiththeMMT/Hectospecfiberspectro- der toperformthiscorrectionaccuratelyitisnecessary tohavea
detectioninatleasttworadiofrequencies.
graph(Papovichetal.2006),andatotalof1587redshiftsarepub-
liclyavailable. Thelargeamountofexistingdataonthisfieldmakesitagood
Therearetwoexisting1.4-GHzradiosurveysofthexFLSre- candidate for a further deep radio survey. We have imaged the
xFLS region at 610 MHz with the Giant Metrewave Radio Tele-
scope (GMRT), reaching an r.m.s. noise level of around 30 m Jy
⋆ E-mail:[email protected] beforeprimarybeamcorrection.Oursurveyhasacomparableres-
2 T. Garnet al.
mosaic,anddiscusstheartefactsseennearbrightsources.InSec-
tion4wecompareourcataloguewiththeVLAsurvey,inorderto
testthepositionalaccuracyofourdata.Furtheranalysesofthedata,
60 30
includingdetailedcomparisionswithSpitzerdatawillbemadein
forthcoming papers. Appendix A presents some technical details
ofcorrectionsmadetotheobservedGMRTdata,andAppendixB
00 givesdetailsofacorrectionmadetotheintegratedfluxdensitiesof
ON (J2000) sources.
DECLINATI59 30
2 OBSERVATIONSANDDATAREDUCTION
00
ObservationsofthexFLSregionweremadeat610MHzwiththe
GMRT (Ananthakrishnan 2005), located near Pune, India, over
four days in March 2004. Seven pointings were observed, in a
58 30 hexagonal grid centred on 17h18m00s, +59 3000 as shown in
◦ ′ ′′
Fig.1.Thepointingswerespacedby43arcmin,approximatelythe
half-power bandwidth of the GMRT primary beam at 610 MHz,
17 25 20 15 10
RIGHT ASCENSION (J2000) whichgivesnearlyuniformnoisecoverageovermostofthexFLS
region.
Figure1.ThesevenpointingsobservedwiththeGMRT.Pointingcentres
Observationsof 3C48or 3C286weremadeatthebeginning
arespacedby43arcmin.Thegreyscalerepresentsther.m.snoiselevelcal-
culatedbySourceExtractor(seeSection3),aftercorrectingeachpointing and end of each observing session in order to calibrate the flux
fortheprimarybeamresponseoftheGMRTandmosaickingthepointings density scale. The AIPS task SETJY was used to calculate flux
together.The45m Jycontour(equivalent tothe23m Jynoiselevelofthe densities at 610 MHz of 29.4 and 21.1 Jy respectively, using an
VLA 1.4 GHz survey, for a source with spectral index of 0.8) has been extensionoftheBaarsetal.(1977)scale.Twosidebandswereob-
plotted. served, eachof 16MHzwith128spectral channels, anda16.9s
integrationtimewasused.Eachfieldwasobservedfor 200min
∼
over four 10 hour observing sessions made between March 23rd
and27th. Theobservations consisted ofinterleaved20minscans
ofeachpointing,inordertoimproveuvcoverage.Anearbyphase
calibrator, J1634+627, was observed for three minutes between
the scans of each pointing to monitor any time-dependant phase
andamplitudefluctuationsofthetelescope.Themeasuredphaseof
J1634+627 variedsmoothlybetweenobservations, withatypical
variation between calibrator observations of lessthan 40 for the
◦
longbaselineantennasandbelow10 fortheshortbaselineanten-
◦
nas.
Standard AIPStasks wereused toflag bad baselines, anten-
nas, channels that were suffering from narrow band interference,
andthefirstandlast16.9sintegrationperiodofeachscan.Aband-
passcorrectionwasappliedusingthefluxcalibrators,foreachan-
tenna.Apseudo-continuumchannelwasthenmadebycombining
thecentraltenchannelstogether,andanantenna-basedamplitude
andphasecalibrationcreatedusingtheobservationsofJ1634+627.
Thiscalibrationwasappliedbacktotheoriginal128channeldata
set,which was compressed into 11channels, eachcontaining ten
oftheoriginalspectralchannels(sothefirstandlastfewchannels,
whichtendedtobethenoisiest,werediscarded).Thesmallwidthof
thesenewchannelsensuresthatbandwidthsmearingisnotaprob-
Figure2.Theuvcoverageforthecentralpointing.Baselineslessthan1kl lem,andall11channelscouldbeindividuallyinspectedtoremove
werenotusedinimagingandhavebeenomitted. additionalinterference.Aftertheflaggingandcalibrationwascom-
pletethetwosidebandswerecombinedintoasingledataset(see
Appendix A1) to improve the uv coverage. The coverage for the
olution (5.8 4.7 compared with 5 ) to the VLA 1.4-GHz sur- centralpointingisshowninFig.2.
′′ ′′ ′′
×
vey,allowingadirectcomparisonbetweensourcepropertiesatthe ThelargefieldofviewoftheGMRTleadstosignificanterrors
twowavelengthstobemade.Ourobservationsaredeeperthanthe ifthewholefieldisimageddirectly,duetothenon-planarnatureof
VLAsurveyovermostoftheimage,foratypicalradiosourcewith thesky.Tominimisetheseerrors,eachpointingwasbrokendown
a = 0.8. into 19 smaller facets which were imaged separately, with a dif-
Section2describestheobservations,anddatareductiontech- ferentassumedphasecentre,andthenrecombinedtodealwiththe
niques. In Section 3 we present the mosaicked image and source transformationfromplanestothesphere.Imagesweremadewith
catalogue.Weshowaselectionofextendedobjectsvisibleinour an elliptical synthesised beam with size 5.8 4.7, position an-
′′ ′′
×
Deep 610-MHzGMRTobservationsof theSpitzerextragalacticFirstLookSurveyfield– I. 3
Figure3.The610-MHzimageoftheSpitzerextragalactic FirstLookSurveyfield.Thegreyscalerangesbetween 0.2and1mJybeam 1.Artefactsare
− −
visiblearoundthebrightestfewsources.
gle+60 ,withapixelsizeof1.5toensurethatthebeamwaswell (ii) The GMRT primary beam centre was shifted by 2.5 ar-
◦ ′′
∼
oversampled. cmintocompensateforaposition-dependantfluxdensityerror.
TheGMRThasalargenumberofsmallbaselines,duetothe
Thetheoretical r.m.s.noiseof eachpointing, before primary
clusterof14antennasinthecentral1kmofthearray.Thisdomi-
beamcorrection,is
natestheuvcoverageandaffectsthebeamshape,sobaselinesless
than1kl wereomittedintheimaging.Theimageswentthrough s = √2Ts (1)
threeiterationsofphaseself-calibrationat10,3and1minutein- Gpn(n 1)NIFD nt
tervals, and then a final round of self-calibration correcting both −
phase and amplitude errors. The overall amplitude gain was held where Ts 92 K is the system temperature, G 0.32 K Jy−1 is
constantinordernottoalterthefluxdensityofsources.Theinitial theantenn≈again–valuestakenfromtheGMRTw≈ebsite1–nisthe
self-calibrationstepcorrectedthephaseerrorsbyupto10 ,with number of working antennas, which was typically 28 during our
◦
later self-calibration making much smaller changes and having a observations,NIF=2isthenumberofsidebands,D n =13.75MHz
smallereffectonther.m.s.imagenoise. istheeffectivebandwidthofeachsideband,andt 12000sisthe
≈
Two problems were identified and corrected during imaging averageintegrationtimeperpointing.
(seeAppendixAforfurtherdetails). The final images have an r.m.s. noise of between 27 and
(i) Anerrorinthetime-stampsoftheuvdata,andhencetheuvw
coordinates,wascorrectedusingacustom-madeAIPStask. 1 http://www.gmrt.ncra.tifr.res.in/gmrthpage/Users/Help/help.html
4 T. Garnet al.
59 00 00 1
0.8
58 58 00
Declination 56 00 fraction of pixels 00..46
0.2
54 00
0
0 20 40 60 80 100 120 140 160 180 200
Noise level / microJy
Figure 8. Pixel noise distibution, from the Source Extractor r.m.s. map.
Thehistogramshowsthefractionofpixelsatthatnoise,whilethedotted
52 00
17 23 15 00 22 45 30 15 00 lineshowsthecumulativefractionofpixelswithnoisebelowaparticular
Right Ascension
value.
Figure 4. Artefacts visible around a 294 mJy source near 17h22m40s,
+58◦56′00′′.Thegreyscalerangesbetween−0.2and1mJybeam−1. 1800
1600
30 m Jy beam 1 before primary beam correction, which is very
−
closetothetheoreticallimitof 26m Jy,althoughdynamicrange 1400
∼
issues limit the quality of the images near bright sources where 1200
the noise is greater. The seven pointings were corrected for the
1000
primary beam of the GMRT, taking into account the offset beam N
position as discussed in Appendix A2. The beam correction was 800
performed using an 8th-order polynomial, withcoefficientstaken
600
fromKantharia&Rao(2001).Thepointingswerethenmosaicked
together,weightingthefinalimagebyther.m.s.noisesofeachin- 400
dividual pointing, and the mosaic was cut off at the point where 200
theprimary beam correction factor dropped to20% of itscentral
0
value.Figure1illustratesthevariationinnoiseacrossthemosaic. -4 -3.5 -3 -2.5 -2
Thenoiselevelissmoothandaround30m Jyacrosstheinteriorof log10(Peak brightness / Jy)
Figure9.Sourcenumbercountsfromthe610-MHzSpitzerextragalactic
themap,andincreasestowardstheedgestoabout150m Jywhere
FirstLookSurveyfield.Thesolidboxesrepresentuncorrectedcounts,while
theprimarybeamcorrectionwas20%.
theopenboxesrepresentthecountsaftercorrectionforthesurveysensitiv-
itylevelforeachsolidangle.
3 RESULTSANDDISCUSSION
andnoiselevelontheimage.Phaseerrorsnearthebrightsources
ThefinalmosaicisshowninFig.3.Therearephaseartefactsvisi- leadtoanincreaseinnoise,butbyusingaboxof16 16pixelsto
×
blearoundthebrightestsources,whichhavenotbeensuccessfully estimatethelocalnoise, thenumber of spuriousdetectionsinthe
removedduringself-calibration.Anenlargedimageofoneofthe finalcataloguewasreducedconsiderably–thismeansthatthefitis
brightsourcesisshowninFig.4.Itisthoughtthattheartefactsare lessdeepnearthebrightestsources.
due to an elevation-dependant error inthe position of the GMRT Table 1 presents a sample of 60 entries in the catalogue,
primarybeam(seeAppendixA2),whichwillleadtoimagedistor- which is sorted by right ascension. The full table, and radio
tionnearthebrightestobjectssincetheobservationsofeachpoint- image of the Spitzer extragalactic First Look Survey field will
ingweretakeninaseriesofscanswithvaryingelevations. beavailableviahttp://www.mrao.cam.ac.uk/surveys/.Col-
AsmallportionofthemosaicisshowninFig.5,demonstrat- umn1givestheIAUdesignation ofthesource, intheformFLS-
ing the quality of the image away from the bright sources, with GMRT Jhhmmss.s+ddmmss, where J represents J2000.0 coordi-
the VLA map of the same area shown in Fig. 6 for comparison. nates, hhmmss.s represents right ascension in hours, minutesand
ThegreyscalehasbeensetonFigs5and6sothatanobjectwith truncatedtenthsofseconds,andddmmssrepresentsthedeclination
aspectral indexof 0.8willappear equally bright inbothimages. indegrees,arcminutesandtruncatedarcseconds.Columns2and3
Mostsourcesareunresolvedinthe610-MHzimage,althoughthere give the right ascension and declination of the source, calculated
aresome objects present withextended structures –wepresent a byfirstmomentsoftherelevantpixelfluxdensitiestogiveacen-
sampleoftheseinFig.7. troidposition. Column4givesthebrightnessofthepeakpixelin
A catalogue of 3944 sources was created using Source Ex- each source, in mJybeam 1, and column 5 gives thelocal r.m.s.
−
tractor(Bertin&Arnouts1996)withpeakbrightnessgreaterthan noiseinm Jybeam 1.Column6givestheintegratedfluxdensityin
−
5s –seeAppendixBforfurtherdetails.SourceExtractorhasa mJy,calculatedfromthemosaicandapplyingthefluxdensitycor-
∼
significantadvantageoverAIPStaskssuchasSADwhencreating rectionfactordescribedinAppendixB.Column7givestheerror
asourcelist,inthatitiscapableofcalculatingthelocalbackground inintegratedfluxdensity,calculatedfromthelocalnoiseleveland
Deep 610-MHzGMRTobservationsof theSpitzerextragalacticFirstLookSurveyfield– I. 5
Figure5.AsamplesectionoftheGMRTimage.Thegreyscalerangesbetween 0.2and1mJybeam 1,andtheresolutionoftheimageis5.8 4.7arcsec2,
− − ×
PA+60◦.Mostobjectsareunresolved,thoughoneclearlyextendedobjectisvisiblenearthemiddle.
Figure6.Theexisting1.4-GHzVLAmapforthesameregion(Condonetal.2003).Thegreyscalerangesbetween 0.1and0.5mJybeam 1,equivalentto
theGMRTimageforobjectswithaspectralindexof0.8.Theimageresolutionis5arcsec2. − −
6 T. Garnet al.
59 15 00 60 04 30
59 47 45
14 45 15
30
30 00
15
15 03 45
00
00 30
17 12 52 50 48 46 44 42 17 14 46 44 42 40 38 36 17 14 42 40 38 36 34 32
(a) FLSGMRTJ171247.5+591428.3 (b) FLSGMRTJ171440.2+600401 (c) FLSGMRT J171437.0+594711 and
J171435.1+594728
59 37 35 60 08 10
30 05
25 00
20 07 55
15 50
10 45
05 40
00 35
36 55 30
17 23 42 40 38 36 34 17 17 38 36 34 32 30
(d) FLSGMRTJ172340.4+593721andJ172336.4+593715 (e) FLSGMRTJ171735.9+600752andJ171731.4+600746
58 45 55 58 52 50 58 45 40
50 45 35
45 40 30
40 35 25
35 30 20
30 25 15
25 20 10
20 15 05
15 10 00
17 13 48 47 46 45 44 43 42 41 40 17 19 58 57 56 55 54 53 52 51 50 17 15 14 13 12 11 10 09 08 07 06
(f) FLSGMRT J171344.4+584533 and (g) FLSGMRT J171954.3+585234 and (h) FLSGMRTJ171509.0+584526
J171341.9+584540 J171953.2+585222
59 48 30
60 04 45
59 21 00
15
30
20 45
00
30 15
47 45
15 00
30
17 14 18 17 16 15 14 13 12 11 10 17 24 42 41 40 39 38 37 36 35 34 17 21 20 19 18 17 16 15 14 13 12
(i) FLSGMRT J171413.8+594800 and (j) FLSGMRT J172439.4+592044, (k) FLSGMRT J172115.4+600432 and
J171414.6+594750 J172435.7+592053andJ172437.3+592017 J172116.7+600406
Figure7.Aselectionofextendedobjects–contoursareplottedat 100m Jybeam 1 1,2,4,8,...Negativecontoursarerepresentedbydashedlines.The
± − ×
resolutionofthebeamisshowninthebottomleftofeachimage,andthedesignationsofeachsourceintheGMRT610-MHzcataloguearegivenbelow.All
sourcesalsoappearextendedintheVLA1.4-GHzcatalogue,andimagesareavailableinCondonetal.(2003).
Deep 610-MHzGMRTobservationsof theSpitzerextragalacticFirstLookSurveyfield– I. 7
59 57 30
00
56 30
00
55 30
00
54 30
17 12 55 50 45 40 35 30 25 20 15
(l) The extended source has been separated into FLSGMRT J171240.3+595659, J171229.1+595639, J171223.0+595551 and
J171222.2+595524.ThetwocompactsourcesareFLSGMRTJ171248.1+595528andJ171216.4+595711.
60 09 00 60 37 30
08 45 15
30 00
15 36 45
00 30
07 45 15
30 00
15 35 45
00 30
06 45 15
17 11 54 52 50 48 46 44 42 40 38 36 17 18 50 45 40 35 30
(m) FLSGMRT J171147.7+600836, J171145.4+600808 and (n)FLSGMRTJ171841.3+603630
FLSGMRTJ171140.8+600731
59 16 00
15 45
30
15
00
14 45
30
17 16 55 50 45 40 35 30 25 20
(o) The extended object has been separated into FLSGMRT J171647.4+591458, J171639.4+151513, J171631.1+591544 and
J171628.4+591527.ThedoublesourcecomponentsareFLSGMRTJ171622.4+591536andJ171623.1+591523.
Figure7–continued
8 T. Garnet al.
sourcesize,andtakingintoaccountthecorrectionfactor.Columns
8and9givetheX,Y pixelcoordinatesfromthemosaicimageof
thesourcecentroid.Column10istheSourceExtractordeblended
objectflag–0formostobjects,but1whenasourcehasbeensplit
upintotwoormorecomponents.Fordeblendedobjectsitisneces-
sarytoexaminetheimageinordertodistinguishbetweenthecase
wheretwoastronomicallydistinctobjectshavebeensplitup, and
whenoneextendedobjecthasbeenrepresentedbymorethanone
entry.Thereare211deblendedsourcesinourcatalogue.
The non-uniform noise characteristics of our survey make it
important to quantify the area that has been surveyed with each
noiselevel.Figure8showsthefractionofpixelswithaparticular
noise level (taken fromtheSourceExtractor r.m.s.map), and the
cumulative fractionofpixelsat that noiselevel. Thepeak bright-
nessdistributionhasbeenplottedinFig.9,alongwithadistribu-
tion that has been corrected for the varying amount of solid an-
glebeingsurveyedtoeachsensitivitylevel.Previousstudieshave
shown(Hopkinsetal.2002)thatSourceExtractorhasafalsede-
Figure10.Sourcepositionsforobjectsdetectedat610MHzandundetected
tectionrateof below 5%,and detectsabove90% ofsources with at1.4GHz.Mostsourcesarelocatednearthecentreofoneofourpointings,
peakbrightnessclosetothedetectionthreshold. wheretheGMRTsensitivityisgreatest.
6
4 COMPARISONWITHTHEVLASURVEY
4
TheVLAsurveyofthexFLSregion(Condonetal.2003)hasauni-
formnoiselevelof23m Jy.Thisisequivalenttoanoiseof 45m Jy
∼
at 610 MHz, assuming a spectral index of 0.8. Our observations 2
havesignificantlylowernoiselevelsthanthisinthecentreofour c)
e
s
mosaic, withthe noise intheoverlap regions between two point- arc
ingsbeingapproximately45m Jy(seeFig.1).Figure8showsthat et ( 0
thenoiseisbelow45m Jyforabouthalftheareaofourmosaic. offs
C
InordertomakeaquantitativecomparisonbetweentheVLA E
D
xFLSsurveyandourGMRTsurvey,wehaverunSourceExtractor -2
on our mosaic and the 1.4-GHz image. At the 5s level, there
∼
were 3826 sources detected at 610 MHz, and 3091 detected at
1.4 GHz, in the region covered by both surveys. Thesource lists -4
werematchedusingapairingradiusof6 ,whichisapproximately
′′
theresolutionofthetwocatalogues,and1580uniquematcheswere
found.Figure10showsthesourcedistributionofobjectsfoundby -6
the GMRT but not by the VLA – the majority of the unmatched -6 -4 -2 0 2 4 6
RA offset (arcsec)
sourcesarefoundinregionswhereourobservationsaredeeperthan
theVLAimage. Figure11.SourcepositionsintheGMRTcataloguerelativetothepositions
foundintheVLAimage,foruniquematcheswithin6 .
It is likely that some of the sources that are not detected at ′′
610 MHz but are detected at 1.4 GHz are flat spectrum objects,
with spectral index 0.5. The VLA noise of 23 m Jy would be
equivalent toa610-M≤Hznoiselevelof 35m Jyfora =0.5,so ACKNOWLEDGMENTS
∼
faint flat spectrum objects detectable throughout the VLA image WethankthestaffoftheGMRTwhohavemadetheseobservations
wouldonlybedetectableinasmallregionofourmosaic. possible.TGthankstheUKPPARCforaStudentship.TheGMRT
Figure 11 shows the position offsets of the matched GMRT isoperated by theNational Centre for Radio Astrophysics of the
sources compared with their VLA counterparts. The offsets have TataInstituteofFundamentalResearch,India.
anapproximatelyGaussiandistribution,withmeanoffsetinright
ascension of 0.4, standard deviation 0.5 and in declination 0.2
′′ ′′ ′′
withastandarddeviationof0.6.Theseoffsetshavenotbeenap-
′′ REFERENCES
pliedtoourcatalogue,sinceitisuncertainastowhichsurveythe
errorscomefrom. Ananthakrishnan S.,2005, Proceedingsofthe29thInternational
The spectral index distribution of objects detected at both CosmicRayConference,10,125
frequencies is shown in Fig. 12. The integrated flux densities of AppletonP.,etal.,2004,ApJS,154,147
sourceshavebeenusedforthecalculation,wherethefluxdensity Baars J.,GenzelR.,Pauliny-TothI.,WitzelA.,1977,A&A,61,
of VLA sources was corrected using the same method as for the 99
GMRTimage(detailsinAppendixB). BertinE.,&ArnoutsS.,1996,A&AS,117,393
Deep 610-MHzGMRTobservationsof theSpitzerextragalacticFirstLookSurveyfield– I. 9
Table1.Asampleof60entriesfromthe610-MHzextragalacticFirstLookSurveycatalogue,sortedbyrightascension.
Name RA Dec Peak LocalNoise Int.FluxDensity Error X Y Flags
J2000.0 J2000.0 mJybeam 1 m Jybeam 1 mJy mJy
− −
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
FLSGMRTJ171751.2+584823 17:17:51.24 +58:48:23.2 0.210 35 0.330 0.070 3486 1752 0
FLSGMRTJ171751.3+594336 17:17:51.31 +59:43:36.2 0.190 34 0.150 0.050 3486 3961 0
FLSGMRTJ171751.3+591436 17:17:51.31 +59:14:36.5 0.210 39 0.140 0.050 3486 2801 0
FLSGMRTJ171751.3+590351 17:17:51.34 +59:03:51.1 0.380 41 0.490 0.080 3486 2371 0
FLSGMRTJ171751.5+590017 17:17:51.57 +59:00:17.8 0.290 42 0.350 0.070 3484 2228 0
FLSGMRTJ171751.6+590931 17:17:51.69 +59:09:31.2 0.310 42 0.350 0.070 3484 2597 0
FLSGMRTJ171751.9+591446 17:17:51.96 +59:14:46.4 0.400 40 0.470 0.070 3483 2807 0
FLSGMRTJ171752.0+603620 17:17:52.00 +60:36:20.1 0.310 57 0.350 0.100 3483 6070 0
FLSGMRTJ171752.4+594157 17:17:52.44 +59:41:57.9 0.280 37 0.310 0.070 3480 3895 0
FLSGMRTJ171752.5+600451 17:17:52.59 +60:04:51.1 0.230 38 0.240 0.060 3480 4811 0
FLSGMRTJ171752.9+591241 17:17:52.95 +59:12:41.7 0.440 43 0.480 0.070 3478 2724 0
FLSGMRTJ171753.0+584608 17:17:53.01 +58:46:08.4 0.320 38 0.610 0.080 3477 1662 0
FLSGMRTJ171753.1+601508 17:17:53.11 +60:15:08.1 0.200 33 0.260 0.060 3477 5222 0
FLSGMRTJ171753.1+592330 17:17:53.14 +59:23:30.3 0.180 32 0.300 0.070 3477 3157 0
FLSGMRTJ171753.3+585502 17:17:53.37 +58:55:02.3 0.290 35 0.370 0.070 3475 2018 0
FLSGMRTJ171753.5+594912 17:17:53.54 +59:49:12.2 0.350 40 0.410 0.070 3475 4185 0
FLSGMRTJ171753.7+594428 17:17:53.74 +59:44:28.2 0.230 41 0.180 0.060 3474 3995 0
FLSGMRTJ171753.7+592117 17:17:53.75 +59:21:18.0 0.180 34 0.190 0.060 3474 3069 0
FLSGMRTJ171753.9+594300 17:17:53.98 +59:43:00.5 0.520 35 0.570 0.060 3473 3937 1
FLSGMRTJ171754.7+593533 17:17:54.73 +59:35:33.4 0.200 35 0.170 0.050 3469 3639 0
FLSGMRTJ171754.7+600914 17:17:54.73 +60:09:14.8 0.670 32 0.890 0.070 3469 4986 0
FLSGMRTJ171754.8+601939 17:17:54.84 +60:19:39.4 0.160 29 0.180 0.050 3469 5403 0
FLSGMRTJ171755.0+585933 17:17:55.03 +58:59:33.7 0.290 44 0.560 0.110 3467 2199 0
FLSGMRTJ171755.0+594300 17:17:55.05 +59:43:00.5 0.190 35 0.130 0.040 3467 3937 1
FLSGMRTJ171755.2+604141 17:17:55.29 +60:41:41.4 0.420 76 0.270 0.100 3467 6284 0
FLSGMRTJ171755.2+585512 17:17:55.29 +58:55:12.8 0.360 33 0.410 0.060 3465 2025 0
FLSGMRTJ171755.3+592846 17:17:55.32 +59:28:46.7 0.280 30 0.440 0.060 3466 3368 0
FLSGMRTJ171755.6+594815 17:17:55.62 +59:48:15.4 0.220 38 0.180 0.050 3464 4147 0
FLSGMRTJ171755.6+593421 17:17:55.63 +59:34:21.6 0.180 29 0.120 0.040 3464 3591 0
FLSGMRTJ171755.8+600519 17:17:55.83 +60:05:19.3 0.190 34 0.260 0.070 3464 4829 1
FLSGMRTJ171756.0+594431 17:17:56.08 +59:44:31.9 0.310 40 0.460 0.080 3462 3998 0
FLSGMRTJ171756.4+592839 17:17:56.40 +59:28:39.6 0.170 31 0.170 0.050 3460 3363 0
FLSGMRTJ171756.4+591744 17:17:56.41 +59:17:44.2 0.310 41 0.400 0.070 3460 2926 0
FLSGMRTJ171756.4+590005 17:17:56.49 +59:00:05.4 0.450 43 0.560 0.080 3459 2220 0
FLSGMRTJ171756.5+584809 17:17:56.51 +58:48:09.1 0.230 39 0.300 0.070 3459 1743 0
FLSGMRTJ171756.8+593631 17:17:56.85 +59:36:31.1 0.470 32 0.580 0.060 3458 3677 0
FLSGMRTJ171756.9+601401 17:17:56.90 +60:14:01.1 0.190 33 0.210 0.050 3459 5177 0
FLSGMRTJ171757.5+602115 17:17:57.54 +60:21:15.7 0.200 32 0.290 0.060 3456 5467 0
FLSGMRTJ171757.6+594247 17:17:57.61 +59:42:47.4 0.310 38 0.310 0.060 3454 3928 0
FLSGMRTJ171757.6+584555 17:17:57.65 +58:45:56.0 0.670 37 0.930 0.080 3453 1654 0
FLSGMRTJ171757.7+593136 17:17:57.80 +59:31:36.3 0.380 33 0.390 0.050 3453 3481 0
FLSGMRTJ171757.7+591527 17:17:57.80 +59:15:27.4 0.200 38 0.140 0.050 3453 2835 0
FLSGMRTJ171757.9+585604 17:17:57.93 +58:56:04.3 0.240 39 0.260 0.070 3452 2059 0
FLSGMRTJ171758.1+604204 17:17:58.18 +60:42:04.6 0.450 77 0.420 0.110 3453 6299 0
FLSGMRTJ171758.2+585811 17:17:58.26 +58:58:11.9 0.230 39 0.360 0.080 3450 2145 0
FLSGMRTJ171758.3+602850 17:17:58.37 +60:28:50.8 0.300 48 0.400 0.090 3452 5770 0
FLSGMRTJ171758.7+590407 17:17:58.76 +59:04:07.4 0.260 45 0.230 0.060 3448 2382 0
FLSGMRTJ171758.9+602307 17:17:58.93 +60:23:07.7 0.260 36 0.410 0.070 3449 5542 0
FLSGMRTJ171758.9+593454 17:17:58.96 +59:34:54.5 0.150 29 0.130 0.040 3447 3613 0
FLSGMRTJ171759.0+592438 17:17:59.08 +59:24:38.9 1.910 33 2.280 0.070 3446 3202 0
FLSGMRTJ171759.1+584435 17:17:59.11 +58:44:35.8 0.440 39 0.680 0.080 3445 1600 0
FLSGMRTJ171759.3+594325 17:17:59.38 +59:43:25.7 0.210 39 0.460 0.100 3445 3954 0
FLSGMRTJ171759.4+590132 17:17:59.40 +59:01:32.1 0.260 38 0.230 0.050 3444 2278 0
FLSGMRTJ171759.3+595246 17:17:59.40 +59:52:46.7 0.980 42 1.250 0.090 3446 4328 0
FLSGMRTJ171759.4+590256 17:17:59.43 +59:02:56.4 0.210 40 0.290 0.070 3444 2334 0
FLSGMRTJ171759.8+594210 17:17:59.84 +59:42:10.8 0.220 37 0.230 0.060 3443 3904 0
FLSGMRTJ171800.0+584604 17:18:00.04 +58:46:04.5 0.250 40 0.260 0.060 3440 1660 0
FLSGMRTJ171800.1+594925 17:18:00.16 +59:49:25.1 0.250 42 0.360 0.080 3442 4193 0
FLSGMRTJ171800.3+590207 17:18:00.31 +59:02:07.8 1.220 40 1.510 0.090 3440 2302 0
FLSGMRTJ171800.3+585142 17:18:00.35 +58:51:42.7 0.190 37 0.330 0.080 3439 1885 0
10 T. Garnet al.
140 2
120
1.8
100
N 6800 Correction factor 11..46
40
1.2
20
0 1
-1 -0.5 0 0.5 1 1.5 2 4 6 8 10 12 14 16 18 20
alpha Peak signal-to-noise ratio
Figure12.Radiospectralindexbetween610MHzand1.4GHz FigureA1.Fluxdensitycorrectionfactorforpointsources
ChapmanS.,BlainA.,SmailI.,IvisonR.,2005,AJ,622,772 data,sothatitwasonthesamefrequencyscaleastheUSBpoints,
CondonJ.,1992,ARA&A,30,575 andsothesidebandscouldbecombinedbeforeimaging.
Condon J., Cotton W., Yin Q., Shupe D., Storrie-Lombardi L., (ii) The coordinates of sources originally found in our GMRT
HelouG.,SoiferB.,WernerM.,2003,AJ,125,2411 imageswereseentobeslightlyrotatedneartheedgeofeachpoint-
Fadda D., Jannuzi B., Ford A., Storrie-Lombardi L., 2004, AJ, ingcomparedwiththeirVLApositions.Thiswasduetoincorrect
128,1 time-stampsbeing usedinthe GMRTonlinesoftware, leading to
FaddaD.,etal.,2006,AJ,131,2859 a slight error in the uv data. We wrote a customised AIPS task,
FazioG.,etal.,2004,ApJS,154,10 UVFXT,toincreasethetime-stampsby7sandcorrectlyrecom-
FrayerD.,etal.,2006,AJ,131,250 putetheuvwco-ordinates.Thiswasperformedbeforethefinalim-
GarrettM.,2002,A&A,384,19 ageswerecreated.
Gruppioni C., Pozzi F., Zamorani G., Ciliegi P., Lari C., Cal-
abreseE.,LaFrancaF.,MatuteI.,2003,MNRAS,341,1
A2 Primarybeamcorrection
HelouG.,SoiferB.,Rowan-RobinsonM.,1985,AJ,298,7
Hopkins A., Miller C., Connolly A., Genovese C., Nichol R., During the preliminary analysis of the data, we compared the
WassermanL.,2002,AJ,123,1086 propertiesofseveral hundred relativelybright sourceswithpeaks
Hughes A., Wong T., Ekers R., Stavely-Smith L., Filipovic M., greaterthan1mJybeam 1,visibleintheoverlappingregionsbe-
−
MaddisonS.,FukuiY.,MizunoN.,2006,MNRAS,370,363 tweentwopointings.Thiscomparisonrevealedasystematicdiffer-
KanthariaN.,&Rao,A.,2001,GMRTTechnicalNoteR00185 encebetweentheapparentbrightnessofsourcesinadjacentpoint-
LacyM.,etal.,2005,ApJS,161,41 ings;sourceslocatedtothenorth-westofapointingwereconsis-
LuoS.,WuX.,2005,ChJAA,5,448 tentlybrighterthanthesamesourcewhenviewedinthesouth-east
Morganti R., Garrett M., Chapman S., Baan W., Helou G., ofanadjacentpointing.Wewereabletomodelthisastheeffective,
SoiferT.,2004,A&A424,371 averagepointingcentreofthetelescopebeingoffsetby 2.5ar-
MurphyE.,etal.,2006,AJ,638,157 cmininanorth-westdirection,comparedwiththenomin∼alpoint-
PapovichC.,etal.,2006,AJ,132,231 ingcentre. Theamount and directionof theoffset wasconsistent
RiekeG.,etal.,2004,ApJS,154,25 between all pairs of pointings. After applying the correction, the
Shim H., Im M., Pak S., Choi P., Fadda D., Helou G., Storrie- systematiceffectswereremovedandther.m.s.fluxdensityerrors
LombardiL.,2006,ApJS,164,435 werebelow10%neartheedgeoftheprimarybeam,comparedwith
StoughtonC.,etal.,2002,AJ,123,485 nearer20%beforethecorrectionwasapplied.
WernerM.,etal.2004,ApJS,154,1 Itisthoughtthattheprimarybeamoffsetdependsontelescope
elevation.Ourscansofeachsourcewerenottakensymmetrically
about sourcetransit, andthedetected offset islikelytobeanav-
erageovertheindividualoffsetsofeachscan.Sourceswillhavea
slightlydifferentpositionandfluxdensityineachscan,duetothis
APPENDIXA: CORRECTIONSTOGMRTDATA
elevation-dependanterror.Self-calibrationassumesthatthesource
A1 Correctionstotheuvdata remainsconstantwithtime,andthisislikelytobethereasonwhy
residualartefactsareseennearthebrightestsources.
TwocorrectionsweremadetotheGMRTuvdatabeforethefinal
imagesweremade.
(i) Theuvcoveragewasincreasedbycombining thetwoside-
APPENDIXB: SOURCEEXTRACTOR
bandsintoasingledatafile.Thefrequencychannelsinthelower
sideband(LSB)haveanegativefrequencyincrement,whiletheup- SourceExtractor calculates thefluxdensity of an object by sum-
persideband(USB)hasapositiveincrement–thismeansthatthe ming all pixels greater than some user-defined threshold. For an
two channels can not be combined directly using standard AIPS object to be included in our catalogue, we required it to have at
tasks.WewroteanAIPStaskcalledUVFLPtore-ordertheLSB least fiveconnected pixels withbrightness above 2s , and a peak