The Last of FIRST: The Final Catalog and Source Identifications

2.6 MB·English

by  David J. Helfand

TOAPPEARINTHEASTROPHYSICALJOURNAL
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
THELASTOFFIRST:THEFINALCATALOGANDSOURCEIDENTIFICATIONS
DAVIDJ.HELFAND1,2,RICHARDL.WHITE3ANDROBERTH.BECKER4,5
ToappearintheAstrophysicalJournal
ABSTRACT
TheFIRST survey, begun over twenty years ago, providesthe definitive high-resolutionmap of the radio
sky. ThisVLAsurveyreachesadetectionsensitivityof1mJyat20cmovera finalfootprintof10,575deg2
5
that is largely coincidentwith the Sloan Digital Sky Survey area. Both the images and a catalog containing
1
946,432sourcesareavailablethroughtheFIRSTwebsite(http://sundog.stsci.edu).Werecordhere
0
the authoritative surveyhistory, including hardware and software changesthat affectthe catalog’s reliability
2
andcompleteness. Inparticular,weuserecentobservationstakenwiththeJVLAtotestvariousaspectsofthe
n
surveydata(astrometry,CLEANbias,andthefluxdensityscale). Wedescribeanew,sophisticatedalgorithm
a
forflaggingpotentialsidelobesinthissnapshotsurvey,andshowthatfewerthan10%ofthecataloguedobjects
J
arelikelysidelobes, andthattheseareheavilyconcentratedatlowfluxdensitiesandin thevicinityofbright
9 sources, asexpected. We also reporta comparisonof the surveywith the NRAO VLA SkySurvey(NVSS),

aswellasamatchoftheFIRSTcatalogtotheSDSSand2MASSskysurveys. TheNVSSmatchshowsvery
]
A good consistency in flux density scale and astrometry between the two surveys. The matches with 2MASS
andSDSSindicateasystematic 10- 20masastrometricerrorwithrespecttotheopticalreferenceframein
G ∼
allVLAdatathathasdisappearedwiththeadventoftheJVLA.Wedemonstratestrikinglydifferentbehavior
. betweentheradiomatchestostellarobjectsandtogalaxiesintheopticalandIRsurveysreflectingthedifferent
h
radiopopulationspresentoverthe flux densityrange 1–1000mJy. As the radioflux density declines, stellar
p
counterparts(quasars)getredderandfainter,whilegalaxiesgetbrighterandhavecolorsthatinitiallyredden
-
o butthenturnbluerneartheFIRSTdetectionlimit.
r Implications for future radio sky surveys are also briefly discussed. In particular, we show that for radio
t
s sourceidentificationatfaintopticalmagnitudes,highangularresolutionobservationsareessential,andcannot
a besacrificedinexchangeforhighsignal-to-noisedata. ThevalueofaJVLAsurveyasacomplementtoSKA
[
precursorsurveysisbrieflydiscussed.

2 Subjectheadings:catalogs—methods:dataanalysis,statistical—radiocontinuum:general—surveys
v
5
5 1. INTRODUCTION throughtheFIRSTwebsite6,theMikulskiArchiveforSpace
5 Telescopes7andotherarchivalsites.
FaintImagesoftheRadioSkyatTwenty-centimeterswere
1 Ahistoryofthehardwareandsoftwarechangesduringthe
collectedoveraperiodofeighteenyearsatthe(now,Jansky)
0 project is provided to alert users to possible systematics in
Very Large Array. The original proposal to use the world’s
. thesurveyimagesandcatalog(§2);wealsonotevariousrare
1 premiere radio telescope for the relatively mundane task of
anomaliesintheVLAsystemthathavebeendiscoveredasa
0 surveyingtheskywassubmittedinAugustof1990.Pilotob-
consequenceof examining over75,000 snapshotimages. In
5 servationsfortheFIRSTsurveybeganinAprilof1993,fifty
1 yearstothemonthafterGroteReber’soriginalradioskysur- some of this work, we make use of matches of the FIRST
: vey(Reber1944). Thefinalobservationswerecompletedin catalogtootherall-skysurveys,thesciencefromwhichisre-
v portedlaterinthepaper. We assesstheastrometricaccuracy
theSpringof2011inanexpansionofthesurveytocoverthe
i
X SDSS3skysurveyarea. Overthisextendedperiod,anumber of the FIRST catalog, which reveals some small systematic
offsetsin the positions (§3). We go on to describe the algo-
r ofVLAhardwarechangeswereimplemented,notleastbeing
a thetransformationoftheentirearrayintotheJVLA.Inaddi- rithmdevelopedtoassigntoeachsourceaprobabilitythatitis
aspuriouscatalogentryresultingfromasidelobeofabright
tion,anumberofchangestothedatareductionsoftwareand
source elsewhere in the field (§4). Various tests of this al-
processingalgorithmsalso took place overthe course of the
gorithmusingradioandopticalcatalogmatchingprovidean
survey. Furthermore,thelower-resolutionNVSSsurveywas
assessmentofthealgorithm’sreliability.
completedduringthisinterval,aswereskysurveysatinfrared
We compare theFIRST surveycatalog with the results of
(2MASS)andoptical(SDSS)wavelengths.
three other major sky surveys that cover most or all of the
In this paper, we describe the final productsof theFIRST
survey. The FIRST images and catalogs are distributed same>10,000deg2ofsky: theNVSS(Condonetal.1998),
the 2MASS survey (Skrutskieetal. 2006), and the SDSS
[email protected] (Yorketal. 2000) plusits morerecentextensions(Ahnetal.
1Department of Astronomy, Columbia University, Mail Code 5233, 2014). Inparticular,weusetheNVSSmatch(§5)toquantify
PupinHall,538West120thStreet,NewYorkCity,NY10027. the accuracyof theFIRSTflux density scale andthe degree
2QuestUniversityCanada,3200UniversityBlvd.,Squamish,BCV8B towhichfaintsourcesaremissedasaconsequenceofthein-
0N8Canada
3SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore, creasednoiselevelnearbrightsources,aswellastheincom-
MD21218.
4DepartmentofPhysics,UniversityofCalifornia,Davis,1ShieldsAv-
enue,Davis,CA95616-8677. 6http://sundog.stsci.edu
5IGPP/LawrenceLivermoreNationalLaboratory 7http://archive.stsci.edu/vlafirst/
2 Helfand,White&Becker
4000
10000
Observing time
Catalog releases
s
r
u
o
h
8000
n 3000 2]
o
g
i
at 70 e
v 60 d
er 50 a [
s 40 6000 e
b r
o a
30
ve 2000 20 ky
ati 10 g s
ul 0 o
4000
m -10 al
u 17 16 15 14 13 12 11 10 9 8 at
c ) 20 C
g
T e 10
S 1000 d
R ( 0
FI ec -10 2000
D
4 3 2 1 0 23 22 21 20
RA (hrs)
0 2011 2009 2003 2001 2000 1999 1998 1997 1995 0
1995 2000 2005 2010 2015
Year
FIG.1.—Thetwenty-yearhistoryoftheFIRSTsurveyshowingtheskycoverage(nearlyverticaldarklines)asafunctionoftime.Therightaxisshowsthesky
areainsquaredegreesandtheleftaxisshowsthecumulativeobservingtime.Theinsertshowstheskycoveragecolor-codedbythecatalogreleasedates,which
arealsoindicatedbycirclescoloredtomatchthecoveragemap. Thefinalproject,representinganinvestmentof4009hoursofVLAtime,generatedasurvey
covering10,575deg2ofsky.
pleteness ofFIRST for very extendedobjects. The 2MASS
(§6) and SDSS (§7) matches provide informationon the as- TABLE1
trometricaccuracyoftherespectivesurveys,aswellasinsight SUMMARYOFFIRSTCATALOGRELEASES
intotheclassesofsourcesthatpopulatetheradioskyatmil-
ReleaseDate Numberof NorthGalactic SouthGalactic Total
liJanskyfluxdensities. Weconcludewithacommentary(§8) Sources CapArea CapArea Area
on the survey’s utility and on the lessons learned from this (deg2) (deg2) (deg2)
undertakingthatcanbeusedtoinformthenextgenerationof (1) (2) (3) (4) (5)
radioskysurveys. Inparticular,weexaminecriticallytheef- 1995Jan06 26,892 300 0 300
fectofsurveyresolutiononoptical/IRidentificationprograms 1995Oct16 138,665 1559 0 1559
forradiosources. 1996May28 138,665 1559 0 1559
1997Feb27 236,040 2576 0 2576
Readersuninterestedinthedetailedtechnicalhistoryand/or
1997Apr24 268,047 2576 350 2926
thesubtlecatalogbiasesusefulonlyforthoseusingFIRSTin 1998Feb04 437,429 4153 611 4764
large, statistical surveys, should skip to the science results, 1999Jul21 549,707 5448 611 6060
whichbeginin§5. 2000Jul05 722,354 7377 611 7988
2001Oct15 771,076 7954 611 8565
2003Apr11 811,117 8422 611 9033
2. THESURVEYHISTORY 2008Jul16 816,331 8444 611 9055
TheVLA8pilotobservationsin1993aidedusindesigning 2012Feb16 946,464 8444 2191 10635
2013Jun05 971,268 8444 2191 10635
the pointinggridfor the survey(also adoptedby the NVSS) 2014Mar04a 946,432 8444 2131 10575
andindevelopingthebasicdatareductionalgorithmsforturn- 2014Dec17a 946,432 8444 2131 10575
ingsnapshotvisibilitiesintofinalsurveyimages. Thesebasic
aThesecatalogversionshavefewersourcesandaslightlyreducedskyarea
attributesoftheprojectaredescribedindetailinBeckeretal. becausetheJVLAimagesarenotco-addedwitholderVLAimages.Seetext
(1995)andthecatalog,constructedasdetailedinWhiteetal. formoredetails.
8 TheKarlG.JanskyVeryLargeArrayisaninstrumentoftheNational (1997); those papers should remain the primary references
RadioAstronomyObservatory,afacilityoftheNationalScienceFoundation whenmakinguseofFIRSTresults. Here,fortherecord,we
operatedundercooperativeagreementbyAssociatedUniversities,Inc. documentthetechnicalhistoryoftheprojectanddescribeall
TheLastofFIRST 3
factorsthatmightaffectthecharacterandintegrityofthedata
products. 12000
Red: 2011 JVLA observations
InFigure1,weshowthetimeallocatedandtheskycover-
ageachievedoverthe18yearsofsurveyobservations.Atotal 10000
ofelevenobservingsessionsintheB-configurationledtothe
accumulationofjustover4000hours(5.5months)ofobserv- ds 8000
ing time that resulted in 10,575 deg2 of sky coverage(8444 Fiel
deg2 inthenorthGalacticcapand2131deg2 inthesouthern of 6000
cap). Acoveragemap, color-codedbyobservationepoch,is er
b
m
displayed in the inset to Figure 1; the catalog-release points
u 4000
N
arecoloredtomatchtherelevantcoverageareas.
Asoriginallyenvisioned,alldatawerereleasedtothepub-
2000
lic archive on the day they were taken, and all images were
fully reduced and put on a public website before a new set
0
of observationscommenced. More than a dozen catalog re-
-3 -2 -1 0 1 2 3
leases were issued; they are located in Figure 1 by date and
Hour Angle of Observation [hrs]
skycoverage(rightaxis). Asummaryofthecatalogreleases
isgiveninTable1. Thefinal2014December17releasecon- FIG.2.—DistributionofhouranglesfortheFIRSTobservations.Thehour
tains946,432entries.Ahistoryofcatalogformatchangesand angleiszeroforfieldsobservedastheypassthemeridian. Theblackline
errorcorrectionscanbefoundonline9. showsthedistribution fortheentiresurvey. Theredlineshowsthedistri-
butionforthe2011JVLAobservations,normalizedtothesameareaasthe
overalldistribution. Thelatterdistributiondiffersowingtoshorter1-minute
2.1. Hardwareandobservingmodechanges integrationsandtheneedtoavoidpointingtowardtheSun.
Theonlymajorhardwarechangeovertheperiodofthesur-
vey was occasioned by the transition of the array from its A7-degreeavoidanceradiuswasusedin2009;thezonewas
originalconfigurationtotheJVLA.Duringthe2009observ- expandedto10degreesin2011owingtoincreasedsolarac-
ingsession,roughlyhalfthearrayhadtheoriginal20cmre- tivity. This,coupledwithnon-optimalschedulingofourtime
ceivers, while half hadbeenchangedto the JVLA receivers. allocations, led to many of the observationsbeingtaken far-
ThecenterfrequenciesofthetwoIFswereheldconstant,but therfromthezeniththanwasdesirable. Theschedulingchal-
the shapes of the receiver bandpasses were different. In the lengescombinedwiththelossofasignificantquantityofdata
2011 session, the JVLA transition was complete. We ob- tointerferencealsoresultedinsomeholesintheskycoverage
served at a different center frequency (1335 MHz vs. two andled tothe disconnectedislandof coveragethatisvisible
narrowbandscentered at 1365MHz and 1435 MHz) with a inFigure1nearδ=15◦inthesouthGalacticcap.
128 MHz total bandwidthcomprised of 64 2-MHz channels Figure2showshowtheobservationsweredistributedwith
(vs. 2 73-MHzchannelsfortheremainderofthe survey); respecttothemeridian.Ideallythefieldswouldallhavebeen
datata×keninasecond128-MHzbandcenteredat1730MHz, observed as they passed the meridian (zero hour angle), but
to be used in deriving source spectral indices, will be re- therealdistributionismorecomplex.90%oftheobservations
ported elsewhere. All the changes in 2011 were motivated for the entire survey were acquired within 1.4 hours of the
byacombinationofthechangingrequirementsoftheJVLA meridian. Thedistributionisnoticeablyspikyandasymmet-
receiver/correlator system, plus the desire to reduce the ob- ricalbecausethe observingtime blocksallocatedwereoften
serving time and to extract additionalscience using the new non-optimal,requiringthatweobservefieldsoffthemeridian.
capabilitiesoftheJVLA. The distribution for the short 1-minute JVLA observations,
While the pilot observations used a 3-second integration shownbytheredline,ismoresymmetrical(theresultofour
time, we quickly adopted 5-second integrations as standard. traveling salesman algorithm) but also has a very extended
For the 2011 JVLA observations, however, the wide band- tail at large hour angles (due to the need to avoid the Sun.)
width required a reduction of the integration time to 1 sec- All these effects conspire to produce point-spread functions
ond. These changes affect the data analysis but do not lead (PSFs) and sidelobe patternsthatvary slightly from year-to-
to appreciablechangesin the surveydata products. The on- year. Formostpurposes,however,thePSFcanbetreatedas
target dwell time was 165-seconds per field until 2011, at ifitwereuniform.OverthenorthernskythePSFisacircular
whichtimethewiderbandwidthavailableallowedustoreach GaussianwithFWHM5.4′′. Southofdeclination+4◦33′21′′
our 1 mJy sensitivity while reducing the observing time to thebeambecomeselliptical,6.4′′ 5.4′′,withthemajoraxis
1 minute per field. The wider bandwidth and different fre- runningnorth-south. Southof- 2◦×30′25′′ theellipticalbeam
quency do have some effects on the data (discussed further sizeincreasesfurtherto6.8′′ 5.4′′.
×
below). The shorter integration time also required a change In the final 2014Dec17 version of the catalog, the JVLA
inthepointingpattern,sincethetimespentperfieldwasthen images are not co-added with older VLA images to avoid
muchlessthanthesiderealrateatwhichtheskypassedover- problems resulting from the different frequencies and noise
head.Atraveling-salesmanalgorithmusingsimulatedanneal- propertiesofthetwodatasets. Thatleadstosmallgapsinsky
ing(Kirkpatricketal.1983)wasdevelopedtominimizeslew coverageat the boundariesbetween the JVLA and VLA re-
timebetweenfields. gions,buthastheadvantagethatitcleanlyseparatessources
During both 2009 and 2011, the Sun was located in the fromtheoldandnewconfigurations. Allsourcesinthefinal
sky region to be observed, requiring us to schedule around catalogthathavefieldnamesendingwith‘W’comefromthe
a zone of avoidance centered on the apparentsolar position. JVLAdata.
9http://sundog.stsci.edu/first/catalogs/history.html
4 Helfand,White&Becker
2.2. VLAanomalies
Unsurprisingly, with 4000 hours of observing time to re- 107
duce and a million sources to catalog, subtle and/or rare ef-
fects, notnoticeablein ordinaryVLA programs, can appear. B) 106
Wesummarizeseveralsuchanomalieshere. T
$/
Misassignment of data blocks: Searches of the FIRST e ( 105
database for source variability led to the discovery of a bug c
intheonlineVLAsoftwarethatappendstothecurrentobser- Pri 104
k
vationafewintegrationsfromthepreviousobservation. This s
wasrevealedwheninvestigatingthe(ultimatelyspurious)de- d Di 103
tectionofvariabilityinabrightsource(Gal-Yametal.2006; ar
Ofeketal.2010). Alluvdatasetsweresubsequentlychecked H 102
for this error and 190 cases of discontinuities in uvw values
withinasinglescanwereidentified(Thyagarajanetal.2011). 101
These data sets were edited and the images re-made. Note 1990 1995 2000 2005 2010
that all versions of the catalog prior to 2012 may contain a Date
fewspurioussourcesasaconsequenceofthiserror.
Misassignment ofarrayconfiguration: In collectingthe FIG.3.—Priceofdiskstorage(dollarsperterabyte, usingconstant2012
dollars)overthedurationoftheFIRSTsurvey.Storagepriceshavedropped
globalobservationrecordstoincludeinthispaper,wenoticed
byafactorofroughly300,000sincetheFIRSTsurveyproposalwassubmit-
thattheVLAarchivesometimesrecordsincorrectarraycon- tedin1990. Computerprocessingandmemoryhavealsobecomedramati-
figurationlabelsforsome observations. Allobservationsfor callylessexpensiveoverthelifetimeofthesurvey.
theFIRSTsurveywere,infact,takenintheBconfiguration.
we adopted an approach of making an initial low-resolution
The VLA archivealso is currentlymissing entries for 1%
∼ map to find brightsources in the image and then using only
oftheFIRSTfields.
a set of small maps centered on those source positions for
Image stretchand rotation: By comparingthe positions
the self-calibration iteration (Beckeretal. 1995). Following
of sources observed in multiple pointings, we discovered
thesix-yearhiatusin2009,wesimplifiedtheprocessfornew
that there exist both a stretch and a rotation of VLA im-
data by repeatedly making full-field images during the self-
ages, which we infer are related to small clock errors at the
calibrationiterationaswellasforproducingthefinalmap.
VLA and small changes in bandpass shape (see footnote 6
Wide-field bright-source mapping: Owing to process-
of Whiteetal. 1997). The astrometric errors introduced are
very small (<0.1′′) for most sources, as the errors tend to ingspeedlimitations,ouroriginalapproachforthefinalmap
wastoaugmenta2048 2048mapcoveringthecentralpri-
cancel out in the co-added images. Only for sources at the
×
mary beam with small satellite maps placed at the positions
very edge of the survey coverage area can the errors rise to
0.3′′.Forthefirstfourepochsofobservation,wesolvedfor of nearby bright sources (from single-dish catalogs) that lie
∼ within 10◦ of each field center (for details see Beckeretal.
the stretch and rotation corrections using multiply observed
1995). Beginning in 2009, we simply made 4096 4096
sources. Since the parameters were quite stable from epoch
×
images and dispensed with special treatment for far off-axis
to epoch, we then fixedthe correctionandappliedit to each
sources. With 1.8 arcsec pixels, these images span 2.0 de-
epoch’simagesthrough2003. The lasttwo epochshavenot
grees,whichisfourtimeslargerthanthe20cmFWHMpri-
had these small corrections applied. Thus, the positions of
mary beam diameter. Note that because these images are
sourcesattheedgeofthecoverageinthesouthGalacticcap
snapshotobservations,thereisnoneedforanyspecialtreat-
shouldbeassignedslightlylargerastrometricuncertainties.
ment of 3-D sky effects in the processing. The 3-D distor-
tionsare removedin post-processingby warpingthe map as
2.3. Datareductionchanges
describedinBeckeretal.(1995).
Computingpowerandstoragecapacityhaveincreaseddra- Also,inthefirstsectionofthesurvey,eachfieldcenterwas
maticallyoverthelifeofthisproject(Fig.3). Inaddition,the shifted by up to 0.9′′ so thatthe brightestsource in the field
AIPS software package has evolved, as have the scripts we fellattheprecisecenterofa pixel. We neverfoundanyevi-
run to process the FIRST data. Finally, the recent upgrade dencethatthissignificantlyaffectedimagequalityand,since
totheJVLAhardwarehasrequiredchangesintheprocessing it complicated the image processing and the final products
pipeline. We briefly note here all significant changes to the (whichotherwisehadapredictablepixelgrid),weabandoned
dataprocessing. thisprocedureinprocessingthefinaltwoepochs.
Dataflagging:Priorto2011,thedatawerecollectedintwo Source extraction: We used our AIPS source extraction
setsofseven3-MHzchannels,andifinterferencewasstrong program HAPPY (Whiteetal. 1997) to identify and mea-
inonechannel,allsevenchannelsofdataforthatintegration surethepropertiesofsourcesintheFIRSTimages. HAPPY
period(5seconds)weredeleted.Withtheadventofthebroad- evolvedoverthecourseoftheprojectasaresultofminoren-
bandwidthJVLA correlator,identifiedinterferencein one2- hancementsandbugfixes. Toensureamoreuniformcatalog,
MHzchannelleadstothedeletionofthatchannelandthetwo in2007wereprocessedallthedatausingthecurrentversion
adjacentchannelsonly. ofHAPPY.Thatledtominorchangesinthesourcelists.
Self-calibration: The self-calibration process (Högbom FluxcalibrationandCLEANbias: Thefluxdensitycal-
1974) requires iteratively CLEANing images and using the ibrators3C286and3C48wereusedfortheNorthandSouth
fluxmodelsfromthoseimagestoimprovetheantennaphase GalacticCaps,respectively. Theco-addingproceduretocre-
(and sometimes amplitude) calibrations. During the initial ate the finalimagesfromthe individualgrid images, as well
years of the FIRST survey, the computing required to con- as the source detection and parameterizationprocedures, re-
structamapwasdaunting.Torenderthecomputingtractable, mainedunchangedthroughoutthesurveyperiod.
TheLastofFIRST 5
2.4. EpochsofObservations
110055
ToenabletheuseofFIRSTdatafortime-domainscience,
All FIRST images
the2014Dec17releaseoftheFIRSTcatalogincludesobser-
110044 JVLA images with higher vation epochs for the sources. Since the catalog is created
CLEAN thresholds from co-added images, each source may have contributions
s
e
ag 110033 from many different pointings taken at different times. For
m some sources all the contributing observations come from a
of I narrow time range of only a few minutes (when one or two
er 110022 consecutivegrid imagesdominatethe co-addedsource posi-
b
m tion),whileothershavesignificantcontributionsfrompoint-
u
N 110011 ingstakenyearsapart(typicallyattheseamsbetweenobserv-
ing seasons and near fields that were re-observed owing to
problemswiththeoriginalobservations).
110000 The catalog includes a mean weighted epoch¯t, defined to
betheaverageoftheepochsofallthecontributingpointings
00..11 11..00 1100..00
Grid Image rms [mJy] atthe sourceposition weightedbythe same weightsused to
combinetheoverlappingmaps. Italsoincludestheweighted
FIG.4.—DistributionofrmsnoisevaluesinindividualFIRSTsurveyim- rmsσ(t)ofthescatterofthepointingepochsaboutthatmean,
ages.ThedarkshadedhistogramshowsthenoisedistributionforJVLAfields
which is a measure of the effective spread in the observing
observedonthreedayswithparticularlybadinterference; theirmeannoise
levelsare∼2.5timeshigher. epoch. Figure5(a)showsthecumulativedistributionofσ(t);
themedianrmsis1.8days,and90%oftheσ(t)valuesareless
thanfourmonths,so mostobservationsarewell-localizedin
One of the systematic errors in VLA images that came ¯
time. Plotting σ(t) versust (Fig. 5b) showsthe tailof much
to light fromFIRST and NVSS was the so-called ‘CLEAN
higher epoch rms values for sources in the overlap regions
bias’. SourcesextractedfromCLEANedsnapshotimagesin-
betweenobservingsessions.
variably lost flux in the CLEANing process. Further analy-
sisoftheFIRSTimagesforstudiesofsub-thresholdsources
throughstacking(Whiteetal.2007)showedthattheCLEAN 3. ASTROMETRY
biaseffectisreallyasnapshotbiasthatreducesthefluxden- InourinitialdescriptionoftheFIRSTsurvey(Beckeretal.
sities of even faint undetected sources. This was quantified 1995) we evaluated the astrometric precision of the source
byBeckeretal.(1995)andWhiteetal.(2007)byaddingar- catalog by comparing the positions of 46 Multi-Element
tificialsourcesintotheUVdataandcomparingtheinputand Radio-LinkedInterferometerNetwork(MERLIN)calibrators
output flux densities for tens of thousands of point sources. lying in the first survey strips and found the systematic er-
We continueto adda CLEAN bias offsetof0.25mJy to the rors to be < 0.05′′. An additional comparison with 4100
peakfluxdensitiesofallsources(seeBeckeretal.1995). optical counterparts from the Automatic Plate Measuring
Since with the change of VLA hardware there was some (APM) machine scans of the Palomar Optical Sky Survey
concernthattheCLEANbiaswouldbedifferent,werepeated plates(McMahon&Irwin1992)foundthesamelimitforthe
these CLEAN bias tests using artificial sourcesfor each day optical-radio frame offset. For a survey in which the indi-
ofdatatakeninthefinal2011JVLAepoch.Wefoundthatthe vidualcatalogentrieshavetypicalpositionaluncertaintiesof
CLEANbiaswasnotstableandseemedtocorrelatewiththe 0.3′′, any systematic positionoffsets of this magnitudewere
amountof interference in the data. In particular, there were insignificantforallenvisionedinitialusagesofthecatalog.In
three days when the CLEAN bias was significantly higher theFIRSTcatalogpaper(Whiteetal.1997)weusedalarger
thanusual.ThehigherCLEANbiasonthosedaysappearedto sampleofradiocalibratorsandthe increasedsize ofthesur-
resultfromCLEANingtoodeeply,i.e.,sincetherewasmore veycoveragearea to concludethat anyoffsetfromthe radio
interference, the rms was higher and hence using the stan- referenceframewas<0.03′′.
dardCLEANlimitwasinappropriate. Forthosedaystheim- With the survey now complete, large optical and infrared
ageswereremadewithashallowerCLEANthreshold,which catalogsof the sky now available, and significantwork over
broughttheCLEANbiasbackintolinewiththenormallevel. the intervening two decades invested in the creation of the
The price of shallower CLEANing for those fields was an International Celestial Reference Frame (ICRF2 – Maetal.
increase in the rms noise levels. Fig. 4 shows the distribu- 2013),wehavetheopportunitytoinvestigatesmallastromet-
tionofthermsnoiselevelsinallFIRST“grid”images(sin- riceffectsinsomedetail. Insubsequentsections,weprovide
glepointings,beforeco-adding),withthermsdistributionfor definitivematchesoftheFIRSTsurvey’sfinalcatalogtothe
imagesonthethreenoisyJVLAdayshighlighted. Themode NVSS,SDSS,and2MASScatalogs;Table2summarizesthe
ofthenoisedistributionis0.132mJy,butthedistributionhas resultsofthosematches,includingthemeanastrometricoff-
atailtowardhigherrmsvalues(mostoftentheresultofbright setsbetweenFIRSTandothercatalogs.
sourcesinthefield). Theextensiontolowerrmsvaluesisthe ThepositionaloffsetforallFIRST-NVSSmatchesinRight
resultofmultiplyobservedfieldsthathavebeencombinedto Ascension is consistent with zero: ∆RA=- 0.009 0.008′′
reducethe noise. The noiselevelsfor JVLA fields observed (avoidingsourceconfusionbyselectingisolatedsour±cesonly,
onthethreebaddaysaretypically2.5timeshigher.Notethat thevalueis∆RA=- 0.011 0.008′′). Thisagreementisun-
theFIRSTcatalogusesthesenoisemeasurementsforallcon- surprising given that both±surveys were conducted for the
tributinggridimagestocomputethermsnoiseasafunction mostpartwiththesameinstrument,butiteliminatesthepos-
ofpositionintheco-addedimages(Whiteetal.1997);conse- sibility that any errors have been introduced by the data re-
quently,thefinalFIRSTcataloghasrmsnoiseestimatesthat ductionprocedures.
correctlyreflecttheelevatednoiseintheseregions. Comparisons with the optical and near-IR catalogs, how-
6 Helfand,White&Becker
1 hr 1 day 1 month 1 yr 5 yrs
1.0
es (a) (b)
urc 66
o 0.8
S
T 55
S
R s]s]
action of FI 00..46 och rms [yroch rms [yr 3434
e Fr EpEp 22
v
ati 0.2
ul 11
m
u
C
0.0 00
0.01 0.10 1.00 10.00 100.00 1000.00 11999955 22000000 22000055 22001100
Epoch rms [days] MMeeaann EEppoocchh [[yyrrss]]
FIG.5.—(a)Cumulativedistributionofobservationepochrangesσ(t)intheFIRSTcatalog. Manysourceshavefluxdensitiesmeasuredoveranintervalof
anhourorless,butsomecomefromcombinedobservationsthatspanyears.(b)Thedistributionofepochrmswiththemeanepochsoftheindividualpointings.
Thesmallarcsfromyear-to-yearoriginateintheoverlappingboundariesbetweenskyareascoveredinconsecutiveobservingseasons,asindividualpointings
fromdifferentyearsarecombinedintheweighted,co-addedimagestoproducearapidlychangingmeanepochwithalargermsvalue. Largerepocharcsand
rmsrangescomefromoverlapsbetweenobservingseasonsthatareseveralyearstodecadesapart;thelongestarcsconnecttothe2009observingseason,which
resumedobservationsofthesouthGalacticcapregionafteralonghiatus.
TABLE2
FIRSTMATCHESWITHEXTERNALCATALOGS
FIRST Subset External Match Match Positionrmsσc Positionmeanoffsetd
Subset Percentagea Catalog Radius Percentageb ∆RA ∆Dec ∆RA ∆Dec
(%) (arcsec) (%) (arcsec) (arcsec) (arcsec) (arcsec)
(1) (2) (3) (4) (5) (6) (7) (8) (9)
all 100.0 NVSS 15 54.9 5.46 5.59 - 0.009±0.008 0.064±0.008
isolatede 73.0 NVSS 15 50.6 4.64 4.89 - 0.011±0.008 0.066±0.008
all 100.0 2MASS 2 8.0 0.42 0.43 0.021±0.002 0.007±0.002
ptf 37.1 2MASS 2 8.3 0.34 0.34 0.020±0.002 0.006±0.002
pt,brightg 4.9 2MASS 2 6.8 0.23 0.23 0.022±0.004 0.009±0.004
all 93.0h SDSSall 2 32.9 0.36 0.37 0.021±0.001 0.016±0.001
all 93.0 SDSSgalaxyi 2 26.5 0.38 0.39 0.021±0.001 0.016±0.001
all 93.0 SDSSstarj 2 6.3 0.26 0.28 0.023±0.001 0.014±0.001
pt 35.3 SDSSall 2 38.2 0.28 0.28 0.020±0.001 0.014±0.001
pt 35.3 SDSSgalaxy 2 29.6 0.30 0.30 0.020±0.001 0.015±0.001
pt 35.3 SDSSstar 2 8.6 0.21 0.22 0.022±0.001 0.012±0.001
pt,bright 4.7 SDSSall 2 41.0 0.17 0.17 0.022±0.001 0.012±0.001
pt,bright 4.7 SDSSgalaxy 2 23.4 0.19 0.19 0.020±0.002 0.013±0.002
pt,bright 4.7 SDSSstar 2 17.7 0.15 0.16 0.025±0.002 0.011±0.002
JVLAk,isolated 4.5 NVSS 15 52.0 4.62 4.99 0.005±0.030 0.179±0.031
JVLA 6.4 2MASS 2 6.7 0.47 0.49 0.000±0.007 - 0.029±0.008
JVLA 99.3 SDSSall 2 28.4 0.40 0.44 0.002±0.003 - 0.009±0.003
aPercentageofFIRSTcatalogsourcesincludedintheselectedsubset.
bPercentageoftheFIRSTsubsetthathasmatchesintheexternalcatalog.
cRMSdifferencebetweenFIRSTandexternalcatalogpositions.ForNVSSthe68.3%percentilewidthofthedistributionisreportedbecause
theNVSSdifferencedistributionishighlynon-Gaussian(Fig.10).
dMeanvalueofFIRSTpositionminusexternalcatalogposition.
eIsolatedFIRSTsourceshavingnoneighborswithin50′′.
fFIRSTpointsourceshavingfittedFWHMmajoraxes<6′′.
gBrightFIRSTsourceshavingpeakfluxdensities>10mJy.
hIncludesonlysourcesinskyareacoveredbySDSSDR10.
iSDSSsourcesclassifiedasgalaxies(type=3).
jSDSSsourcesclassifiedasstars(type=6).
kIncludesonlysourcesfrom2011JVLAobservations.
TheLastofFIRST 7
ever, reveal an apparent discrepancy. In Table 2, the mean oblique decision tree classifier10 (Murthy,Kasif&Salzberg
offsetin RA betweenFIRSTandeither SDSS or 2MASS is 1994) that was trained using a set of sidelobes identified by
consistent with 0.021′′, and the offset is determined with a visualexaminationofsomeFIRSTimages.
verysmalluncertaintyrangingfrom1to4masforthevarious While this approach had some value in identifying poten-
subsets. WenotethatanA-configurationsurveyoftheSDSS tiallyspurioussources,itwas notveryaccurate. Itwaseasy
Stripe82region(Hodgeetal.2011)alsofinds∆RA=0.020′′. tofindcaseswheresidelobeswerenotflaggedorrealsources
Since the offset between the ICRF2 and optical reference were incorrectly flagged. In addition, the use of a binary
frames has been established to be less than 0.003′′ in RA yes/noflag forsidelobeflaggingdidnotprovidemuchguid-
(Orosz&Frey2013;Assafinetal.2013),itappearsthatVLA ance as to the actual likelihood that a source was spurious.
datasufferfroma+20masoffset. Consequently, this catalog entry was considered somewhat
However,usingonlyFIRSTdatatakenwiththeJVLAand unreliableandsawrelativelylittleuse.
matchingtotheSDSScatalog,wefind∆RA=0.002′′ 0.003. Whiteetal.(2005)developedamoresophisticatedanduse-
±
Thetimingsystem,correlator,anddataacquisitionhardware fulvariationonthisalgorithm. Weuseddeepobservationsof
and software are different between the VLA and the JVLA. aportionoftheGalacticplanefromadifferentsurveytode-
Thus,wefindstrongevidencethatpositionsderivedfromthe termineobjectivelywhichsourcesinacatalogwerespurious
oldVLAsystemhaveasystematicoffsetinRAof +20mas (notseeninthemuchdeeperdata)andwhichwerereal(con-
∼
withrespecttotheradioandopticalreferenceframes. firmedinthe deeperdata). Thatproduceda reliable training
Notethatthissystematicerrorisverymuchsmallerthanthe set for the decision tree classifier. We then created multiple
positionaluncertaintiesonsourcesintheFIRSTcatalog;for independentdecisiontreeswhoseoutputwascombinedtoob-
even the brightestsources, the randomposition errorsare tainasidelobeprobabilityestimateforeachsourceratherthan
∼
100masorlarger. Thiserrorissmallenoughtobeirrelevant a simple binary classification. This voting decision tree ap-
exceptinlarge-scalecross-matchesthatcompareastrometric proachisdescribedinmoredetailbyWhiteetal.(2000)and
systems. White(2008).
The declination offsets also show some systematic varia- For the 2008 and later releases of the FIRST catalog, we
tions in Table 2, although different external catalogs have adopted a similar approach to computing sidelobe probabil-
different offsets. The SDSS matches have a typical decli- ities for the FIRST sources. We created a training set of
nation offset of 15 mas, while the 2MASS offset is smaller, 1905sources(including120sidelobes). We matchedFIRST
7mas. TheNVSS-FIRSTdeclinationoffsetstandsinstark with two deep radio surveys, the Spitzer First Look Survey
∼
contrast with a difference of 60 mas. As for Right Ascen- (416FIRST sources, 15 sidelobes; Condonetal. 2003) and
sion, in the JVLA region the remaining declination offsets the COSMOS deep survey pilot area (72 FIRST sources, 6
comparedwith SDSS are consistent with zero. We discount sidelobes;Schinnereretal. 2004)11. We also includeda less
thelargediscrepanciesfromtheNVSSmatch,whichwefind reliablebutlargersamplefromourownsurveyof theDeep-
in §5tohavehighlynon-Gaussianpositiondifferencedistri- range area (1356 FIRST sources, 99 sidelobes; Whiteetal.
butions with long tails. With that caveat, we conclude from 2003). Finally we augmented the set with a sample of 61
the SDSS and 2MASS matches that systematic errorsin the brightsources(F >2 Jy), none of which are sidelobes; this
FIRSTdeclinationsatallepochsareoforder0.02′′orless. improvedtheperformanceoftheclassifierforbrightobjects.
The 15 parametersused for the classification include nine
4. THESIDELOBEFLAGGINGALGORITHM source properties (peak-to-integrated flux ratio, rms noise
TheoriginalversionsoftheFIRSTcatalog(beginningwith level, source major and minor axes compared with the syn-
the1995October16release)includedasidelobewarningflag thesizedbeam,sourcepositionangle,ratioofsourcepeakand
thatindicateda likelihoodthatthe sourceentrywas actually integrated flux densities to the corresponding values in the
a sidelobeof abrightnearbysourceratherthana realobject “island” to which the source belongs12), and six properties
(Whiteetal.1997).Thealgorithmforsettingthisflagusedan of the nearest bright source that could be creating sidelobes
(positionaloffsets, flux ratios, and directions). Ten indepen-
dent oblique decision trees were created using this training
set. Their outputsare combinedas described in Whiteetal.
0.05 (2000) to estimate a sidelobe probability, P(S), for every
FIRSTsource.
0.76
s 0.04 Ahistogramoftheresultingsidelobeprobabilitiesisshown
ce in Figure6. Thevastmajorityofthe sourceshave lowside-
our lobeprobabilities;infact, 70%ofthe objectshavethemini-
s
T 0.03 mumP(S)valueof0.014. Themeansidelobeprobabilityfor
S
R the catalog is 0.097, which is an estimate of the fraction of
of FI 0.02 sidelobesinthecatalog.
n Figure 7 shows the spatial distribution of objects in the
o
cti vicinity of bright radio sources, which frequently give rise
Fra 0.01 to nearby sidelobes. Applying a probability cut P(S)<0.1
(Fig.7b)eliminatesthevastmajorityoftheobvioussidelobe
0.00 10 TheOC1oblique decision tree software isavailable fordownload at
0.0 0.2 0.4 0.6 0.8 1.0 http://www.cbcb.umd.edu/~{}salzberg/announce-oc1.html.
P(S) Sidelobe Probability 11 ThedeeperimagingfromSchinnereretal.(2007)wasnotavailableat
thetime.
FIG.6.—DistributionofsidelobeprobabilityP(S)intheFIRSTcatalog. 12 The islands are rectangular pixel regions surrounding sources as de-
76%ofthesourcesareinthelowestbin,P(S)<0.05. scribedin§3ofWhiteetal.(1997).
8 Helfand,White&Becker
1155 1155
FF((bbrriigghhtt)) >> 330000 mmJJyy FF((bbrriigghhtt)) >> 330000 mmJJyy
P(S) > 0.10 P(S) < 0.10
1100 1100
55 55
n] n]
mi mi
c c
ar ar
c [ 00 c [ 00
e e
D D
a a
elt elt
D D
--55 --55
--1100 --1100
--1155 --1155
--1155 --1100 --55 00 55 1100 1155 --1155 --1100 --55 00 55 1100 1155
(a) Delta RA [arcmin] (b) Delta RA [arcmin]
FIG.7.—Distributionofsidelobesinthevicinityofbrightsources.TheoffsetsofallFIRSTcatalogentriesinthevicinityofsourceswithF(peak)>300mJy
areshown. (a)ThetypicalsnapshotsidelobepatternthatresultsfromthethreearmsoftheVLAisclearlyvisibleinsourceswithhighersidelobeprobabilities,
P(S)>0.1.(b)Thestrongsidelobepatternisalmostabsentforthesourceswithlowsidelobeprobabilities,P(S)<0.1.
doesseparatelikelysidelobesfromothersourcesinthevicin-
0.5 ityofbrightobjects.
F(bright) > 1.0 Jy Werelyonmatchestoexternalcatalogsforquantitativeas-
y 0.4 0.5 < F(bright) < 1.0 Jy sessments of the accuracy of the FIRST sidelobe probabili-
bilit 0.3 < F(bright) < 0.5 Jy ties. The NVSS-FIRST cross-match (§5) provides one ob-
a 0.1 < F(bright) < 0.3 Jy vious test of P(S). It is not definitive, however, because the
b
pro 0.3 2.5mJyNVSSdetectionlimitissignificantlyshallowerthan
e FIRST. The greatmajorityof sourceswith high P(S)values
b
sidelo 0.2 awriethfaPin(tSi)n>th0e.1raidsioo:ntlhye1m.3e5anmpJeya,kanflduxondleynsity4%forosfouthroceses
n objectshavefluxdensitiesabovetheNVSSde∼tectionthresh-
a
Me 0.1 old. ThatmeansthatNVSScan onlybe usedto confirmthe
accuracy of P(S) for bright sources; it does not provide any
Far from bright sources: mean P(S) = 0.068 information about the much more common faint sidelobes.
0.0 Nonetheless,thecomparisonisuseful. Figure9(a)showsthe
0 5 10 15 20 25
NVSSdetectionfractionforFIRSTsourcesasa functionof
Distance from bright source [arcmin]
sidelobeprobability.WeincludeonlyFIRSTsourcesthatare
FIG.8.—VariationofmeansidelobeprobabilityP(S)withdistancefrom brightenough that they oughtto be detected by NVSS. The
brightsources. Thesidelobelikelihoodgenerallydeclineswiththefluxden- detectionfractiondeclinesasexpectedasP(S)increases. The
sityofthebrightsourceandwithdistancefromthebrightsource,asexpected. generaltrendtoward fewer detectionsat higherP(S)is clear
ThedashedlineshowsthemeanP(S)forallthecatalogsourcesthatarenot
and confirms that the sidelobe probabilities are reliable for
within25arcminofasourcebrighterthan100mJy.
brighterFIRSTsources.
Thecross-matchbetweenFIRSTand SDSS(§7) provides
pattern.Theeffectofthebrightsourcecanalsobeseeninthe
amorepowerfultest ofthesidelobeprobabilities. Thatmay
radialdependenceofthemeanP(S)(Fig.8). Theprobability
be surprising since the optical counterparts of most FIRST
islowerintheclosestbinsbecauseofthetendencyofbright
sourcesaretoofainttobedetectedbySDSS;theabsenceofan
radio sourcesto have multiple components(doubles, triples,
opticalcounterpartdoesnotrevealmuchabouttherealityof
etc.); away from the center the probability declines with ra-
anindividualradiosource.However,theSDSSmatchfraction
diusandwiththebrightsourcefluxdensity.
providesan accurate statistical measurementof the sidelobe
The sidelobes are heavily concentrated in the vicinity of
fractiondowntothedetectionlimitoftheFIRSTsurvey.The
brightsources. The 10% ofFIRST catalog sources that fall
complicationin this case is that the fraction of sources with
within 10 arcmin of a bright object (F >100 mJy) have a
opticalcounterpartsdependsontheradiofluxdensity,butthat
mean sidelobe fraction P(S)=0.24 and account for 25% of
variationisrelativelysmoothandcanbeeasilymodeled.
allthesidelobesinthecatalog.Theremaining90%ofFIRST
Figure 9(b) displays the fraction of FIRST sources with
sources(notnearabrightobject)haveamuchlowersidelobe
an SDSS counterpart as a function of the sidelobe proba-
fraction,P(S)=0.08.Weconcludethatthesidelobeprobabil-
bility. The distribution is well-behaved and clearly shows
itybehavesqualitativelyasexpected,andthattheP(S)value
TheLastofFIRST 9
1.0
Isolated FIRST F(peak) > 3.0 mJy
All FIRST-NVSS matches
Isolated FIRST sources
0.8
10000
n
o
h fracti 0.6 (a) matches
c
at of
S m 0.4 ber 1000
S m
V u
N N
0.2
0.0 100
0.4 -20 -10 0 10 20
FIRST-NVSS Delta RA [arcsec]
n
o
cti FIG.10.—ThepositionaloffsetsinRAforallFIRST-NVSSradiosource
a 0.3 matches (in black) and for isolated FIRSTsources defined as there being
Fr onlyonesuchsourcewithin50′′(theNVSSsynthesizedbeamFWHM)ofan
h NVSSsource.Thedistributionishighlysymmetricalandcenteredveryclose
c
at tozero(seeTable2forquantitativedetails). Thedistributionforallsources
M (b) issignificantlywiderassingleNVSSsourcesareoftenresolvedintomultiple
S 0.2 componentsbyFIRST.
S
D
S theexpecteddeclinewith the increasingfractionofspurious
d sources(whichnaturallydonothaveSDSScounterparts). In
e
erv 0.1 thisfigurenocorrectionshavebeenmadeforthedependence
bs of the SDSS match fraction on radio flux density. For Fig-
O
ure9(c),wehavebothcorrectedforthatfluxdependenceand
0.0 alsohaveinvertedthedistribution,usingthecorrectedSDSS
1.0 matchfractionasameasurementofthesidelobeprobability.
Thiscanthenbecompareddirectlywiththecatalogvaluefor
n P(S). Thepointswouldfallalongthediagonallineifthecat-
o
cti 0.8 alogP(S)valueswereperfectpredictorsofthesidelobefrac-
a
Fr tion. The actual sidelobe probabilities appear to be slightly
h higherthanthecatalogestimates,butthecatalogP(S)values
c
at 0.6 doappeartobereasonablyaccurate.
M (c)
S In summary, the sidelobe probabilities P(S) in the FIRST
S cataloggiveausefulmeasureofthelikelihoodthatanysource
D 0.4
S is spurious. The probabilities have been shown to be suf-
m
o ficiently accurate to be useful, and they do a good job of
S) fr 0.2 eliminatingthespuriousdetectionsthattendtoclusteraround
P( brightradiosources.
0.0 5. COMPARISONSWITHNVSS:COMPLETENESSAND
0.0 0.2 0.4 0.6 0.8 1.0 RELIABILITY
FIRST Sidelobe Probability
The NRAO VLA Sky Survey (NVSS — Condonetal.
1998), also conducted at 20 cm, covered over 3π steradians
FIG.9.—TestsoftheaccuracyofthesidelobeprobabilityP(S).(a)FIRST- oftheskynorthofδ=- 40◦toacompletenesslimitoutsideof
NVSSmatchrateasafunctionofP(S). OnlyisolatedFIRSTsourceswith
integrated fluxdensities brighter than3.0mJyareincluded. Forlow P(S) theGalacticplaneofroughly2.5mJy. Thesynthesizedbeam
values,95%oftheFIRSTsourcesaredetectedinNVSS.(b)FIRST-SDSS size was 45′′ leadingto rmspositionaluncertaintiesof 7′′
matchrateasafunctionofP(S).Thedeclineinthematchratewithincreasing for point∼sources at the catalog detection limit, with errors
sidelobeprobabilityisroughlyconsistentwithexpectationsassumingthatthe
fractionofrealsourcesineachbinisproportionalto1- P(S).Thedistribution forbrightersourcesdecreasinginverselywithfluxdensityto
isnotexpectedtobeexactlylinearsincetheradiofluxdensitydistribution 1′′forthebrightestsources13. Thelargebeamsizeallowed
varieswithP(S),andtheobservedSDSSmatchratedependsonradioflux ∼thedetectionofextended,low-surface-brightnessobjectsthat
(Fig.16). (c)SidelobeprobabilityesimatedusingtheSDSSmatchratesin canberesolvedoutbythehigh-resolution(5′′)FIRSTbeam;
(abfi).ttTohteheefdfiescttriobfutviaorniaitnioFnigo.f1m6.aTtchherpaoteinwtsitwhoraudldiolifleuaxlowngasthceordrieacgtoendaulsliinnge in addition, for sources with angular sizes between 10′′
ifthecatalogP(S)valueswereperfectpredictorsofthesidelobefraction.The and60′′, theFIRSTfluxdensitiesunderestimatethe tr∼uein-
sidelobeprobabilityP(S)quotedinthecatalog(x-axis)appearstobeslightly tegratedsourceintensity.
underestimatedformoderateprobabilities(0.1<P(S)<0.6).
In Figure 10 we show the positional offsets in RA for all
FIRST-NVSS radio source matches (in black) and for all
isolated FIRST sources defined such that there is only one
13 Butsee§8.3foradiscussionofmorerealisticpositionaluncertainties
foropticalmatching.
10 Helfand,White&Becker
11..44 Sources not in JVLA area (Dec < 15o) 1.4 Sources in JVLA area
11..22 1.2
FF F
S S 11..00 S 1.0
SS S
VV V
NN N
nt/nt/ 00..88 nt/ 0.8
FiFi Fi
T T 00..66 T 0.6
SS S
RR R
FIFI 00..44 FI 0.4
00..22 0.2
(a) (b)
00..00 0.0
00..0011 00..1100 11..0000 0.01 0.10 1.00
NNVVSSSS FFlluuxx DDeennssiittyy [[JJyy]] NVSS Flux Density [Jy]
FIG.11.—TheratioofFIRSTsourcefluxdensitytoNVSSfluxdensityasafunctionofNVSSfluxdensity(a)forallmatchingsourcesatdeclinations<15◦
and(b)forsourcesobservedin2011exclusivelywiththeJVLA.ThebrighterofthepeakorintegratedFIRSTfluxdensityisused.Thebluebarshowsaratioof
unity,whilethereddotsshowthemedianvaluesforthebinsindicatedbythehorizontalerrorbars(verticalerrorbarsaremainlysmallerthanthepoints). The
2.0mJycutoffontheleftistheNVSSthreshold;thecurvingcutoffinthelowerleftrepresentstheFIRSTthresholdof1.0mJy.
suchsourcewithin50′′oftheNVSSsourceposition(roughly theNVSSthreshold;thecurvingcutoffinthelowerleftrep-
equaltotheNVSSsynthesizedbeamFWHM).Thedistribu- resentstheFIRSTthresholdof1.0mJy. The 1- 5%deficit
∼
tionishighlysymmetricalandcenteredveryclosetozero(see inFIRST flux density between 2.0 and 20 mJy is likely the
Table 2 for quantitative details); the distribution of offsets result of diffuse flux from extended sources resolved out by
in declination is indistinguishable from the RA distribution. theFIRSTbeam;theturn-upforthelowesttwofluxdensity
Thedistributionforallsourcesissignificantlywiderassingle pointsarisesfromtheimpositionofthe1mJyFIRSTthresh-
NVSSsourcesareoftenresolvedintomultiplecomponentsby oldthatbiasesthedistributionupward.Theriseaboveunityat
FIRST.Bothdistributionsaredistinctlynon-Gaussian.Thus, NVSSfluxdensities>50mJycouldarisefromacalibration
ratherthanquoteanrmsinTable2,werecordthe68.3%per- offsetof 2%plusa(possiblydominant)contributionfrom
∼
centileoftheabsolutevaluedistribution,whichistheequiva- the different bandwidths used in the two surveys convolved
lentofthermsforaGaussiandistribution. withthesourcespectralindexdistribution.
InFigure11(a)weplottheratioofFIRSTsourcefluxden- Figure 11(b) displays the same plot as Figure 11(a) for
sity to NVSS flux density for all matching sources at decli- sourcesobservedwiththeJVLAin2011.FortheJVLAdata,
nations <15◦ (imposed for comparison with Fig. 11b) as a the agreement with NVSS fluxes is actually somewhat bet-
functionofNVSSfluxdensity.Thebrighterofthepeakorin- ter for flux densities greater than 4 mJy. In Figure 12, we
tegratedFIRSTfluxdensityisused. Thebluebarshowsara- plot the ratio of the flux density offsets (FIRST divided by
tioofunity,whilethereddotsshowthemedianvaluesforthe NVSS) between FIRST data taken with the VLA and with
binsindicatedbythehorizontalerrorbars(verticalerrorbars the JVLA. All data are for sources with δ < 15◦ to elimi-
aresmallerthanthepoints). The2.0mJycutoffontheleftis
1.0
1.15
0.8
n
o
1.10 cti
x a
A flu 1.05 on fr 0.6
VL cti All FIRST sources
J e
A flux / non- 01..9050 NVSS det 00..24 NVIsSoSla dteedte FcItRioSn Tth sroeushrcoelds
L
V
J
0.90
0.0
1 10 100 1000
0.85 FIRST F(int) [mJy]
0.01 0.10 1.00
NVSS Flux Density [Jy] FIG.13.—FractionofFIRSTsourcesthataredetectedbyNVSSasafunc-
tionoftheFIRSTintegratedfluxdensityusinga15′′matchingradius. The
FIG.12.—Theratioofthefluxdensityoffsets(FIRSTdividedbyNVSS) verticaldashedlineshowstheNVSS2.5mJydetectionlimit.Theblackdot-
betweenFIRSTdata taken with the VLA and the JVLA.All data are for tedlineshowsthedistributionforallFIRSTsources. Thesmallfractionof
sourceswithδ<15◦toeliminatedeclinationeffects.Thehorizontaldashed brightunmatchedsourcesareextendedobjectsthatareresolvedintomultiple
lineat+3.3%representstheexpectedoffsetforsourceswithameanspectral components byFIRST.Thatisdemonstrated clearly bythesolidredline,
indexofα=- 0.7,giventhedifferenceineffectivefrequencyforthetwoset whichshowsthedistributionforisolatedFIRSTsources(havingnoneigh-
ofobservations. borswithin50′′).