Search for Compact Extragalactic Radio Sources Near Massive(4)

We have used the Very Large Array to search for compact milliarcsecond-size radio sources near methanol masers in high-mass star-forming regions. Such sources are required for Very Long Baseline Interferometry phase-referencing observations. We conducted p

acompactcomponentthatcouldbeusedasaphase-referencecalibratoratlowerfrequencies,wealsoretaintheminTable4.

Already,source023016+620937inTable4hasproventobeagoodcalibratorforVeryLongBaselineArrayobservations(VLBA;Xuetal.2006).Thissourceprobablyhasapositionuncertaintyofaboutafewmilliarcseconds(mas).Usingthisandtwoothersourceswehavemeasuredaparallax(totheW3OHmasers)withanaccuracyof0.007mas.ThreeofoursourcesareinclucedinthethirdextensiontotheVeryLongBaselineArray(VLBI)CalibratorSurvey(VCS3;Petrovetal.2005)listedtherewith(sub)masaccuracies.Bycomparingtheresultsfromdi erentobservationslistedinTable5we ndthatourC-arraypositionsdeviatefromtheVCS3positionsby0.25,0.91,and0.14arcsecandourA-arraypositionsby51and11mas.We,thus,estimatethatthepositionuncertaintyisabout0.5′′insnap-shotC-con gurationobservationsand0.05′′–0.15′′inVLAA-andB-con gurationobservations,respectively,forhighdeclinationsources.Positionalaccuracyislikelytodegradeconsiderablyatlowdeclinationsandforweaksources(lessthan10mJy).Wecalculatedthetwo-pointspectralindexfrom5to15GHzfromC-con gurationobservations.MostofthesourcesinTable4haveasteep-spectrumwithanaveragespectralindexofα=0.96(Sν～ν α).OnemustkeepinmindthatsincethesewereoftenmeasuredwithobservationswithinoneVLAcon guration,the5GHzbeamsizewasaboutafactorof10largerinsolidanglethanthe15GHzbeamsize.Thus,ifasourceisunresolvedat5GHz,butpartiallyresolvedat15GHz,thespectralindexmayappearoverlysteep.Ulvestadetal.(1981)foundamedianspectralindexofα=0.09between1.4and5GHzforunresolvedsourcesina ux-limitedsample.Zhang&Fan(2003)classi edthissamplebypropertiesofgalaxies.Theyfoundanaveragespectralindexof0.22forblazers,0.28forQSOs,and1.01forgalaxies,respectively.Asacomparison,weobtainedanaveragespectralindexofα=0.83between1.4and5GHz.Ofcourse,sincebothobservationsspannedmanyyears,timevariabilitycould,inprinciple,a ectspectralindexresults.Howevergeneralspeaking,mostofsourcesarelikelytoberadiogalaxies.

4.Conclusions

AVLAsearchhasresultedintheidenti cationof92compactsourcesmostlikelyofextragalacticnature.Thesesourcesarelikelytobesuitablecalibratorsforphase-referencingVLBIobservationsof12.2GHzCH3OHor/andlowerfrequencymasers.