Infrared Properties of Star Forming Dwarf Galaxies Blue Compact Dwarfs in the Virgo Cluster

http://aries.phys.yorku.ca/∼ovidiuv/Paper3.ps.gz(5MB)

InfraredPropertiesofStarFormingDwarfGalaxies:II.BlueCompactDwarfGalaxiesintheVirgoCluster1

arXiv:astro-ph/0510325v1 11 Oct 2005Vaduvescu,Ovidiu

YorkUniversity,DepartmentofPhysicsandAstronomy4700KeeleStreet,M3J1P3,Toronto,ON,Canada

email:ovidiuv@yorku.ca

Richer,MichaelG.

ObservatorioAstronomicoNacional,InstitutodeAstronomia,UNAM,

POBox439027,SanDiego,CA92143-9027,USA

email:richer@astrosen.unam.mx

McCall,MarshallL.

YorkUniversity,DepartmentofPhysicsandAstronomy4700KeeleStreet,M3J1P3,Toronto,ON,Canada

email:mccall@yorku.ca

ABSTRACT

Asampleof16bluecompactdwarfgalaxies(BCDs)intheVirgoClusterhasbeenimagedinthenear-infrared(NIR)inJandKsonthe2.1mtelescopeatOAN-SPMinMexico.IsophotesasfaintasµJ=24magarcsec−2andµKs=23magarcsec−2havebeenreachedinmostofthetargets.Surfacebrightnessprofilescanbefittedacrossthewholerangeofradiibythesumoftwocompo-nents:ahyperbolicsecant(sech)function,whichisknowntofitthelightprofilesofdIs,andaGaussiancomponent,whichquantifiesthestarburstnearthecen-tre.IsophotalandtotalfittedNIRmagnitudeshavebeencalculated,alongwithsemimajoraxesatµJ=23magarcsec−2andµKs=22magarcsec−2.Thediffuseunderlyingcomponentandtheyoungstarbursthavebeenquantifiedusingtheprofilefitting.Mostcolorprofilesshowaconstantcolor,betweenJ−Ks=0.7to0.9mag.ThediffusecomponentrepresentstheoverwhelmingmajorityoftheNIRlightformostBCDs,thestarburstenhancingthefluxbylessthanabout0.3mag.Linearcorrelationswerefoundbetweenthesechscalelengthandthe

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sechmagnitude,andbetweenthesechsemimajoraxisandthesechmagnitude.Overall,galaxieswithmoreluminousdiffusecomponentsarelargerandbrighterinthecentre.Thecentralburstcorrelateswiththediffusecomponent,withbrighterBCDshavingstrongerstar-bursts,suggestingthatmoremassiveobjectsareformingstarsmoreefficiently.BCDslieonthe“fundamentalplane”definedbydwarfirregulars(dISs)inPaperI,followingthesamerelationbetweensechabsolutemagnitude,sechcentralsurfacebrightness,andthehydrogenline-widthW20,althoughthescatterislargerthanforthedIs.Ontheotherhand,correla-tionsbetweenthesechabsolutemagnitudeandthesechcentralsurfacebrightnessinKsforBCDsanddIsareequallygood,indicatingthatBCDlinewidthsmaybeenhancedbyturbulenceorwinds.

Subjectheadings:galaxies:bluecompactdwarfs;near-infrared;surfacebright-nessprofiles;directimaging;photometry;

1.Introduction

AccordingtoKunth&Ostlin(2000),theconceptof“compactgalaxies”wasintroducedbyZwicky(1965),inordertorefertogalaxiesbarelydistinguishablefromstarsonthePalomarSkySurveyplates.Inasubsequentpaper,Zwicky(1970)refinedthedefinitionofa“compactgalaxy”tobeanygalaxy(orpartofagalaxy)whosesurfacebrightnessisbrighterthan20magarcsec−2inanychosenwavelengthrange(DePaz,MadoreandPevunova2003).Theterm“blue”,asusedbyZwicky(1970),referstothosegalaxiessatisfyingthepreviousconditiononbothblueandredplates(Zwicky&Zwicky1971).Lateron,Thuan&Martin(1981)introducedtheterm“bluecompactdwarf”(BCD)referringtothosecompactgalaxiescharacterizedbythreemainproperties:lowluminosity(MB󰀂−18),smallsizes(diameterslessthan1kpc),andstrongnarrowemissionlinessuperposedonanearlyflatcontinuum,similartoHIIregionsinspiralgalaxies.Astheirnamesuggests,thevisiblecolorsofBCDsappeartobeblue.ThestarformingratesinBCDsarehugeincomparisonwiththoseindIs(between0.1and1M⊙yr−1,e.g.Fanelli,O’ConnellandThuan1988),andmetallicitiesarelow(oxygenabundancebetweenZ⊙/50andZ⊙/2,e.g.Kunth&Ostlin2000),suggestingyoungchemicalages.

Thuan&Martin(1981)undertookthefirstsystematicapproachtostudyBCDs,ob-servingHIfluxesforalargesample(115targets)forwhichtheyderivedHIprofiles,neutral

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hydrogenmassesandtotalmasses.UsingtheIRTF3mtelescope,Thuan(1983)observedinthenear-inrared(NIR)asmallerBCDsample(36targets),derivingacolor-metallicityrelationandawelldefinedcolor-luminosityrelationwhichsuggestedthatstarformationprocessesareintimatelylinkedtototalmasses.

James(1994)pioneeredNIRsurfacephotometry,observing13dwarfsinVirgoincludingafewBCDs.TheauthorconcludedthattheNIRcoloursoftheBCDsareparadoxicallyred,possiblyindicatinganintermediate-agestellarpopulation.NosimpleevolutionarylinkbetweenBCDs,dIsanddEsinthesamplecouldbediscerned.

Usinga3-componentdecompositionschemeappliedtothesurfacebrightnessprofilesof12BCDs,Papaderosetal.(1996)foundthatthesizeandluminosityofthepurportedstarburstcomponentdependsupontheluminosityoftheunderlyingcomponent,aswellasontheHImassoftheBCD.ComparingthestructuralpropertiesofBCDswiththoseofothertypesofdwarfs,theauthorsfoundthatforthesameBluminosity,theunderlyingcomponentofaBCDhasacentralbrightnessbrighterby∼1.5magandanexponentialscalelengthsmallerbyafactorof∼2thanthatofdIsanddEs.

Employingsomelargersamples,otherauthorshavederivedtotalmagnitudes,surfacebrightnessprofilesandprofiledecompositionsinthevisible(Doublieretal.1997;Doublier,Caulet,&Comte1999;Cairosetal.2001)andmorerecentlyintheNIR(Bosellietal.1997;Gavazzi,etal.2000;Cairosetal.2003;Noeskeetal.2003,2004).Baseduponasmallsample(12targets)observedintheNIR,Doublier,Caulet,&Comte(2001)foundasystematicexcessoflightintheKbandwithrespecttoHandJandtheirpreviousobservationsinthevisible,indicatingthatastellarpopulationofredgiantsdominatestheflux.Moreover,comparisonsofthemetallicity-colorrelationsofBCDsandglobularclustersshowverylittledifference,indicatingthatBCDsaremostprobablyold,incontradictionwithpreviousresults.Employingasmallsample(9targets)observedintheNIR,Cairosetal.(2003)foundthatthemorphologyofBCDsisbasicallythesameasintheoptical.UsingHSTandground-basedopticalandNIRdata,Papaderosetal.(2002)investi-gatedIZw18,knowntobetheleastchemicallyevolvedstar-formingBCDgalaxyinthelocalUniverse,withanoxygenabundanceof12+log(O/H)∼7.2,or1/50oftheSun(Searle&Sargent1972;Izotov&Thuan1999;Kunth&Ostlin2000).Intheoptical,itsexponentialprofileisnotduetoanevolvedstellardiscunderlyingitsstar-formationregions,butrather,duetoextendedionizedgasemission.Theunresolvedstellarcomponent,thoughverycom-pact,isnotexceptionalwithrespecttostructuralpropertiesforintrinsicallyfaintdwarfs.However,itdiffersstrikinglyfromtheredlowsurfacebrightnesshostofstandardBCDs.Also,unlikemoreevolvedBCDs,thestellarcomponentofIZw18ismuchmorecompactthantheionizedgasenvelope.

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Despitethreedecadesofstudyatvariouswavelengths,atleasttwobasicquestionsremainopenaboutBCDs,namelytheiragesandpossibleevolutionarylinksbetweenBCDs,dIsanddEs.Inparticular,howtodecouplethediffuseunderlyingcomponentfromtheyoungstarburstregionsremainsafundamentalprobleminstudyingdwarfgalaxies.ThesolutioniseasiertoachieveintheNIR,whichhastheadvantageofbettertracingthediffusecomponentofBCDs,whosefluxinthevisibleisoverwhelmedbyyoungstarbursts.Aboutfouryearsagowestartedalong-termresearchprogrammetoinvestigatetheNIRpropertiesofdIsandBCDs(Vaduvescu2005).Thefirstpaperofthisstudy,hereafterPaperI,addressedthedIs(Vaduvescuetal.2005).Basedupondeepdataderivedfromasampleof34fielddIsobservedatCFHT,wehaverecentlyfoundanuniversallawfittingtheirNIRsurfacebrightnessprofiles.WealsofoundthatthesedatapermitthedefinitionofafundamentalplaneforthedIs(PaperI).ThesecondpaperofthisstudyapproachestheBCDs.Tothisend,wehaveacquiredNIRdeepimagesof16BCDsintheVirgocluster.UsingthesedataandourinsightsaboutdIs,weemploysurfacephotometrytoaddresstheNIRmorphologyofBCDsinanattempttoprovidenewinsightregardingpossibleevolutionarylinksbetweendIsandBCDs.

TheorganizationofthepresentpaperfollowstheoneofPaperI.In§2wepresentthesampleselectioncriteria,andin§3ourobservations.Theimagereductionmethodispresentedin§4.Surfacephotometrywillbeaddressedin§5.Photometricdataareusedin§6tostudytheNIRpropertiesofBCDs.Conclusionsarepresentedin§7.

2.TheSample

TheBCDcandidateswereselectedaccordingtothefollowingcriteria:

1.commondistance,i.e.membersoftheVirgoClusterasdefinedbytheVirgoClus-terCatalog(VCC;Binggeli,SandageandTammann1985;Binggeli,TammannandSandage1987;Binggeli,PopescuandTammann1993);2.classifiedasBCDordI/BCD,accordingtotheliterature(i.e.,NED);

3.availabiltyofoxygenabundances,preferablyfromthe[OIII]λ4363emissionline;4.low-densityenvironment,asdelineatedbytheX-raymapprovidedbytheROSATAll-Sky-Survey(Lee&Schindler2001);5.absolutemagnitudebetween−12󰀁MB󰀁−16.

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ByrestrictingtargetstotheVirgoCluster,wehopedtoreducescatterindistance-dependentparameters.Byminimizingthedensityofintergalacticgas,wewouldavoidperturbationswhichmightbecausedbyram-pressurestripping(Lee,McCallandRicher2003).Finally,byrestrictingtheluminosityrange,wehopedtobeabletomakeastatisticallyrelevantcomparisonwithdIs.Weidentified16BCDswhichsatisfytheseconditions.TheyarelistedinTable2.

InPaperI,itwasshownthatNIRsurfacebrightnessprofilesofdIsflattentowardthecentre.However,twonearbygalaxiesclassifiedasdIs,namelyNGC1569andNGC3738,displaycentrally-peakedsurfacebrightnessprofiles.NGC1569,whichhasadistancemodu-lusofDM=26.37(PaperI),iswellknownforitscentralstarburstactivity(e.g.,Greggio,etal.1998;Aloisi,etal.2001).NGC3738,whichislocatedintheCanesVenaticiIcloudatDM=29.27,alsohasbeenpreviouslysuggestedtoharbourstochasticstarformationprocesses(Hunter,GallagherandRautenkranz1990).DuetotheirNIRfluxexcessesatsmallradii,weaddNGC1569andNGC3738totheVirgoBCDsampleinthispaper.WewillshowthattheirsurfacebrightnessprofilessuggestthattheyareBCDs,notdIs.

3.Observations

Between2001and2004wehad4observingruns(21nightsintotal)onthe2.1mtelescopeattheObservatorioAstron´omicoNacionalintheSierraSanPedroM´artir(OAN-SPM)inBajaCalifornia,Mexico.TheprimaryobjectiveoftherunswastoobtaindeepimagingofasampleofVirgoBCDsintheNIR,allofwhichhaveknownorsoon-to-be-knownmetallicities.

AllgalaxieswereobservedinJandKsbandsusingtheCAMILANIRcameraatthef/13focusofthe2.1mtelescope.CAMILAisequippedwithaNICMOS3256×256pixelarray(Cruz-Gonzalesetal.1994).Thescalewas0′′.85pix−1,yieldingafieldofviewFOV′′3.6×3.6.TheweatherduringFeb-MaratOAN-SPMwasunstablethroughoutthefourruns,withclearskieslessthanhalftheallocatedtimeandonlyonenightwithphotometricconditions.Mostoftheallocatedobservingtimewasbright(Moonwithin5daysoffull),butthisislessimportantwhenobservingintheNIR,asmoonlightbrightenstheskybyabout0.3magarcsec−2inKsandabout0.9magarcsec−2inJ2.

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Duringthefourruns,weimagedinJandKs16BCDsinVirgo.Wetookthedatastartingaroundlocaltime23hwhentheairmasswasbelow∼1.8.Becauseofcirruscloudsmostofthetime,variableexposuretimesof20–70mininKsand15–40mininJwereusedinordertoreachµKs=23magarcsec−2andµJ=24magarcsec−2.Theseexposureswereobtainedbycombiningsequencesofone-minuteframesinbothKsandJ.Theseoneminutesub-exposureswereacquiredunguided.Duringthefirstrun,wecycledthetargets(VCC24andVCC324inKs)throughthefourquadrantsofthechip.Duringthenextthreeruns,wesampledtheskyhalfasoftenasthetargetsinregionschosenrandomlytobe4to5arcminawayfromthegalaxies.Eachgalaxyframewasditheredbyabout3′′.Inshort,weadoptedthefollowingobservingsequence:

sky−target−target−sky−target−target−sky−...−sky−target−target−sky(1)WepresenttheobservinglogforallfourrunsinTable2.Inthelastcolumnweincludethefullwidthathalfmaximum(FWHM),measuredonafewstarsineachreducedimage.WeobservedNGC1569andNGC3738ontheCanada-France-HawaiiTelescope(CFHT)inMar2002,usingexposurestunedtomatchthesamesurfacebrightnesslimitasourOAN-SPMVirgoobservations.DetailsoftheobservationsanddatareductionofthesetwogalaxiesaregiveninPaperI.

4.DataReductions

Onebadpixelmapwasbuiltforeachrunusingflatfieldimagestakenwithtwodifferentexposuretimes.Itwasappliedasthefirststepinthedatareductionprocesstoallourrawimages(usingtheBADPIXtaskofIRAF).

Foreachfilter,aflatfieldwasbuiltforeachrunfromtwoseriesoftwilightskyimagestakenwithlongandshortexposuretimes.Thelow-signalflatwassubtractedfromthehigh-signalflatinordertoproducethefinalflatfieldimage.

RemovalofthebackgroundrepresentsthemostimportantstepinNIRobservations,es-peciallywhenobservingtheouterregionsoffaintgalaxies(e.g.,Vaduvescu&McCall2004).Inordertoremovethevariableairglowandthermalcontributions,theskyframeobservedclosetoeachscienceframewassubtractedfromthescienceframe.Thesky-subtractedgalaxyframesweredividedbytheflatfield,thenleveledadditivelybyforcingthecornerstozero.Finally,theywerecombinedusingtheIMCOMBINEtaskinmedianmode,whicheliminatedthehotpixels,cosmicrays,andnegativeresidualsthatappearinthereducedframesafterskysubtraction.

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ThecollectionofIRAFscriptscreatedtoreduceourdataisavailableonline3.ThereducedJ-bandimagesofthe16VirgoBCDsobservedarepresentedintheleftpanelofFig.7.

Throughoutourruns,weplannedtoobservePerssonreferencestars(Perssonetal.1998)tocalibrateourobservations.Asonlyonenightturnedouttobephotometric,wederivedthezero-pointforeachreducedimageusing3to122MASSstarsappearingineachfield.Formostimages,weestimatethezero-pointerrorstobelessthan0.05mag,withthemaximumreachingabout0.2maginthreesparselypopulatedcases:VCC641(includingonlyone2MASSstarinthefield),andVCC144andVCC1313(withonlytwo2MASSstarseach).

5.SurfacePhotometry

Duetotherelativelysmalltelescopeapertureandpoorspatialresolution,itwasnotpossibletoresolvestarsatthedistanceofVirgo.WeemployedthetaskELLIPSEoftheSTSDASpackageunderIRAFtoperformsurfacephotometry.KsobservationsweremoreaffectedthanJbythepoorweatherconditions(highhumidityandcirrus),sowepreferredtouseimagesinJtodeterminetheellipsefittingparameters.

WeusedtheELLIPSEtaskintwostages.First,weapproximatedtheinitialellipsecentres,ellipticitiesandpositionangles,butallowedthemtovaryfreelywithradiusduringthefitting.ThenweplottedthefitsusingISOPALLtoanalyzetheouterisophoteswheretheunderlyingdiffusecomponentisexpectedtoreflectthegeometryofthegalaxy.Atlargeradii(e.g.,closetothesemimajoraxisgivenbyNED4),thefittingparameterscanberegardedasconstant,noneofthemshowingisophotaltwistsortidaldistortionsintheouterregions.Usingthesevalues,inthesecondstagewefixedthecentre,ellipticity,andthepositionangleandrepeatedthefittingprocessforbothbandpasses.Settingthefittingparameterstobeconstantisamorerobustapproachtomodelagalaxy’sbrightnessinitsouterregions.InTable1,weincludethemeasuredeccentricitiesandpositionanglesofourtargets.

InFig.7,theuppergraphsintherightpanelbesideeachgalaxypresentsurfacebright-nessprofilesinKsandJfortheBCDsobservedatOAN-SPM.TheprofilesofNGC1569andNGC3738areincludedinthelasttwopanels.Theformaluncertaintiesinthesurface

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photometryasgivenbyELLIPSEareplottedaserrorbars.Mostoftheerrorsarelessthan0.1magarcsec−2.EllipticityerrorslistedbyELLIPSEarelessthan0.05inmostcases,andpositionanglesareuncertainbyabout2degrees.Forthefaintestorthemostcompacttar-gets(VCC428,VCC1313),weestimatethaterrorsreach0.1inellipticityand5-10degreesinpositionangle.

5.1.IsophotalMagnitudes

UsingtheISOPLOTtaskofSTSDAS,wehavemeasuredtheisophotalmagnitude(mI)ofeachBCDfromthetotalfluxintegratedouttothefaintestvisibleisophote,i.e.,aboutµKs=23magarcsec−2orµJ=24magarcsec−2.WeincludetheisophotalmagnitudesinTable4.Theerrorsintheisophotalmagnitudesaredeterminedmainlybythezeropoints,whosetypicaluncertaintiesarelessthan0.1mag(see§4).

The2MASS5extendedsourcecatalogincludes8BCDsincommonwithoursample.Also,Goldmine6(Gavazzi,etal.2003)listsNIRdatafor9BCDsobservedbyus.InTable3wecomparethetotalmagnitudesfromthesesourceswithourown.Inmostcases,both2MASSandGOLDMinefluxesarebelowourfluxes,likelyduetoourfainterdetection.Thedifferenceislargerfor2MASS,asmuchas1magforVCC848(forwhichGOLDMineKsdataismuchclosertoourresult).

5.2.

Theexponentiallaw,

ProfileDecomposition

I=I0exp(−r/r0)

andthedeVaucouleurslaw,

󰀆󰀆1

I=Ieexp−7.67(r/re)

(2)

56

2MASSGATORCataloghttp://irsa.ipac.caltech.edu/applications/Gator

GOLDMine-GalaxyOnlineDatabaseMilanoNetworkhttp://goldmine.min.infn.it

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James(1994)fittedanexponentialinordertomodeltheluminosityprofilesoffiveVirgoBCDsobservedintheNIR,derivingexponentialscalelengthsandextrapolatedcen-tralsurfacebrightnesses.Analysingasampleof44BCDsobservedinthevisible,Doublieretal.(1997)andDoublier,Caulet,&Comte(1999)foundthat25%oftheSBPsfitapureexponentialprofile,18%haveadominatingexponentialcomponent,20%followapuredeVaucouleursprofile,27%presentadominantdeVaucouleursdistribution,andtheremain-ing7%areunclassifiable.Usingasubsampleof12BCDsobservedinthenear-infrared,Doublier,Caulet,&Comte(2001)concludethattheNIRlightdistributionsaregenerallyconsistentwiththoseintheoptical,althoughinsomepeculiarcasesthebrightnessprofilesdiffersignificantly.

Acombinationoftwoorthreedifferentmodelshasbeenemployedbysomeauthorstoimprovethefitstothesurfacebrightnessprofiles.Gavazzi,etal.(2000)usedtheexponentiallaw,thedeVaucouleurslaw,oracombinationofthetwoinordertofitthelightprofilesofdIsandBCDsobservedinHandKsinfivenearbyclusters.Usingobservationsinthevisibleof12BCDsand2otherbrightstarburstgalaxies,Papaderosetal.(1996)decomposedprofilesintothreecomponents:anexponentialatlargeradii,aplateauatintermediateradii,andaGaussianatsmallradii,withthelasttwosupposedlydescribingthecurrentstarburstsuperimposedontheolderstellarcomponent.

Cairosetal.(2001)performeddeepbroadbandobservationsinthevisibleonasampleof28BCDs.Over70%ofthegalaxiesshowedcomplexprofilesthatprecludedfittingwithasinglestandardlaw(exponentialordeVaucouleurs),withextrastructureathightoin-termediateintensitylevels.Usingasubsampleof9BCDs,Cairosetal.(2003)performedNIRsurfacephotometry,findingthatthemorphologyisbasicallythesameasinthevisi-ble,withtheinnerregionsdominatedbythestarburstcomponent.FitsofSersiclawswereverysensitivetotheselectedradialinterval.Fittinganexponentialmodelgavemorestableresults.

Noeskeetal.(2003)andNoeskeetal.(2004)tookdeepNIR(J,HandKs)imagesofasampleof23fieldBCDswiththeCalarAlto3.6mtelescopeintheNorthandtheESO3.6mNTTintheSouth.Toperformthesurfacephotometry,theauthorsemployedacomplicatedmethod(Papaderosetal.2002)whichusesisophotalmasksdefining“polygonalapertures”whichfollowtheflux.Thismethodissensitivetotheskynoiseatlargeradii,totheimperfectmanualcleaningofthefieldstars,andalsotothestarburstregionatsmallradiiwheremostoftheSBPsareseentobrightenabruptly.Nevertheless,exponentialfittingwasfoundtoapproximatewelltheintensitydistributionatlargeradii.ThesurfacebrightnessprofilesofsomeBCDsshowacentralflattening,theso-called“typeV”profile,followingthenomenclatureofBinggeli&Cameron(1991).Todescribethisdistribution,theauthors

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employedthe“modifiedexponentialdistribution”-themedlaw(Papaderosetal.1996a)-acomplicatedfunctioninvolvingtwomoreconstants,bandq,additionaltothecentralintensity,I0,andtheexponentialscalelength,α.However,aSersiclawwasfoundtomodeltheflatteningatsmallradii,too,althoughapureormodifiedexponentialformulamaybepreferredtofittheunderlyinglowsurfacebrightnesscomponentofBCDs.

Asmentionedpreviously,wederivedoursurfacebrightnessprofilesbyfixingtheellipseparameters.TyingtheprofilestothegeometryofthesmoothextendedcomponentinsteadofthelightaffectedbytheoutburstregionisamorerobustapproachtostudyingwhatunderliesBCDs.WepresentourprofilesintherightpanelsofFig.7.Mostgalaxiesshowanexponentialprofileintheouterregions,extendingaslowinsurfacebrightnessasµKs=23magarcsec−2andµJ=24magarcsec−2.Atsmallradii,superposedontopoftheexponential,mostprofilesshowsomeexcessoveranexponentialduetothestarburst.Profilesoftenappeartoleveloffatradiiveryclosetozero.ThesetrendshavebeenfoundpreviouslyindIs,suggestingsomestructurallinkswithprobableevolutionaryimplicationsforthetwotypesofdwarfs(Papaderosetal.1996).

AsshowninPaperI,theNIRsurfacebrightnessprofilesofdIscanbemodeledusingthefollowinghyperbolicsecant(sech)law:

IS=I0Ssech(r/r0S)=

I0S

er/r0S+e−r/r0S

(4)

HereISrepresentsthefittedsechfluxatradiusr,thedistancefromthecentrealongthesemimajoraxis.I0Sisthesechcentralsurfacebrightness(m0Sinmagnitudeunits),andr0Srepresentsthesechscalelength.

WeattemptedtofittheprofilesofBCDsusingEquation4incombinationwiththefollowingGaussiancomponenttomodelthestarburst:

󰀊

1

r0G

󰀉2󰀂

IG=I0Gexp−(5)

HereIGrepresentsthefittedGaussianfluxatradiusr,whileI0GistheGaussiancentralsurfacebrightness(m0Ginmagnitudeunits),andr0GrepresentstheGaussianscalelength.Thetotalfittedfluxatradiusrcanbeexpressedasthesumofthetwocomponents:

IT=IS+IG

(6)

Integratedoverallradii,Equation4definesthesechmagnitude,mS,whichweinterpretasrepresentativeofthediffuseunderlyingcomponent.Towardthecentre,thesechprofile

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flattensandlevelsoutatthesechcentralsurfacebrightness,µ0S.TheintegralofEquation5determinesthemagnitudeoftheoutburst,mG.

ToperformthefittingoftheprofilesgeneratedbyELLIPSE,weemployedthetaskNFIT1D(intheFITTINGpackageofSTSDAS),enteringEquation(6)asouruserspeci-fiedfunctionfortheUSERPAR/FUNCTIONparameter.Fittedsurfacebrightnesseswereconvertedintomagnitudeunits(magarcsec−2)usingthezero-pointsfortheframes.ThefitstooursurfacebrightnessprofilesinJandKsareshownontheuppergraphsintherightpanelsofFig.7.NGC1569andNGC3738areincludedinthelasttwopanels.ForKsonly,thetwocomponentsareplottedindividuallyaswell,withadashedlineforthesechcomponentandwithadottedlinefortheGaussian.Thefits(solidlinespassingthroughthepoints)matchtheprofilesveryaccuratelyfromthecoretothefarthestregions.Thesechcomponentmatchesthetotalatlargeradii(wherethesolidandthedashedlinessuper-pose),whiletheGaussianburstcontributesmostlyinthecores.NotethatthepeaksurfacebrightnessoftheGaussianisashighasthatoftheunderlyingsechinmostcases.FitstoNGC1569andNGC3738aremuchimprovedoverthosewithapuresechfunction(PaperI).AdoptedsolutionsforparametersaresummarizedinTables4and5.

Duetothepoorresolutionandseeing,mostoftheGaussianscalelengthsarecomparablewiththeseeing(columnsr0GfromTables2andFWHMfrom4,respectively).Inthesecases,theGaussianscalelengthshavetoberegardedwithcaution.Forafewcompactgalaxies,sechfittingwasbiasedbyacentralspikeinsurfacebrightness,headingtoanover-estimateofthetotalflux.Inthesecases,topreventaninaccuratefitoftheexponentialatlargeradiiandtoaccountfortheseeing(2to3′′,cf.withTable2),werestrictedthefittoregionsbeyondabout3′′fromthecentre.SuchintervalsaremarkedbyhorizontallinesinourplotsinFig.7.Inonlyonecase(VCC848),weuseddifferentinnerlimitsforthetwobands;bothlimitsareshown,correspondingtoKs(top)andJ(bottom).

Therearesixgalaxieswhicheitherwereobservedinpoorweatherconditions,haveverysmallsizes(and,thus,poorsampling),orhaveunsureBCDclassification.SomeofthesehaveprofilesresemblingthoseofdIs,whichcanbewellfittedusingthesechlawalone(PaperI).FourBCDsgivenegativeGaussiancentralsurfacebrightnesses,impossiblephysically.Were-fittedthesurfacebrightnessprofilesofthesesixgalaxiesusingasechcomponentalone.InTable6,wecomparethesechmodel(denotedbySinColumn3)withthetwo-componentmodel(sech+Gaussian,denotedbyS+G).

ImagesinKsforthefollowingtargetswereespeciallyaffectedbythepoorweatherconditions:VCC428,VCC1313,VCC2033,andVCC848.Thefirstthreearealsocompact,soprofilesarepoorlysampled.Asaresultofthelossofsensitivity,twoconsequencesare

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likely.First,welikelyhavelessextenttofitthesechcomponentduetothelossofthefainterisophotes.Second,combinedwithourlowspatialresolution,sharpfeaturesmaybeblurredsomewhat,resultinginGaussianparametersthatmaybelessreliable.

VCC428isaverycompactgalaxyobservedunderverypoorconditions.ApuresechmodelmatchesmTbetteranditsrmserrorsareslightlysmallerthanthoseforthetwo-componentmodel.Thus,weadoptthesechfit.KsimagesforVCC848alsowereobservedunderpoorconditions,althoughthermserrorsinthetwo-componentfitsuggestabetterfitthaninthesechcase.VCC1313isaverycompactBCD,soitisnotwellsampled.Thetwo-componentmodelprovidesasmallerrmsthanthesechalone,soweadoptit.However,giventhegalaxy’scompactsizeandasurfacebrightnessprofilethatisshallowerthandesirable,itislikelythatthesechparametersofthiscompositefitarepoorlydeterminedandmoreuncertainthattheformalerrorsindicate.

ForVCC1374andVCC1725,thefittingderivesanegativeGaussiancentralsurfacebrightness(µ0Gmarkedas“n/a”inTable6).AccordingtoNED,thesetwogalaxieshaveambiguousBCDclassification,VCC1374beinglistedas“BCD?,IBm:sp”,VCC1725as“SmIII/BCD”.Forbothgalaxies,apuresechmodelproducesbetterfits(chi-squareandrmsvaluesaresmaller),soweadoptit.VCC1699hasasurfacebrightnessprofileresemblingthatofadI,butitsNEDclassificationlistsitas“SBmIII”,althoughitislistedasaBCDbyDePaz,MadoreandPevunova(2003)andhasbeenincludedasadwarfHIIgalaxybyVilchezandIglesias-Paramo(2003).ItspuresechmodelprovidesabettermatchbetweentheJandKsscaleradii,soweprefertoadoptthisfitinsteadofthetwo-componentone.InFig.7weshowthefitofthepuresechmodelforVCC428,VCC1374,VCC1699,andVCC1725,andthefitforthetwocomponentmodelforVCC848andVCC1313.

Thestructuralparametersforatleasttwogalaxies,NGC3738andVCC848,appeartobeunusual.TheradiusoftheirGaussiancomponentinbothKsandJisatleastafactoroftwolargerthanthatfoundforalltheothers.ItisthereforenotclearwhethertheirGaussiancomponentsrepresentthesamephenomenonasintheothergalaxiesandmayexplaintheirsometimesanomalouspositionsinsomeofthediagramsthatfollow.

IncludedinTables4and5arethesechcentralsurfacebrightness,µ0S,theGaussiancentralsurfacebrightness,µ0G,inmagarcsec−2,thesechscalelength,r0S,andtheGaussianscalelength,r0G,inarcsec,forbothJandKs.BasedupontheNFIT1Derroranalysis,theuncertaintiesinµ0Sandµ0Garelessthan∼0.1magarcsec−2.Thesechscalelengthsinthetwobandsagreetowithinabout0.5arcsec(about5%)formostgalaxies,buttheGaussianscalelengthsshowlargerdifferences,possiblyduetovariationsinimagequality.

–13–

5.3.

SechandGaussianMagnitudes

TheintegratedmagnitudeofthesechcomponentofaBCDcanbecalculatedfromEquation(4)byintegratingthesurfacebrightnessoverradiifromzerotoinfinity(PaperI):

󰀁󰀃2

mS=zps−2.5log11.51I0Sr0(1−e)(7)SHereI0Sisthesechcentralsurfacebrightness,r0Sisthesechscalelength,erepresentsthe

galaxyellipticity,andzpsistheframezero-point.

ThefluxduetothestarburstcanbequantifiedsimilarlybyintegratingEquation(5).Letaandbbethesemimajorandsemiminoraxisoftheisophotes,andIG(a)theaveragesurfacebrightnessoftheGaussiancomponentinanellipticalannuluswithareadA:

dA=d(πab)=d(πa2(1−e))=2πa(1−e)da

Then,thetotalfluxIGduetotheGaussiancomponentisgivenby:

󰀅∞󰀅∞

󰀁󰀃

IG=IG(a)dA=2π(1−e)I0Gaexp−0.5(a/r0G)2da

0

0

(8)

(9)

Bysubstitutingx=a/r0Ginthelastintegral,theintegratedGaussianfluxbecomes:

󰀈2󰀅∞

−x2

IG=2π(1−e)I0Gr0xexpG

0

–14–

5.4.

IsophotalRadii

UsingEquation(4),wehavecalculatedthesemimajoraxisr22correspondingtoKs=22magarcsec−2aswellasr23correspondingtoJ=23magarcsec−2.Thepositivesolutionforthe2ndorderequationinr22comesto

whereA22isgivenby:

r1+

22=r0Sln

󰀄A22logA2.5logI22=

zps−22−0S

(12)–15–

typicaluncertaintiesineachparameterareplottedasanerrorcross.FortheVirgogalaxies,absolutemagnitudesandscalelengthsarecalculatedassumingacommondistancemodulus,DM=30.62(Freedmanetal.2001),whichistheHSTKeyProjectdistancetotheVirgoClusteranchoredtothemaserdistanceforNGC4258(PaperI).

6.1.1.SizeandBrightness

Inthevisible(Bband),emission-linegalaxies(mostlyBCDs)andgasrichdwarfsdisplayalinearrelationshipbetweenthelogofthediskscalelengthandtheabsolutemagnitude(Vennik,HoppandPopescu2000;Bergvalletal.1999).AsimilartrendhasbeenfoundintheNIRfordwarfgalaxies,includingBCDs(Papaderosetal.1996,2002).ThecorrelationfortheBCDsappearstobesteeperthanthecorrelationforotherdwarfs,inthesensethat,foragivenBluminosity,BCDshavesmallerscalelengthsbyafactorof∼2thanotherdwarfgalaxies(Papaderosetal.1996).AsimilartrendwasfoundbyBosellietal.(1997)betweentheNIRconcentrationindexc31(definedastheratiobetweentheradiithatcontain75%and25%oflight)andthemagnitude(seetheirFig.10)forlate-typegalaxiesinVirgo.InFig.4,weplotthesechscalelengthr0SversusthesechabsolutemagnitudeMSforourBCDsample(filledsymbols).dIsfromPaperIareshownasopensymbols,andthelinearfittothedIsfromPaperIisshownasadottedline.ThestarburstdIsNGC1569andNGC3738arelabeled.VCC848isanoutlier,whosedatawasacquiredunderpoorweatherconditions.VCC1699,VCC1725andVCC1374werefittedusingthesechcomponentalone.ForthebrighterBCDs,scalelengthdoesappearsystematicallysmallercomparedtodIsatagivenabsolutemagnitude.

UsingtwoBCDsamplesobservedinthevisible(22and23targets)andasubsampleobservedintheNIR(11targets),Doublieretal.(1997)andDoublier,Caulet,&Comte(1999,2001)foundalineartrendbetweenthelogoftheeffectiveradiusandtheabsolutemagnitude,inthesensethatbrightergalaxieshavelargersizes.AsimilarrelationwasfoundbyVennik,HoppandPopescu(2000)inthevisibleforalargersamplewhichincludedabout115BCDs.Fig.5plotsthesemimajoraxisr22versustheabsoluteisophotalmagnitudeMIinKs.BCDsaremarkedwithfilledsymbolsanddIswithopencircles.ThefittothedIsderivedinPaperIisshownasadashedline.AsinFig.4,VCC848andVCC1699areoutliers.TheBCDsdisplayatrendsimilartothatofdIs,albeitwithmorescatter.TheenhancedscatterisgreaterthancanbeexplainedbytheinclussionofthestarburstinMI.

–16–

6.1.2.CentralSurfaceBrightness

Vennik,HoppandPopescu(2000)didnotfindanycorrelationbetweentheextrapolatedcentralsurfacebrightnessofanexponentialandtheabsoluteBmagnitudeforalargesampleofemission-linegalaxies(mostlyBCDs)observedinthevisible.However,usinganotherlargesampleofdwarfgalaxiesincludingBCDs,dIsanddEs,Papaderosetal.(1996)andPapaderosetal.(2002)foundatrendbetweenthecentralsurfacebrightnessoftheexponentialandthetotalBmagnitude,althoughthecorrelationwasveryloose.

InFig.6,wepresentthecorrelationbetweenthesechcentralsurfacebrightnessµ0SandtheabsolutesechmagnitudeMS,KinKs.BCDsandthetwostarburstdIsareplottedwithfilledsymbolsanddIsfromPaperIareplottedwithopensymbols.BCDsanddIsfollowsimilartrends,althoughluminousBCDsmighthavesystematicallybrightercentralsurfacebrightnessesthanluminousdIs.ThetwostarburstfielddIsarelabeled.VCC848isanoutlierobservedinpoorweather,whileVCC1725andVCC1699werefittedusingsechcomponentalone.Using38dIsandBCDsinourtwosamples,thefollowingrelationisderived:

MS,K=(1.52±0.08)µ0S,K−(46.51±1.35)(14)ThefitisshownasadottedlineinFig.6.Thermsdeviationofpointsaboutthefitis0.84

mag.

6.1.3.Starbursts

Fig.7plotstheGaussianmagnitudeMGversusthesechmagnitudeMSforourBCDsample.Objectswithbrighterunderlyingcomponentsseemtohavelargerbursts.VCC1313’spositionmaywellbetheresultofthepoorerqualityofthedataavailableforit.Fig.8showsthestrengthoftheburstrelativetothediffusecomponent(MG−MS)versusMS.Thereisnoobviouscorellation,whichindicatesthatburststrenghtscaleslinearlywithmass.Inotherwords,thereisnoevidencefortriggeringofstarformationinmoremassivehostsoverandabovewhatisexpectedforalargereservoirofmatter.VCC1313’spositionmaywellbetheresultofthepoorerqualityofthedataavailabletoit.

6.1.4.FundamentalPlane

Forspiralgalaxies,absolutemagnitudesaretightlycorrelatedwiththelogoftheHIline-widthW20aftercorrectingforprojection(Tully&Fisher1977).However,thecorrelation

–17–

ispoorforlate-typegalaxies,evenwhenNIRdataisemployed(Pierini&Tuffs1999,PaperI).InPaperIitwasdiscoveredthatresidualsintheTully-FisherrelationfordIswerelinkedtothecentralsurfacebrightnessofthesechprofile,µ0S.A“fundamentalplane”wasdefinedforthedIs,relatingMStoµ0SandlogW20.InFig.9,wereproducethisfundamentalplane,representingdIsfromPaperIasemptysymbolsandtheBCDsasfilledsymbols.AlthoughBCDsshowmorescatterthanthedIs,theyareconcentratedaroundtheplane,suggestingthatBCDsaredynamicallysimilartodIs.ThetwostarburstfielddIs,NGC1569andNGC3738,arelabeled,alongwiththemostoutlierVirgoBCDs.ThelargerscatterofBCDsabouttheplaneismainlyduetothelargeuncertaintiesinW20,ashighas13km/s,whichtranslatesinto4.5maginthefirsttermoftheXaxis.Alsoitcouldbeduetointrinsicfactorsaffectingthelinewidths,suchasturbulenceorwindsandtodifferencesindistancesduetotheclusterdepth.

7.Conclusions

Bluecompactdwarfs(BCDs)anddwarfirregulars(dIs)areimportantprobesforstudy-ingtheformationandevolutionofgalaxies.Inanefforttoextractinformationaboutoldstellarpopulations,asampleof16BCDsintheVirgoClusterhavebeenimagedusingaNIRarrayattheOAN-SPMinMexico.

Surfacebrightnessprofilesweresuccessfullymodeledusingonlytwofunctions(fourfreeparameters):ahyperbolicsecant(sech)tracingthediffusecomponent(responsibleformostofthelight),andaGaussianforthecentralstarburst.Ahyperbolicsecantisknowntofitthenear-IRprofilesofdIs.Isophotal,sech,andtotal(sechplusGaussian)NIRmagnitudeswerecalculatedforallgalaxies.Also,semimajoraxesatµJ=23magarcsec−2andµKs=22magarcsec−2weredeterminedfromthesechfit.

Wesearchedforrelationsbetweensemimajoraxes,scalelength,absolutemagnitude(sech,isophotal,total,andGaussian),centralsurfacebrightness,andcolor.Correlationsbetweenthesechscalelengthandthesechmagnitude,sechsurfacebrightnessandsechmagnitude,andbetweenthesechsemimajoraxisandthetotalmagnitudeoverlapthoseofdIs,thoughwithmorescatter.Overall,BCDswithmoreluminousdiffusecomponentsarelargerandhavebrightercores,asisalsofoundfordIs.Thecentralburstappearstocorrelatewiththeluminosityofthediffusecomponent,withbrighterBCDshavingstrongerstarbursts.However,thestrengthoftheburstrelativetothediffusecomponentshowsnotrendwithluminosity,indicatingthatmoreluminousburstsaresimplyaconsequenceofalargerreservoirofmatter,notextratriggering.ColorprofilesshowarelativelyconstantJ−Ks=0.7to0.9magatallradii.Thediffusecomponentrepresentstheoverwhelming

–18–

majorityoftheNIRlightformostBCDs,thestarburstenhancingthefluxbylessthanabout0.3mag.

BCDslieonthefundamentalplaneofdIs(PaperI),showingthesamerelationbetweenthesechabsolutemagnitude,thesechcentralsurfacebrightness,andthehydrogenline-width.However,thescatteraboutthisplaneislargerthanforthedIs,perhapsduetoenhancedturbulenceorwinds.ItisconcludedthatBCDsanddIsaresimilarstructurallyanddynamically.

WethanktheOAN-SPMtimeallocationcommitteesforgrantingustheopportunitytoobserve.MGRacknowledgesfinancialsupportfromCONACyTgrant37214-EandDGAPA-UNAMgrantIN114199.SpecialthankstoF.Montalvo,S.Monrroy,andG.Melgoza,fortheirhelpwiththeobservations.MLMthankstheNaturalSciencesandEngineeringCouncilofCanadaforitscontinuingsupport.Forourdatareductions,weusedIRAF,distributedbytheNationalOpticalAstronomyObservatories,whichareoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.,undercooperativeagreementwiththeNationalScienceFoundation.ThisresearchhasmadeuseoftheGOLDMineDatabaseinMilanoandtheNASA/IPACExtragalacticDatabase(NED)whichisoperatedbytheJPL,CALTECH,undercontractwithNASA.

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–20–

Pierini,D.,&Tuffs,R.J.1999,A&A,343,751

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Press)Zwicky,F.,&Zwicky,M.A.,1971,CatalogueofSelectedCompactGalaxiesandofPost-EruptiveGalaxies(Zurich:Offsetdruck,L.Speich)

–21–

Table1.GalaxySample

Galaxy(1)

α(J2000)

(2)

δ(J2000)

(3)

e(4)

PA(5)

Note.—Col.(1):Galaxyname;Col.(2):Rightascension(listedbyNED);Col.(3):Declination(listedbyNED);Col.(4):Ellipticity(1-b/a),fromthispaper;Col.(5):Positionangleofmajoraxis(NorththroughEast),fromthispaper;

–22–

Table2.ObservingLog

Galaxy

Date(UT)

Filter

ExpTime(sec)

FWHM(arcsec)

–23–

Table3.ComparisonofApparentMagnitudes

Galaxy(1)

∆mJ(2)

∆mK(3)

Reference

(4)

Note.—Col.(1):Galaxyname;Col.(2):DifferenceinJmagnitude(referenceminusthispaper);Col.(3):DifferenceinKsmagnitude(referenceminusthispaper);Col.(4):Refer-ence.

–24–

Table4.PhotometricParameters

Galaxy(1)

Filter(2)

mI(mag)(3)

mS(mag)(4)

mG(mag)(5)

mT(mag)(6)

µ0S

(mag/sq.′′)

(7)

r0S(′′)(8)

µ0G

(mag/sq.′′)

(9)

r0G(′′)(10)

r23(22)(′′)(11)

(a)

Gaussiancomponentnotfitted

Note.—Col.(1):Galaxyname;Col.(2):Filter;Col.(3):Isophotalmagnitude;Col.(4):Sechmagnitude(fromsechlaw);Col.(5):Gaussianmagnitude(fromGaussianlaw);Col.(6):Total(Sech+Gaussian)magnitude;Col.(7):Centralsurfacebrightnessofsechcomponent;Col.(8):Scalelengthofsechcomponent;Col.(9):CentralsurfacebrightnessofGaussiancomponent;Col.(10):ScalelengthofGaussiancomponent;Col.(11):Semimajoraxisr23correspondingtotheJ=23magarcsec−2;semimajoraxisr22correspondingtoKs=22magarcsec−2.

–25–

Table5.LocalVolumedIswithBCD-likeprofiles

Galaxy(1)

Filter(2)

mI(mag)(3)

mS(mag)(4)

mG(mag)(5)

mT(mag)(6)

µ0S

(mag/sq.′′)

(7)

r0S(′′)(8)

µ0G

(mag/sq.′′)

(9)

r0G(′′)(10)

r23(22)(′′)(11)

Note.—Col.(1):Galaxyname;Col.(2):Filter;Col.(3):Isophotalmagnitude;Col.(4):Sechmagnitude(fromsechlaw);Col.(5):Gaussianmagnitude(fromGaussianlaw);Col.(6):Total(Sech+Gaussian)magnitude;Col.(7):Centralsurfacebrightnessofsechcomponent;Col.(8):Scalelengthofsechcomponent;Col.(9):CentralsurfacebrightnessofGaussiancomponent;Col.(10):ScalelengthofGaussiancomponent;Col.(11):Semimajoraxisr23correspondingtotheJ=23magarcsec−2;semimajoraxisr22correspondingtoKs=22magarcsec−2.

–26–

Table6.SechandSech+GaussianSBPfittingforsixBCDswithdI-likeprofiles

Galaxy(1)

Filter(2)

Fit(3)

rms(4)

mI(mag)(5)

mT(mag)(6)

µ0S

(mag/sq.′′)

(7)

r0S(′′)(8)

µ0G

(mag/sq.′′)

(9)

r0G(′′)(10)

r23(22)(′′)(11)

Note.—Col.(1):Galaxyname;Col.(2):Filter;Col.(3):Fit(S=sech;S+G=sech+Gaussian);Col.(4):rmserroroffit;Col.(5):Isophotalmagnitude;Col.(6):Totalmagnitude(sechorsech+Gaussian);Col.(7):Centralsurfacebrightnessofsechcomponent;Col.(8):Scalelengthofsechcomponent;Col.(9):CentralsurfacebrightnessofGaussiancomponent;Col.(10):ScalelengthofGaussiancomponent;Col.(11):Semimajoraxisr23correspondingtotheJ=23magarcsec−2,semimajoraxisr22correspondingtoKs=22magarcsec−2.

–27–

Fig.1.—VirgoBCDgalaxiesobservedatOAN-SPM.Leftpanel:Jimages(Northup,

′′

Easttoleft,field3.0×3.0).Rightpanel:surfacebrightnessprofilesinJandKs(uppergraphs),andJ−Kscolorprofiles(lowergraphs).Tomakethemmorevisible,thesurfacebrightnessprofilesinJhavebeenshiftedbyonemagnitude.Solidlinespassingthroughthepointsrepresenttotal(sechplusGaussian)fits.ForKsonly,sechandGaussiancomponentsareplottedindividuallyaswell,withadashedlineandadottedline,respectively.ForVCC428,VCC1374,VCC1699andVCC1725,thesechcomponentisplottedasasolidline,becausethatwastheonlycomponentfitted.ErrorbarsarethosecalculatedbyISOPLOT.NGC1569andNGC3738,starburstdIswithBCD-likeprofilesobservedatCFHT(PaperI)areincludedinthelasttwopanels.

–28–

Fig.2.—ThedifferencebetweentheapparenttotalmagnitudemTandtheapparentisopho-talmagnitudemIversusmIfortheKsband.ThethreeVirgoBCDswithresidualslargerthan0.1magwereallobservedinpoorweather.

–29–

Fig.3.—ThedifferencebetweentheapparentsechmagnitudemSandtheapparentisophotalmagnitudemIversusmIfortheKsband.ThegraphrevealstheeffectofstarburstsonKsmagnitudes.VCC1313hasaverysmallsize,whichlimitedthenumberofsamplesforfitting.

–30–

Fig.4.—Thescalelengthr0SofthesechprofileversustheabsolutemagnitudeMSofthesechcomponentinKs.BCDsandthetwostarburstdIsarerepresentedasfilledcircles,anddIsfromPaperIasopencircles.ThedashedlineisalinearfittothedIsalone.VCC848appearsasanoutlier,whileVCC1699,VCC1725andVCC1374werefittedusingthesechcomponentalone.

–31–

Fig.5.—Thesemimajoraxisr22oftheisophoteKS=22magarcsec−1ofthesechcomponent,versustheabsoluteisophotalmagnitudeMIinKs.BCDsandthetwostarburstdIsarerepresentedasfilledcircles,whiledIsfromPaperIasopencircles.Twotwostarburstfieldgalaxies,NGC1569andNGC3738,arelabeled.VCC1699,VCC1725,andVCC1374wereallfittedwithsechcomponentalone.VCC848isanoutlierobservedinpoorconditions.AlinearfittothedIsaloneisshownasadashedline.

–32–

Fig.6.—Thesechcentralsurfacebrightnessµ0SversusthesechabsolutemagnitudeMSfortheKsband.BCDsandthetwostarburstdIsareplottedwithfilledcircles,whiledIsfromPaperIwithopencircles.ThetwostarburstdIsarelabeled.VCC848isanoutlierobservedinpoorweather,whileVCC1725andVCC1699werefittedusingsechcomponentalone.Afitthroughtherestofthepoints(dIsandBCDs)isplottedasadashedline.

–33–

Fig.7.—TheGaussianmagnitudeMGversusthesechmagnitudeMSforourBCDsampleandtwostarburstdIs,inKs.Thereisaroughcorrelationbetweenthestrengthofthestarburstandtheluminosityofthediffusecomponent,inthesensethatobjectswithbrighterunderlyingcomponentshavemoreluminousbursts.VCC1313’spositionmaywellbetheresultofthepoorerqualityofthedataavailableforit.

–34–

Fig.8.—MG−MSversusMS.Thereisnoevidencethattherelativestrengthofburstsgrowswithluminosity.VCC1313wasobservedinpoorweather,beingalsoaverycompactobject.

–35–

Fig.9.—ThedIfundamentalplaneinKS,asdescribedbytherelationbetweentheabsolutesechmagnitudeMS,HIline-widthW20,andthecentralsechsurfacebrightnessµ0(PaperI).dIsarerepresentedbyopencircles,whileBCDsandthetwostarburstdIsaremarkedbyfilledcircles.TheBCDsshowmorespreadthanthedIs,buttheyscatteraroundthesamerelation.TheerrorbarintheXaxisrefferstoaverageBCDdataandisduemainlytouncertaintiesinW20.NGC1569andNGC3738arelabeled,alongwiththemostdeviantVirgoBCDs,whichhavethelargestuncertaintiesinW20(ashighas13km/s,whichtrtanslatesinto4.5maginthefirsttermoftheFP).