1985ApJS...58..533H luminous, lowerinopticalsurfacebrightness,andlessmassive most numeroustypeofstar-forminggalaxy(Zwicky1957; properties ofgas-richdiskgalaxies.Althoughtheyarethe high-surface brightnessMagellanic-typeirregulars,which we de Vaucouleurs1959h;vandenBergh1966,1977;Kraan- will genericallyrefertoasgiantIrr’s.Despitedifferences in (cf. SandageandTammann1981).Theygenerallyare less usually haveuninspiringappearancesonopticalphotographs Korteweg andTammann1979),dwarfirregular(Irr)systems e.g., chaoticopticalmorphologieswithlittleornospiral scale, manykeypropertiesofgiantanddwarfIrr’saresimilar; structure, lowrotationvelocities,blueopticalcolors,large than eitherspiralgalaxiesortheirstructuralcousins, the © 1985.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. The AstrophysicalJournalSupplementSeries,58:533-560,1985August omy, Inc.,undercontractwiththe NationalScienceFoundation. 1 Dwarf irregulargalaxiesrepresentoneextremeofthe OperatedbytheAssociationofUniversities forResearchinAstron- indicative ofthephysicalhomogeneitynormalirregulargalacticstructuralclass:(1)Despitelocal irregular galaxies. irregulars potentiallycansustainconstantstarformationratesforverylongtimes(>100Gyrinsomecases). luminosities, andsurfacebrightnessesofHiiregions.(3)Starformationhistoriesareestimatedfromtheratio points tolocalregulationofstarformationprocesses,andwenotethepossibleimportancealow-density irregular galaxies.(4)Inameansense,galaxiesformstarsatnearlyconstantrates.Thesepointsare nonuniformities, irregularshaveapproximatelyexponential,disklikeradialbrightnessprofiles.(2)Population The dwarfirregularsdomakelargestar-formingregions,butonaveragetheyaresmallerthanthoseinthegiant emission lines.(5)Imagesprovideinformationontheopticallightdistributions. cutoff forgastoeffectivelyparticipateinstarformation. discussed intermsofphysicalmechanismswhichmightcontrolevolutionaryprocessesgalaxies.Wesuggest directly associatedwiththelevelsofstar-formingactivity,dwarfandgiantirregularsaresimilar,whichis formed starsatapproximatelyconstantratesoverthelastfewGyr.BecauseoftheirlargeHigascontents,dwarf Ha toblueluminositiesandfromthestellarcontentmodels.(4)Themetallicityofgascomesoxygen deduced fromHaluminosities.(2)Propertiesoftypicalstar-formingcentersarederivedthesizes, ments ofstar-formingcharacteristicsdwarfirregulargalaxies:(1)Totalcurrentstarformationratesare images takenwithaCCDdetector;flux-calibrated,Ha,narrow-banddigitalimagery;large-aperturespectropho- Subject headings:galaxies:stellarcontent—stars:formation Oxygen abundancesinHnregionsfallbetweenthoseoftheSmallandLargeMagellanicCloudsformost synthesis modelingyieldsequivalentstellartemperaturemixesinsamplesofdwarfandgiantirregulars.(3) optical surfacebrightnessdwarfirregulargalaxies.NewobservationsmadeforthisstudyincludeBandR tometry; andspectrophotometryofindividualHnregions.Thesedataprovidethebasisforseveralmeasure- that theexistenceofconstantstarformationratesingalaxiescoveringawiderangeluminosityandmass 102 STAR-FORMING PROPERTIESANDHISTORIESOFDWARFIRREGULARGALAXIES: A comparisonismadebetweenpropertiesofdwarfandgiantirregulargalaxies.Asidefromcharacteristics The dwarfirregularsinoursamplehavelowstellarproductionratesof-10“Myr“pc“and Star formationprocessesandtheirrelationshipstoothergalacticpropertiesareinvestigatedinasampleoflow 0 © American Astronomical Society • Provided by theNASA Astrophysics Data System I. introduction 1 Kitt PeakNationalObservatory,OpticalAstronomyObservatories Deidre A.HunterandJohnS.GallagherIII Received 1984October31;accepted1985January28 DOWN BUTNOTOUT ABSTRACT levels ofOBstarformationcould besustainedinsuchsimple were ledinourstudiesofgiant Irrgalaxiestoaskhowblatant with thoseofdwarfspresented hereandbyFisherTully brightness galaxies(FisherandTully1975;etal.1978; visibility ofdwarfIrrgalaxiesisfurtherreducedby the but coverawiderangeinmassandluminosity. obvious andcommonfeaturesingiantIrr’slate-type de Vaucouleurs,andButa1981,1983). The spiral galaxies(compareimagesofgiantIrr’sinHunter1982 h central surfacedensitiesofluminousmattercoupledwith their Irr’s asastructuralsubclassstemlargelyfromtheir low 1979). Thispointleadstoan interesting conundrum:Whilewe absence ofnumerouslargestar-formingcomplexes,which are small totalmasses,whichresultinopticallytiny,low-surface therein). TheIrrgalaxiesthusbelonginonestructuralfamily, Feitzinger 1980;GallagherandHunter1984,references (see Hodge1971;deVaucouleursandFreeman1972; fractional Hicontent,andmodestgasoxygenabundances The characteristicswhichenableustodistinguishdwarf 1985ApJS...58..533H galaxies (Hunter,Gallagher,andRautenkranz1982,hereafter are sounspectacularintheirproductionofOBstars. HGR; Gallagher,Hunter,andTutukov1984),inexamining the structurallysimilardwarfIrr’s,weinsteadaskwhythey rates andstellarpopulations,characteristicsofstar-forming pirical probesofgaschemicalabundances,starformation parallels thatofourearlierworkongiantIrrgalaxies We alsoareinterestedinexploringvariationstheproperties 534 § IV,andadditionaldiscussionisinV. are reviewedinthenextsection,derivedpropertiespre- centers, andspatialdistributionsofthestars.Theobservations row-band digitalimagery,whichweanalyzetoprovideem- comparable withthosestudies.Thedatabaseconsistsoflarge- (HGR; Hunter1982a,b)andyieldsresultswhicharedirectly formation rates. of star-formingregionsasfunctionsgalaxysizeandstar mation histories,orarethedwarfsmerelyscaled-downver- differentiate evencloselyrelatedtypesofgalaxieslikedwarf sented in§III,acomparisonismadewiththegiantIrr’s and smallaperturespectrophotometrybroad-nar- Irr’s, wehaveundertakenanobservationalprogramwhich short-lived periodsofenhancedstarformation,or“bursts.” rates insimplegalaxies.Forexsimple,dogiantanddwarf included inthespectrophotometry ofindividualHnregions. ary relationshipswiththemoreactivelystar-forminggiant formation processesindwarfIrrgalaxiesandtheirevolution- 1984), fortheviewthatdwarfsystemsaredominatedby etal. 1982)andtheoretical(Gerola,Seiden,Schulman (Searle, Sargent,andBagnuolo1973;Huebra1911b;Stewart sight intothefactorswhichinfluenceglobalstarformation and giantIrr’sholdsconsiderablepromiseforprovidingin- higher-surface brightness,smaller-angularsizesystems were by outsideforcesandthestarformationprocessesreflect only number) andwereselectedformFisherTully’sHisurvey sions ofthegiants?Thereissomesupport,bothobservational systems showsystematicevidencefordifferencesinstarfor- The programgalaxiesaregiven inTable1. conditions internaltothesystems.Thecriterionthat the (no nearbycompanions)sothatthegalaxiesareundisturbed Irr’s. Preferencewasgivenforsystemscloserthan10Mpc, (1975) onthebasisoftheirmorphologicalclassification as 1980; Seiden,Schulman,andFeitzinger1982;Comins1983, eters forwhichpublished photographs areavailablewere galaxies beobservablealsoplacedconstraints,sothat the addition, programdwarfswerechosentobenoninteracting although severalmoredistantsystemsarealsoincluded. In Bergh’s (1959)surveyofdwarfs(designatedbyhisDDO and imaging.Severalnearby galaxies withlargeangulardiam- somewhat preferredforthelarge-aperturespectrophotometry In ordertofurtherexploretheissuesassociatedwithstar Understanding theunderlyingphysicalmechanismswhich The programgalaxiesareprimarilycatalogedinvanden © American Astronomical Society • Provided by theNASA Astrophysics Data System a) TheObjects II. THEDATA HUNTER ANDGALLAGHER pixels at0755perpixelforafieldofview~2'X2'. were typically14minutes.Theresultingimagesare256X wavelength 6612ÁandFWHMof72À)thenearby centered at6204ÀandFWHMof149À).Integrationtimes continuum (centeredat7096ÀandFWHMof83À;or (central wavelength6577AandFWHMof54A;orcentral Digital imageswereobtainedthroughfilterscenteredonHa system wasusedonthe2.1mtelescopein1982November. videocamera imagesareshowninFigure1(Plates19-25). in twilight.Geometricaldistortionsintroducedbytheimage NGC 604,and2363,forwhichHameasurementswere corrected withobservationsofaquartzlampandblanksky redshift differencesbetweenthecalibratingandprogram calibration wasfoundthroughobservationsofNGC595, Kitt Peak.AnHaemissionpicturewasproducedbyaligning by observingastaratdifferentairmassesasitroseand set. through cirrus,andtheseareflaggedinsubsequenttables.The used forextinctioncorrections.Someobjectswereobserved galaxies. MeanatmosphericcoefficientsforKittPeakwere unpublished). TheHa-onfiltersalsopassed[Nn]\6584,and available (Kennicutt,Balick,andHeckman1980;Hunter, Á, andaquasi-Rfilter(6204FWHM149Á)whichalso was usedatf/7.5ontheKPNONo.10.9mtelescope.The for thedisplacementofHawithinfilterbandpassesdueto galaxies themselves.Inaddition,acorrectionhasbeenmade although nocorrectionhasbeenmadefor[Nn]intheobject corrections tothecalibrationweremadeaccountforthis, subtracting toremovethestellarcontinuum.Thescalingfac- the Ha-onandHa-offimages,scalingonetoother, The extinctioncoefficientsfoundwerehigherthanusual for minutes. Thenightswereclear,andstandardstarsforB and resulting digitalarrayhasascaleof0786perpixelandfield tor wasdeterminedfromstandardstarfields.TheHaflux tube wereremovedusingastandardreductionprogramat Kitt Peak(K=—0.4). R wereobserved.Theatmosphericextinctionwasdetermined avoids emissionlines.Integrationtimeswereusually30-60 of view7'3(E-W)X4'6(N-S).Thegalaxieswereobserved filter butavoidsthemajoremissionlinesat3727Àand4861 through twofilters:onewhichissimilartotheJohnsonB pixels, werereplacedwithan interpolationbetweenthegood near therightedgeofpictures(E-W),aswellafew bad overscan segmentdiscarded.Thenbiasframesweresubtracted overscan segment.Thiswassubtractedfromthearrayand the columns. Finallytheimages were invertedsoastoconform are shownaspartofFigure1. Table2givestheBmagnitudes and theimagesdividedbydomeflats.Afewbadcolumns the usualastronomicalorientation ofeast.TheCCDimages B The KittPeakNationalObservatory(KPNO)videocamera Small- andlarge-scalepixel-to-pixelgainvariationswere In 1982NovembertheRCACCDdirectimagingcamera The electricalDCoffsetoftheCCDwasmeasuredinan i) Videocamera b) Images ii) CCD Vol. 58 1985ApJS...58..533H pixel sizeofthevideocamerais0755 andthatoftheCCDis0786.Insomeimages(cf.DDO 26)onecanseewhereseveralbadrowshavebeen removed andreplacedwithaninterpolation acrosstheremainingrows.Northisattopandeastto theleft. Hunter andGallagher {seepage534) Fig. 1.—VideocameraH«emission imagesofdwarfirregulargalaxiesshowthecurrentstar-forming regions,andCCDBimagesshowthestars.The © American Astronomical Society •Provided bythe NASAAstrophysics Data System PLATE 19 1985ApJS...58..533H PLATE 20 Hunter andGallagher {seepage534) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 1—Continued 1985ApJS...58..533H Hunter andGallagher {seepage534) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 1—Continued PLATE 21 1985ApJS...58..533H PLATE 22 hunter ÂSftiôïiomical Society •Provided bythe NASA Astrophysics Data System Fig. 1 Continued 1985ApJS...58..533H Hunter AD (S)Ä'irimcan ÄlffonoVnical Society •Provided bythe NASAAstrophysics DataSystem Fig. 1—Continued PLATE 23 1985ApJS...58..533H PLATE 24 Hunter andGallagher{seepage 534) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 1—Continued 1985ApJS...58..533H Hunter andGallagher{seepage534) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 1—Continued PLATE 25 1985ApJS...58..533H No. 4,1985 _1- observed butnotdetected. corrected forexternalreddeningusingE(B—V)fromcol.(5)andA=4.1. determined fromrecessionalvelocitiesinFisherandTully1975,1981assuming/7=50kmsMpc. NGC 7292. and the1RSorIIDSforspectrophotometry.A“D”indicatesthatgalaxywasonlyjustdetected,“ND”meansit DDO 236... A2228+33 . ICIO DDO 218... DDO 214... DDO 209... DDO 155... DDO 140... DDO 94.... the bluemajoraxisgivenbyNilson1973(hereafterUGC)usingrelationshipsFisherandTully1981. DDO 168... DDO 89.... DDO 83.... DDO 75... DDO 68.... DDO 64.... DDO 53.... DDO 50... DDO 49... DDO 47... DDO 43... DDO 42... DDO 39... DDO 26... DDO 11... DDO 8 B 0 © American Astronomical Society • Provided by theNASA Astrophysics Data System Cols. (9)-(12).—An“X”indicatesthatthegalaxywasobservedwithinstrument:videocameraorCCDforimaging Col. (8).—AbsoluteBmagnitude.istakenfromtheRC2ordeVaucouleurs,andButa1981 Col. (7).—Ratioofminortomajoraxis(FisherandTully1975). Col. (6).—MajoraxisofthegalaxyinkpconHolmberg1958systemtakenfromFisherandTully1975orconstructed Col. (5).—ColorexcessduetoGalacticreddening,determinedfromBursteinandHeiles1978,1984. Col. {4).—DistancetothegalaxyinMpc.DistancesaretakenfromKraan-KortewegandTammann1979wheneverpossibleor Col. (3).—MorphologicaltypetakenfromdeVaucouleurs,andCorwin1976,(hereafterRC2). Cols. (l)-(2).—Galaxyidentification. T Galaxy a) DDO 21814.02 0.77 DDO 21413.45 0.97 DDO 6415.83-0.02 DDO 5314.87-0.08 DDO 4913.810.78 DDO 4314.910.66 DDO 2614.980.62 IC 1011.84 1.68 DDO 64-Hii219.1 -0.1 DDO 64-HH118.90.7 Parameters fromCCDImages Galaxy B-R Ho II,Arp268,VIIZw223 TABLE 2 VII Zw238 NGC 3109 NGC 6822 NGC 2366 Arp 267 IC 3617 IC 1613 Names Arp 3 Sex A Other Gr 8 (2) DWARF IRREGULARGALAXIES IBm IBm? Im IB(s)m IBm Im Im IBm Imp SB(s)m IBm SA(s)m IB(s)m Imp Im IBm Im Im Im IB(s)m Im IB(s)m Im Im SBm IAB(s)m Type D(Mpc)E(B-V)a(kpc)b/aMVCCCD1RSIIDS B (3) (4)(5)(6)(7)(8)(9)(10)(11)(12) Galaxies Observed 40.8 22.3 25.4 29.5 23.5 45.6 31.6 37.4 11.0 23.0 19.4 0.68 3.5 5.5 3.5 2.9 1.3 9.0 0.85 9.5 3.5 3.5 3.4 7.3 3.5 TABLE 1 a IIIa-Jphotographicplate throughaGG385filter.The Schmidt telescopeatKittPeak wasusedtoimageDDO50on images ofthedwarfsalsoaregiveninFigure1. in thesamewayasforvideocameraimages.Thesenights 4214, and4449,wasobservedinB,Ha-on(6564À,FWHM in Bonly.NGC2366,aswellthegiantIrr’s3738, radiation events,manyofwhichhavebeenremoved.These were partlycloudy,andthischipwasaffectedbynumerous described above).TheHaemissionimageswereconstructed March. Afewdwarfs(DDO39,140,155,168)wereobserved aperture photometry. and B—Rcolorsofthegalaxies,determinedbysimulating = 27Aso[Nii]isexcluded),andHa-off(thequasi-Rfilter 0.03 0.03: 0.08 0.04 0.22 0.09 0.4 0.00 0.01 0.00 0.03 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.02 0.06 0.04 0.12 0.09 0.07 0.01 In 1982NovembertheCase WesternReserveUniversity Additional datawereobtainedwiththesystemin1983 23.9 22.7 26.6 34.8 24.9 31.8 17.5 16.4 10.0 11.2 38.5 23.4 10.2 36.7 6.4 4.0 9.1 1.9 9.9 7.8 9.4 3.0 6.4 6.2 5.7 0.83 0.87 0.33 0.80 0.89 0.45 0.83 0.62 0.64 0.61 0.64 0.92 0.48 0.42 0.85 0.81 0.95 0.82 1.00 1.00 0.70 0.53 0.82 0.60 0.62 1.00 iii) SchmidtPhotography -15.5 -18.6 -19.5 -15.7 -16.0 -13.3 -18.7 -19 -17.1 -15 -15.5 -15.0 -15.5 -13 -16.6 -19.2 -14.2 -14.6 -16.5 -17.0 -14.7 ND X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X D D X X X D X X X X D D X D X X X X X X X X X X X X X 535 1985ApJS...58..533H -1 -1 mm. TheimageisshowninFigure2(Plate26). polynomial fitbutwasdivideddirectlyintotheobjectspectra. exposure timewas55minutes,andtheplatescaleis96'/6 labeled “SumofDwarfs”isthesumdwarfsDDO42, except thatthequartzspectrumwasnotnormalizedbya near afeatureofinterest,thescanwasdiscontinued.Data resolution. Theaperturediameterswere22",andsetupson grating (No.26)wasusedtocover3500-5800Áwith-15 on thewhitespectrographofNo.10.9mtelescopeatKitt cooled dual-beamintensifiedReticonscanner(1RS)mounted reduction proceduresweresimilartothosedescribedinHGR sionally appearinthespectra.Ifaspuriouseventappeared Large spuriousemissionandabsorptionfeatureswouldocca- Peak during1981OctoberandDecember.A600linesmm 536 and synthesizedUBVcolors. flux at5500À.InTable3areparametersoftheobservations the programgalaxieswerebyblindoffsetfromnearbystars. 50, 53,64,83,155,and236,eachnormalizedtohavethesame Dwarfs” includesDDO42,50,53, 64,83,155,and236. Spectrophotometric observationswereobtainedwiththe The observationsarepresentedinFigure3.spectrum Fig. 3.—Large-aperture(22")spectra ofdwarfirregularsobtainedwiththeKPNO1RSon0.9 mtelescope.“D”refersto“DDO.”“Sumof © American Astronomical Society • Provided by theNASA Astrophysics Data System c) Spectrophotometry i) 1RS HUNTER ANDGALLAGHER -1 2 -1 night tonight.Weestimatetheuncertaintyinmeasured lines mmgrating(No.240)whichcovered4800-6900Á were observed.In1983DecembertheIIDSwasusedon nights weremainlycloudy,andonlyDDO26218 (IIDS) wasusedtoobtainspectraofindividualHuregionsin night skyatKittPeak*. fluxes oftheemissionlinestobeabout15%.TheaverageV with 11Àresolution.Theapertureswere874indiameter. arcsec. Thisrepresentsabout50%ofthebrightness surface brightnessdetectedintheaperturewas22magfor1 2.1 mtelescope.TwonightswerespentwithgratingNo.26 3500-5800 Àat-10resolutionwitha474aperture.These flagged insubsequenttables.Blindoffsetsfromnearbystarsto again, andonenightwasusedforredobservationsusinga500 telescope andgratingNo.26(600linesmm)tocover the dwarfs.In1983Novemberweobservedwithiton4m sized colorsofthestandardstarsrepeatedto+0.01from ure 4. CCD images.Datareductionproceduresaresimilartothose Some cirruswaspresentattimes,andtheseobservationsare for the1RS.AfewrepresentativespectraareshowninFig- the Hiiregionshadbeenmeasuredfromvideocameraand The KittPeakdual-beamintensifiedimagedissectorscanner The uncertaintyinReticondataishardtoestimate.Synthe- n) IIDS 1985ApJS...58..533H PLATE 26 Hunter andGallagher {seepage536) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 2.—DDO50fromaIIIa-Jplate+ GG385filterobtainedwiththeCWRUSchmidttelescope # maJ-GG385 DDO 50 1985ApJS...58..533H © American Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 3—Continued 1985ApJS...58..533H emission featureswerefound byfittingGaussianstothelines data proceededinthemanner describedbyHGR.Fluxesof Analysis oftheemissionlines inthespectrophotometric © American Astronomical Society • Provided by theNASA Astrophysics Data System a) MetallicityoftheGas III. DATAANALYSIS HUNTER ANDGALLAGHER Fig. 3—Continued be deconvolvedfromtheunderlying stellarabsorptionfeature. using aninteractivecodedevelopedbyS.Kenyon,D.Hunter, This wasdoneusinganiterative populationsynthesisproce- Because thelarge1RSaperture includedmuchmoreofthe D. Shaw,andH.Bushouse attheUniversityofIllinois. dure basedonO’Connell’s(1973) bandpasses.Onlyafewof stellar backgroundofthegalaxy, theHßemissionfluxhadto 6000 Vol. 58 1985ApJS...58..533H s// s 4 S S between twoN-SstarsandslightlyE;DDO75—5E64S;140 were positionedasfollows:IC10—3.4Wtoavoidastar:DDO49—10"S NGC NGC NGC obtained throughthewestandeastaperturesrespectively.Usually DDO DDO DDO IC 10- [see Table1forE(B-V)\. “N” indicatesnebular;i.e.,thegalaxywasinbothaperturesandobserva- DDO DDO DDO DDO DDO DDO IC 10- 0.2 X10“andthelatterin.5x10“. (Saraph andSeaton1970),whereas([On]+[0in])/Hßgives to avoidastar;DDO50—25E;53—13"N;64—centered aperture wascenteredonthepubhshedpositionofgalaxy,butseveral DDO DDO proper sideofthecurves. 6400 K(Pageletal.1979).TheformerresultsinanO/H of French (1980)yieldaTof16,300Kfrom[Om]X4363 [O in]X4363.Forexample,theobservationsofIZw18 by regions.” Onecanseethatthiscurveisdouble-valued; the tions ofthegalaxywereinterspersedwithoffsetstosky. He intheregionofhighO/H butlow[On]/[0in]inFigure erly placedontherighthalfofcurveinFigure5bwould emission isnotdetectedshowninFigure5a(fromHunter O/H. TheproblemariseswhenTisnotdeterminedfrom same sumoftheoxygenratioscanresultintwovalues of objects, includingsomeverymetalpoor“intergalacticHn of thedwarfs,soweconclude thatwehaveplacedthemonthe from the1RSdatafordwarfIrr’s,giantandother of O/Hforlow-abundanceobjects(Hunter1982a).Figure5b are presentedinTable4andtheIIDSdataTables56. —centered byeyeatU"N;DDO236—36"Nand5.6W. stand outaspossiblyhaving improperlyassignedoxygen 5a. Thisdoesnotseemtobe the casefor1RSobservations shows aplotof([Oii]+[0ni])/H/?versusO/Hdetermined and T.Thereresultsapotentialambiguityinthederivation et al.(1979)between([Oil]X3727+[Oin]XX4959,5007)/Hß gas. Wehaveused,therefore,therelationshipfoundbyPagel which issocrucialtomeasuringthetemperatureofionized the IIDSobservationsrequiredthiscorrection.The1RSdata 1982 a).Althoughthescatterislarge,ameasurementimprop- No. 4,1985 e e e Col. (1).—Galaxyidentification.“W”and“E”refertothespectra Cols. {4)-{6).—Synthesizedcolors,correctedforGalacticreddening Col. (3).—Modeofobservation.“S”indicatesstellarbeamswitching. Col. (2).—Totalintegrationtimeinminutes. Among theIIDSobservations, however,twoHnregions A possiblewayofresolvingthisambiguitywhenX4363 In noneofthespectrahavewedetected[Om]\4363, Galaxy ■E 7292 .. 2366 .. 784 ... 49 64 236-W. 218 ... 94 50 W 236-E . 155 ... 140 ... 53 83 (i) Intensified RjeticonScannerObservations © American Astronomical Society • Provided by theNASA Astrophysics Data System t (2) 48 48 42 48 66 24 72 96 72 84 18 36 84 80 56 18 Mode N, S S, N TABLE 3 (3) N N N N N N s s s S S S s S 13.36 15.57 15.27 15.58 15.91 16.61 15.85 13.27 15.18 15.18 13.50 15.18 15.56 16.49 14.07 14.63 (4) V DWARF IRREGULARGALAXIES B —V -0.22 (5) 0.35 0.38 0.36 0.17 0.42 0.27 0.78 0.51 0.34 0.28 0.47 0.35 0.27 0.50 0.40 U-B -0.29 -0.50 -0.42 -0.44 -0.58 -0.35 -0.35 -0.60 -0.57 -0.55 -0.67 -0.15 -0.49 -0.29 -0.46 -0.26 (6) 3-12 -4 -4 -4 4- -4 -4 ing singlestar-formingunits. gions ingiantIrr’stothesame surfacebrightnesscontourlevel distant than—10Mpcwecannot besurethatweareresolv- presented forallofthegalaxies observed,forsystemsmore brightnesses arepresentedin Table7.Althoughthesedataare included intheaperture.Sincedifferentemissionfinesmay luminosities, correspondingnumberofOstars,andHasurface level correspondingto2.5XlOergsspc.Thesizes, Ha diameters oftheregionsweremeasuredtoacommoncontour galaxies. ContourmapsweremadeoftheHnregions, and what largerthanthatfoundbyPageletal.(1978)forthe larger thanthatfoundbyTalentforNGC6822,andsome- which appeartobenearlysolarinvalue.(SeealsoPeimbert lower-excitation diffuseemissionwhichthelarger1RSaper- errors intheemissionratios(Rosa1983).Theremayalsobe data wasasacheckonthelarge-aperturespectra.Infact,we observations. Oneofthepurposessmall-apertureIIDS determine variouspropertiesofthestar-formingregionsin the detected (seeEdmundsandPagel1984). including datafromTalent(1980),is3X10;for12galaxies is thatwehavesystematicallyundercorrectedthe1RSHß arise fromdifferentregionsinanebula,thiscouldintroduce One problemwiththesmall-aperturespectrophotometryis average O/HratiosoftheindividualHnregionsinagalaxy. systematically higherbyanaverageof40%comparedwiththe shown graphicallyinFigure6,includingbothIIDSand1RS values ofO/Hforthoseregionsandwewouldcalculate LMC andSMC.Atleastsomeofthescattercouldbedueto for thesixgalaxieswithmultipleobservations.Thisismuch Ruotsalainen 1982forstellarmetalhcities.)ScatterinO/H emission finesforunderlyingabsorption. that thereisagreaterchancetheentirenebulanot find thattheO/Hratiosderivedfrom1RSspectraare abundance determinationswillstillservetheirpurpose. Pagel etal.’smethod.Asastatisticalsurvey,however,these some ideaoftheuncertaintyresultingfromourapphcation from hisresultsbyanaverageof+0.5X10.Thisgives from hisdatawithoutusingX4363showsthatwewoulddiffer among Hnregionswithinagalaxyrangesbyasmuch50% and Spinrad1970;Pagel,Edmunds,Smith1980;see galaxies istheexcitationratio([On]/[0m])in1RS the methodusedincalculatingTwhen[Oin]X4363is not SMC-like tosomewhatgreaterthanLMC-fike,withafew spectrum higherthanfortheIIDSspectra.Anotherpossibility the 1RSmeanis4x10.Mostofmetalhcitiesrangefrom ture mightpickup.However,inonlyfouroftheseven calculated O/Hvaluesof9xl0~and15X10,buttheir H nregionsinthreedwarfIrr’s.Acomparisonbetweenhis [O n]/[0in]ratiosare0.3and0.9,respectively.Infact, and 1X10. abundances. TheseareNos.2and6inDDO68.Wehave therefore, thetrueO/Hratiosareprobablycloserto0.2X10 e For comparisonpurposeswe neededtomeasureHnre- The meanO/Hratiofor37HnregionsindwarfIrr’s, The distributionofthemetalhcitiesforeachgalaxyis The videocameraHaemission-fineimagescanbeused to Talent (1980)hasmeasured[Oni]X4363strengthsforsix b) CharacteristicsoftheStar-formingRegions 539 1985ApJS...58..533H 3-12 34-12 -162_ in Table10.ThetotalHaluminosities arefoundbyaddingup A bettercontourlevelfordistinguishingregionsinthegiant regions whichcouldbemeasuredinnearby(D<10Mpc) be followedouttoamuchlowercontourlevelbecause the regions becauseofthebrightHabackgroundandcrowding are UstedinTable9. giant Irr’stothissurfacebrightnessarealsogiveninTable 8, giant Irr’sto2.5XlOergsspcarepresentedinTable 8. ground isofmuchlowersurfacebrightness.ThoseH n regions aremoredistinctandwellseparatedtheback- and thefewthatcouldbemeasuredindwarfsatthis level Irr’s is10ergsspc'.PropertiesofHnregionsin the H iiregionsinthegiants.Thendwarfscould as inthedwarfs.ThiswasnotpossibleformanyofHn total starformationrateofa galaxy.Thesedataarepresented the luminositiesofindividual Hnregions(col.[3])andby F (10ergscmsÀ)vs.X. 540 x The videocameraHaimages canbeusedtoestimatethe Fig. 4.—AfewexamplesofthespectraHnregionsindwarfsobtainedwithKPNOIIDS.See1forregionidentifications. Theseare 0) O' w_ © American Astronomical Society • Provided by theNASA Astrophysics Data System 3400 3800420046005000 c) StarFormationRates HUNTER ANDGALLAGHER WAVELENGTH (A) 6 synthesis techniqueswereused withthe1RSspectratode- stellar populationsinlarge regionsofdwarfs.Population The latteraregenerallylarger,probablybecauseofdiffuse where theaperturewasplaced.Nevertheless,theseparameters Table 11.Theseparametersreferonlytotheregionof the galaxy (-20%ofHght). galaxy coveredbythe1RSapertureandthereforedepend on mass function)andanaverageUfetimeofHnregion of number ofOstarsusingaMillerandScalo(1979)IMF(initial emission, andarethevaluesusedtocalculatenumber of O are usefulforcomparingwhatisusuallyalargepieceof each Hß emissionlineinthe1RSspectraandarepresented in 6 X10yr(fordetailsseeHGR). simulating large-aperturephotometryontheimage(col.[4]). stars. Thestarformationrate(col.[6])isderivedfrom the The 1RSspectracanalso give someideaofthetypical The starformationrateshavealsobeencalculatedfromthe 3800 4200460050005400 d) StellarPopulations Vol. 58 1985ApJS...58..533H 9 No. 4,1985 HGR fordetails).Sincetrade-offsbetweenstellartemperature luminosity classes.Thestellartemperaturemixesfortheopti- termine theroughstellartemperaturemixofgalaxies(see necessarily agreewiththatfrom theinnerregions;peakof The majoraxiswaschosenfrom outerisophotesanddoesnot on theKittPeakinteractive picture processingsystem(IPPS). plotted theresidualsbetweenmodelsandobservationsat mal solutionsareshowninFigures7and8.InFigure9 result fromthestellarpopulationsynthesismodelshavebeen groups cantakeplace,thecontributingspectralclasseswhich surface brightnessprofilesalong themajoraxisusingsoftware each ofthebandpassesused. current synthesismodelsprovidenoinformationonstellar summed intothreebroadstellarbins:OB,AF,G-M.The the brightnessdistributionoften isoffsetfromthecentersof check onthemodelM^/LisprovidedbyLMC,where which wehaveroughlyestimatedthemassinstars.Although of 0.6.ThiscombinedwithLgivesM*(col.[5])and with we usethetotalmassofM=5X10fromFeitzinger the massingasgivesestimatedfraction(col.[8]). A tionary modelstoestimateanaverageM*/Lforthedwarfs the methodisfraughtwithproblems,weuse,asinHGR, (see Gallagher,Hunter,andTutukov1984). (1980) incombinationwithagasmassoftwicetheHi from McGeeandMilton(1966)tofind(M*/L)<1± 0.5 UBV colorscombinedwithLarsonandTinsley’s(1978)evolu- v v 0 v klmc For galaxieshavinggoodCCD imagingdata,weobtained In Table12aregivenvarioustotalgalaxyparametersfrom NGC NGC NGC absorption. DDO DDO DDO DDO DDO DDO DDO IC 10- IC 10- Sum DDO DDO DDO DDO 4 14-2 Col. (1).—Galaxyidentification. Col. (11).—10timestheelementaloxygenabundancerelativetohydrogen. Col. (10).—ElectrontemperatureinK;estimatedfromthemethodofPageletal.1979. Col. (P).—Ratioof[Oil]X3727tom]XX4959+5007lines. Col. (8).—RatioofX5007toX4959[Om]emissionlines. Col. (7).—FluxinHß,correctedforreddeningandunderlyingabsorption,unitsof10~ergscms; Cols. (2)-(6).—Observedemissionfluxes,correctedforGalacticreddening,relativetoH/Î,whichhasbeenstellar Galaxy ■E 7292.. . 2366.. . 784 .... 236-E.. 236-W 218.... 94 64 49 155 .... 140 .... W 83 53 50 © American Astronomical Society • Provided by theNASA Astrophysics Data System (1) e) GalacticStructuralProperties \3727 [O ii] 2.75 2.34 2.61 2.44 2.44 3.82 3.42 3.32 3.04 3.32 2.62 2.40 2.61 2.46 2.79 1.69 1.96 (2) \4340 0.22 0.38 0.23 0.39 0.30 0.32 Hy (3) X4959 [O III] 0.27 0.99 0.50 0.60 0.95 0.53 0.65 0.64 0.80 0.93 0.91 0.95 0.47 0.54 1.12 1.16: 1.70 (4) Intensified ReticonScannerEmissionData DWARF IRREGULARGALAXIES X5007 [O Hi] 0.85 2.62 2.90 2.08 2.97 2.20 2.19 1.65 3.84 2.43 2.46 1.18 1.61 2.63 2.77 1.80 1.56 (5) TABLE 4 0.86 0.92 0.45 0.67 0.85 0.70 0.81 0.69 0.36 0.70 0.94 0.96 0.81 0.65 0.52 1.03 (6) (7)(8)(9)(10)(11) profile andisdisplacedfromthepredictionsforanexponen- part duetothepresenceof luminous clumpsofyoungstars de VaucouleursandMoss1983;Karachentseva,Schmidt, and Norman 1979)althoughisophotesinspiralsnormallyimply light orplumelikefeatures,evenatlowbrightnesslevels. presence ofoff-centerstellarbars,whicharecommoninIrr indicative oftruenonexponential distributionsofstellar nearly circularsymmetries. in Figure11.ThegiantIrrNGC4449liesverynearthe galaxies arepresentedinFigure10(Plate27). Further complicationsarisesincedwarfIrrgalaxieshavea galaxies (deVaucouleursandFreeman1972;Feitzinger1980). extreme dwarfIn’sstudied by Karachentseva,Schmidt,and (cf. deVaucouleursandMoss 1983),buttheyalsocouldbe Richter 1984).Deviationsfromanexponentialareatleast in dwarf Insystems:themajoraxishasacomplexbrightness (de Vaucouleurs1960;Hodge1971)andthestellardisksof despite itsunusualrectangularisophotes(Hitchcockand Examples ofhigh-contrastimagesforprogramdwarfIn- outer isophotes.Bothofthesefeaturesprobablyareduetothe surface densitiesindwarfIn* systems. 1971, 1977,1978;Abies1971;and1977; that ofanexponentialdisk(deVaucouleurs1959û;Hodge spirals (Freeman1970;Schweizer1976;Kormendy and Hodge 1968).InthisrespectthenitresemblesothergiantIn’s tial model,whilethemeanradialbrightnessprofileisnearer theoretical expectationsforanidealizedexponentialdisk, tendency toshowstructures,suchasdisconnectedblotchesof In thisregard,itisinteresting thatthetwoM81Group Two samplesofmajoraxisbrightnessprofilesareillustrated DDO 43providesamoretypicalexampleoftheresultsfor F(Hß) X5007/X4959[Oii]/[0Hi]TO/H e 2.91 2.37 9.50 4.54 4.62 8.67 3.68 3.79 3.66 1.43 1.40 1.65 3.61 5.48 9.62 8.20 2.75 2.23 2.58 2.66 3.37 3.23 3.09 2.27 2.86 3.01 3.30 1.89 3.04 2.65 2.90 2.91 3.84 0.84 0.65 0.86 0.95 0.73 0.31 0.81 0.71 1.01 3.05 1.41 1.50 0.55 0.66 1.52 1.23 1.24 7600 7500 9800 9400 8500 8100 8200 8000 8200 8100 9300 7700 8400 8400 8400 8600 8000 4.98 4.67 4.46 2.83 4.47 4.34 6.29 2.56 4.03 3.79 5.83 3.16 3.90 3.56 3.65 5.00 5.27 541 1985ApJS...58..533H is atthetop;easttoleft. Hunter andGallagher(seepage 541) Fig. 10.—High-contrastversionsoffourrepresentativeCCDimages showthefainterregionsofdwarfgalaxies.ComparewithimagesinFig.1.North © American Astronomical Society •Provided bythe NASAAstrophysics Data System PLATE 27 1985ApJS...58..533H result ofatriaxialformrather thanathindisk(seealsoTully Richter (1984)alsoshowdeviationsfromstandardexponen- sistent withtheirrotationproperties (Tullyetal1978),al- galaxies have“disklike”photometric characteristics,con- et al1978).Areasonableview forthepresent,then,isthatIn- tial diskmodels,whichtheseauthorspointoutmaybe the 542 a a DDO 218 was used. (Table 6)reddening.AbsolutefluxesofH/?arenotgivenbecause nonphotometricobservingconditions. IC 1613A10 IC 10 DDO 83... DDO 214. underlying stellarabsorptionusingpopulationsynthesistechniques. Ratios arecorrectedforexternalGalactic(Table1)andinternal M33 the continuum.ForIC1613-A17andM33-47correctionis15%. DDO 68.. DDO 64.. DDO 53. DDO 50. DDO 49. DDO 47.. DDO 43. DDO 26 b a Hß emissionfluxesinthesespectrawerecorrectedforunderlyingstellar absorption.InDDO49-cen,Hßwasnotobservedabove ObservedwiththeKPNO4m,soaperturesizewas474.Theother observationsweremadeonthe2.1m,andan874aperture Cols. (3)-(l5).—Emission-linefluxesrelativetoHßemission.In afewcasestheHßemissionfluxhasbeencorrectedfor Col. (2).—Hiiregionidentification,asnumberedinFig.1. Col. (7).—Galaxyidentification. Galaxy © American Astronomical Society • Provided by theNASA Astrophysics Data System (1) b A17 Region \3727 b H il[Oii] 47 10 15 14 11 (2) (3) 6 4 2 2 2 6 1 4.18: 2.73 4.61 2.62 1.51 1.98 1.58 3.76 3.44 0.93: 3.76 1.69 0.52 2.06 2.59 3.21 3.49 4.91 4.85 4.97 1.73 2.14 2.01 3.73 2.36 3.99 1.98 3.47 1.00: 1.73 [Ne in] A 3869 0.11 0.16: 0.21: 0.50 0.97: 0.37: 0.40: 0.26: 0.16: 0.48 0.200.280.45 (4) Intensified ImageDissectorScannerEmissionData A3889 0.20 0.17: 0.22: 0.21 0.22 0.280.29 0.36: 0.20: 0.15: 0.13: H8 (5) HUNTER ANDGALLAGHER A3970 A4101 0.17: 0.29: 0.25 0.30: 0.51: 0.36: 0.19: 0.21: 0.23: 0.28 0.24 0.29 0.42 0.34 h£ m (6) (7) 0.24 0.20 0.39 0.18 TABLE 5 A4340 A4959A5007 0.30 0.47 0.51 0.54: 0.47 0.48: 0.47 0.47 0.53 0.46 0.39 0.45 0.44 0.50 0.41 0.49 0.25 0.47 0.65: 0.44 Hy [OIII]ill] (8) (9)(10) B CCDimagesaresummarized inTable13.Circularaper- derived frommajoraxisprofiles andaperturephotometryon tures werecenteredbyeye fromouterisophotesandused to derivehalf-lightdiameters (takendirectlyfromthe though theirdetailedstructuralpropertiesarenotunderstood. Central surfacebrightnessesandangularsizesofdwarfIn’s 0.53 0.63 0.26 0.99 0.40 0.60: 0.43 0.70 1.16 1.04 1.32 1.41 1.42 0.84 0.66 1.11 1.66 4.57 1.67 0.94 0.60 1.23 2.18 1.35 1.10 1.20 1.15 1.26 1.06 2.64 0.83 0.78 0.21 4.36 2.08 2.78 2.21 1.50 3.52 3.99 1.39 3.72 1.12: 0.79: 2.15 4.69 3.21 1.30 4.58 2.76 6.18 3.80 3.37 2.10 2.14 2.39 3.65 1.60 3.91 Ha/Kß A6584A6678A6717A6731 2.91 0.170.04:0.11:0.66: 2.87 0.180.06:0.29:0.19: 3.10 0.140.03:0.09:0.06: 2.89 0.24 2.86 2.87 2.85 2.86 2.85 2.85 2.86 2.47 5.57: (11) (12)(13)(14)(15) [N ii]Hei[Sil] 0.06: 0.09 0.05 0.16 0.33: 0.24: 0.18: 0.13: 0.09: 0.08: 0.13: 0.11: 0.10: 0.16: Vol. 58 1985ApJS...58..533H probably reflectsthedifficulties inherentinworkonfaint brightnesses. ThedatainTable13comparewellwith the objects, whichcan,forexample, leadtodifferentchoicesof de Vaucouleurs,andButa(1981)conven- circular-aperture data),totalmagnitudes,andmeansurface tween thetworscalesand betweenthetwoii(r)scales tional aperturephotometry. The possiblemodestoffsetbe- No. 4,1985 e Be © American Astronomical Society • Provided by theNASA Astrophysics Data System a M33 . 1979. IC 1613 IC 10 DDO 218. DDO 214. DDO 83.. DDO 68 DDO 64. DDO 53. DDO 50. DDO 49. DDO 47. DDO 43. DDO 26. a + -3 Theseabundancesareprobablyoverestimates;seetext. Col. (5).--Theratioofionizednitrogentosulfur. Col. (7).—Theneonabundance. Col. (6).—TheionizationcorrectionfactorusedingoingfromNe toNe. Col. (5).—Theoxygenabundancecalculatedassuminganelectrondensity of100cm. Col. (4).—Electrontemperaturecalculatedfrom([On]+[0iii])/H/? andtherelationshipdeterminedbyPageletal. Col. (3).—InternalreddeningbasedprimarilyonHa/Hß.Whenthis wasnotavailable,0.2adopted. Cols. (7)-(2).—GalaxyandHiiregionidentification. Galaxy (1) Region A17 A10 H ii cen (2) 47 15 10 14 11 4 4 7 5 4 1 3 7 6 2 2 2 6 4 1 2 1 1 2 1 Intensified ImageDissectorScannerDerivedParameters E(B-V) 0.2 0.13 0.00 0.2 0.2 0.2 0.38 0.2 0.50 0.21 0.18 0.39 0.2 0.21 0.2 0.1 0.17 0.24 0.2 0.13 0.15 0.00 0.2 0.2 0.20 1.0 0.25 0.2 0.2 0.2 (3) DWARF IRREGULARGALAXIES 11000 10000 12000 12000 10000 10000 11000 10000 7400 7100 7300 8100 8300 9100 8200 8600 8000 8200 9800 9600 5500: 5800: 8500 8300 9300 8800 8000 9200 9100 9700 (4) T e TABLE 6 4 galaxies havingappropriate data inTables1and13.Two well-known pointsareillustrated bythisdiagram:(1)The of 1.13forpurelyexponentialdisks. 10(O/H) dwarf Irrsamplewaschosen onthebasisofirregularmor- central positions.UsingtheCCDdatawefind/¿(0)- = 1.1±0.1,inexcellentagreementwiththetheoreticalvalue 5 15 :: In Figure12weplotsurface brightnessesversusMfor B 4.3 4.9 6.9 4.7 4.2: 5.2 7.4 3.6 3.2 3.9 2.8 2.5 2.7 2.2 4.3 1.8 8.7: 3.7 2.8 3.2 2.3 2.3 1.6 1.7 (5) 3.5 1.6: 2.4 2.8 2.8 + 0/0 2.9 1.6 2.1 1.5 1.9 2.7 1.8 1.5 1.7 2.1 (6) 5 10(Ne/H) 4.9 4.8: 7.8 6.0 6.2: 9.0 5.3: 9.0 19: 11: (7) +4 N/S 7.5: 8.1: 3.3 1.6: 2.8: 1.6: (8) 543 1985ApJS...58..533H phology andlowsurfacebrightness,butasThuanSeitzer (1979zz, h)haveemphasized,thosecriteriadonotuniquely been truncatedbecauseofexhaustionorremovalthe bulk These galaxiesarethoughttohaveatmostlowlevels of by thediffusedwarfellipticalmembersofVirgoCluster of (see below).Thishastheeffect ofmovingpointsinFigure12 of theinterstellargas(WirthandGallagher1984).If star considered tobedwarfIrr-likesystemsinwhichevolution has dwarf Irrsystems,surfacebrightnessandluminosityare not to theupperleftalong45° or steepertracks.Itisthenclear Bagnuolo 1973)andperhaps appearsomewhatmorediffuse formation weretoceaseinthedwarfIrr’s,theywouldfade as galaxies studiedbyBinggeli,Sandage,andTarenghi(1984). correlated (deVaucouleurs,deandButa1983). conventions, truedwarfshaveM<—16.(2)AmongDDO select galaxiesoflowintrinsicluminosity.Followingstandard that mostofthedwarfIrr’s, which alreadyheatlowermean their stellarpopulationsage(e.g.,Searle,Sargent, and star-forming activity(Caldwell1983)andthusaresometimes The dashedlineinFig.5bisdrawnbyeye.“HSBIrr’s”referstothegiantIrrsampleofHGR.“OtherObjects”aretakenfromLequeux etal.(1979), 544 French (1980),andKinmanDavidson(1981). B Also showninFigure12istheapproximateregionoccupied Fig. 5.—[Oil]X3727and[OXX4959,5007emission-linesumratioasafunctionoftheO/Hcomputedfromtheselines(fromHunter 1982û). © American Astronomical Society • Provided by theNASA Astrophysics Data System O 0 1=1 O 1.0 4 0 b “ 2 H QQ. 6 X O O + 2.0 12 10 8 0 ■i -t. ,/é> •.*.•^ A *• : Xx A • ^S . X* HUNTER ANDGALLAGHER ^ x Q . ^n* x x >*< XA C 4 6 4 10 0/H X A X a DwarfIrrs HS Birrs • OtherObjects A Z^A A fight growthcurves.Thisresult isshowninFigure13,where ongoing starformation.IngalaxieswithintenseHaemission, H iiregionsasindicatorsofthelocationsandlevels of forming activity.Wehaveextendedthesedatabyutilizing an HaCCDimageandcan be compareddirectlywiththeB the HaandBfightmagnitude growthcurvesagreetowithin such asNGC4449,simulated photometrywascarriedouton within dwarfIrrgalaxiestotheaveragedistributions of (de Vaucouleurs,deandButa1981,1983; also comparing theaverageradialprofilesofstar-formingactivity usually revealsaslightcentralbluewardcolorgradient Hodge 1978),whichsuggestsacentralconcentrationof star- stars. MultiapertureUBVphotometryofdwarfIrrgalaxies liptical galaxiesthanasimpleremovalofgas. involved indistinguishingdwarfIrr’sfromdiffuse el- fi(r) thandiffuseellipticalsofsimilarabsolutemagnitude, found intheVirgoCluster.Thisimpliesthatmoremaybe systems existforwhichelliptical-likecounterpartsarenot can neverevolveintosuchsystems.ThussomedwarfIrr A Be ‘ A The aperturephotometrydataalsoprovideameansfor TS- A x (b)- (a) 10 Vol. 58 1985ApJS...58..533H No. 4,1985 included morethanoneionizedregion.AverageabundancesfortheLMC, measurements weremadewithalarge(22")apertureandsomayhave surements ofdifferentHnregionswithinagalaxy(IIDS).The1RS SMC, andtheSunareshown(Smith1975;Kaler1980). O 4 i Fig. 6.—Theoxygenabundanceforeachgalaxy.Linesconnectmea- 6 8 0 5 7 2 3 I © American Astronomical Society • Provided by theNASA Astrophysics Data System a a a a DDO II. DDO 26. DDO 43.. DDO 42.. DDO 47. DDO 49.. DDO 50. Galaxy (1) Region H II (2) 13 11 10 17 16 14 13 12 19 3 2 1 1 2 2 1 4 3 1 2 1 6 3 1 9 5 8 DWARF IRREGULARGALAXIES NGC 2363 HK 10-12 HK 13 HK 16 HK 15 HK 14 Videocamera HiiRegionData Other HK4 HK 5 HK 2 HK1 HK 6 (3) ID TABLE 7 dn 41.3 23.8 13.8 16.7 12.3 10.6 12 12 (4) 4.5 2.9 2.6 2.9 2.2 5.4 4.1 3. 3.3 6.2 9.0 5.1 3.6 6.2 5.4 5.1 8.1 8.2 be verysimilar,whichimpliesthatstarformationisnot profiles ofHaemissionandBfightinNGC4449thenmust centrally concentratedtoanymajordegreeascomparedwith the basisofBgiveninRC2.Onaverage,radial the existingstars. between Bfight,whichoriginatesfromstarsformedatleast videocamera datahavebeenmergedtoimprovethestatistics. introduces difficultiesinmakingapropercomparison.At problems areillustratedbytheHaandBprofilesforDDO introduce statisticalsamplingproblems,particularlyindwarfs larger radiitheHa-to-bluefluxratioisroughlyconstant; regions requiredtofittheaveragetrendissmall.These over afewGyrago(Gallagher,Hunter,andTutukov1984), dwarf IrrgalaxieswheretheHagrowthprofilesfrom effect. ThisapproachhasbeenextendedinFigure14tofour formation, thereisnotanexactone-to-onecorrespondence apparent central“hole”inHaismostlikelyastatistical 214 inFigure13.AlthoughrichHnregionsforadwarfIrr, and intheouterregionsofgalaxies,wherenumberHn and Haemission,whichcomesfromyoungerstars.Thiscan the individualHnregionsdetectedonvideocameraimages the discontinuousnatureofHafluxcurvederivedfrom T -10% outtoaradiuswhichcontains90%oftheBlighton Of course,becauseofthespatiallylocalizednaturestar d (pc) 2700 (5) 290 330 510 700 280 700 100 190 400 230 200 150 200 100 180 140 140 100 60 90 36 54 70 90 90 270 L(Ha) 93 15 4.2 0.2 0.05 0.1 1.5 (6) 4.4 0.6 0.1 1.4 0.6 2.1 3.4 1.1 0.2 0.3 0.5 0.3 0.3 0.2 0.8 2.3 3.4 0.6 V(0*) 1300 450 (7) 20 20 20 20 70 10 10 0.3 7 7 0.5 5 1 1 4 3 2 1 3 1 2 2 3 1 2.4 2.4 2.3 2.4 4.1 (8) 6.6 3.0 3.5 1.7 5.0 SB 7.7 5.8 4.3 2.0 3.5 8.0 4.2 8.6 5.6 3.5 5.5 1.7 24 14 12 12 12 545 1985ApJS...58..533H a a -2 a a a DDO 53. DDO 64 DDO 68 photons perstar.Thiscorresponds toadoptingameanmassof~30MperOstar(seeHGR). ionizing photonsandusingaMiller andScalo1979initialmassfunction-weightedaveragenumber of bandpass, transformationtofluxunits,andGalacticreddening. pc inHa. Regions inDDO209givenasIIIandIIfromSandageTammann 1974arealsoVandIIIinHubble IC 10 DDO 218. DDO 214 DDO 209. 1925. DDO 83 0 a 31-2 3g-1 3-1 Thesegalaxieswereobservedthrough thinclouds,sofluxesarelowerlimits. Col. (8).—TheHasurfacebrightness, i.e.,luminosity/area,inunitsof10ergss~pc. Col. (7).—Thenumberofstarswithmassesgreaterthan15A/, calculatedfromthenumberof Col. (6).—Haluminosityinunitsof10ergss,correctedfor extinction,redshiftinthefilter Col. (5).—Thediametersinpcatthegalaxybasedondistancesgiven inTable1. Col. {4).—ThediameteroftheHnregionsinarcsecmeasuredtoacontour levelof2.5X10ergss Col. (3).—AlternateidentificationoftheHiiregions.“HK”refers toHodgeandKennicutt1983. Col. (2).—Hiiregionidentification(seeFig.1). Col. (7).—Galaxyidentification. Galaxy 0 (1) © American Astronomical Society • Provided by theNASA Astrophysics Data System Region H ii (2) 20 22 21 23 24 25 . 2 III 11 2 II 1 3 2 1 2 1 4 3 6 5 4 4 4 6 2 9 7 2 7 5 1 3 1 8 5 3 3 2 1 1 Hubble III Hubble V HK 17 HK 18 HK 19 Other ID (3) TABLE 7—Continued 14 18.1 25.6 33.0 14.6 14.5 12.7 10.6 11.6 7.2 5.3 4.9 3.0 4.6 1.9 6.2 4.8 4.3 5.2 dn 6.0 7.3 1.9 5.6 3.1 4.3 4.3 4.8 4.8 4.4 4 6.4 546 6 7.5 3.3 6 6 3.8 3 3.5 3.7 3 3 5.1 (4) d (pc) 1700 1800 1400 1200 (5) 270 210 290 240 260 200 150 360 100 310 120 630 600 690 600 600 670 500 780 200 290 780 150 870 110 30 50 90 93 80 80 40 40 40 30 90 90 80 84 280 L(Ha) 66 13 79 81 14 15 12 36 16 0.2 0.9 4.3 0.2 0.6 0.1 0.2 0.05 0.06 0.8 0.4 0.5 2.2 3.4 1.2 3.0 1.3 (6) 0.1 4.4 0.2 0.2 0.2 4.9 4.1 0.4 0.4 3.3 7.6 6.8 7.9 7.9 3.7 1.4 1400 400 400 300 200 (7) 20 20 10 15 40 40 40 20 20 20 60 60 20 30 70 70 80 4 0.2 0.3 4 6 6 2 2 1 1 3 1 1 2 2 2 1 1 1 1 7 66 18 26 79 50 15 19 12 15 4.6 4.6 4.1 4.6 4.8 2.5 2.5 2.3 2.8 2.0 7.6 7.4 7.4 3.1 3.4 1.4 3.7 (8) SB 4.3 2.7 6.1 2.8 6.3 3.2 3.9 5.0 5.0 3.4 3.4 3.8 6.0 1.9 8.6 3.7 1985ApJS...58..533H NGC 4449. NGC 4214. A1004+10. NGC 3738. NGC 3510., NGC 3274., b 34_1-2 a 3_1-2 38_1 3_1-2 The sameascols.(3)-(5)butfor a contourlimitof10ergsspc. The parametersincols.(3)-(5)are measuredtoacontourlevelof2.5X10ergsspc. Col. (5).—Haluminosityinunits of10ergss. Col. (4).—Hasurfacebrightnessin unitsof10ergsspc. Col. (3).—SizeoftheHnregion in pcusingdistancesadoptedHGR. Col. (2).—Hiiregionidentifications arefromHunter19826. Col. (7).—Onlygalaxieswithin~13Mpcareused. Galaxy (1) Comparison HiiRegionDataforHigh-SurfaceBrightnessIrregularGalaxies © American Astronomical Society • Provided by theNASA Astrophysics Data System Region I +5 29 27 26 H ii 23 22 20 19 22 21 20 17 16 15 12 II 10 15 14 12 (2) 6 2 4 3 9 2 1 8 7 3 2 6 4 3 2 4 1 5 3 6 2 1 9 7 8 1 d (pc) 200 200 420 400 380 250 220 250 260 160 320 260 260 250 420 130 260 240 220 190 560 170 (3) 3 30 2.5 X10 TABLE 8 12a 14 11 13 ergs spc (4) SB 4.9 9.2 2.3 2.7 3.8 5.5 9.9 3.8 5.1 3.7 7.2 5.5 7.7 8.3 5.6 5.9 8.3 5.7 547 L(Ha) 16 17 19 (5) 0.9 0.2 1.6 1.4 3.8 4.1 1.7 2.8 2.2 1.1 3.9 5.6 1.5 5.4 1.2 3.3 3.3 2.2 2.3 1.2 d (pc) 200 190 190 no 300 120 120 160 120 100 no 120 270 no 110 120 160 150 150 180 180 190 210 160 100 170 180 150 210 120 120 120 3412b (6) 90 90 60 80 60 60 60 90 90 10 ergsspc 48 22 23 21 18 18 17 34 (7) (8) 30 55 19 33 33 70 32 73 56 20 SB L(Ha) 11 16 18 55 26 23 16 13 85 56 35 34 27 32 11 18 19 14 13 13 15 32 16 48 10 22 32 14 13 4.6 0.6 6.0 0.6 4.1 2.3 4.9 1.2 5.7 3.7 2.2 1.4 5.9 3.9 3.7 2.9 2.3 4.9 0.2 0.6 0.6 1.3 1.1 3.7 3.2 5.9 2.8 1.8 1.7 1.5 2.6 1.4 1.6 1.6 1.3 1985ApJS...58..533H 34-12 Again weseethattoaradiusenclosingmostofthelight 3_1-28 ( ~70%),theHa-to-Æfluxratioisroughlyconstant. 548 Table 1andforgiantIrr’sfromdatainHGRHunter be measuredtothehighercontourlevelof10ergsspc.SBisin DDO 49 DDO 42 units of10ergsspc,andluminosityisin. DDO 64. DDO 53. DDO 50 Ha luminosities(col.[5])forHnregionsinthedwarfIrr’swhichcould DDO 218 DDO 214 Videocamera DataofHiiRegionsataHigherContourLevel Mean propertiesofdwarfIrr’sderivedfromthegalaxiesin Note.—Diameters (col.[3]),Hasurfacebrightnesses[4]),and Galaxy (1) IV. COMPARISONSBETWEENDWARFANDGIANT © American Astronomical Society • Provided by theNASA Astrophysics Data System NGC 2363 Region H II (2) 11 4 6 2 2 5 3 1 7 1 IRREGULAR GALAXIES a) GlobalStructure 38 1 38-1 galaxies whichwerelargerthanthefieldofviewimage. clouds, sofluxesandstarformationratesarelowerlimits. apertures wereusedinsteadofone largeone,andsosomeofthefluxcouldhavebeenmissed. Ha images;unitsof10ergs s“. Numbersinparenthesesindicatethatseveralsmaller IC 10*.... DDO 218.. DDO 214* DDO 83* DDO 68* DDO 64* DDO 53... DDO 50* DDO 49... DDO 47* DDO 43... DDO 42... DDO 26* 10 ergss. DDO 11* -1 -12 TABLE 9 Col. (6).—Thestarformationrate, Myr. Col. (2).—ThenumberofHnregionsidentifiedinFig.1.Numbers inparenthesesreferto Col. (1).—Galaxyname.Anasteriskindicatesthatthegalaxywas observedthroughthin Col. (7).—Logofthestarformation rateperunitarea,Myrpc. Col. (5).—Numberofstarswithmasses greaterthan15Af(seecol.[7]ofTable5). Col. (4).—ThetotalHaluminosity foundfromperforminglarge-aperturephotometryonthe Col. (3).—ThesumoftheHaluminositiesHnregionsgiven inTable5,unitsof Q Q 0 Galaxy (1) d (pc) 490 640 460 580 110 110 580 150 150 120 (3) 60 V(H ii) (18) (10) (25) (2) 11 4 6 4 2 2 3 3 3 3 1 (4) SB 20 68 12 25 13 15 13 34 17 15 18 HUNTER ANDGALLAGHER Total StarFormationRates F(Ha) E 260 280 380 13 33 (3) L(Ha) 4.5 4.2 0.2 9.3 1.3 7.1 110 60 29 20 32 (5) 0.4 6.5 2.7 1.3 1.6 1.6 TABLE 10 F(Ha) r (400) 720 590 (15) 21 19 17 59 (4) (5.5) (1.9) blue, compactdwarfIrr’swhicharenotwellrepresentedin (1982û, b)aresummarizedinTable14.Mostofthedwarf probably mosttypicalofthenumeroussmallgalaxieswhich ever, thattherangesinluminosityforeachsampleoverlap, and secondarilyintermsofopticalluminosities.Note,how- from thegiantsprimarilyintermsofbluesurfacebrightness specifically selectedgalaxiesoflowopticalsurfacebrightness. Irr’s arefromtheDDOcatalog(vandenBergh1959),which 6.9 Barbiéri, Bonoli,andRafanelli1979;Tullyetal.1981).Still, our selectionofgalaxies(Sargent1970;Huchra1911a,b; thus thebiasistowardhigheropticalsurfacebrightnesses.It properties arenoteasilyderivedfromeitherHioroptical lar disksinthemajorityofgalaxies(Freeman1970),andmany logical class.AllIrr’shaveroughlyexponentialradialoptical underlying physicalunityofthenormalIrrgalaxymorpho- and thatthereexisthigh-surfacebrightness,low-luminosity, then isnotsurprisingthatthedwarfsampledistinguished as partofaninvestigationstar-bursteventsingalaxies,and On theotherhand,ourgiantIrrsampleoriginallywaschosen 1.0 have circularaxialsymmetries,andasaresulttruerotation low peakrotationvelocities(seeGallagherandHunter1984), Freeman 1972).Ascomparedwithspirals,allIrrgalaxieshave Irr’s showevidenceforstellarbars(deVaucouleursand support starformation. estimated fromglobalHiprofilewidths.Inclinationcorrec- differentiated fromgiantsonthebasisofrotationvelocitiesas surface brightnessdistributionsthatarecharacteristicofstel- the low-surfacebrightness,intrinsicallyfaintdwarfIrr’sare although fromTable14itisnotclearwhetherdwarfscanbe tions presentasevereprobleminIrr’s,manyofwhichmaynot The propertiesofdwarfandgiantIrr’salsoillustratethe N(0) 3500 2000 2900 (5) 290 100 90 30 30 85 10 70 5 0.0032 0.325 0.181 0.0025 0.0086 0.0077 0.267 0.0005 0.0008 0.0065 SFR (6) log SFR a -10.3 -10.7 -10.6 -10.4 -9.1 -9.6 -9.3 -9.7 -9.6 -9.6 (7) Vol. 58 1985ApJS...58..533H holds betweenthestellarand HIsurfacedensitiesintheouter regions ofstellardisksinlate spiralandIn*galaxies.Atafixed pencil-beam andsynthesis maps inGallagherandHunter be centrallypeakedinIrr’sandtohaveaflatterradialfalloff into stars(seeTruranandCameron1971;Tinsley1980;Pagel ness betweenourdwarfandgiantIn*galaxysamples. gas intostarsthanIrr’s.Bythesamelogic,therethenappears integrated sense,spiralshavebeenmoreeffectiveinconverting metal valuesoftenfoundinspiralsthenestablishthat an values of—0.5notbeinguncommon.Thelargerelativegas is againnocleardistinctionbetweenthedwarfandgiant structure onsmallerscales,theHisurfacedensityisfound to and Edmunds1981).Thereducedgascontentnear-solar mass fractionofbothgiantanddwarfIrr’sisconsistentwith 1984) . Therealsoappearstobeageneralrelationshipwhich than thestellarlight(see references towell-sampledHi approximately standardform.Despitetheexpectedchaotic fraction oftheavailablegassupplyhasnotbeenprocessed H imasstoopticalluminosityishigherthaninspirals,with categories. Onaglobalscale,thedistance-independentratioof fact, likelytoexist(deVaucouleurs,deandButa readily obscuredifferencesinglobalkinematicsbetweensmall No. 4,1985 to beonlyamodestdifferenceinglobalstar-formingeffective- the modestgasmetallicitiesinimplyingthatasignificant 1983) . statistical correlationbetweenluminosityandrotationis,in samples ofgiantanddwarfIrr’s,eventhoughasystematic Fisher andTully1981).Theseoperationaldifficultiescould spectral observations(seeTullyetal1978;Feitzinger1980; The spatialdistributionofHiinIrr’salsofollowsan As aclass,Irrgalaxiestendtoberichingas,althoughthere © American Astronomical Society • Provided by theNASA Astrophysics Data System 2 corrected foremissionUnes. observed withthe1RS. NGC 7292.. NGC 2366.. NGC784 ... DD0 64.... DD0 49.... 1 arcsec. DDO 236-E. DDO 218... DD0155 ... DDO 140... DD0 94.... DDOS3 .... DD0 53.... DDO50 .... IC 10-E IC10-W .... DDO 236-W. -2 -1 Col. (6).—LogoftheluminosityinBdeterminedfromVand-Table3, Col. (5).—AbsoluteVmagnitude. Col. (4).—SurfacebrightnessatK,givenbyF+6.73,whichisthemagnitudeof Col. (3).—LogoftheSFRperunitarea,inMyr*pc,forregion Col. (2).—TotalSFRinthe1RSaperture,unitsofMqyr. Col. (1).—Galaxyidentification. ö Galaxy (1) Parameters fortheRegionsObservedwith1RS DWARF IRREGULARGALAXIES 0.063 0.00035 0.0029 0.0014 0.00041 0.054 0.00053 0.029 0.0146 0.0028 0.0041 0.00069 0.00084 0.123 0.00019 0.00016 SFR (2) log SFR a TABLE 11 -8.07 -8.61 -8.32 -8.00 -8.54 -8.33 -8.42 -8.82 -8.84 -8.70 -8.41 -8.31 -8.23 -8.29 -8.01 -8.09 (3) 20-n -2 -2 or 311 20- histories, itislikelythatmass-to-light ratiosareapproximately below thatmostIrr’shave experienced similarevolutionary properties areconsidered,as aresultofthepreselectedlower while Huchtmeier,Seiradakis,andMateme(1981)havefound by VanWoerden,Bosma,andMebold(1975)to-5R , average bluesurfacebrightness ofthedwarfs.Aswewillargue stands outfromourearlier giantIn*samplewhenintegral 3109). influence, ifany,onopticalcorepropertiesremainsunclear. similar featuresinintrinsicallyfaintdwarfIrr’s(suchasNGC most late-typegalaxies,theHiandstellarmasssurface is -4+2X10cm,orMpc.ThusatRoimi populations ofmoderateage(LarsonandTinsley1978).The brightness is~1.3Lpc,whichcorrespondsroughlytoa isophotally (i.e.,toastandardstellarsurfacebrightness)de- For example,thegiantIrrNGC4449hasbeenmappedin Hi optical dimensionsofeitherdwarforgiantIrr’s.Afraction of of Irr’s,whereweestimatemeangassurfacedensities are densities areroughlyequal,incontrasttothecentralregions mass surfacedensityof~4Mpcfortypicalstellar later spiralsinBosma’s(1981a,b)surveyhaveaverageextents all Irr’sinfacthaveveryextendedHicomponents,whose H isurfacedensityatÆoim^late-spiralsample of 1.5±0.5Holmbergradii(#Hoim)-^equivalentwaytosee fined Holmberg(1958)radius.Atthisradiusthebluesurface Irr’s (seereferencesinGallagherandHunter1984),Scor H icolumndensityof~1.8xl0cm,dwarfIrr’s,giant this correlationistofindtheHicolumndensityat H olm 0H 0 0 H <0.1 ofthestellarsurfacedensitymassvalues. In summary,weseethatthecurrentdwarfIrrsample Not alloftheHiisconfinedtowithinapproximate 20.80 21.91 21.91 21.91 23.34 20.23 21.36 22.29 22.30 22.00 23.22 22.31 22.64 22.58 20.09 20.00 SB y (4) -17.95 -12.54 -14.11 -14.22 -13.09 -17.32 -12.16 -17.48 -17.08 -13.72 -14.31 -11.81 -11.11 -17.44 -12.21 -12.30 My (5) log L b 6.79 6.62 6.37 6.57 7.38 7.26 6.92 7.48 6.43 6.64 8.90 8.67 7.20 8.72 8.53 8.78 (6) 549 x en ^ 550 HUNTER AND GALLAGHER Vol. 58
Fig. 7.—Results of population synthesis fits to the 1RS spectra. We have plotted the percent contribution of three broad stellar temperature groups to the galaxian light at the normalizing wavelength 5050 A. “ GM” refers to G, K and M stars. Fig. 8.—Population synthesis model for the summed dwarf spectrum
the same in all Irr’s, and thus dwarfs have lower mean stellar are only — 0.01-0.001 of the star formation rates found for densities than giants. That evolutionary histories, mean stellar luminous Sc spirals by Kennicutt (1983; multiply his rates by densities, global gas contents, and total optical luminosities do 0.79 to convert to our scale). Yet since dwarf Irr’s contain not correlate with one another is a remarkable and puzzling considerable reserves of H i gas, their minimal lifetimes for feature of the normal Irr galaxy family. star formation as given by AfH ,/SFR can be incredibly long. For example, DDO 26 and DDO 83 have sufficient interstellar b) Global Star Formation Rates matter to sustain their sparse star formation levels for - 200 The comparison of integral star formation rates for the Gyr, or -10 Hubble times! Giant Irr’s, although rich in H i, dwarf and giant Irr samples in Table 14 serves to underscore have gas for — 1 Hubble time (HGR), while the star forma- an obvious point: dwarfs have few complexes of luminous OB tion lifetimes of spirals may be even more limited (Larson and stars and in a relative sense are making stars slowly. Average Tinsley 1978; Kennicutt 1983). Dwarf Irr galaxies are evolu- stellar production rates are <0.1 of those in giant Irr’s and tionary plodders.
© American Astronomical Society • Provided by the NASA Astrophysics Data System oox S No. 4,1985 DWARF IRREGULAR GALAXIES 551
Fig. 9.—The percent residuals at each O’Connell (1973) passband fit by the population synthesis models
Although dwarf and giant Irr’s are forming stars at different provide an estimate of the current ( < 107 yr) star formation absolute rates, it is less obvious that these two classes of rates, while the blue luminosity can yield a star formation rate galaxies have experienced different evolutionary histories. The averaged over several billion years. The ratio between stellar UBV colors of blue galaxies are primarily indicative of time production rates averaged over these two time scales is then variations in star formation rates (Searle, Sargent, and sensitive to the long-term star-forming history. This informa- Bagnuolo 1973; Larson and Tinsley 1978), and the similarities tion is contained in the ratio (SFR/LÄ), the blue luminosity between dwarf and giant Irr’s in this regard (de Vaucouleurs, normalized star formation rate, which provides an index of de Vaucouleurs, and Buta 1981) then point to parallel recent relative star formation rates over the past several billion years. histories. Furthermore, on the basis of available stellar popula- Figure 15 shows the distribution of the Irr galaxies in terms of tion models, such ás those by Searle et al. and Larson and (SFR/Lß) versus LB. Although there is significant scatter, no Tinsley, Irr’s have integrated UBV colors which are consistent strong correlation exists between normalized star formation with near-constant rates of star formation over a Hubble time. rates and luminosity over a factor of ~ 103 in blue luminosity. Finally we have found that the stellar population temperature Nor is there a clear-cut dependence on Irr galaxy structural mixes yielded by the population synthesis models are similar subclass. These remarkable results imply that Irr galaxies are for dwarf and giant Irr’s; thus there is no evidence for characterized by similar star-forming histories, independent of systematic differences in stellar content. their family branches and luminosities. Huebra (19776) and Gallagher, Hunter, and Tutukov (1984) A crude time scale for constant star formation rates can have also explored star-forming histories of blue galaxies using also be determined by noticing that the time required to a slightly different approach. Briefly, H Balmer emission fluxes approximately produce the present mass of stars at the current
© American Astronomical Society • Provided by the NASA Astrophysics Data System 1985ApJS...58..533H explanation. exponential diskmodelisdrawn forcomparison.Seetextfurther radius inthedwarfIrrDDO43 andthegiantIn*NGC4449.The 552 Fig. 11.—Bluesurfacebrightness asafunctionofnormalizedspatial © American Astronomical Society • Provided by theNASA Astrophysics Data System Vaucouleurs, andButa1981.ColorsofDDO209(NGC6822)comefromHodge1977. molecules. sample. Thecolorsarecorrectedby0.1magforthelowermetallicityofirregulars. latter comesfromthemodelsofLarsonandTinsley1978(theirTable1)foraverageUBVcolorsgalaxiesinthis computed fromMinTable1and(B-V)col.(2). NGC 7292. IC 10 DDO 218. DDO 214. DDO 209. DDO 168. DDO 155. DDO 140. DDO 94.. DDO 89.. DDO 39.. DDO 83.. DDO 75.. DDO 68.. DDO 64.. DDO 53.. DDO 50.. DDO 49.. DDO 47.. DDO 43.. DDO 42.. DD0 26.. DDO 8... BQ Col. (P).—Thedynamicalmass,Munits,fromFisherandTully1975,1981,Shostak1974. Col. {8).—Thegasfraction,¡x=M/M. Col. (7).—Thetotalmassofthegalaxy,AT©units;M=+M*.TheA/=1.34A/toaccountforHeand Col. (6).—Themassinhydrogengas,M©units,takenfromFisherandTully1975,1981,Shostak1974. Col. (5).—LogofthemassgalaxyinstarsunitsA/©.Thisiscomputedfromcol.(4)andM*/L=0.6.The Cols. (2)-(3).—TotalUBVcolors,correctedforGalacticreddening.ValuesaretakenfromRC2ordeVaucouleurs, Col. (7).—Galaxyidentification. Col. (4).—VisualluminosityintheconventionalunitsofSun’svisual(~0.1itsbolometricluminosity), Q T TgäsgasH Y y Galaxy (i) {B~V) 0 R/R (1/2) 0.44 0.1 0.34 0.2 0.23 0.59 0.37 0.39 0.51 0.29 0.25 0.42 0.40 0.24 0.17 0.31 0.30 0.44 0.33 0.42 1.03 (2) (U~B) 0 -0.27 -0.10 -0.17 -0.24 -0.52 -0.22 -0.28 -0.21 -0.5 -0.33 -0.58 -0.46 -0.35 -0.33 -0.11 -0.26 -0.41 (3) 0.00 0.0 HUNTER ANDGALLAGHER Global GalaxyParameters log Ly 8.53 9.50 9.86 8.40 7.34 9.51 9.6 7.4 7.77 8.49 8.0 8.66 9.74 7.82 8.90 8.27 8.02 8.20 8.60 8.84 8.03 (4) TABLE 12 scale. TherewillbestatisticalfluctuationsasparticularH log M* log (SFR/L)=—9.7,which isalsoconsistentwithnearly bursts ofstarformationwhereglobalratesvarybyfactors of5 regions comeandgo,especiallyinthedwarfswithfewer i.e., thepropertiesofIrrgalaxiesareconsistentwithnear extend ourstatementandconclude thatconstantstarforma- constant recentstarproduction rates.Thusweshouldperhaps more extensiveinvestigationofthegiantIrrsample,and thus or more(see,forexample,Searle,Sargent,andBagnuolo overall regions(see,forexample,Ruotsalainen1982), but {factor of3)constantratesstarformationoveracosmictime (M/L) -1fortypicalIrrgalaxies,wesee~Hubbletime; star formationrateis~(L/SFR)(M/L).Taking sample withM<—20have, onoursystem,amean family. Forcomparison,Sc spiralsinKennicutfs(1983) formation tobeageneral characteristic oftheIrrgalaxy clusion ofconstantstarformationratesfromasomewhat from thepreviousdiscussionwecanruleoutgalaxy-wide stellar populationonewouldexpectfromapostburstsystem. 1973). Furthermore,wehavenotyetseenanIrrwith the there arestrongreasonstoconsiderconstantratesof star fi b ßB B 7.12 9.28 9.64 9.29 9.4 7.8 7.80 7.98 7.2 7.55 8.31 8.18 8.27 8.05 8.44 9.52 7.60 7.81 8.68 8.38 8.62 (5) Gallagher, Hunter,andTutukov(1984)arrivedatthecon- log M Hl 9.53 9.54 9.73 7.40 8.27 8.17 8.75 9.47 9.71 8.65 8.91 9.76 9.46 7.78 8.97 8.59 8.78 8.56 8.25 8.19 8.91 9.21 (6) log M t 10.06 10.04 9.82 7.67 9.77 8.66 8.54 8.97 9.97 9.24 8.82 9.14 9.12 8.80 8.94 8.76 8.44 8.39 9.41 8.16 (7) 0.5 0.7 0.6 0.8 0.7 0.9 0.6 0.7 0.7 0.7 0.8 0.9 0.8 0.7 0.9 0.8 0.6 0.8 0.8 0.8 (8) log M [ 10.17 10.14 10.46 9.37 7.96 9.74 9.93 9.37 9.46 9.17 9.24 9.11 9.77 8.86 9.22 8.49 (9) Vol. 58 1985ApJS...58..533H No. 4,1985 constant raredrawn.Errorbars areshownononeparticularlyuncertaindatapoint. profiles ofHaemissionandbluelightareapproximately galactic disks.Wefoundthatthemeanradialbrightness region occupiedbyVirgoClusterdiffuse Egalaxies(Binggeh,Sandage,andTarenghi1984).Arrowsshowing theeffectofdistanceerrorsandevolutionata tomatic ofthisnear-equilibriumpropertystarformationin contain sufficientamountsofgas. tion ratesareanintrinsicfeatureofgalacticdiskswhich € Another featureoftheIrrgalaxiesalsomaybesymp- Fig. 12.—Bluesurfacebrightness attheeffectiveradiusasafunctionofblueabsolutemagnitudefor dwarfandgiantIrr’s.Thehatchedareaisthe © American Astronomical Society • Provided by theNASA Astrophysics Data System NGC 3738., NGC 7800.. NGC 1156. NGC 1140. DDO 214 DDO 43. DDO 218 DDO 49. DDO 26. photometry. KPNO 0.9mtelescopeinBlight.DataforNGC3738from1983springobserva- Vaucouleurs, andButa1981ortheRC2. and Buta1981orintheRC2. axis radialprofiles;typicalprecisionis±0.2mag. tions withsimilarequipment. -2 Col. (2).—EstimatedcentralsurfacebrightnessinBmagarcsecfrommajor Col. (5).—EffectiveradiusinpcbasedonCCDdataanddistancesTable1. Col. (S).—EffectiveradiusinarcsecasgivenbydeVaucouleurs, Col. (7).—Galaxies,bothdwarfsandgiants,observedwithanRCACCDonthe Comparisons ofSurfaceBrightnessesandSizesforIrregularGalaxies Col. (7).—MeanBsurfacebrightnesswithinr'fromCCDdata. Col. (6).—MeanBsurfacebrightnesswithinrasgivenbydeVaucouleurs, Col. {4).—EffectiveradiusinarcsecderivedfromCCDimagesviasimulated e e Galaxy (1) 20.5 20.9 20.6 21.3 22.1 21.5: 22.3 22.7 18.3 DWARF IRREGULARGALAXIES (2) 19±4 (3) 49 25 26 26 25 TABLE 13 (4) 40 25 23 24 21 23 37 10 19 whether thisrelationshipholdsintheoutermostregionsof radial profilesofstar-formingactivityinIrr’sremainroughly Irr’s (wherestatisticalsamplingproblemsaremostsevere), constant overtime.Notethatwearenotabletoascertain similar informbothdwarfandgiantIrr’s.Itthenfollows from ourprecedingdiscussionthatinanaveragesensethe 4700 2900 7300 2600 1700 5300 1400 (5) K 600 740 20.5 23.0 23.8 23.3 24.1 24.2 (6) r ^BÍé) 23.3 22.0 20.7 22.1 22.3 22.7 23.5 23.7 19.8 (V) 553 1985ApJS...58..533H centrally concentratedasIrrgalaxiesevolve(seealsoHodge and thusitispossiblethatstar-formingactivitybecomesmore and Lucke1970). high-surface brightnessbackgroundofdiffuseHaemission giant Irr. 554 measuring propertiesofHIIregionsindwarfsandgiantsto the sameHasurfacebrightnesslevel.Thegiantsoftenhavea In §IIIZ?wepointedoutafewofthedifficultiesin Fig. 13.—Surfacebrightnessvs.normalizedspatialdiameterinB(stars)andHaemission(currentstar-formingregions)lightforadwarf Irranda © American Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 14.—Surfacebrightnessvs.normalized diameterforfourdwarfIrr’sandthemeanHnregionprofile c) Star-formingRegions HUNTER ANDGALLAGHER D/DU/Z) These arealloftheHnregionsinnearby(<12Mpc) Tables 7,8,and9,summarizedinFigures16,17,18. level. OnecanseethatHIIregionsinbothgroupsofgalaxies galaxies andareconsistentwiththeloweroverallstarforma- These areimportantdifferencesbetweenthetwogroupsofIrr Irr’s whichcouldbemeasuredtoagivensurfacebrightness cover thesamerangeofsizesandluminosities.DwarfIrr’s tion activityinthedwarfs. and manyHregionswhicharecloselycrowdedtogether. Properties ofindividualstar-formingregionsaregivenin Vol. 58 1985ApJS...58..533H -2 3-12 3-1 No. 4,1985 diameters (vandenBergh1981).Partofthedifferencemay also bestatistical;thedwarfIrr’shavefewerHiiregionsand in detail,thedistributionofHregionsizesturnsupatlower Irr’s. Forexample,intheLMC,whichiscloseenoughtosee regions arehardertoidentifyasdistinctunitsinthegiant may beduetoselectioneffectsalreadymentioned;thesmaller However, onaveragetheHiiregionsmeasuredindwarfs can makelargeHIIregions(seealsoRuotsalainen1982). pc (Tables7and8).Arrowsmark themedianvaluesforeachsample. appear tobesmaller(fewerOstars).Partofthisdifference measurements toanHasurfacebrightness levelof10X10ergsspc(seeTables8and9);thebottom twopanelsareforalevelof2.5X10ergss o o u_ co cr Fig. 16.—Thedistributionofsizes ofHnregionsforthegiant(HSB)anddwarf(LSB)irregular galaxysamples.Thetoptwopanelsarefor Fig. 15.—Totalstarformationrate perblueluminosityvs.globalforgiantanddwarf Irr’s -10 - -9 - I 2346810204080200400800 © American Astronomical Society • Provided by theNASA Astrophysics Data System “i inTTTT 1 * LocalDwarfIrrSample ° Distant'DwarfIrrSample • GiantIrrSample H iicomplexes: Evolutionary parameters: Global structuralcharacteristics: AI/ Average PropertiesofGiantandDwarfIrregularGalaxySamplesforD<15Megaparsecs ¡‘■(’’e) M M (median) Ha surfacebrightness(median). O/H gasabundances Estimated gasmassfraction HI lifetime,M,/SFR(median). Typical log(SFR) Ha luminosity(median) Diameter (median) Global log(SFR/L*) SFR/area (median) (hi)2o* (median) Major axis b Hl H l/ioV Fig. 15 b Parameter ii ¡ii DWARF IRREGULARGALAXIES Haro 22 -10.0 +0.3MqyrX.Q 9 1 7 10“ Myr 110 kms“(80,220) 0 TABLE 14 -1±1 M c lOGyr (0.1,16) - 4 8 Lpc 9 0 5 Giant In- 4X10" 10 M 10 Ln 0 225 pc 15 kpc -18 0.5 Irr’s bedoingalargepartoftheirstarformationinsmaller 22 there isadistributioninsizesofregions;butcouldthedwarf hence arelesslikelytocontainagiantstar-formingcomplexat clearly (seealsoFeitzingerandBraunsfurth1984). LMC (Davies,Elliott,andMeabum1976)showsthisproblem any giventime.Anotherinherentproblemisindefininga single star-formingunit.AnexaminationofHaimagesthe regions ingiantsanddwarfsmaybereal.Inanygivengalaxy In spiteoftheseproblemsthedifferencesinsizesHn pc 1 102 -10.2 +0.4Mqyr“i.^ _1 10' Mqyr'pc' 90 kms(45,160) 1 -2 +1Mqyr' 36 Gyr(11,270) s 4 1.5 Lpc" 0 4 x10Mq 4 3 X10L, Fig. 16 Dwarf In 4x 10' 100 pc -15.5 7 kpc 0.8 24 d(pc) 'G 555 1985ApJS...58..533H which isthepredictionofstrongly decliningSFRsinclosed- insights intosomebasicevolutionarypossibilitiesforgalaxies we seemanyHnregionspoweredbyonlyafewOstars(e.g., like Hiiregions(KillenandDufour1982).IntheMilkyWay difficulties inapplyingthetechnique togalacticdisks(Larson (e.g., Larson1975;Gottand Thuan 1976),thereisavarietyof is temptingtothinkofthedwarfIrr’sasformingalarge not uniqueevents(Hodge1977).Throughouttherestofthis its currentlargestar-formingcomplexesinthenorthernpart have steeperHiiregionsizedistributions(fewerlargeregions). galaxies. Inthesemodelsinterstellarmatterofdensityp is difficulties forgalacticevolutionarytheories.Oneapproach to Hodge 1977,1978). dwarf Irr,however,thereare25smallnebulaeand14stellar- of thegalaxy,yetpresencenumerousOBassociations This pointisillustratedbyNGC6822,whichhasallthreeof units? Hodge(1983)hasfoundthatthelessluminousgalaxies assumed tobesmoothlydistributed,andtheSFRisgiven by Milky WaytoobtainaprescriptionforSFRsinidealized fraction ofstarsinthislessspectacularmanner(seealso Orion), whichwouldbehardtodetectinothergalaxies,andit and starclustersovertheentiregalaxyshowsthattheseare Tutukov 1984). 1976; Smith,Biermann,and Mezger1978),nottheleastof this problemhasinvolvedanextensionofresultsfrom 556 system galaxieshavingfixed sizes(Gallagher,Hunter,and SFR =Ap”withn—2.Whilethisdescriptionprovidesuseful Schmidt’s (1959)investigationofstarformationratesin the giant anddwarfirregulargalaxysamples.SeeFig.16forfurtherexplana- tion. g The descriptionofstarformationprocessespresentsmajor Fig. 17.—ThedistributionofHaluminositiesforHnregionsinthe © American Astronomical Society • Provided by theNASA Astrophysics Data System a) SimpleEvolutionaryModels V. DISCUSSION F,o38ers Ha (g"^ HUNTER ANDGALLAGHER 2 devised thatisbasedonthe concept ofstochasticself-propa- giant anddwarfirregulargalaxysamples.SeeFig.16forfurtherexplana- relationship initsusualSFRapformdoesnotproperly galaxies. describe currentstarformation processesinthesetypesof tion. gests thatinternalmechanismsmustcontrolratesof star reservoirs couldsupplysufficientfueltoallowactive star uniformity ofstar-forminghistoriesamongIrr’sstrongly sug- formation, andthatanextensionoftheidealizedSchmidt formation tocontinueovercosmologicaltimescales.But the the opticalbodiesofIrr’sisunimportant;e.g.,such gas tion ratesinIrrgalaxies.Thisisnottosaythatgasoutside that gasinfallorotherexternaldrivescannotsetstarforma- be stabilizedifpcouldkeptatnear-constantlevelsvia infall fromgaseoushalos(seealsoTinsley1980andShore radial dependencesindiskstar-forminghistorieswithinIn- over thepastseveralbillionyears,SFRswithingalactic cases exceedreasonableexpectationsforaccretionfromhalos. more massivegalaxieswouldberequiredtohavelowervalues near-constant SFRsareapropertyofmostIrrgalaxiesand galaxies. disks arelocallyinequilibrium;i.e.,whytherenotobvious of theirequilibriumSFR.Yettheobservationsclearlyshow only weaklyongalacticmass(Tutukov1983),andtherefore gravitationally boundgashalos,theaccretionratewilldepend mass. Forsimplemodelsofgasaccretionfromisothermal, Similarly theinfallconceptdoesnotnaturallyexplainwhy that SFRsincreasewithmassinspiralsandIrr’smany thus donotobviouslydependonglobalparameters,suchas 1981). Thisidearunsintoproblemssincewenowseethat s g Recently aclassofgalactic evolutionmodelshasbeen These shortcomingsintheinfallmodelleadustoconclude Fig. 18.—ThedistributionofHnregionsurfacebrightnessesforthe Gallagher etal.suggesttheSFRinsmallergalaxiesmight b) StochasticSelf-propagating StarFormation 12 SBOO ergss"pc") Vol. 58 1985ApJS...58..533H in aregionwhichhasjustundergonestar-formingevent.The which thepossibilityfornewstarformationisgreatlyreduced refractory timemightphysicallycorrespondtothere- gating starformation(SSPSF;MuellerandArnett1976; for starformation. quired foragivenareatorefillwithgasinanappropriatestate formation canspatiallypropagatethroughoutagalaxy.Atime star formationinadjacentregions,andthisway region isassumedtoconsiderablyenhancetheprobabilityfor Gerola andSeiden1978;seealsoreviewby properties ofdwarfIrr’spredictedbythesemodelsdependon have beenpublishedbyGerola,Seiden,andSchulman(1980), scale isbuiltintotheprocessthrougharefractorytimein 1982). Inthesemodelsthepresenceofstarformationinone we seelittleevidencefortheoccurrenceofglobalbursts The SSPSFmodelsthenpredictcorrelationsbetweengalaxy No. 4,1985 size andcertainindicatorsoftheoverallstarformationhistory show thewidestrangeinobservablepropertiesthataresensi- Seiden, Schulman,andFeitzinger(1982),Comins(1983, largest giantIrr,NGC4449,hasamuchhigherrate.Models process thatcontrolsSFRsandislargelyindependentof is anuncertainfeatureofthemodels(cf.Freedmanand This importantpropertyoftheSSPSFmodelsisnotdirectly models havetheverynicecharacteristicofproducingstarsat star formation,andneitherourdatanorthephotometric such asmetallicityandgasfraction.Inoursampleofgalaxies a seriesofdisconnectedstar-formingburstsandthusshould the galaxysizerelativetoofstarformationcells. SSPSF simulationsofsmallIrrgalaxiesforus.Ingeneralthe 1984). Inaddition,Seiden(1982)kindlyranaseriesofspecific propagation ofstarformation betweenspatialcells.Asdis- by Seiden(1982)demonstratethatitispossiblewithin the interpret theconstantstarformationratesingalacticdisksas which controlstarformationrates.Itthereforeistemptingto roughly constantrates(