1992ApJ. . .388. .310K where theprojectedaperturediameteristypicallyseveralkilo- directly comparedtotheobservationsofdistantgalaxies, regions areavailableforlargesamples,butsuchdatacannot be spectra ofverynearbygalaxies.Spectracoveringthenuclear objects, surprisinglylittleisknownabouttheintegrated mediate sampledata,withtypical projectedaperturesofafew (1982), Bothun&Dressier(1986),andSilva(1991),whileinter- parsecs ormore.Large-aperturespectrophotometryhasbeen kiloparsecs, areavailablefor largersamples(Petersonetal. obtained forafewobjectsbyWells(1972),Dressier&Gunn The AstrophysicalJournal,388:310-327,1992April1 virtually theonlyinformationontheirstellarpopulations. properties. Forhigh-redshiftgalaxies,whicharetoodistantto produced anextensivedatabaseonthespectroscopicproper- Gunn 1983;Couch&Sharpies1987;Broadhurst,Ellis, spatially resolvefromtheground,integratedspectraprovide of theirstellarcontent,starformationrates,andevolutionary ties ofgalaxiesatmoderateredshifts(z~0.2-0.7),anda few © 1992.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. observations havebeenmadetoz~2(e.g.,Elstonetal.1991). Shanks 1988;Lavery&HenryCollessetal.1990)have Surveys offaintgalaxiesbyseveralgroups(e.g.,Dressier& ment withtheNationalScienceFoundation. versities forResearchinAstronomy, Inc.(AURA),undercooperativeagree- Astronomical Observatories,which are operatedbytheAssociationofUni- 1 Given thebroadinterestinspectralpropertiesofdistant The integratedspectraofgalaxiesarepowerfuldiagnostics VisitingAstronomer,KittPeakNational Observatory,NationalOptical for spectroscopicsurveysofmoredistantgalaxies. emission-line spectrainnormalandpeculiargalaxies.Thedataareespeciallyusefulasacomparisonsample Intermediate- andlow-resolutionspectrophotometryoverthe3650-7000Árangehasbeenusedtocompilea line galaxies,duetovariationsinstellarabsorptionandexcitation,respectively.Methodsfordistinguishing cessible. TheH/?and[Om]linesareunsatisfactoryasstarformationtracersinallbutthestrongestemission- spiral andirregulargalaxiesexhibitdetectableemissionlinesofHa,[On],[N[SsometimesH/l spectral atlasofgalaxies(publishedinacompanionpaper)andtoinvestigatethesystematicbehavior brate ameanrelationbetween[On]luminosityandthetotalstarformationrate.Starrates between distantstarburstgalaxiesandactivenucleiarediscussed. star formationrate,butthe[On]lineprovidesagoodsubstituteinhigh-redshiftgalaxies,whereHaisinac- and [Oin].TheHalineisthestrongestemissionmostreliablequantitativetracerofmassive for blue-selectedgalaxiesatintermediateredshift.The[On]distributionfaintisverysimilarto rates derivedfromthecontinuumfluxesandcolors.However,[On]dataareveryusefulforcomparing derived fromthe[On]linearelessaccuratethanthoseHa,andcomparableinaccuracyto that whichisobservedfornearbyMarkariangalaxies. sample isingoodagreementwithprevioussurveysofnearbygalaxiesandverydifferentfromthatobserved the starformationpropertiesoflargesamplesgalaxies.Thedistribution[On]equivalentwidthsinour Subject headings:galaxies:ISM—photometrystellarcontent This paperpresentsthefirstresultsfromasurveyofintegratedspectra90nearbygalaxies. The integratedabsorption-andemission-linespectrashowasmoothprogressionwithHubbletype.Most The newdataareusedtoexaminethestatisticalpropertiesof[On]emissioningalaxies,andcali- © American Astronomical Society • Provided by the NASA Astrophysics Data System THE INTEGRATEDSPECTRAOFNEARBYGALAXIES:GENERALPROPERTIESAND 1. INTRODUCTION Steward Observatory,UniversityofArizona,Tucson,AZ85721 Received 1991July29;acceptedSeptember11 EMISSION-LINE SPECTRA 1 Robert C.Kennicutt,Jr. ABSTRACT 310 (z <0.03)galaxies,coveringabroadrangeinmorphological luminosities. rest frame),andanincompleterangeofgalaxytypes limited spectralcoverage(typically~3700-5000Âinthe these dataislimitedbyanumberoffactors,however,including grated spectrophotometryhasbeenobtainedfor90nearby spatial undersampling(usuallyasmallfractionofthedisk), noise ratiowhichmatchesorexceedstheavailableobserva- Â), witharangeinspectralresolution(5-25Â)andsignal-to- types. Thespectracoverthefullvisiblespectrum(3700-6800 types, luminosities,starformationproperties,andnuclear spectral propertiesofgalaxiesatvisiblewavelengths,newinte- workers haveusedtheH/?or [On]linestostudytheevolu- excellent starformationdiagnostic(e.g.,Kennicutt1983), but galaxies. FornearbyobjectstheHaemission-linefluxis an tions ofmoredistantgalaxies. for quantitativeapplications has neverbeenfullytested.This et al.1990;Elston1991), buttheaccuracyoftheselines Ha redshiftsoutofthevisiblewindowbeyondz~0.4.Several titative tracersofthetotalmassivestarformationrate in sion linesintheblue(H/?,[Om]25007,n]23727)asquan- One objectivewastoevaluatethereliabilityofstrongemis- tionary propertiesofmoredistant galaxies(e.g.,Dressieretal. survey providesameansofcomparing thebluelinesdirectly 1986; Gallagher,Bushouse,&Hunter1989).Theutilityof 1985; Couch&Sharpies1987; Broadhurstetal.1988;Colless In ordertoprovideamorecomprehensivedatasetonthe This surveywasmotivatedbytwoprimaryscientificgoals. 1992ApJ. . .388. .310K paper (Kennicutt1992,hereafterPaperII).Thiscontains fication ofunresolvedgalaxiesobservedatlargedistances,the library ofspectraforgalaxiesdifferenttypes,whichcanbe fluxes andthestarformationrate. the integratedemissionlinespectra.Furtherpapersin infrared, radio,andX-raysurveys,rudimentaryspectral used foravarietyofotherapplications,suchastheclassi- against Haandforcalibratingthemeanrelationsbetweenline peculiar galaxies. series willdealwiththeabsorptionlinespectrainnormaland a generaldescriptionofthesurvey,anddetailedanalysis synthesis applications.Aspectrophotometricatlasofgalaxies spectroscopic identificationofextendedsourcesdetectedin measuring starformationratesinmoredistantgalaxies. describes theobservationaltechniques,reductionmethods, selected fromthissurveywillbepublishedinacompanion moderate-resolution (15-25Â)spectrophotometrywere cussed in§4.In5theseresultsareappliedtotheproblemof properties oftheemission-linespectraarepresentedanddis- presents examplesforbothnormalandpeculiargalaxies.The and observationalerrors.Section3providesasynopsisofthe National Observatory(KPNO).Thisprovidedabasicsetof spectral propertiesofgalaxiesalongtheHubblesequenceand continuum energydistributionsandemission-linefluxesfora logical typeswereobserved.Theseincludedgalaxieswithsmall Hubble sequence,objectscoveringthefullrangeofmorpho- enhance thevalueofdatabase.Subsequentlyhigher ratio observations,sufficienttoresolvetheprincipalstellar became apparentthathigherresolutionandsignal-to-noise broad rangeofgalaxytypes.Asthissurveyprogressed,it obtained for70galaxiesin1985-1987,usingtheIntensified 45" aperturesinthelow-resolutionsurvey(below),andlarger tory 2.3mtelescope. resolution (5-7Â)datawereobtainedfor44galaxiesin1989- absorption featuresandfaintemissionlines,wouldgreatly Reticon Scanner(1RS)onthe0.92mtelescopeatKittPeak resolution datawithsufficientsignal-to-noiseratiotomeasure drift-scanning thegalaxiesacrossasingleaperture.High- galaxies whichwereobservedwithmultipleapertures,or by angular diameterswhichcouldbeadequatelysampledwith the fraction oftheobservedrangespectralproperties.Conse- three galaxieseachoftypesE,SO,Sa,Sb,Sc,Im,and10. the primaryabsorptionfeatureswereobtainedforatleast were selectedfromtheHuchra(1977)sampleofstar-forming nearby emissionlinegalaxiesandotherpeculiarsystems.Most quently aspecialeffortwasmadetomeasurediverseset of ([O ii]23727,H/?)isavailable for75bluegalaxiesfrom Clusters, takenfromthesurvey ofKennicutt,Bothun,& and afewemission-linegalaxies intheAbell1367andComa axies selectedfromtheHasurvey ofKennicuttetal.(1987), Markarian galaxies.Theremainder consistofinteractinggal- Gallagher etal.(1989). 1991, usingtheCCDspectrographonStewardObserva- Schommer (1984).Additional spectrophotometryintheblue A secondobjectiveofthesurveywastocompileareference The remainderofthispaperisorganizedasfollows:§2 The surveywascarriedoutintwostages.Large-aperture, In ordertocharacterizethespectralpropertiesof As demonstratedlater,normalgalaxiesoccupyonlyasmall © American Astronomical Society • Provided by the NASA Astrophysics Data System 2.1. GalaxySample 2. OBSERVATIONS INTEGRATED SPECTRA OF NEARBYGALAXIES311 included Seyfert1and2galaxiesfromthecompilationof when onlyanintegratedspectrumisavailable.Thesample were includedinthesurvey,ordertoassesseffectof nucleus ontheintegratedspectrum,andtotestwhethersuch dual beam1RSspectrometerontheKPNONo.20.92m nuclei fromKennicutt,Keel,&Blaha(1989). Weedman (1977),andgalaxieswithHnregion-likestarburst objects canbereadilydistinguishedfromstar-forminggalaxies, integrated spectrawereonlyobtainedforgalaxieswithdiam- telescope (f/7.5focus).Thisinstrumentiswellsuitedformea- erally toofaint(B^14-16)tomeasureona1mtelescope. this condition,butnormalgalaxieswithdiameteraregen- eters lessthan45".Mostofthebluegalaxiesinsamplemet tures, withcentersseparatedby61"2onaneast-westline.True spectra withlargeentranceapertures. and fastcameramakeitpossibletoobtainmoderateresolution surements ofextendedobjects,becauseitscompactimagescale 70 galaxiesduringthreeobservingrunsin1987-1989,usingthe Instead severalgalaxieswithdiametersof45"-90"(E-W)were individual spectrawithappropriateweightingfactors. pseudo-integrated spectrumwasconstructedbyaddingthe nebular mode,i.e.,bothaperturesonobject,withseparatesky addition, 14verylargegalaxies(D=2-10')wereobservedin photometric conditions. brightness ofthegalaxyandrelativestrengthsemis- Total integrationtimeswere4-180minutes,dependingonthe alternating apertures,withthebeamswitching(orskyoff- tions (pairs),usuallyspacedalonganeast-westline,anda observations. Eachgalaxywasmeasuredattwotofiveposi- observed, withtheaperturecoveringcentral45"region.In resolution formostofthedatais15-20ÂFWHM.Additional point source(suchasanemission-linenucleus)to25Âfor distributions ofthegalaxies,rangingfrom12ÂFWHMfora tures, thespectralresolutionwasmodulatedbyluminosity photometric coverageover370-6900Â.Withthelargeaper- sion lines.Thedataweretakeninphotometricornear- setting fornebularmode)performedundercomputercontrol. uniform illuminationofthe45"apertures.Theactual mated tobeapproximately±5%-10%,basedoncomparison observations intheblue(3600-5500Â)wereobtained for 5500 AwithaWG-360UV-blockingfilter,whichprovided calibration. ent configurations,andcomparisons withpublishedscanner verify theuniformityofresponseoverlarge-entranceaper- details canbefoundinPaperII.Specialtestsweremade to This providedsomewhatbetterresolution(typically9-13 Â), grating blazedat800ÂinsecondorderbehindaCuS0filter. uncertainties ofafewpercent inthespectrophotometric the 1RS(Barnes,Massey,& Carder1986),combinedwith between thetwoReticonchannels, causedbyscatteredlightin tures. Theoverallaccuracyofthespectrophotometryis esti- and highersensitivityshortwardof4000Â. approximately halfoftheprogramgalaxies,usinga400gpm observations. Thelargestsources oferrorarecrosstalk of independentmeasurementsthesameobjectswithdiffer- 4 Finally, avarietyofgalaxieswithluminousactivenuclei Spectra coveringtherange3650-6900Âwereobtainedfor Measurements weremadewithapairof45"5circularaper- All galaxieswereobservedusinga500gpmgratingblazedat Each observationconsistedofaseriesintegrationsin The spectrawerecalibratedusingstandardtechniques,and 2.2. Low-ResolutionSpectra 1992ApJ. . .388. .310K was trailedacrossalongslitseveraltimes.Thespectral (5-7 Â)datafor44galaxieswithdiametersofT-14'.The drift scanningtechniquewasusedtoobtainhigherresolution width isaseverelimitationforfixedpointingobservationsof length ofthedriftscaninperpendiculardirection. resolution wasdictatedbytheslitwidth,whileapertureon galaxies withlargeangulardiameters.Tosolvethisproblem,a (with 4200Âblockingfilter)coveredthe4950-7150region. trum. A600gpmgratingblazedat3568Âcoveredthe Two gratingsettingswereusedtocoverthefullvisiblespec- TI CCD,whichwasUV-floodedforenhancedblueresponse. the skywasdefinedbyslitlengthinonedirection,and over 10nightsin1989-1991.Eachmeasurementconsistedof spectrograph ontheStewardObservatory2.3mtelescope, spectra wereobtainedusingtheBoiler&Chivens(B&C)CCD minor axis,anddrift-scanningthetelescopealongmajor inclined galaxiesweremeasuredbyaligningtheslitwith program galaxies(D<2!5)andadjacentsky.Severallarger, red. Theusableslitlengthwasroughly3!5andincludedthe izing therespectivefluxesin[Om]25007emissionline.A mean fluxesinthe4950-5150Âoverlapregion,orbynormal- The blueandredspectraweretiedtogetherbynormalizingthe one ormoreintegrations,duringwhichtheimageofgalaxy 2'.'5 slityieldedaresolutionof4.5Âintheblueand7 minutes andwereofsufficientlengthtoensurethatskynoise drift speedortransparency.Integrationtimeswere20-60 at leastfourtimes,tominimizeerrorsduesmallvariationsin axis. Driftrateswereadjustedsothattheimagecrossedslit 3650-5150 Âregion,anda400gpmgratingblazedat7500 package. Eachobservationproducedatwo-dimensionalspec- dominated overchip-readnoiseinallcases. were summedintooneormoreone-dimensional,sky- most truespatialinformationislost.Consequentlythedata trum, butsincethetelescopewasdriftedduringexposure, CCDRED andLONGSLITpackagesintheIRAFsoftware was evaluatedinternallyusingresidualsofthestandardstar published multichannelscannerobservations.Thesetestsshow distributions to1RSobservationsincommon,and calibrations andexternallybycomparingthegalaxyenergy subtracted spectra.Theaccuracyofthespectrophotometry better onlargescales.Theerrorsaredominatedbytheaccu- 312 detector introducessmallvariationsinlinepositionandwidth racy oftheskysubtraction.CrinklingthinnedCCD that thespectrophotometryisaccuratetobetterthan±5% along theslit,andthismainlyaffectsskysubtraction near over smallwavelengthregions,andabout±10%-15% or resolution spectra.Ofthese, 80 possessedmeasureableemis- total of90galaxies,including24withbothhigh-andlow- spectrophotometric accuracyisgiveninPaperII. strong nightskyemissionlines.Adetaileddiscussionof the proj were measuredbydirectnumerical integration,usingthe properties ofthatsubsample. sion lines,andtheremainder ofthispaperdealswiththe SPLOT programinIRAF.The continuumlevelsandintegra- The degradationofspectralresolutionwithincreasingslit The B&Cspectrographwasusedwithathinned800x Data reductionfollowedstandardtechniques,usingthe Acceptable qualityspectrophotometrywasobtainedfor a The equivalentwidths(EWs) and fluxesoftheemissionlines © American Astronomical Society • Provided by the NASA Astrophysics Data System 2.4. Emission-LineMeasurements 2.3. High-ResolutionSpectra KENNICUTT lines (3c)measured.Inaddition,anumberofsubtlesystematic noise andskysubtraction,withuncertaintiesrangingfrom (1977). Column(3)liststheaperturesizeinarcseconds, evaluation oftheobservationaluncertainties.Linesmeasured lines weremeasuredwiththe1RSatleastonceeachdisper- surements madeindifficultcases.Galaxieswithweakemission tion limitsforthelinesweresetinteractively,withrepeatmea- effects wereencountered,andthesemeritfurtherdiscussion,as about ±5%-10%inbrightgalaxiesto30%theweakest de Vaucouleurs,&Corwin(1976),orHuchra type. ThelatterwastakenfromSandage&Tammann(1981), index wasmeasuredbymeasuringthemeancontinuuminten- included theHa+[Nn]226548,6583blend,H/?,[O they maybeimportantsourcesoferrorinotherspectroscopic emission featuresaregivenincolumns(5)-(9),asdefined uum colorasdefinedabove.Equivalentwidthsfortheprimary drift-scanned observations.Column(4)liststhe41-50contin- diameter foracircularaperture,andEWxNSnebularor separately, byusingtheGaussiandeblendingprogramin In mostcasestheHaand[Nn]linescouldbemeasured 23727, [Om]25007,andwhenavailable[Sn]226717,6731. sion, toprovidetwoindependentmeasurementsandadirect low-resolution data,thecomponentsareblended,andone high-resolution spectra,thebroadstellarabsorptionlinescan (except for[Nn]26583/Ha),arelistedincolumns(10)-(14). sities atrestwavelengthsof4050-4250and4900-5100Â: measures aweakeremissionline,ornoatall.Since stellar absorptionfeatures,especiallyfortheBalmerlines.In surveys ofgalaxies. SPLOT package.Inadditionacontinuum“41-50”color ments willbediscussedindetail§3. primary objectiveofthisstudyistoaidintheinterpretation be easilyseensurroundingthenarrowemissionlines,butin above. Therespectivelinefluxes,normalizedtoHa+[Nn] here refertotherawdata,includinganysuperposedabsorp- resolution andsignal/noise,thelinefluxesEWsreported spectra ofdistantgalaxies,whichareusuallyobtainedatlow Remarks aregivenincolumn(15). tion. TheeffectsofthisabsorptionontheBalmerlinemeasure- 1 and2.Columns(1)(2)listthegalaxynameHubble when thelineEWsbecamemuchsmallerthanspectral lines. Alloftheemissionlinesaresuperposedonachoppy tral resolutiononthedetectionandmeasurementofweak ways. Inoneexperiment,thefluxesofeachcomponents resolution, orcomparabletothestrengthsofadjacentabsorp- absorption spectrum,andmeasuringerrorsincreaserapidly weaker line.Athighsignal-to-noise ratio,thisratioshowedthe (theoretical value0.33)was plotted againsttheEWof low-resolution 1RSsurvey,andtheobserved4959/5007ratio tion features.Thiseffectwasanalyzedquantitativelyin two of the[Om]224959,5007doubletweremeasuredfrom the expected uncertainty,butfor EW(4959) <5Àtheerrorsin cally fromthetheoreticalvalue. Thesameeffectwassimulated doublet ratioincreaseddramatically, anddeviatedsystemati- by smoothinghighresolution spectratoeffectiveresolutionsof The randomerrorsinthesedataaredominatedbyshot A completelistingoftheemission-linedataisgiveninTables The largestsystematiceffectsarecausedbysuperposed Another systematicproblemisthenonlineareffectofspec- 41-50 =2.5 log /v(4100) /v(5000) Vol. 388 (1) 1992ApJ. . .388. .310K No. 2,1992 fected bythedegradedresolution,whileveryweaklines(few lines. Strongemissionlines(EW>30Â)wereessentiallyunaf- line fluxesincreasedrapidlyasresolutionwasdegraded, and the meanratiossometimesdeviatedsystematicallyfrom the EW) werecompletelylostwhentheresolutionexceeded 7-30 ÂandremeasuringtheEWsfluxesofemission (full spatialcoverage).InmostcasesthelinefluxesandEWs importance ofthiseffectbyobservingseverallargegalaxies absorption features. true value,dependingonwhetherthelinesfellnearstrong EW byfactorsofafew.Atintermediateresolutiontheerrors in measured inthe1RSaperturewererepresentativeofinte- with boththe1RS(central45"only)and2.3mtelescope apertures wasanotherpotentialsourceoferror.Wetested the cally weakerinthe1RSspectra. Thismainlyoccursbecause grated spectrum,outsideofanabsolutescalefactor,but in concentrated thanthenebular diskemissioninmanygalaxies. continuum emission(froma disk andbulge)ismorecentrally several oftheSb-Scgalaxies emissionlinesweresystemati- emission linesinroughlyequal proportion(withthepossible This undersamplingtendsto reduce thestrengthsofall Peculiar galaxy,probablemerger.(6)MemberofAbell1367cluster. exception of[Om]),sothe relativelinefluxesinTable2 should berelatively insensitivetotheseaperture effects.Never- NGC 1357. NGC488 ... NGC1832 . NGC1569 . NGC2798 . NGC2775 . NGC2276 . NGC3034 NGC2903 NGC2799 . NGC3227 NGC3147 , NGC3077 . NGC3368 NGC3310 NGC 3921 NGC 3627 NGC 3623 NGC3516 NGC 4485 NGC 4449 NGC 4775 NGC 4750 NGC 4670 NGC 4631 NGC 5248 NGC 5195 NGC 6181 NGC 5548 NGC 6643 NGC 6240 NGC 6217 NGC 7714 UGC 6697 Mrk 3 Mrk270 ... Notes.—(1) Stronglyinteractinggalaxy.(2)Containsstarburstnucleus.(3)Longaperturedimensionalongmajoraxisof(4)Seyfert nucleus.(5) Spatial undersamplingofthelargergalaxiesby45"1RS Name (1) © American Astronomical Society • Provided by the NASA Astrophysics Data System Sa Sab Sc SBb Sm/Im 10 10 Sm Sap Sa Sc Sb Sb Sa SO Sab Sbcp Sm/Im SOp Sb Sc Sm/Im lOp Sc Sbp SBp SO/a Sbc lOp SBbc Sc Sc SO Sp Sp SO Type (2) 314 x120 120" x180" 120 120 120 x 120 x 150 x360 800 x110 120 120 x 150 x180 120 x180 180 x120 150 x180 120 120 Aperture 90 90 40 60 90 90 x600 90 x 60 x 60 x45 90 90 90 90 60 40 90 60 60 60 60 (3) x 90 x 60 x 90 x 45 x 90 x 45 x 90 90 90 90 65 80 45 45 90 45 90 60 45 20 45 INTEGRATED SPECTRAOFNEARBYGALAXIES 41-50 0.64 0.77 0.43 0.66 0.54 0.87 0.26 0.46 0.46 0.69 0.43 0.57 0.77 0.15 0.60 0.72 0.77 0.25 0.60 0.49 0.67 0.30 0.33 0.29 0.96 0.28 0.53 0.48 0.55 0.53 0.52 0.75 0.95 0.92 0.19 0.26 (4) [On] 150 49 26 33 22 (5) 63 17 40 48 17 62 55 35 10 44 67 11 59 33 15 12 11 4 7 8 3 7 1.6 5 9 6 7 5 1.5 -2.5 -4 -4 -3.5 -1.5 -1 -3.5 -3 -3 -3 -2 -5 H/? [Oin] 35 20 (6) (7) 22 22 11 10 16 0 7 2 7 High-Resolution Data 4 4 7 1 0 6 0 0.5 Equivalent Width(Â) 3 8 1 1.5 182 <0.3 <0.5 <0.5 <0.6 <0.5 <0.5 <0.5 <0.5 <0.4 157 <0.5 <0.5 TABLE 1 48 10 33 33 26 54 13 46 22 20 15 14 11 2 3 7 5 5 1.5 2 3.5 1 1.5 Ha +[Nil] be seriouslyinfluencedbyundersamplingwereremovedfrom theless, theabsolutevaluesofemission-lineEWscanbe noted inTable2. the sample,anddonotappearinTable2.Borderlinecasesare strongly affected,andforthisreasonobservationswhichmight observed at5-7Âresolutiononthe2.3mtelescope(90"x 60" alter thecharacterofagalaxyspectrum.Asanexample,Figure nently inbothspectra,buttheweakerlinessuchasH/? and aperture). ThestrongHa+[Nii]featureshowsuppromi- aperture), andat20Âresolutionwiththe1RSscanner (45" due toacombinationofundersamplingthediskin 45" coverage areimperativeforthe studyofnormalgalaxies(type illustrates thathighspectralresolutionandadequateaperture resolution. InSa-Sbgalaxies,theeffectsareevenstronger.This 1RS aperture,andblendingofweaklinesatlowspectral [O in]arevirtuallylostinthe1RSspectrum.Thedifference is 1 showstwospectraofthesameScgalaxy,NGC6181, Sc andearlier)atanyredshift. emission-line spectra,itisuseful toexaminethegeneralvaria- tion inspectra along theHubblesequence,and amongvarious Taken together,thesesystematiceffectscanqualitatively 202 Before discussingthequantitative propertiesofthe 153 3. GENERALSPECTRALCHARACTERISTICS OFGALAXIES 114 113 172 104 147 (8) 45 84 34 28 53 38 53 63 32 20 65 84 60 11 40 47 56 31 15 77 35 16 17 9 2 2 4 2.4 3.5 [Su] 25 21 34 11 21 13 26 15 31 39 30 18 17 10 18 (9) 7 9 5 4 1.5 6 5 6 2 3 1.5 1 8 7 8 [O ii]Hßm][S 0.18 0.45 0.34 0.13 0.3 0.39 0.17 0.39 0.18 0.37 0.06 (10) (11)(12)(13) 0.14 0.13 0.08 0.36 0.73 0.97 0.30 0.16 0.2 0.75 0.79 0.52 0.90 0.17 0.22 0.05 0.17 0.22 0.27 0.26 0.27 0.65 0.65 0.64 1.07 Flux (Ha+[Nn]=1) 0.02 0.16 0.10 0.10 0.02 0.07 0.20 0.10 0.12 0.19 0.25 0.09 0.16 0.03 0.12 0.03 0.02 0.10 0.14 0.14 0.03 <0.05 <0.02 <0.03 <0.02 <0.03 0.07 0.91 0.15 0.12 0.05 0.14 0.03 0.46 0.21 0.22 0.57 0.75 0.66 0.36 0.03 0.06 0.35 0.10 0.06 0.07 0.26 0.34 0.34 0.05 0.92 1.13 0.13 0.19 0.25 0.21 0.24 0.21 0.21 0.17 0.17 0.22 0.17 0.09 0.34 0.25 0.20 0.3 0.22 0.31 0.29 0.18 0.11 0.21 0.21 0.18 0.47 0.21 0.31 0.21 0.41 0.26 0.21 [N n]/HaNotes >1 (14) (15) 10.3 0.66 0.66 0.81 0.30 0.43 0.47 0.28 0.27 0.27 0.58 0.21 0.47 1.72 0.12 0.77 0.46 0.10 2.4 0.22 0.21 0.22 0.54 0.41 0.42 0.23 0.23 0.38 0.53 1.35 0.90 1 1,2 4 4 3,6 1,2 313 1992ApJ. . .388. .310K types ofpeculiargalaxies.Several examplesareillustratedin Figures 2-3.Amorecomplete libraryispresentedinPaperII. along theHubblesequence.These datawereobtainedwiththe 2.3 mtelescope,withresolutions of4.5-5Âintheblueand7-8 314 (5) Peculiargalaxy,probablemerger.(6)ContainsSeyfertnucleus.(7)Member ofComaCluster.(8)Datamaybeinfluencedbyspatialundersampling;seetext.(9) Member ofAbell1367cluster.(10)AperturecenteredondominantHnregion inlargegalaxy. NGC 5953 NGC 4736 NGC 4670 NGC 4631 NGC 4156 NGC 3995 NGC 3991 NGC 3516 NGC 3395 NGC 3303 NGC 2881 NGC 7714 NGC 7469 NGC 7448 NGC 6764 NGC 6240 NGC 5954 NGC 5929 NGC 5548 NGC 5055 NGC 4900 NGC 4848 NGC 4826 NGC 4704 NGC 4449 NGC 3994 NGC 3690 NGC 3310 NGC 2841 NGC 2798 Z130-008 . UGC 6697 IC 1236.... IC 883 NGC 5194 Mrk 71.... Mrk 35.... Mrk297 ... Mrk286 ... Mrk 201... Mrk 181... Mrk 161... Mrk 158... Mrk 111... Mrk 59.... Mrk 33.... Mrk487 ... Mrk432 ... Mrk 296... Mrk270 ... Mrk 256... Mrk 188... Mrk 691... Mrk 685... Mrk479 ... Figure 2illustratestheprogression ofintegratedspectra Notes.—(1) Stronglyinteractinggalaxy.(2)Containsstarburstnucleus.(3) Long-aperturedimensionalongmajoraxisofgalaxy.(4)Observedinnebularmode. Name (i) © American Astronomical Society • Provided by the NASA Astrophysics Data System Sc Sab Sc Smp Im Scd S Sap Im Im Im Im SBb lOp Sedp Sap Sbcp SO/a Sbe Sc Sab SBe SBp Sm/Im SBe Sep Sep SO Sbcp P Sb Im Im SBe Im Im Im Sp Sc Sp Sa Sc Sbe Sc SBe SBdmp Smp Scd SO Sd Smp Sc SBe Sab SBd Type Aperture (2) (3) 3.1. NormalGalaxies 230 x45 230 x 170 x 170 x45 110 x165 110 x165 105 x165 45 45" 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 41-50 [Oii] -0.14 (4) (5) 0.36 0.17 0.07 0.12 0.33 0.55 0.25 0.21 0.37 0.34 0.36 0.58 0.63 0.47 0.56 0.78 0.44 0.67 0.38 0.47 0.76 0.73 0.53 0.22 0.28 0.60 0.48 0.30 0.76 0.09 0.87 0.95 0.23 0.17 0.17 0.23 0.98 0.13 0.35 0.46 0.28 0.30 0.54 0.24 0.47 0.34 0.41 0.55 0.19 0.28 0.19 108 <2 <1 43 44 45 40 40 48 29 43 47 23 22 25 62 24 60 99 91 24 28 21 60 62 23 76 77 36 35 24 27 28 36 85 62 36 58 50 54 16 13 18 12 10 12 11 10 15 4 4 2 8 8 5 299 115 -2 -5 -1 -2 -1 -3 -1.5 <1 H/? 47: 20 28 22 23 21 22 70 27 21 21 11 11 (6) 17 11 10 15 11 10 17 11 19 10 4 0 4 0 0 4 0 4 4 2 2 2 5 5 8 7 Equivalent Width(Â) 1 1 1 1 1 Low-Resolution Data [Om] KENNICUTT 2490 (7) 304 750 115 <1 <1 <2 <2 43 41 25 23 43 67 65 79 53 22 21 33 30 30 51 57 30 19 14 12 14 11 12 11 17 TABLE 2 4 9 4 2 9 9 6 7 2 6 6 8 6 3 1 1 Ha +[Nii] -1 -1 1350 ward of5500Â.Inbothcases theordinateshowsflux intensity. Figure2bshowsexpanded plotsoftheregionblue- (assuming H=75kms Mpc). Twoversionsofeach Table 3summarizesthemain propertiesofthesegalaxies age, withthedifferentspectra scaledtoroughlythesamepeak spectrum areplotted.Figure 2a showsthefullspectralcover-  longwardof5100Â,andsmoothed witha4Âsquarefilter. 0 203 234 796 134 155 161 181 124 148 167 156 196 126 131 160 103 115 143 (8) 42 41 92 92 41 92 24 71 24 73 61 68 93 94 65 69 22 86 73 29 54 54 72 83 81 33 51 54 34 32 15 17 18 7 5 1.5 [Su] (9) 46 40 41 21 61 23 20 23 26 32 35 31 33 32 24 30 37 30 37 86 34 50 33 13 14 13 10 16 16 11 10 13 12 9 4 9 9 2 2 6 7 5 5 1 <0.2 <0.2 [On] (10) 0.24 0.56 0.18 0.21 0.34 0.37 0.22 0.61 0.33 0.73 0.06 0.64 0.6 0.52 0.08 0.40 0.23 0.25 0.24 0.41 0.17 0.29 0.63 0.55 0.6 0.40 0.06 0.33 0.27 0.23 0.13 0.02 0.11 0.36 0.27 0.65 0.57 0.18 0.58 0.27 0.51: 0.32 0.36 0.40 0.53 0.22 0.35 0.19 0.22 0.25 0.08 1.08 1.2: Flux (Ha+[Nn]=1) <0.03 <0.04 <0.01 (11) H/? 0.04 0.31 0.22 0.19 0.02 0.22 0.05 0.17 0.16 0.16 0.04 0.4: 0.12 0.14 0.38: 0.06 0.22 0.10 0.16 0.06 0.12 0.28 0.17 0.13 0.05 0.06 0.02 0.22 0.08 0.19 0.03 0.06 0.21 0.10 0.16 0.15 0.23 0.13 0.14 0.16 0.02 <0.03 <0.1 <0.06 <0.14 <0.02 <0.1 <0.04 <0.03 <0.06 <0.01 [Om] <0.03 (12) 0.27 0.39 0.86 0.12 0.26 0.08 0.65 0.27 0.57 0.37 0.08 0.62 0.14 0.33 0.32 0.12 0.16 0.02 0.16 0.95 0.16 0.21 0.81: 0.13 0.40 0.13 0.18 0.06 0,35 2.00 0.35 0.16 0.13 0.08 0.08 0.03 0.10 0.34 0.16 0.44 0.17 0.39 1.78: 1.65 [Su] 0.47 0.27 0.15: 0.39 0.23 0.18 0.25 0.04 0.09 0.14 0.17 0.3 0.17 0.13 0.20 0.2 0.30 0.32 0.33 0.22 0.22 0.11 0.33 0.27 0.16 0.15 0.23 0.27 0.18 0.38 0.26 0.1 0.21 0.24 0.28 0.18 0.20 0.45 0.22 0.21 0.19 0.3 0.14 (13) [N n]/HaNotes 0.02 0.35 0.32 0.38 0.20 0.14 0.21 0.45 0.35 0.49 0.51 0.16 0.04 0.12 0.22 0.37 0.68 0.88 0.56 0.53 0.64 0.11 0.69 0.15 0.39 0.18 0.26 0.44 0.36 0.26 0.21 0.45 0.40 0.48 0.39 0.55 (14) (15) 1.02 Vol. 388 10 10 9 9 4 4 4 4 6 6 8 7 3,4 6 8 5 3,4 5 3,4 1,5 1,2 1 1,6 1 1 1 1 1,2 1,6 1,6 1 1 1992ApJ. . .388. .310K features suchasMgbandNaDtendtobesomewhatweaker integrated spectraobtainedhereareverysimilar(e.g.,Pickles with EWsintherange0.3-3Â,aredetectedmanyof continuum structureisreal,producedinlargepartbyblended present aswell,butitisalmostalwayslostinthestellar detectable emissionlinesatall,butfaint[On]and[Nlines, are dominatedbyabsorptionfeatures.NGC4889showsno redshift). near arealspectralfeatureinthegalaxy(e.g.,NaDatlow from brightnightskylines(mainly[Oi]A5577,6300,6364) have beenclippedforclarity,unlessthenightskyartifactfell rected forGalacticorintrinsicreddening.Residualfeatures normalized tounityatareferencewavelengthof5500Â.The & Visvanathan1985).Whilethestrengthsofmetal-sensitive absorption intheintegratedspectra.Notethatchoppy E/S0 galaxies,includingNGC4262.Balmeremissionmaybe Osterbrock (1969).EllipticalandSOgalaxies(NGC4889,4262) originally byMorgan&Mayall(1957)and spectra arecorrectedforatmosphericextinctionbutuncor- studied extensivelyforspectralsynthesisapplications,andthe stellar absorptionlinesintheKgiantdominatedcontinuum. roughly 20Âresolution. bottom spectrumisasinglepointedobservationofthecentral45",with scanning anarrowslitover90"x60"region.Resolutionis5-7Â.The resolution andspatialsampling.Thetopspectrumwasobtainedbydrift- No. 2,1992INTEGRATEDSPECTRA NGC 3034(M82)3 10 -18.5IRluminous NGC 42622SB0-18.4 NGC 48892E4-22.1ComaCluster NGC 62403 10 —21IRluminous,merger NGC 77143 Sdmp —19.7Nuclearstarburst NGC 33103 Sbcp -19.9Globalstarburst NGC 35163 SB0 -20.6Seyfert1 NGC 44492 Sm -17.9 NGC 61812Sc-20.7 NGC 47502Sb-20.2 NGC 27752Sa-19.9 Mrk 2703 SO -18.5Seyfert2 The integratedspectrafollowthegeneralpatternsdiscussed Spectra ofthenuclearregionsE-S0galaxieshavebeen Fig. 1.—ComparisonoftwospectraNGC6181,showingtheeffects Properties ofGalaxiesIllustratedinFigures2and3 Name FigureTypeM{B)Remarks © American Astronomical Society • Provided by the NASA Astrophysics Data System TABLE 3 OF NEARBYGALAXIES315 cases thespectralappearance isdominatedbythediskand extreme starburstgalaxies)the nuclearregionscontributeonly uum ortheemissionlines(Kennicutt &Kent1983),soinmost what earlier(i.e.,meanstellartype)thanontheYerkes except thatthebest-fittingspectraltypesheretendtobesome- a fewpercentorlessoftheintegrated flux,ineitherthecontin- are available.Inmostnormal galaxies(excludingSeyfertand sampled thenuclearregions, whereashereintegratedspectra that theMorgantypeswerebasedonspectrawhichprimarily system. Thisdifferencecanbeattributedentirelytothe fact parallels theYerkesspectralclassification(Morgan1958), mally highlevelsofstarformationand/ornuclearactivity. blue almostcertainlywillbeapeculiargalaxy,withabnor- which doesexhibitaprominentemission-linespectrumin the lation orgalaxytypewithinthisrange.Conversely,any (SBm/Im), theintegratedspectrumbecomesincreasinglydomi- the [Oii]linewouldsufficetocharacterizestellarpopu- der ofthespectrum.IfobservedshortwardHaatmoderate why Haisusedforquantitativemeasurementsofstarforma- population overtheentirerangeoftypes;itisnotsurprising composite spectrumbetweenHubbletypesEandSb.Mostof in stellarpopulationsandstarformationratesalongthe emission andabsorptionspectrawithgalaxytype(compare features, butlinesat(indescendingorder)[Sn],[OH/?, emission lines.Haand[Nn]remainthedominant (Sb) andNGC6181(Sc),arecharacterizedbymuchbluercon- shape shortwardof4500Â,orhigh-resolutionobservations of alone. Onlyaprecise(~10%)measurementofthecontinuum surveys, thespectraofgalaxiesrangingintypefromEtoearly spectral resolution(>10Â),typicalparametersforfaintgalaxy tion ratesingalaxies!Thisismarkedcontrasttotheremain- Ha +[Nii]emissionfeature,whichissensitivetostellar the varianceinspectralproperties,especiallyblueregion, revealing resultinFigure2isthesubtletyofchanges radically overtherangeofHubblesequence,most Hubble sequence. emission lines.Thesmoothmonotonieprogressioninboththe nated bythecontinuaofB-Astars,andstrongnebular spirals andMagellanicirregulars,asillustratedbyNGC4449 and [Oin]begintoappear.Asoneprogressesthebluest tinua, moreprominentBalmerabsorptionlines,andnebular tions areinvariablygalaxieswithactivenuclei,suchasSeyfert nebular linesaredetectedintheintegratedspectrum.Excep- weak Haand[Nn]emission,atthelevelofafewÂorlessin Sc arenearlyindistinguishablefromqualitativeinspection occurs amongSc-Irrgalaxies.Thesoleexceptionisthe Figs. 2aand2b)showsclearevidenceofthesystematicchanges or starburstgalaxies. in thebluecontinuum,seenmosteasilyasadecrease EW. Exceptforoccasionalweak[On]emission,noother strength ofthe4000ÂH+Kbreak,andappearance the spectraofE-S0galaxies.Themainchangesareanincrease cussion oftheabsorptionspectraisdeferredtofuturepapers. with thesamemeanEWasnucleistudiedbyPickles& also dominatedbylate-typestarsanddifferonlysubtlyfrom Visvanathan (1985).Thislendssomesupportfortheefficacyof in theintegratedspectra,EWofmetallicity-insensitive our skysubtractionandreductionprocedures.Furtherdis- Ca il(H+K)doubletisvirtuallyconstantoveroursample, In generalthespectralsequenceshowninFigure2closely Although thecharacterofintegratedspectrumchanges The spectraofearly-typespirals,suchasNGC2775(Sa),are Intermediate- tolate-typespirals,illustratedbyNGC4750 1992ApJ. . .388. .310K Same spectraasin (a), butexpandedinthebluetoshow thepropertiesofabsorption-line spectra. Fig. 2.—(a)ThespectralHubblesequence. GalaxiesareshowninorderofincreasingHubbletypefromtop to bottom.SeeTable3fordataonthegalaxies,(b) © American Astronomical Society • Provided by the NASA Astrophysics Data System 4000 450050005500 6000 65007000 Wavelength (A) Fig. 2a 316 4000 450050005500 6000 65007000 Wavelength (A) 1992ApJ. . .388. .310K 3600 40004400 4800 5200360040004400 48005200 © American Astronomical Society • Provided by the NASA Astrophysics Data System Wavelength (A) Wavelength (A) Fig. 2b 317 1992ApJ. . .388. .310K properties oftheabsorption-linespectra. Forclaritythespectrahavebeenalignedinwavelength. Â, sorelativelinestrengthscannotbe inferreddirectlyfromtheplots.SeeTable3fordataongalaxies,{b)Same spectraasin{a),butexpandedthebluetoshow Fig. 3.—(a)Spectraforsixpeculiargalaxies withactivenucleiorstarformationbursts.Notethatthespectralresolution changesshortwardandlongwardof5200 © American Astronomical Society • Provided by the NASA Astrophysics Data System 318 4000 450050005500 6000 65007000 Wavelength (A) 1992ApJ. . .388. .310K 3600 40004400 4800 5200360040004400 48005200 © American Astronomical Society • Provided by the NASA Astrophysics Data System Wavelength (A) Wavelength (A) Fig. 3b 319 1992ApJ. . .388. .310K interstellar inorigin.ThespectrumofNGC6240isextremely Note theunusuallystrongNaDabsorption,whichisprobably NGC 6240.M82exhibitsstronglineemissionandaheavily the otherhand,isfairlytypicalofanormallate-type,actively A6300 and[Sn]226717,6731.Theabsorptionspectrum,on peculiar, withabnormallystrongandbroadlinesof[Oi] karian samplefallintothiscategory,withanemissionspec- star-forming galaxy(cf.Fig.2). reddened continuumdominatedbyBalmerabsorptionlines. galaxies, NGC3034(M82)andtheinfrared-luminousgalaxy very similar(Kennicuttetal.1989),soitisvirtuallyimpossible alone. to distinguishbetweenthetwoclassesfromintegratedspectra The emission-lineratiosforthenuclearanddiskstarburstsare galaxy withaglobaldiskstarformationburst,whileNGC trum characteristicofmoderatetohigh-excitationHnregions, emission-line galaxies.MostofthebluegalaxiesinourMar- 7714 istheprototypicalexampleofanuclearstarburstgalaxy. and lineEWsoforder10-100Â.NGC3310isanexamplea two typesofgalaxiesarediscussedin§4.4. forward, duetotheunmistakablebroadBalmeremissionlines, identification ofluminousSeyfert1nucleiisusuallystraight- superposed. Variousquantitativemeansfordistinguishingthe this spectrum,especiallywhenstrongBalmerabsorptionis is oftenmoredifficult.Thelatterareusuallyidentifiedonthe but recognizingaSeyfert2nucleusintheintegratedspectrum properties ofthesegalaxiesarelistedinTable3.TheSeyfert al. 1985),buthigh-excitationstar-forminggalaxiescanmimic basis ofanabnormallyhigh[Om]/H/?ratio(e.g.,Baldwin, obtained forseveralotherSeyfertgalaxies(PaperII)showthat galaxies NGC3516(Sy1)andMrk2702)areclearlyrec- Phillips, &Terlevich1981;DressierGunn1983;et ognizable assuch,eveninanintegratedspectrum.Data again infullscaleandexpandedform,respectively.The those objects. is underrepresentedinverylowsurfacebrightnessgalaxies,so 0.5-1.2 inmostcases),sothespectramaybeslightlybiasedto the conclusionsofthispapermaynotnecessarilyapplyto the highestsurfacebrightnessregions.Thegalaxysamplealso generally didnotextendtothefaintestisophotes(A/D= bulge. Ontheotherhand,aperturesusedinthisstudy more practicaltomeasurethe emissionlinesintheblue. embedded stars,andthismakesitpossibletousetheBalmer 320 KENNICUTTVol.388 doublet asastarformation indexfordistantgalaxies(e.g., into thenearinfrared,andfor manyapplicationsitbecomes line forsuchapplications,butbeyondz~0.2-0.3,Haredshifts lines toderivequantitativestarformationrates(SFRs)in gal- line luminositiesscaledirectlywiththeionizingfluxesof the al. 1990).Gallagheret(1989) havederivedanapproximate Dressier &Gunn1982; etal.1985;Peterson axies (e.g.,Kennicutt1983;Gallagher,Hunter,&Tutukov 25 1984). InspectionofFigure2showsthatHaisclearlythe best 1986; Broadhurstetal.1988; Lavery &Henry1988;Collesset NGC 3310and7714areexamplesofstarburst Finally, Figure3displaysspectraoftwopeculiarirregular Figures 3aand3bshowspectraofseveralpeculiargalaxies, For ionization-boundedHnregions,theBalmeremission- Several workershaveapplied theEWof[On]23727 4. EMISSION-LINESPECTRAANDSTARFORMATION © American Astronomical Society • Provided by the NASA Astrophysics Data System 3.2. GalaxieswithPeculiarSpectra DIAGNOSTICS /(Ha +Nn)//(H/)=6,aflatcontinuum(f)betweenHßand ment, asdiscussedinthetext. thermal nuclearemission.Thelineis asimplemodelwithconstantstellar nated bydiskemission,andopensymbols denotegalaxiesdominatedbynon- absorption equivalentwidthatH/?,and aconstantvaluefortheBalmerdecre- Ha +[Nii]emission-lineequivalent widths.Solidpointsaregalaxiesdomi- most usefullinesin§5. calibrations basedonthelinefluxesarethenderivedfor more reliablyinfaintsources.Thissectionexaminesthe evaluate theirgeneralutilityasSFRtracers.Approximate relationships betweentheEWsofdifferentfeatures,inorderto measure infaintgalaxies,duetoeffectsofspatialunder- based onemission-lineEWs,whichcanbedeterminedmuch quently, mostanalysesofdistantgalaxyspectrahavebeen to determineSFRs,butabsolutelinefluxesaredifficult nuclear properties. work bytestingthereliabilityofblueemissionlinesdirectly À sampling, variableseeing,andatmosphericdispersion.Conse- against Ha,overthefullrangeofgalaxytypes,SFRs,and H/? inanearbybluegalaxies.Inthissectionweextendthat perature andionizationleveloftheemittinggas.Figure4 data followthesamegeneraltrendsasshown. regions anddiffuseinterstellaremission,whileopendiamonds absorption). SolidsymbolsindicatespectradominatedbyHn components, somanygalaxiesexhibitnegativeEWs(net both thenebularemissionandunderlyingstellarabsorption shows therelationbetweenEWsofH/?andHa+[Nn]. the massiveSFRandarenearlyindependentoftem- Balmer lines,sincethefluxesoftheselines-scaledirectlywith EW(Ha +Nn)>160ÂareoffthescaleofFigure4,butthose denote galaxiesdominatedbyactivenuclei.Afewpointswith star formationtracersintheblueshouldbehigherorder Ha +[Nn]EWsisapparentinFigure4,especiallyamongthe Positive EWsdenotenetemission.Themeasurementsinclude [O il]versusSFRcalibrationfromobservationsofn]and Ha, andameanstellarabsorptionEWof5Â.Notethatthe galaxies dominatedbystar-formingHnregions.Thesolidline shows therelationexpectedforasimplemodelwith Fig. 4.—Relationbetweentheequivalent widthsoftheHßand Strictly speakingthefluxesofemissionlinesshouldbeused From purelyastrophysicalconsiderations,themostreliable A strong,roughlylinearcorrelationbetweenH/?and 4.1. HßandOtherBalmerLines 1992ApJ. . .388. .310K mean extinctionA~1mag.Thisisconsistentwiththe emission. Thedifferencecanbeattributedtoreddening.Foran Ha/H/? ^3whichisexpectedforHnregionswithT~ emission line,andthenetH/?EWrangesfrom—5 normal galaxies(typesE-Sc),whicharecharacterizedby Â. ThisrangeincludesthebluestMagellanicirregularsand galaxies, thosewithEW(Ha+Nn)>60Â,andEW(H/?)5 the sample. reddening, [Nn]/Ha,andnuclearversusdiskemissionwithin absorption-corrected BalmerdecrementHcc/Hß~4,anda average [Nn]/Ha^0.5(§5.3),Figure4impliesamean slope ofthislineissubstantiallydifferentfromthedecrement magnitude estimate(orlowerlimit)totheSFR.Thehigher blended, andthelinewillprovideatbestonlyanorder-of- be particularlyresolvedfromthebroaderabsorptionfeatures, equivalent widthismeasuredwithveryhighprecision (absorption) to+3Â.Forsuchgalaxies,H/?isonlyusefulifits EW(Ha +Nn)<50Â(Kennicutt&Kent1983;Romanishin starburst galaxies.UnfortunatelyH/?ismuchlessusefulin serve asareliablestarformationtracerinstrongemissionline tion isroughly±30%,whichpresumablyreflectsvariationsin optical extinctionderivedfromothermethods(e.g.,Kennicutt No. 2,1992INTEGRATEDSPECTRA tions, however,especiallythoserelevanttostudiesoffaintgal- resolution andsignal-to-noiseratio,theemission-linecorescan tion lineisknowntocomparableaccuracy.Athighspectral 5000-10,000 K,evenwhenallowanceismadeforthe[Nn] effects ofabsorptionbecomeprogressivelyworsefartherupthe order Balmerlines(Hy,etc.)areevenlessreliable,becausethe axies, theemissionandabsorptioncomponentswillbe and suchacorrectionmightbepossible.Formostapplica- 25007 andHa+[Nn]lines.Thesymbolsarethesameasin Balmer series. [a(EW) <1Â],andtheintrinsicstrengthofstellarabsorp- Figure 4,exceptthatticksbelowafewpointsdenoteupper 1990). Therestellarabsorptiondominatesoverthenebular 1983; Kennicuttetal.1989).Thescatteraboutthemeanrela- v e upper limitsin[Om]. sion lineandHa+[Nn].Notation the sameasinFig.4.Verticallinesdenote The correlationinFigure4confirmsthattheH/?linecan Figure 5showstherelationbetweenEWsof[Om] Fig. 5.—Relationbetweentheequivalent widthsofthe[Om]25007emis- © American Astronomical Society • Provided by the NASA Astrophysics Data System 4.2. [Oin]25007 OF NEARBYGALAXIES321 is alargevariationinmeannebularexcitationdisks,pos- with thelargedispersioninexcitationseenspectroscopic The spiralgalaxiesobservedathighresolutionshowalarge not asaquantitativeSFRtracer. value asanexcitationdiagnosticinhigh-redshiftsystems,but range intheintegrated[Om]/H/?ratio,whichisconsistent ive SFRindicators.AmonggalaxieswithnormalSFRs, very loosecorrelationbetween[Om]andHa+[Nn]EWsin limits in[Om].Herethesituationisevenworse.Therea Elston, &Hill1990).Clearlythe[Om]linehasconsiderable surveys ofHnregionsinnearbygalaxies(e.g.,Zaritsky, sibly combinedwithvariationsinthelevelofnuclearactivity. detected atall(EW<1-2Â). roughly EW(Ha+Nn)<40Â,the[Om]linesarerarely relation rendersthe[Om]fluxesvirtuallyuselessasquantitat- the strongestemission-linegalaxies,butdispersioninthis tracer intheblue.Figure6showsrelationbetween with luminousactivenuclei(AGNs).Thesegalaxiesfallinto relation EW(0n)=0.4EW(Ha+Nn),withanrmsdisper- stellar photoionization(solidpoints)thedatafollowamean emission in[On].TheformermostoftenareSeyfert2or independent ofBalmerlinestrength,andthosewithstrong the studiesbyDressier&Gunn(1982),etal.(1985), addition tobeingthebestbehavedemissionlineinblue, galaxies withverystrongemissionlines(HaEW>100Â).In sion of~50%.Thisdispersionmaybesomewhatlargerfor Balmer, [Nn],and[Om]emissionbutweak(orundetected) two classes,thosewithabnormallystrong[On]emission, bration ofthelineisderivedinnextsection. substitute forHaindistantgalaxies.Anapproximatecali- al. (1989),whichmaintainedthatthe[On]linecanserveasa Peterson etal.(1986),Broadhurst(1988),andGallagher [O ii]andHa+[Nn]EWs.Amonggalaxiesdominatedby [O ii]isthestrongestfeatureafterHa+[Nn].Thisconfirms simple modelwithEW(0n)=0.4EW (Ha+Nii). Ha +[Nii]emissionlines.Notation thesameasinFig.4.Thelineshowsa The mostlikelyexplanationforthedispersionin[Oin]/Ha The [On]23727doubletisthemostusefulstarformation The [Oii]versusHacorrelationbreaksdownforgalaxies Fig. 6.—Relationbetweentheequivalent widthsofthe[On]23727and 4.3. [Oii]23727 1992ApJ. . .388. .310K mean relationforHnregionsintheBaldwin etal.study.The[Ohi]/H/?ratios indicated here. not beencorrectedforreddening,so mostpointsliefarthertotherightthan have beencorrectedforstellarBalmer absorption.Thedatashownherehave following theconventionofBaldwin, Phillips,&Terlevich(1981).Solidpoints extend to~7000Â,itisquiteeasyidentifyanAGN- axies withemissiondominatedbya nonthermalnucleus.Thelineshowsthe denote galaxiesdominatedbydiskemission, andopensymbolsindicategal- ratio diagrams(e.g.,Baldwinetal.1981).Inourdata,which imate correctionfortheeffectsofstellarabsorptionon in individualHnregionsandnuclei. compare thediagnosticratiosinintegratedspectratothose provide someinterestinginformation,anditisusefulto mination complicatesthetaskfurther.Neverthelesstheselines mation isavailable).Thelackofanaccuratereddeningdeter- longer accessible,sotheclassificationmustbebasedon however, whenonlythebluespectrumisavailable.Themost tion EWof5Â(seeFig.4).Neitherratiohasbeencorrectedfor convention ofBaldwinetal.(1981).Inthiscaseanapprox- H/? and[On]/[0m]fluxratiosforoursample,followingthe powerful diagnosticfeatures(e.g.,[Oi],[Nn],[Sn])areno emission lines.Theidentificationismuchmoredifficult, dominated spectrum,eitherbythepresenceofstrong[Nn]/ ionized bystarsandnonthermalprocessesarediagnosticline Ha relativeto[Om]/H/?,orbythedetectionofbroadBalmer explore themostreliablemeansofseparatingtwoclasses detected athighredshift(cf.Dressier&Gunn1983).Herewe emission) whichcloselymimicstarburstingemission-linegal- [O m]/H/?ratiohasbeenapplied,byassuminganHßabsorp- objects, whenonlyanintegratedspectrumisavailable. use thespectraofnearbystarburstinggalaxiesandAGNsto AGNs havespectralsignatures(bluecontinuum,strong[On] regions fromothersourcessuchasactivenuclei.Manydistant though thereareexceptionstothisdivision. [O n],in],andH/?lines(pluswhatevercontinuuminfor- axies, andasaresultsuchobjectsmaybepreferentially to beabledistinguishemissionproducedbystar-forming LINER nuclei,andthelattertendtobeSeyfert1galaxies, 322 KENNICUTTVol.388 Fig. 7.—Diagnosticdiagramforthe integrated oxygenemission-linefluxes, The conventionalmeansfordistinguishingbetweengas Figure 7showstherelationbetweenintegrated[Om]/ When interpretingtheemission-linespectra,itisimportant © American Astronomical Society • Provided by the NASA Astrophysics Data System 4.4. EmissionDiagnostics redder colorsthanstar-forming emissionlinegalaxies.Many relation, whichisexpectedbecause theSeyfertnucleitendto Huchra 1977;Kennicutt&Kent 1983).Thereisalsoaclear occur inearlytointermediate-type spirals,whichhavemuch AGNs. Seyfert2galaxiestend tofalltherightofmain Balmer EWversuscolorrelationforgalaxies(Cohen1976; strength andcolor(cf.Dressieretal.1985),whichparallels the separation betweenthenormal galaxiesandmostofthe to extendtherelationforstar-forminggalaxiesbynearly an dominated galaxiesfromthissurvey,whilesolidcircles are (1985). SolidandopendiamondsdenotenormalAGN- (1982). Figure8showsthelogarithmof[On]EWplotted show EW(0ii)>30Â. upper limits)fromDressieretal.(1985).Thenewdataallow us data for32nearbyfieldandclustergalaxies(crossesdenote against the41-50colorindex,asdefinedinequation(1).Note separating starburstsfromAGNs,sincemanyofthelatter order ofmagnitudehigherinEW(0n),andthisiscritical for that thisdefinitiondiffersfromusedbyDressieret al. tinuum color,asproposedoriginallybyDressier&Gunn star-forming galaxiesistocorrelate[Oii]strengthwithcon- most oftheSeyfert2galaxiesinoursample,aresufficiently 26300 areoftenrequiredforanunambiguousseparation. bases oftheselineratiosalone.Thisisnotsurprising;analyses close tothemainrelationthattheycannotbeidentifiedon low [On]/[0in].MostoftheAGNs,however,including forming galaxies,especiallythethreeSeyfert1galaxieswith minimize theeffectofspuriousmeasurements,samplewas strate thatadditionalfeaturessuchas[Nn]26583or[Oi] of nearbygalaxiesbyBaldwinetal.(1981)andothersdemon- 4449,4485) showextensivediffusenebulosityindirectimages. regions (e.g.,Hunter1984;&Gallagher1990).Manyof lower ionizationandstronger[On]emissionthanHn toionized Hnregions,takenfromBaldwinetal.(1981). restricted togalaxieswithEW(0n)>10Á.ThelineinFigure the galaxiesinoursamplewithstrongest[On](e.g.,NGC excess in[On]/[0m]isprobablyreal.Inspectionofthe toward therightinthisdiagram,suggestingthatapparent (not possiblewiththeavailabledata),woulddrivepoints marginal, butcorrectingthe[On]/[0m]ratiosforreddening 7 showsthemeanreddening-correctedrelationforstellarpho- sion fromdiffuseinterstellargas,whichtypicallyexhibitsmuch bright Hiiregions.Wetentativelyattributethisexcesstoemis- H/? ratios(ratherthanlow[Om]/H/?)whencomparedto spectra showsthatthisexcessisduetounusuallyhigh[On]/ mates wouldrequireaccuratemeasurementsofthe[Om], expected forHnregions.Thissuggeststhatsuchintegrated H/? >1)lietotherightofHnregionrelation(high[On]/ tendency formostofthehigh-excitationpoints([Om]/ posed byEdmunds&Pagel(1984).Anysuchabundanceesti- using anempiricalabundancecalibrationsuchastheonepro- tion, andperhapseventhemeanmetalabundanceofagalaxy, by nuclearemissionareindicatedopensymbols.Inorderto which aredominatedbystarformation,whilethose only possibledeviationfromthemainHnregionrelationisa spectra couldbeusedtocrudelyderiveacharacteristicexcita- reddening, however.Asbefore,solidsymbolsdenotegalaxies [O m]).Theformalsignificanceofthetrendinrawdatais [O ii],andHßfluxes,correctedforstellarabsorption.The Overall thereisawell-definedrelationbetween[On] A moreeffectivemeansofdiscriminatingdistantAGNsfrom A fewoftheAGNsliewelloffmeanrelationforstar- The star-forminggalaxiesgenerallyfollowtherelation 1992ApJ. . .388. .310K cation insurveysoffaintobjects. Wefirstdiscussthestatistical means ofstudyingthesystematicsgalacticstarformation to usable asaquantitativeindicatoroftheSFRindistant the AGNsinthosesurveys. that thedividinglinebetweenAGNsandstarburstgalaxiesin conclusions reachedearlierbyDressier&Gunn(1982)and emission lineratios(Fig.7).The[On]versuscolorrelation properties ofthe[On]EWs inthelocalsample,andthen and Gallagheretal.(1989). vious workbyDressier&Gunn (1982),Petersonetal.(1986), cosmological redshiftsandlookbacktimes,confirmingthe pre- galaxy, inmuchthesamewaythatHaisusednearby gal- distant AGNsisnotanewresult;itmerelyconfirmsthe relation atall.Seyfert1galaxiescanusuallybeidentifiedby sion innearbygalaxieswhich aremostrelevanttotheirappli- ground toredshiftsofatleastunity,theyprovideapowerful axies. Sincethe[On]featurecanbereadilyobservedfrom the EWs (Fig.6)suggeststhatthe[On]luminosityoughtto be the previouswork.Thiswillalteridentificationsofsome of Dressier etal.(1985).Itisimportanttopointout,however, Figure 8isquitedifferentfromthatwhichwasextrapolated in offers anespeciallypowerfulmeansofdiscriminatingbetween other criteria,however,suchasbroademissionlinesorbythe separation islessclean,andsomedonotdeviatefromthemain of theSeyfert1galaxiesliebelowmainrelation,but trum alone. are oftendifficulttoidentifyonthebasisofemissionspec- star-forming emission-linegalaxiesandSeyfert2nuclei,which thermal nucleus. symbols aregalaxiesfromthissurveywithemissiondominatedbyanon- al. (1985),respectively.PlusmarksareupperlimitsfromDressieretOpen width andcontinuumcolor,asdefinedinthetext.Soliddiamondspoints are galaxiesdominatedbydiskemission,fromthissurveyandDressieret No. 2,1992 In thissectionwediscussseveral aspectsofthe[On]emis- The strongcorrelationbetween[On]andHa+[N The utilityoftheEWversuscolorrelationforidentifying Fig. 8.—Relationbetweenthelogarithmof[On]/13727equivalent w c o Ü0 © American Astronomical Society • Provided by the NASA Astrophysics Data System 5. DISCUSSION 41 -50 INTEGRATED SPECTRAOFNEARBYGALAXIES distribution clearlyisnosubstitutefortheactual[On]mea- portional tothedistributionofmorphologicaltypesin the nicutt &Kent(1983),andalocaldistributionofHa+[Nn] EW(Ha +Nn)ratioof0.4(§4.3).Thisrescalingthe Ha RSA. Thiswasthenrescaledto[On],usingameanEW(0 n)/ Hubble typeseparately,andaveragingthesewithweightspro- data areavailableforroughly140nearbygalaxiesfromKen- between [On]andHa+[NEWs(Fig.6). (see Table4)matchestheRSAtowithinafewpercent.Henceit EW wasderivedbycompilinganhistogramforeach derived ifonetakesadvantageofthestrongcorrelation evolution inthe[On]EWdistributions.Althoughsample errors, ofalocalmagnitude-limitedEWdistribution. galaxies inthebinsE-S0,Sa-Sab,Sb-Sbs,Sc-Sd,andSm-Im ted basedonbluecolors,nuclearactivity,etc.Thefractionof excludes 10galaxiesandpeculiarobjectswhichhadbeenselec- nine E/S0galaxieswhicharenotlistedinTables1-2),and tical, SO,spiral,andMagellanicirregulargalaxies(including the RevisedShapley-AmesCatalog(Sandage&Tammann provide anindependentmeasurementofthelocal[On]EW of galaxiesobservedhereismuchsmallerthantheDARS,its should provideareasonableapproximation,withinsampling approximately thesamedistributionofmorphologicaltypesas sample ofnormalE-Irrgalaxieswasdeliberatelyselectedwith distribution, forcomparisonwiththeDARSresults. selection propertiesarewellunderstood,anditcanbeusedto expressed byBroadhurstetal.(1988)andColless(1990) which undersamplethegalaxies,andsomeconcernhasbeen (1986). TheDARSobservationsweremadewithnarrowslits limited “DARS”surveyof340fieldgalaxiesbyPetersonetal. reference sampleofnearbygalaxieswhichisfreeselection with cosmologicallookbacktime.Suchcomparisonsrequirea evidence forsubtantialevolutioninthefieldgalaxypopulation as towhetherthiseffectcouldproducepartoftheapparent In thestudiescitedabove,referencesamplewasB=11 biases ineitherthesampleitselforspectrophotometry. high-redshift samplesisoneofthestrongestpiecesempirical z ^0.05.Theapparentexcessofstrong[On]galaxiesinthe the distributionfunctionofrest-frame[On]EWstocompare the starformationpropertiesofgalaxiesatz^0.2-0.3to of distantgalaxies. strategy forestimatingSFRsfromtheUVandopticalspectra the integratedSFR.Weconcludebydiscussingoptimal derive approximatecalibrationsbetweenthe[On]fluxesand 1981, hereafterRSA).Thissampleconsistsof37normalellip- Sa, Sab2.3(7) 5.9(11) 8.5(24) E, SO1.3(9) 23.9 (14)2.7(55) Sm,Im 48.3(2) 54.1 (21)29.6(4) Sc, Scd,Sd17.4(10) 29.7(22)9.4(36) Sb, Sbc6.7(9) 16.1 (15)10.8(38) A secondestimateofthelocal[On]distributioncanbe The presentsurveyisbynomeanscomplete,butthesub- Broadhurst etal.(1988)andColless(1990)haveused Type NormalGalaxies All GalaxiesDARS 5.1. Distributionof[0n]EquivalentWidths Mean [Oii]EquivalentWidths TABLE 4