1983AJ 88.1094K a) analysis ofthestatisticalproperties ofHaemissionin the observations,acomplete listingofthedata,andan in normalspirals.Thispaper presentsadescriptionof obtain quantitativeestimatesofthestarformationrates activity amonggalaxiesofthesametype,tocompare cated inrichclusters,andtousetheBalmerfluxes star formationpropertiesoffieldspiralswiththoselo- cise enoughtostudythevariationsinstarformation properties, toobtainemissionlinefluxeswhicharepre- more completesampleofgalaxymorphologiesand investigate thesystematicpropertiesofHaemissionina surveys wereundertakenforanumberofreasons,to as adirectindexofstarformationactivity.Thepresent to itstypeandotherintegratedproperties,value the sensitivityofagalaxy’sglobalBalmerlineemission Bagnuolo (1976),andHuchra(1977),whodemonstrated were outgrowthsofearliersurveysbyCohen(1976), and Virgoclustergalaxies,mostlyspirals.Theprograms Presentaddress. surveys ofintegratedHaemissionin—200nearbyfield calculations ofgalaxyevolution. body ofobservationssmall-scalestarformationpro- cesses inourownGalaxy,andascriticalinputdatafor 1094 Astron.J.88(8),August1983 0004-6256/83/081094-14$00.90 for anumberofpurposes,ascomplementtothelarge on SFR’singalaxiesofdifferenttypeswouldbevaluable axies isonlybeginningtobeexploited.Goodharddata star formation,butitsfullpotentialinprovidingquanti- demonstrated thevalueofionizedgasasatracer tative informationonstarformationrates(SFR’s)ingal- THE ASTRONOMICALJOURNAL This paperpresentsresultsfromtwophotoelectric Photographic surveysofH11regionsingalaxieshave © American Astronomical Society • Provided by the NASA Astrophysics Data System a) and CenterforSpaceResearch,DepartmentofPhysics,MassachusettsInstituteTechnology,Cambridge,02139 is moreimportant. weakly correlatedwithgalaxyluminosityorHIcontent,suggestingthatsomeotherparameter and inmostcasesisduetoarealdispersionstarformationactivity.Thisactivityonly earlier conclusionsofCohen.Emissionamonggalaxiesagiventypevariesgreatly,however, emission ofagalaxyisstronglycorrelatedwithitsHubbletypeandcolor,confirmingthe effects, nuclearemission,[Nn]andextinctionbydustareevaluated.Theintegrated normal diskHnregions,andconsequentlytheintegratedBalmerfluxofagalaxycanbeused We presentthecombinedresultsofphotometricandspectrophotometricsurveysHaemis- as aquantitativeindexofitscurrentOBstarformationrate.Uncertaintiesduetoinstrumental sion in200fieldandVirgoclustergalaxies.Inmostspiralgalaxiestheemissionisdominatedby I. INTRODUCTION Department ofAstronomy,CaliforniaInstituteTechnology,Pasadena,91125 A SURVEYOFHaEMISSIONINNORMALGALAXIES Department ofAstronomy,UniversityMinnesota,Minneapolis,Minnesota55455 Received 28January1983;revised29April1983 Robert C.Kennicutt,Jr. Stephen M.Kent VOLUME 88,NUMBER8 ABSTRACT this purposethecompilationofRoberts(1975)wasused. in morphologicaltype. Objects intheVirgoclusterweregenerallyavoided.Fin- common withherlist.Alsosomeattentionwaspaidto ally, anattemptwasmadetoobtainareasonablespread selecting objectsthathave21-cmmeasurements,andfor ments withthoseofCohen(1976),manyobjectsarein cause itwasdesiredtocomparethepresentmeasure- was thatthegalaxiesbebrighterthanB=12.5.Be- leurs, andCorwin(1976)].Theonlygeneralrequirement after referredtoasRC2,deVaucouleurs,Vaucou- sistency oftheseobservations. were repeatedondifferentnights asacheckonthecon- the SecondReferenceCatalogofBrightGalaxies[here- field galaxieswhichwereobservedwasobtainedfrom spectrum wasobtainedforeach object,althoughafew sure timesweretypically20 minutes. Usuallyonlyone pixel. Thescalealongtheslit was2"2perpixel.Expo- covered was5850-7400Aataresolutionof3.2Áper lomar 1.5-mtelescopeandtheDigitalSITSpectrograph axies during1978November-1979AugustwiththePa- tained during1978-1979(Kent1980).Thesampleof tered onthenucleusofagalaxy.Thewavelengthregion the slitwasalwaysorientedinanE-Wdirectioncen- (Kent 1979).Aslitsizeof3"X3'wasgenerallyused,and Schommer 1983). spirals (Kennicutt1983a;Kennicutt,Bothun,Kent,and SFR calculations(Kennicutt1983b)andwiththecluster the programgalaxies.Otherpaperswilldealwith T Long-slit spectrawereobtainedforthissampleofgal- Long-slit spectrophotometryfor57galaxieswasob- II. SPECTROPHOTOMETRICOBSERVATIONS © 1983Am.Astron. Soc.1094 AUGUST 1983 1983AJ 88.1094K /I 7136present. These lastthreegalaxiesaretheonlyonesinwhich[Oi] was detected. Weedman (1977). with linesofHeI/U5876,6678,7065,and[Arm] fert. galaxy areextremelybroadandtheHa+[Nn] disk (deVaucouleurs,deandPence1974). going extremelyactivestarformationacrosstheentire much strongerthisgalaxymightbeclassifiedasaSey- Near thenucleusahighexcitationHnregionwasfound further comment. measurements istheirhigh spectral resolution,which [S n]linepairsareblended.Iftheintensitieswere the continuumdistributionalongslitinawave- illustrated inFig.1.Foreachobjectthetopprofilegives a galaxy.Thishasbeendoneforallthegalaxiesandis possible toexaminethespatialdistributionofHaacross lines maytake. included heretoillustratetherangeinvaluesthatthese The [Nn]and[Slineswillnotbeusedfurtherbutare width. Anyotheremissionlinespresentwerealsomea- mension forthepurposeofmeasuringHaequivalent were usedtocalibratethespectraandconvertthem trailing ofthegalaxyalongslit. file inthecontinuumwascausedbyaslightirregular net Hadistribution.Inafewcasesanasymmetricpro- length regionnearHa,andthebottomprofileshows brightness levelsatthesedistancesfromthenucleus, contaminated bythegalaxy.Becauseoflowsurface of thespectrumwhichareusedforskysubtractiontobe length oftheslit,thuscausingstripsalongedge sured. Inafewcasesthegalaxyextendedbeyond3' relative fluxes.Thespectrawerecompressedtoonedi- emission itisestimatedthatanyabsorptionS2K. ing absorption;fromthespectrawithnodetectableHa than 1A.Nocorrectionhasbeenmadeforanyunderly- strength relativetoHawhenmeasured.Incaseswhere relative toHawhenmeasured;(5)[Sil]/I6717+6731 number; (2)morphologicaltype“T”fromRC2;(3)Ha Ha wasnotdetected,theupperlimitistypicallyless equivalent widthinÀ;(4)[Nn]X6548+6583strength along withthefollowinginformation:(1)NGCorIC tempt atcorrectionwasmade. this contaminationisprobablynotseriousandnoat- 1095 R.C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES NGC 3310:HeIA5876and[Oi]X6300present. NGC 3227:ThisgalaxyisclassifiedasaSeyfertby NGC 2782:[Oi]X6300present. NGC 5005:Theemissionlinesinthenucleusofthis NGC 1569\Thisunusualgalaxyappearstobeunder- The primaryadvantageof the spectrophotometric A fewgalaxiesareofsufficientinteresttomerit Because ofthetwo-dimensionalnaturedataitis Standard observationsandreductionprocedures The emissionlinemeasurementsaregiveninTableI © American Astronomical Society • Provided by the NASA Astrophysics Data System III. LARGE-APERTUREPHOTOMETRY a) Observations c b a d d .N5448 jor disadvantageisthatthesmall entranceaperturesad- XX 6717,31/Halineratiosas diagnosticsofthephysical conditions andabundancesin theionizedgas.Theirma- [Sn]/Ha +[Nii]. EquivalentwidthHa+[Nil]. Equivalentwidth[Nil]. N5585 N6503 N6015 sion lines,andtoapplythe [Nn]/Haand[Sn] Exposureinterruptedbyearthquake. N1569 1342 N3556 N5033 N4605 N3726 N3359 enables ustoresolvethecontiguous Haand[Nn]emis- N3294 N3198 N3079 N2976 N2776 N2525 N1961 N1637 N5248 N5055 N5005 N3521 N2276 N5371 N5194 N3646 N3310 N3344 N2964 N2903 N2268 N5746 N3627 N3351 N2841 N2683 N2613 N7217 N5566 N3504 N5377 N4995 N2146 N3227 N3190 N3169 N2782 N2775 N2639 N4753 N5701 N5363 N2681 N2655 (1) NGC/IC N3166 Type 10 (2) 6 7 6 6 6 5 5 5 5 5 4 4 4 4 4 4 4 5 5 4 4 4 4 5 5 5 5 5 5 0 0 3 2 2 0 0 0 0 3 3 3 3 3 3 3 2 2 1 1 1 1 1 1 1 1 Table I.Fieldspirals. b 125.0 (3) 135.0 wAm 66.0 28.0 35.0 40.0 50.0 62.0 27.0 43.0 43.0 13.0 15.0 19.0 10.0 52.0 14.0 11.0 85.0 19.0 19.0 15.5 13.0 17.0 13.0 17.7 4.3 8.5 0.5 3.2 8.1 6.8 2.2 6.6 3.0 5.0 7.1 6.8 1.5 1.2- 1.9 < < < < < < < < < < < < < a a a a a a a 0.28 0.25 0.11 0.33 0.57 0.48 0.42 0.50 0.31 0.32 0.36 0.81 0.37 0.35 0.47 0.85 0.80 0.64 0.53 0.39 0.45 0.71 4.3 0.69 0.49 2.6 0.94 4.9 2.0 4.0 1.87 5.1 2.1 2.8 (4) 3.00 5.8 1.08 [N n]/Ha 1.85 1.43 1.20 1.93 1.60 c 0.14 0.16 0.44 0.35 0.26 0.98 0.65 0.23 0.36 0.31 0.39 0.26 0.23 0.19 0.31 0.33 0.43 0.32 (5) [S ii]/Ha 1.6 1095 1983AJ 88.1094K 1096 Ha. Thelengthofeachprofileis4!7. Westistotheleft. Fig. 1.Distributionofcontinuumand Halightalongtheslitforfieldgalaxies.Ineachcasetopprofile is thecontinuum,andbottom NGC 3227 NGC 3294 NGC 3310 © American Astronomical Society • Provided by the NASA Astrophysics Data System R. C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES NGC 3344 NGC 3351 NGC 3359 NGC 3504 NGC 3521 NGC 3556 _X NGC 3627 NGC 3646 NGC 277528m29763169 NGC 4605 NGC 4753 NGC 4995 _/L NGC 500551945371 NGC 5055 NGC 5377 NGC 5448 1096 1983AJ 88.1094K -1 useful asaSFRindicator Haalone.Theothertwo program galaxiessothatthe combinedfluxisnearlyas (Ha +[Nn])ratioisprobably constantenoughinthe Ha andthe[Nn]lines.Aswill beseenbelow,theHato broad (100-ÂFWHM)filter was usedtotransmitboth ÀÀ 6548,6584emissionforallgalaxieswithradialveloc- to thepresent[Nil]errorsanyway.Consequentlya certainties (Kennicutt1979a)comparableinmagnitude and wouldhaveintroducedphotometriccalibrationun- quired alargenumberofnarrowbandpassfilters photometry, photometryofHaalonewouldhavere- tamination complicatestheinterpretationof (FWHM 520Âspacedevery10-15Áinwavelength), sion inthebandpasswasacompromise.Whilecon- ities under3000kms.Theinclusionof[Nn]emis- measurement ofthecombinedHaandsatellite[Nn] in Fig.2.Thecentralbandpassat6580Ápermitted the nightskybrightnessweredominantsourceof signal oftheredwavelengthsmeasured.Fluctuationsin substantial, usuallycomprising70%-95%ofthetotal mize itseffectwithintheconstraintsimposedby error inthephotometry,andpainsweretakentomini- on theuncertainties. were usedwithsingle-channelphotometers,equipped panas ObservatoryandthePalomar0.5-mtelescope in 1979-1980.TheSwope1.0-mtelescopeatLasCam- served on3-4differentoccasionsasanabsolutecheck completely cloudlessconditions,andmostwereob- typically reducedinthiswaytowithin1.5-2timesthe tainties intherawcountsduetoskyvariationswere consisted ofapair30-sintegrationsineachfilter, available instrumentation.Atypicalobservingsequence regions beyondthisradiusisusuallynegligible. shows thattheamountofemissionfromresolvedHn surement oftheintegratedemission.Thesamestudy with asetof1-in.interferencefilterstoisolatethe photometric Hasurveywasinitiatedbyoneofus(RCK) valent widths.Consequentlyasecondlargeaperture rate measurementofthetotallinefluxes,onlytheirequi- disk emissionandinanycaseitdoesnotyieldanaccu- cleus) mayornotberepresentativeoftheintegrated cases. Thissmallfraction(usuallycenteredonthenu- repeated through4—8sky-switchingsequences.Uncer- sizes arerequiredinordertoobtainanaccuratemea- produced intheouterdisk,andhencelargeaperture show thataconsiderablefractionoftheHaemissionis as listedintheRC2.Studiesofradialdistribution the entirevisiblediskdowntoalimitingisophoteof25/i, Ha +[Nn]emissionlinesandtheadjacentredcontin- mit onlyasmallfractionofthegalacticlightinmost Poisson error.Photometrywasonlyobtainedunder H iiregionsingalaxies(e.g.,HodgeandKenncutt1983) most galaxiestheaperturesizewaschosentocontain ing 0-3'diameterwasusedonthe1-mtelescope.For used onthe0.5-mtelescope,whileavariableiriscover- uum. Fixedaperturesranginginsizefrom1to7'were o 1097 R.C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES The skycontaminationintheselargeapertureswas The filtersystemusedinthemeasurementsisshown © American Astronomical Society • Provided by the NASA Astrophysics Data System -21 RC2 wasavailable.Thisresults inbasicallyamagni- pley-Ames catalog,andthat UBV photometryfromthe lection criteriawerethatgalaxies beincludedintheSha- and irregularsystemsforcomparison.Theprimaryse- cluster galaxies,mostlyspirals,withafewelliptical,SO, ies areclearlyundersampled. Nearbygalaxieswithlarge tude-limited sample,andhence intrinsicallyfaintgalax- of order±O^S—0.1. type observed,alongwithanobservationaluncertainty range in(V-R)colorsobservedamonggalaxiesofthe between themeanobservedandpredictedmagnitudesis satisfactory, andthedispersionisconsistentwith Wood’s (1966)filterphotometry.Theagreement sured fromTumrose’s(1975)spectrophotometryand expected mean(V—6600Á)color(0.3mag),asmea- magnitudes fromtheRC2.Thezeropointrepresents sured at6600AarecomparedwiththepublishedV shown inFig.3,wherethecontinuummagnitudesmea- bands oneithersideofHainordertoaccuratelydefine the continuum.Anexternalcheckonfluxscaleis standard stars,soitwasimportanttoselectcontinuum galaxy continuaintheredareusuallyredderthan bration ofHayesandLatham(1975),G-starenergydis- as interpolatedfromthe6400-Áand6800-Àfluxes.The flux inthe6580-ÁfilterfromcontinuumatHa, line fluxesweredeterminedbysubtractingtheobserved curves onaCary14spectrometer.Thefinalemission son (1980),andmeasurementsofthefiltertransmission absorption equivalentwidthsfromMendozaandJohn- tributions fromBreger(1971)andTumrose(1975),Ha fluxes (ergcmsA~^wasdoneusingtheVegacali- nightly. Conversionoftherawmagnitudestoabsolute atmospheric extinctionandinstrumentalzeropoint used inconventionalbroadbandphotometry.Dwarf continuum atHa. known Haabsorption)weremeasuredtodeterminethe filters sampledemission-freecontinuumbandpassesat G0-G5 stars(chosenfornonvariabilityandsmall, 6400 and6800Áinordertosubtracttheunderlying Photometry wasobtainedfor175fieldandVirgo The calibrationprocedureswereidenticaltothose Fig. 2.Thefiltersystemusedfortheaperturephotometry. 1097 1983AJ 88.1094K -2 1098 R.C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES measured hereandobservedJohnsonVmagnitudesfromtheRC2. Fig. 3.Histogramofdifferencesbetweentheredcontinuumfluxes The 0.3-magoffsetisthemeanexpecteddifference,basedongal- consistent withtheexpecteddispersioningalaxycolor. axy colorsandthedifferenceinaperturesizes.Theobservedspreadis Table IIareasfollows: ies aredenotedinaremarkscolumn.Specificentries taken fromtheRevisedShapley-AmesCatalogofBright continuum fluxat6570A.Anumberofunusualgalax- width, definedastheratioofemissionlinefluxto Ha +[Nn]lineflux,andtheemissionequivalent ic extinction.Uncertaintiesare notlisted,buttheymay flux, inunitsofergcms~ \ correctedforatmospher- larger ofthetwoislisted. tainties; incaseswherethey differedsignificantlythe relevant emissionlinequantitiesarelisted,theabsolute earlier, but25galaxiesweremeasuredincommon. most casesthetwomethodsyieldedcomparableuncer- ment ofdifferentnights’observationsthegalaxy.In individual observations,andfromtheinternalagree- two ways,fromthestatisticsofskynoiseduring width, inangstroms.Theuncertaintieswerederived Galaxies (hereafterRSA,SandageandTammann1981). overlap withthespectrophotometricsampledescribed few casescrudeequivalentwidthsfortheinnerregions angular diameterscouldnotbesurveyed,thoughina were obtained.Noattemptwasmadetocompletely Column 2.MorphologicaltypeintheHubblesystem, Column 1.GalaxyNGCorICdesignation. A summaryofthedataiscompiledinTableII.Two © American Astronomical Society • Provided by the NASA Astrophysics Data System Column 3.Aperturediameterused,inarcminutes. Column 4.ObservedHa+[Nn]emissionequivalent Column 5.Logarithmofthe observedemissionline 1,v,v,v -0.6 -0.4-0.20.00.20.40.6 o Iri"' -m ^6600“ (0-3) -1 noted byV.Ndenotesgalaxieswithevidenceforstrong be determinedusingtheequivalentwidthuncertainties. lation, basedonthetotalgalaxymagnitudeandob- the measuredregion,andflux(iflisted)isanextrapo- used. Inthosecasestheequivalentwidthonlyappliesto The notation/denotesgalaxieslargerthantheaperture contribute 20%-100%oftheobservedtotalemission. nuclear emission.Inthosecasesthenucleusmay served equivalentwidth. dard deviationsofzero,thefluxisnotlisted. the spectrometermeasurements. by circles.Thescatterisalmostentirely duetothelimitedsamplingin corrected forforegroundgalacticextinctionusing: RSA (i.e.,H—50kmsMpc).Thesefluxesare from theobservedfluxesanddistancestaken data fromthispaperaredenotedby crosses, datafromCohen’ssurvey selves hasbeenmade,however. In caseswheretheequivalentwidthiswithintwostan- aperture photometryandspectrophotometry. Spectrophotometric between thetwosurveys.Spectrometermeasurements tainty intheequivalentwidthswhichisintroducedby using bothtechniques,inordertoestimatetheuncer- interest tocomparetheresultsforgalaxiesmeasured the integratedaperturephotometry,anditisofsome No correctionforextinctionwithinthegalaxiesthem- measurements fromTableIfor24galaxiesincommon the limitedgalaxysampling.InFig.4aperturephotom- equivalent widthsareconsiderablyeasiertoobtainthan Fig. 4.ComparisonofHa+[Nn] equivalent widthsmeasuredfrom etry fromTableIIiscomparedwithspectrophotometric 0 A (6570)-0.08mag(csc6-1). Column 7.Remarks.Virgoclustermembersarede- Column 6.AbsoluteHa+[Nn]luminosity,derived Spectrometric measurementsoftheemissionline b) ComparisonwithSpectrophotometry 1098 1983AJ 88.1094K NGC/IC 1099 R.C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES 3115 2276 2139 1058 2976 2841 2775 2763 2681 1079 1073 1068 1022 1087 3379 3368 3351 3310 3504 3389 3384 3185 3081 3034 1357 1302 1291 1232 1156 1140 1084 3486 1569 1518 1385 4237 3627 3623 3521 1637 4189 4178 4027 1832 4254 4214 4212 4152 3810 3726 3631 3938 3955 221 450 428 278 488 337 357 628 949 718 681 672 584 157 150 © American Astronomical Society • Provided by the NASA Astrophysics Data System Type Sc She Sc(p) She E Sc SBa Sc Sc SO Sab Sab SBc Sc Sa Sc SBa(p) Sc Sa Sa Sm Sb(p) Sc Sc SBc Sb Sc Sa SBa Sm Sc Sc SBc SBb SBc Sa Sc Sb/SBb Sc SB0 E Sab SBb Sbc(p) SO Am Sd Sb Sa Sc SBa SBa Sb Sa Sb Sc Sc Sc SBc SBc Sc S(P) Sc Sc SBm Sc Sc Sc Aperture 3 3 3 3 5 7 7 3 3 3 3 7 3 3 3 3 3 . 3 3 2 3 3 7 3 3 2 3 3 3 3 3 3 3 3 7 3 5 3 2 5 5 7 3 3 7 7 7 3 3 7 7 3 3 3 3 7 3 7 5 5 5 3 3 5 3 3 3 5 Table ILLargeapertureHa+[Nh]photometry. EW -1± -1+ 3 -3+ 3 -3+ 5 -1+ 2 -2 + -1+ 1 149 +15 — Í+2 -1+ 1 113+ 4 28 + 49 + 24 + 36 + 24+2 38+4 41+2 21+3 20+3 20+3 21+ 3 99 +20 46+ 3 46+2 35+3 50+ 1 42+4 28+3 20+3 18+3 33+4 80+4 29 + 26 + 56+ 1 16+2 35+3 35+4 35+4 47 + 20 + 20 + 23 + 12+2 13+3 31+7 24 + 36 + 30 + 32+ 1 33+8 18+ 1 Í4+ 2 17 + 19+4 4+ 2 2+ 2 8+ 2 5+ 2 1± 1 2 + 4+ 3 5+ 2 1+ 3 1 + 6+ 2 1± 1 3 3 5 3 11.33 11.64 11.52 11.17 11.90 11.85 a 11.56 10.89 11.75: 11.88 12.14 11.45 11.82 11.17: 11.64: 11.70 11.18 10.60 11.77 11.37 -log/ 11.62 11.68 11.49 11.54 11.27 11.91 11.34 10.91 12.18 11.56 12.7 11.46 11.19 11.78 10.95: 10.94: 11.29 11.50 11.31 12.50: 11.48 11.93: 10.91 10.84: 11.70 12.00 11.72 11.32 11.85 41.86 41.45 4165 41.44 41.36 41.06 40.74 41.67 40.54 40.93 40.72 41.36 41.65 41.83 41.17: 42.00: 40.53: 40.37 40.83 41.89 42.03 41.59 40.70 41.21 41.59 41.83 41.25 40.95 40.1 log F 40.88 39.89 41.56 41.09 41.00 41.28: 41.32: 40.25: 40.75 41.25 41.09 41.55 41.84 40.76: 41.03 41.45 41.50 41.21 41.05 40.80: I N N I N Remarks N N, I N I I V V V V V 1099 1983AJ 88.1094K NGC/IC 1100 R.C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES 4298 4294 4293 4274 4321 4303 4299 4394 4382 4374 4365 4472 4449 4420 4406 4487 4486 4941 4826 4569 4567/8 4548 4526 4501 4496 4594 4571 4561 4536 4535 4504 4900 4808 4790 4781 4866 4775 4689 4631 4597 4595 4579 5005 4736 4713 4666 4654 4651 4602 5055 5033 4658 4632 5194 5962 5866 5806 5746 5701 5585 5364 5248 5204 5676 5633 5485 5474 5457 © American Astronomical Society • Provided by the NASA Astrophysics Data System Type Sc Sab Sab Sc 4- SBc SBp Sc S0 Sc Sc Sc SBc Sa/Sb Sc SBc SBc SBc Sa(p) Sa Sbc Sbc SBc Sc Sc Sd Se Sc SO Sc Sbc Sd Sbc Sb Sab Sc Sa Sab Sc SBc Sc S0(p) Scd(p) Sc Sc Sd Sc Sab Sc Sbc SBc Sc SBb S0(p) E E Sb Sb SBa Sc Sbc Sd SBc SBc Sm Sc SO/E E E/S0 Aperture Table IL(continued) EW -2+ 3 -1± 3 -1± 3 27 + 40+ 4 24 + 24+2 43+7 33+5 25+7 37+ 8 22 + 23 + 21+ 1 28+2 20 + 39 + 37+3 56 + 31 + 34 + 30+3 18+ 4 14 + 17 ± H± 14+2 18+3 17+ 1 17+2 10 ± 15 + 12 + 16+2 12+3 13 + 17 + 18 + 18 + 19 + 19 + 40 ±5 71 ±7 21+5 63+ 1 55 ±5 34 ±3 '3 + 18± 1 11± 2 0 + 0 + 4+ 1 4 + 3+ 1 6+ 2 6 +1 2 + 6 +2 7 + 8 + 4± 4 2± 2 0+ 2 2+ 3 3± 2 1+3 1± 3 2 5 3 12.05: 11.77 11.50 11.69 11.48 11.43 11.70 10.52: 10.83: 11.36: 11.65 11.92 11.52 11.32 11.64 11.97 11.64 10.71 12.03: 11.83: 12.5: 12.07 11.62 12.29 11.96 11.47 11.33 11.59 11.68 10.85 11.52 11.85 11.55 11.67 12.1: 11.62 12.04 11.72 11.71 11.28 12.15: 11.54 11.70 11.05 12.02 11.71 10.59 10.85 11.71 11.87 11.82 -log/ 40.71 41.25 41.06 40.7: 41.63 41.32 40.99 40.61 40.71 41.53 40.2: 40.46 41.62: 41.01 40.64 40.42 40.83 41.27 41.39 40.90 41.73 41.20 41.39: 40.68 41.13 41.54 41.32 41.39 41.33: 41.28: 40.89 41.21 41.36 40.70: 40.92 41.54 40.9: 41.76 41.41 40.12 40.25 41.21 40.13 41.70 41.90 40.73 41.04 40.64 41.04 40.88 41.22 log/- I N N I I I N NJ I V VJ K N VJ K N V K N V V V VJ Remarks V V V K N V VJ VJ Kl V, N,I Kl K N 1100 1983AJ 88.1094K b a grossly overestimatedinthe slit data. bright Hiiregions,andarelessreliable.Thisproblemis roughly the50%levelforgalaxieswithstrongemission. timate oftheintegratedemissionthatisaccurateat emitting nucleuscausedthe integratedemissiontobe large diskHiiregionsaltogether, andnoemissionatall especially noticeableforthenarrow-slitmeasurements was detected,whileinafew otherinstancesastrongly show inFig.4;severalgalaxies theslitmissed For galaxieswithrelativelylittlediskemission 20-30 A,thespectrometermeasurementsprovideanes- Ha +(Nii)equivalentwidthinthesampleisoforder reflecting errorsinbothsetsofdata.Sincetheaverage trophotometry differsby±11Àonaverage,probably dersampling inthenarrowslitdata,whileCohen’sspec- dominated bythepropertiesofnucleusand/orafew ( <10À),thespectrometermeasurementsareoften a meandispersionof±18À,almostallduetotheun- NGC/IC dispersion issubstantial.Kent’sequivalentwidthsshow 1101 R.C.KENNICÜTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES the twosetsofmeasurementsarewellcorrelated,but ies incommon. from Cohen’s(1976)surveyarealsoplottedfor23galax- 6106 6070 6015 6412 6340 6217 6207 6181 5970 6503 6643 6574 7764 7743 7742 7741 7727 7723 7716 7590 7448 7392 7218 7217 6946 6814 6654 15271 14662 7479 7371 7213 7177 7137 NotinRSACatalog.Classifiedbyauthors. Hiiregionsdetectedinthisgalaxy.Theemissionmeasuredhereisprobablyreal. For galaxieswithsubstantialemission(W^\0k) © American Astronomical Society • Provided by the NASA Astrophysics Data System Ä b Type Sc Sc Sc SBbc SBa Sab Sc SBc Sa(p) SBb Sc SBbc Sc Sbc SBbc Sc Sb SBm Sa SBa Sb SBc/Sc Sa Im Sc Sc Sc Sbc Sc Sa Sab Sc Sbc SBa Aperture Table IL(continued) EW -2+ 2 29+2 41+7 24+5 40 + 26 + 24 + 29 + 21+3 25+5 27 + 33 + 35+4 38+3 33+3 24+2 53 + 35 + 51 +11 11 ± 19+2 12 ± 17 ± 14+4 10+2 10+3 14+2 0+ 2 6 + 7 + 6 + 5 + 3 _ 1 + i 3 among galaxiesofthesame Hubble typeisverylarge. Before goingontointerpretthese results,itisimportant the Hubblesequence,but dispersioninemission the aperturephotometrysample issummarizedinFig. lution makesitpossibletoseparatetheHáemission, mation inearly-typegalaxies,andthewavelengthReso- more sensitivefordetectingverylowlevelsofstarfor- able totherealscatteringalaxies,butthismethodis instrumental scatterinthespectrometerdatäiscompar- tics ofstarformationinvarioustypesgalaxies.The equivalent widthswhichcanbeusedtostudythestatis- 5. Lineemissionincreasessmoothly onaveragealong Both setsofcapabilitieswillbeexploitedhere. (N ii)andHaabsorptioncontributiontothétotalflux. photometry providesaccurate,absoluteline,fluxesand complement oneanothertoalargeextent.Theaperture posses differentadvantagesandhandicaps,they IV. INTERPRETINGTHEEMISSIONLINEDATA:DISK The distributionofHa+[Nii]equivalentwidthsin In summary,thetwosetsofemissionlinephotometry 11.78 11.59 11.58 Í 1.79 11.80 11.58 12.11 11.83 11.67: 11.99 10.5: 11.6 11.44 11.8: 11.31: 11.92 11.61 11.42: 12.57 11.96 11.98 11.71 11.98 11.49 11.§6: 12.18 11.1 12.36 -log/ STAR FORMATION 41.84 41.21 41.47 41.27: 41.83 41.53 41.35 41.27 41.1: 41.72 40.33: 41.15 41.Ö8 41.53 41.66: 41.89 41.2: 40.80 41.5 41.6: 41.49 40.99 41.19 40.58 40.3 41.16 41.25 41.29 log/” I N N N N Remarks 1101 1983AJ 88.1094K is probablyinstrumental.Thisconsistentwiththe dispersion of1.5Áobservedintheaperturephotometry the emisssiOnequivalentwidthisreallyzero,sothat galaxies listedinTableII. average estimateduncertaintyof±2ÁfortheE-SO trometer dátaindicatethatinvirtuallyallsuchgalaxies with armsdispersionof1.5A.Themoresensitivespec- best externalcheckontheuncertaintiesisprobably and lenticulargalaxies.Aperturephotometryfor14E- be verysmall,however.Theintegratedspectraofearly- S0 systemsyieldedameanÈW(Ha+[Nil])=0Â, observed dispersioninequivalentwidthsforelliptical and wewishtoverifythevalidityofthoseestimates.The with veryweakHaemission (EWS3Â),wheretheob- by late-typediskgiants,stars whichnormallypossess servations arepoorattyway. tion linecontributionwillonly beimportantingalaxies emission orinabsorption,atthe0*5-1-Alevel(NGC the equivalentwidthscaleispresenceofunderlying measure therelativeratesofmassivestarformationin only weakBalmerabsorption lines.Hencetheabsorp- red continuaofmostluminousgalaxiesaredominated type galaxiesgenerallypossessnomeasurableHa,in Ha inabsorptionfromthestellarlight.Thiseffectmust try listedinTableIIaremuchlower,2-3Atypically, substáritial. Theuncertaintiesintheaperturephotome- data werediscussedearlier,andtheyfoundtobe instrumental errors,nuclearemission,[Nn]/Havaria- the galacticdisks,andnotextraneousfactorssuchas to verifythattheemissionlinefluxespredominantly 2681 isanotableexception).Thisnotsurprising;the tions, orvariationsinextinctionbydust. by RSAHubbletype.ThesymbolSdenotesaSeyfertgalaxy. Fig. 5.DistributionofHa+[Nri]emissionequivalentwidth,binned 1102 R.C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES The instrumentaluncertaintiesinthespectrometer A possiblesmallsourceofsystematicuncertaintyin © American Astronomical Society • Provided by the NASA Astrophysics Data System a) InstrumentalUncertainties -21 the small-slitspectrometerdata,itisrarelydominantin the integratedphotometry.Figure6comparesnuclear fluxes fromTable2,for37galaxiesincommon.Four Ha-h (Nn]fluxesfromKeel(1982)withintegrated broadened. Thisemissionisonlyseeninearly-typegal- or “starburst-nuclei”(Balzano1982),consistoflarge nents, knownvariouslyas“hotspots”(Morgan1958), components intheHaemission,someexamplesbeing vey, onecanoftenidentifydistinctnuclearanddisk emission alongtheslitinspectrophotometricsur- to beexcitedbysomesourceotherthannewlyformed weak [W{Ha)S10A].Thelinesareoftennoticeably is characterizedbyhaying[Ñn]£Haandusuallybeing scribed byBurbidgeand(1965).Suchemission is anadditionaltypeofnuclearemissionthekindde- they areeasytopickoutassuch(e.g.,NGC2782).There emission invariablyturnouttohavesuchnuclei,and gions ofthesegalaxies.Sa-SabgalaxieswithstrongHa H iiregioncomplexesintheinnerdiskandbulge-re- NGC 1637,2903,and5248.Thestrongnuclearcompo- stars. axies withalargespheroidalcomponent,anditislikely erg cms. with integratedfluxesfromthis paper. Unitsinbothcasesare Fig. 6.ComparisonofnuclearHa+ [Nn]fluxesfromKeel’ssurvey À While nuclearemissioncansignificantlyinfluence From Fig.1,whichillustratesthedistributionofHa b) NuclearEmission 1102 1983AJ 88.1094K fluxes ofthelargestthreeHiiregions. ÜnitsarethesameasinFig.6. Fig. 7.ComparisonofintegratedHa +[Nn]fluxwiththecombined gest threeHnregions,in36galaxiesforwhich grated galacticemissionwiththetotalfluxesoflar- regions (Kennicutt1978).Figure7comparestheinte- sion whichisobservedinthepropertiesofgiantHn small numberstatistics,combinedwiththelargedisper- Ha emissionevidentinFig.5couldbeexplainedby one ortwolargeHnregioncomplexes,thescatterin not thenuclei. late-type galaxiesisapropertyofthestar-formingdisks, large dispersionintheemissionlinepropertiesof from normalHnregionsintheinnerdisk.Hence among thegalaxiessurveyed. tion betweendiskandnuclearemissionproperties cases, becauseKeel’saperturesalsoincludeemission nuclear fractionsarealsoonlyupperlimitsinmost were measuredinthetotalsampleof175galaxies.The with brightnuclei;virtuallyallofthestarburstsystems The sampleshowninFig.6isheavilybiasedtogalaxies note inpassingthatthereisnoevidenceforanycorrela- nuclei possess0.2S//50.5.Itisinterestingto and NGC6814),anumberofgalaxieswithstarburst time ofthesurveywereexcluded,exceptforNGC1068 galaxies provedtobeSeyferts(knownatthe less ofthetotaldiskemission.Exceptionsdoexist:two cases thenuclearemissionamountstoafewpercentor nuclear fluxesmeasuredbyusarealsoplotted.Inmost Nuctotal 1103 R.C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES © American Astronomical Society • Provided by the NASA Astrophysics Data System If thestarformationingalaxiesweredominatedby In general,thenuclearemissionisrarelysignificant. c) LargestH//RegionEmission served dispersionofemission ingalaxies. consider thepossibleeffects ofextinctionontheob- in detailelsewhere(Kennicutt 1983).Hereweshallonly problem ofcorrectingforextinction willbeconsidered uncertainty ininterpretingthe Balmerlinefluxes.The tial, anddustisprobablythesinglelargestsourceof strength. pared totheobserveddispersioningalacticemission of valuesderivedabove,butinanycaseitissmallcom- The actualscattermorelikelyliesbetweenthetwosets galaxies, however,withhigherexcitationHiiregions. sample maybeweightedsomewhattowardlate-type iations issmall,+5%-20%intheextremes.Thislatter artificial dispersioninFig.5arisingfrom[Nn]/Havar- change significantly.Ifthesedataarerepresentative,the are addedtothoseshowninFig.8,theresultsdonot extensive spectrophotometricobservationsofMcCall (1982) werenotavailableatthetime,butwhenhisdata in Fig.8,for14spiralandsevenirregulargalaxies.The tribution. Resultsofsuchacompilationaresummarized galaxy, weightedroughlybytheradialHnregiondis- and Heckman1980)computinganaverageforeach and Joly1979;Dufouretal.1980;Kennicutt,Balick, Strom 1976;Pageletal.1979;Alloin,Collin-Souffrin, Peimbert 1974,1976;Dufour1975;Jensen,Strom,and ture (Searle1971;Smith1975;PeimbertandTorres- of individualextragalacticHnregionsfromthelitera- values canbeobtainedbycompilingspectrophotometry a factor2. tion. ForspiralgalaxiesoftypeSabandlaterthemedian ratio Ha(Ha+[NIt])is0.64,witharangeofO.4-0.8,or large inthenuclearregions),buttheyprovideanindica- emission (inparticularthe[Nn]/Haratiotendstobe ratios arenotentirelyrepresentativeoftheintegrated ly weightedbythenucleus,soderived[Nn]/Ha the galaxieslistedinTableI.Theseslitspectraareheavi- available. Lineratiosweremeasuredforamajorityof properties. Twosourcesofdataon[Nn]/Haratiosare count forthelargeobserveddispersioninemission large andvariablefromgalaxytogalaxy,thiscouldac- and thesatellite[Nn]emissionlines,ifn]/Hais lars, however(Hunter1982). This situationmaynotholdforlowerluminosityirregu- the giantHnregiondispersionwillnotbeimportant. high luminosityspirals,andforthosegalaxiesatleast total emission.ThesampleinFig.7isdominatedby small andsurprisinglyuniform,atabout10%ofthe The contributionofthelargestHnregionsisgenerally region photometryisavailable(Kennicutt1979,1981). Israel andKennicutt(1980) combined ratiocontin- The extinctioningiantHnregionsisoftensubstan- A morerepresentativesetofintegrated[Nn]/Ha The interferencefilterphotometrymeasuresbothHa e) ExtinctionbyDust d) [NII]/HaVariation 1103 1104 R. C. KENNICUTT, JR. AND S. M. KENT: Ha EMISSION IN GALAXIES 1104

this section are small in comparison, though dust may play a significant role, and the other effects may be im- portant in very early-type galaxies with only a small amount of star formation. Not mentioned above are the strong correlations which are often observed between the line emission and galaxy morphology in specific in- stances. Galaxies with very high equivalent widths (>50 A) invariably prove to be galaxies which appear to be undergoing intense star formation bursts; galaxies with low emission almost always possess morphologies and integrated colors which are consistent with a low star formation rate. Quite clearly the Ha + [N n] emission of a galaxy predominantly measures the current rate of massive star formation.

V. STATISTICAL PROPERTIES OF Ha EMISSION IN NORMAL GALAXIES In order to shed light on the possible factors which do influence the Ha emission and the star formation rate, we have investigated the systematic properties of the emission, and its dependence on galactic morphology, luminosity, and stellar and gaseous content. Here we are primarily interested in the statistical relationships Fig. 8. Distribution of integrated Ha to Ha + [N il] ratio in galaxies between various integrated properties of the galaxies, with published H H region spectrophotometry. independent of any (model-dependent) quantitative in- terpretation of the star formation rates derived from the uum and Ha measurements of 30 extragalactic H n re- emission line fluxes. The latter will be considered separ- gions to derive visual extinctions. The mean absorption ately (Kennicutt 1983). found in this way was Av = 1.7 +0.8 mag. The ob- Before proceeding we should emphasize that no at- served dispersion includes a substantial contribution tempt was made to define a precisely distance-limited or from uncertainties of the individual radio fluxes, so that magnitude-limited sample of galaxies. In particular, in- the variation in extinction is probably somewhat lower. trinsically faint and low surface brightness galaxies are A more relevant comparison between the integrated ga- severely undersampled, a deficiency which we hope to lactic Ha + [N n] emission of galaxies in this sample correct eventually. The data should provide, however, a with unpublished 10-GHz thermal radio fluxes has been reasonable representation of the star formation proper- m made by Kennicutt (1983), and yields a mean AUa ties of normal luminous (MB < — 19 ) spiral galaxies. In = 1.1 ±0.5 mag. Again the radio fluxes are very uncer- the following comparisons only the large aperture filter tain, and the observed dispersion is really only a crude photometer data will be plotted, since the individual upper limit. measurements are more reliable. Both studies confirm that the extinction is consider- able in most galaxies, and that extinction variations may a) Dependence on Hubble Type and Luminosity produce considerable variations in Ha emission inde- pendent of the star formation rate. Again, however, the The type dependence of the equivalent width is shown effect can hardly explain the very large dispersion in in Fig. 4. The qualitative features of this plot have al- emission properties which is observed here, and among ready been discussed. The median equivalent widths the small sample studied there is little or no anti-correla- within each RSA type provide a quantitative indication tion observed between the absorption and the of how relative star formation activity increases along Ha + [N n] equivalent width. Clearly dust is a very im- the Hubble sequence. portant factor to be considered in quantitatively inter- The dispersion in emission among galaxies of the preting the emission line fluxes, but the major portion of same type may partly reflect the relative coarseness of the variation in observed emission is real. the classification bins, but at least one other parameter is probably involved. There exists a large range in galaxy ß Summary: Real Variations in Star Formation luminosity within each type, but the emission does not appear to be strongly coupled to luminosity, at least For spiral galaxies of type Sab-Sb and later, the large among galaxies of the same type. This is illustrated in dispersion in observed emission properties observed Fig. 9, where the equivalent widths of all Sc and SBc must reflect a real dispersion in relative star formation galaxies surveyed are plotted as a function of absolute activity. The extraneous sources of error discussed in blue magnitude, the latter taken from the RSA. A gen-

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1983AJ 88.1094K 78 order 10-10years. ability inthestarformation activityontimescalesof fects discussedinthelastsection,alongwitharealvari- flects thecombinedcontributionsofextraneousef- formation rateandtheintegratedstarhistory of agivencolorisstillsubstantial,anditprobablyre- of agalaxy.Thedispersioninemissionamonggalaxies emission pointtoacouplingbetweenthecurrentstar correlations observedbetweengalaxycolorandHa antly influencedbytheirstarformationhistories (Searle, Sargent,andBagnuolo1973),thestrong is eventighter. the correlationwithemissionlinestrength(notshown) only availableforfractionofthegalaxiessurveyed,but cluded thecorrelationiseventighter.U—Bcolorsare galaxies withactivenuclei,andifthoseareex- B —Vcolor,thelattertakenfromRC2.Mostof points withabnormallyhighemissionfortheircolorare relationship betweenequivalentwidthandcorrected confirmed inourlargerdataset.Figure10showsthe galaxy color.ThiswasnotedbyCohen(1976),andis of meanluminosityonHubbletype. ered, butthisappearstomainlyreflectthedependence emission doesexistwhentheentiresampleisconsid- eral correlationbetweengalaxyluminosityandHa Sc andSBcgalaxies. Fig. 9.LuminositydependenceofemissionequivalentwidthforRSA 1105 One ofthemoresurprisingresults ofoursurveywas The integratedcolorsofdiskgalaxiesarepredomin- © American Astronomical Society • Provided by the NASA Astrophysics Data System The integratedemissionisstronglycorrelatedwith R. C.KENNICUTT,JR.ANDS.M.KENT:HaEMISSIONINGALAXIES c) RelationshipwithHIContent b) ColorDependence er, Gallagher,andRautenkranz (1982)inastudyofstar of agalaxy.Thesameconclusion wasreachedbyHunt- activity isonlydirectlycoupled tothetotalHIcontent data confirmthegeneralimpression thatstarformation Bothun, Schommer,andSullivan 1982),andthepresent between gascontentandlineemission,butthescatteris color andHIcontenthasbeennoticedinthepast(e.g., same type.Asimilarlyraggedrelationbetweengalaxy implied starformationactivityamonggalaxiesofthe huge, withoveranorderofmagnitudevariationinthe veyed inHa. all buteightofthe160spiralandirregulargalaxiessur- data setsarenearlycomplete,withHIavailablefor absolute blueluminositiesfromtheRSA.Thecombined tance-independent M/Lratios,usingdistancesand Huchtmeier (1982),andcomputingHimassesdis- Bottinelli, Goughenheim,andPaturel(1982), surveys andcompilationsofFisherTully(1981), compiling the21-cmHifluxintegralsfromrecent luminosity ratio(M/L).Thelatterwereobtainedby Ha +[Nn]equivalentwidthwiththeHImasstoblue strength andHicontent.Figure11showsaplotof the relativelypoorcorrelationbetweenemissionline hb HB are +3,4. ed colorsarefromtheRC2.Typicaluncertaintiesinequivalentwidth Fig. 10.ColordependenceofHa+[Nn]equivalentwidths.Correct- Figure 11showsadefinitepositivecorrelation 1105 1106 R. C. KENNICUTT, JR. AND S. M. KENT: Ha EMISSION IN GALAXIES 1106

is proportional to a high power of the gas mass or den- sity. The Schmidt law, regardless of whether it is valid locally, cannot be applied as a global relationship between the total rate of star formation and total gas content (at least the H i content) of a galaxy.

VI. SUMMARY We have presented the results of two surveys of Ha emission in normal disk galaxies, along with a minimal interpretation. Besides being useful in their own right as qualitatively delineating the star formation properties of normal galaxies, we hope that the results will prove use- ful as reference data for comparisons with other types of galaxies with less normal star formation properties. Our principal conclusions are as follows: (1) Photoelectric photometry can be used to measure the integrated Ha -f [N n] flux of a galaxy, to an accu- racy of typically ± 10%. Spectrophotometry can pro- vide ± 50% line strengths for large samples, and is probably preferable in very early-type galaxies. (2) Emission line strength varies markedly among dif- ferent galaxies. Most of this variation can be attributed to the galaxy type or to real variations in star formation activity among galaxies of the same type. Extraneous effects such as nuclear emission, etc. are usually unim- log(MH/LB) portant, though the effects of extinction by dust are cer- tainly significant. Fig. 11. Emission equivalent width vs hydrogen content. The MH/LB (3) The Ha + [N n] equivalent widths are strongly values were derived from published catalogs, as described in the text. correlated with galaxy color, as noted by Cohen (1976). Points to the lower left part of the diagram possess large uncertainties The emission is only weakly correlated with total H I in both parameters. content, however. Taken together these results suggest formation and H i in irregular galaxies. The results in that gas density is probably not the primary causative this paper indicate a similar behavior in spirals. parameter which determines the global rate of star for- This result is surprising because for some time it has mation. been assumed that the local rate of star formation is The observations presented in this paper were ob- directly related to some power of the gas density, follow- tained while SK was supported by a Hertz Fellowship at ing Schmidt’s (1959) classic study of the star formation the California Institute of Technology, and RK held a history in the Galactic disk. It is tempting to use the data postdoctoral fellowship at the Mt. Wilson and Las Cam- in Fig. 11 to determine a best fitting power-law relation panas Observatories, Carnegie Institution of Washing- between the total star formation rate and total H i mass, ton. The support and generous allocation of telescope but such an exercise is dangerous. While formally a lin- time by both of these institutions is gratefully acknowl- ear (i.e., log-log) fit yields a slope of about one (i.e., a edged. Part of this work was supported by the National linear dependence of star formation rate on gas mass), Science Foundation through Grant AST81-11711. RK the fitting is dominated by a few very poorly measured would also like to express his thanks to the staff of the points with low emission and low gas content. Further- Leiden Observatory and the Dutch National Science more, the very large scatter in the Ha-H i relation indi- Foundation for their hospitality and support while this cates that at best such a power-law relation is only paper was being written. Bill Keel, Frank Israel, Thijs meaningful on the average. Clearly other parameters are van der Hulst, and Ule Klein kindly provided and dis- at least as important as H i mass. The data clearly ques- cussed unpublished data with us, and their generosity is tion the notion that the rate of star formation in a galaxy greatly appreciated.

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