Constraints on the star formation history of the L

arXiv:astro-ph/0412530v120Dec2004Astronomy&Astrophysicsmanuscriptno.1057˙xxxFebruary2,2008(DOI:willbeinsertedbyhandlater)ConstraintsonthestarformationhistoryoftheLargeMagellanicCloudS.C.Javiel,B.X.Santiago,andL.O.KerberUniversidadeFederaldoRioGrandedoSul,IF,CP15051,PortoAlegre91501–970,RS,BrazilReceived/Accepted1October2004Abstract.Wepresenttheanalysisofdeepcolour-magnitudediagrams(CMDs)of6stellarfieldsintheLMC.ThedatawereobtainedusingHST/WFPC2intheF814W(∼I)andF555W(∼V)filters,reachingV555∼26.5.WediscussandapplyamethodofcorrectingCMDsforphotometricincompleteness.AmethodtogenerateartificialCMDsbasedonamodelstarformationhistoryisalsodeveloped.Thismethodincorporatesphotometricerroreffects,unresolvedbinaries,reddeningandallowsuseofdifferentformsoftheinitialmassfunctionandoftheSFHitself.WeusethePartialModelsMethod,aspresentedbyGallartandothers,forCMDmodelling,andincludecontrolexperimentstoproveitsvalidityinasearchforconstraintsontheLargeMagellanicCloudstarformationhistoryindifferentregions.Reliablestarformationhistoriesforeachfieldarerecoveredbythismethod.Inallfields,agapinstarformationwithτ∼700Myrsisobserved.Field-to-fieldvariationshavealsobeenobserved.ThetwofieldsneartheLMCbarpresentsomesignificantstarformingevents,havingformedbothyoung(τ∼1Gyr)andold(τ∼10Gyr)stars,withacleargapfrom3−6Gyrs.TwootherfieldsdisplayquitesimilarSFHs,withincreasedstarformationhavingtakenplaceatτ≃2−3Gyrsand6∼τ∼10Gyrs.Theremainingtwofieldspresentstarformationhistoriesclosertouniform,withnocleareventofenhancedstarformation.Keywords.galaxies:galaxies:formation,galaxies:evolution;stars:statistics1.IntroductionThestudyofthestarformationhistory(SFH)inneargalaxiescancontributetoourunderstandingoftheevo-lutionoftheseobjects,themechanismsinvolvedinthestarformationbursts,theirassociatedtimescales,andtheinitialstellarmassfunction(IMF).StarformationtimescalesandIMFsareparticularlyimportant,beingusedasinputforpopulationsynthesismodelswhichtrytointerpretintegratedlightfromdistantgalaxiestoob-taininformationabouttheircomponentstars.Moreover,bymatchingSFHtodynamicalmodels,itispossibletoinvestigatetheinfluenceofgravitationalinteractionsonstarformation.TheLargeMagellanicCloud(LMC)isasatelliteoftheGalaxy,beingatthesametimequitedistinctfromitandyetnearenoughtoittoallowastudyofitsstructure,internalkinematicsandstellarpopulations(Westerlund1990).Byitsproximityandnature,theLMCprovidesanexcellentlaboratoryforthestudyoftheprocessesinvolvedinstarformation,suchasinteractionswithneighboringgalaxies(especiallytheGalaxyandtheSmallMagellanicSendoffprintrequeststo:S.C.Javiel,e-mail:[e-mail](SMC)),theinfluenceofbardynamicsandchemicalenrichment(Gardineretal.1998).ThemainadvantageofLMCstellarpopulationstud-iesistheabilitytobuilddetailedcolour-magnitudedia-grams(CMDs)invariousregionswithinit.Furthermore,theLMCsuppliesauniquechancetocomparetheinferredSFHthroughthestudyoffieldstarswiththeSFHfoundfromstudiesofitsclustersystem.ThereasonisthattheLMCpresentsarichclusterpopulationoflargelyvaryingages.ClusterCMDshavetheadvantageofbeingcom-posedofasinglepopulation,makingtheiranalysismuchsimpler.StudiesusingLMCclustersshowthatthemajor-ityofthemarerelativelyyoung,withagesτ∼4Gyr,asmallnumberhasτ∼10Gyrandonlyoneisofinterme-diateage(τ∼8Gyr)(Mateo,Hodge&Schommer1986).However,onlyasmallfractionofLMCstarsbelongtoclusters,renderingthefieldCMDanalysesofmoregen-eralinterestandmorerepresentativeoftheglobalSFH.Ground-basedCMDstudieshavefavouredapredom-inanceofyoungorintermediate-agepopulationsintheLMC.Bertellietal.(1992),forexample,haveanalyzedCMDsinafewLMCouterregionsandmodelledthemwithasingle-burstSFHwithτ∼2–4Gyr,withafactorof∼10increaserelativetotheaveragequiescentstarfor-mation,theexactburstagebeingdependentonthedetails2S.C.Javiel:StarFormationHistoryoftheLMCofstellarevolutionarytheoriesavailablethen.Thesamegroupalsofoundsomeevidenceforfield-to-fieldvariationsinSFH(Vallenarietal.1996).Earlierground-basedworkdidnotreachdeepenoughmagnitudestodirectlyaccessthemainsequenceturn-offsofstellarpopulationswithagesτ∼6Gyr.ThisbecamepossibleonlywiththeuseoftheHubbleSpaceTelescope(HST),whoseimageswerefreeofatmosphericturbulenceand,therefore,muchsharperanddeeper.Gallagheretal.(1996),basedonasingleHSTWideFieldandPlanetaryCamera2(WFPC2)field,suggestedaroughlyconstantstarformationrate(SFR)inthepastfewGyr,exceptforasmallperiodofenhancedstarformationabout2Gyrago.Holtzmanetal.(1997)analyzedtheluminosityfunc-tionofthesameHSTfield,locatedsome4◦fromLMCbar,andfavouredaconstantSFRfor10GyrintheLMC,followedbyanincreasefactorof3inthelast2Gyr,yield-ingroughlythesamenumberofstarsyoungerandolderthan4Gyr.Theyalsoidentifiedanintermediate-ageturn-offintheirfieldCMD.Elson,Gilmore&Santiago(1997)studiedanHSTfieldclosertotheLMCbarandidenti-fiedtwomainpopulations,withagesτ∼1–2Gyrandτ∼2–4Gyr,theexactagesdependingontheirmetal-licity.Gehaetal.(1998)havestudied3HSTfieldsintheouterregionoftheLMCandalsosuggestedamodelwithroughlyequalnumbersofstarsyoungerandolderthanτ=4Gyr.StudyingfieldsinsidetheLMCbar,Olsen(1999)suggestedthatthestar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