Testing Intermittency of the Galactic Star Formati

arXiv:astro-ph/0004111v17Apr2000Apr.02,2000TESTINGINTERMITTENCYOFTHEGALACTICSTARFORMATIONHISTORYALONGWITHTHEINFALLMODELTsutomuT.Takeuchi1andHiroyukiHirashita1DepartmentofAstronomy,FacultyofScience,KyotoUniversity,Sakyo-ku,Kyoto606-8502,JapanE-mail:takeuchi,[e-mail]arformationhistory(SFH)oftheGalacticdiskbyusinganin-fallmodel.BasedontheobservedSFHoftheGalacticdisk,wefirstdeterminethetimescalesofthegasinfallintotheGalacticdisk(tin)andthatofthegasconsumptiontoformstars(tsf).Sinceeachofthetwotimescalesdoesnotprovetobedeterminedin-dependentlyfromtheSFH,wefirstfixtsf.Then,tinisdeterminedsothatweminimizeχ2.Consequently,wechoosethreeparametersets:(tsf[Gyr],tin[Gyr])=(6.0,23),(11,12),and(15,9.0),wherewesettheGalacticageas15Gyr.Allofthethreecasespredictalmostidenticalstarformationhistory.Next,wetesttheintermittency(orvariabil-ity)ofthestarformationrate(SFR)alongwiththesmoothSFHsuggestedfromtheinfallmodel.Thelargevalueoftheχ2statisticsupportstheviolenttime-variationoftheSFH.IfweinterprettheobservedSFHwithsmoothandvariablecomponents,theamplitudeofthevariablecomponentiscomparabletothesmoothcomponent.Thus,intermittentSFHoftheGalacticdiskisstronglysuggested.WealsoexaminedthemetallicitydistributionofG-dwarfs.Wefoundthatthetrueparametersetliesbetween(tsf[Gyr],tin[Gyr])=(6,23)and(11,12),thoughweshouldneedamoresophisticatedmodelincludingtheprocessofmetalenrichmentwithintheGalactichalo.Subjectheadings:Galaxy:evolution—Galaxy:stellarcontent—methods:statistical—stars:abundances—stars:formation1.INTRODUCTIONInthefieldoftheGalacticchemicalevolution,Eggen,Lynden-Bell,&Sandage(1962)haveinspiredthemodeling.Fromthecorrelationbetweentheultravioletexcessandorbitaleccentricity1ResearchFellowsoftheJapanSocietyforthePromotionofScience.–2–ofstars,theyconcludedthattheGalaxyformedbycollapseonafree-falltimescalefromasingleprotogalacticcloud.AnalternativepictureofhaloformationisproposedbySearle&Zinn(1978).TheyarguedthatGalacticsystemisformedfromthecaptureoffragmentssuchasdwarfgalaxiesoveralongertimescalethanthatproposedbyEggenetal.Inanycase,determiningthetimescaleoftheinfallofmatterandthechemicalenrichmentisanimportantproblemtoresolvetheformationmechanismoftheGalacticsystem.Indeed,therehavebeenanumberofpapersthatinvestigatedtheformationandchemicalevolutionoftheGalaxy(e.g.,Matteucci&Fran¸cois1989)andspiralgalaxies(e.g.,Lynden-Bell1975;Sommer-Larsen1996).Manymodelsofthestarformationhistory(SFH)oftheGalaxyincludestheformationoftheGalacticdiskbygasinfall.Thisscenario(so-calledtheinfallmodel)isconsistentwiththeage-metallicityrelationofthediskstars(e.g.,Twarog1980),ifareasonableSFHisapplied.Moreover,theinfallmodelprovidesaphysicallyreasonablewayofsolvingtheG-dwarfproblem(e.g.,Pagel1997,p.236),contrarytotheclosed-boxmodelwhichtendstooverpredictthenumberofthelow-metallicitystars.Oneofthefactorsthatdeterminethestarformationrate(SFR)isthegascontentofgalaxies.Indeed,theSFRandthegasdensityiscloselyrelatedtoeachother(Kennicutt1998),andthisrelationisgenerallycalledtheSchmidtlaw(Schmidt1959).TheSFRandthegasdensity,ρ,arerelatedasSFR∝ρn,wheren=1−2.Intheone-zonemodel,therelationissimplyassumedtobedescribedwithSFR∝Mng,whereMgisthegasmassinthegalaxy.Thoughthe“classical”infallmodeliswidelyaccepted,thereareobservationaldatathatsuggesttheintermittentstarformationactivitiesinspiralgalaxies.Kennicutt,Tamblyn,&Congdon(1994)showedtheratioofpresent-to-pastSFRinspiralsamplehasasignificantscatter.Morerecently,Tomita,Tomita&Sait¯o(1996)analyzedthefar-infraredtoB-bandfluxratiofFIR/fBof1681spiralgalaxies(seealsoDevereux&Hameed1997).TheindicatorfFIR/fBalsorepresentstheratiobetweenthepresentSFRandtheaveragedSFRduringtherecentGyr.Theyshowedtheorder-of-magnitudespreadoffFIR/fBandsuggestedtheviolenttemporalvariationoftheSFR.TheintermittencyoftheSFHintheGalacticdiskisrecentlysuggestedbyRocha-Pintoetal.(2000;hereafterR00).TheyderivedtheSFHoftheGalacticdiskfromobservedagedistributionofthelate-typestarsandsuggestedthatthestarformationactivityofthediskisintermittentorviolentlyvariable.Theytestedtoshowthatthevariationisofstatisticalsignificance.TheirtestarebasedonthenullhypothesisthattheSFRoftheGalaxyistemporally“constant”,buttheirderivedSFHindicatesthattheSFRaround10–15Gyragoissignificantlysmallerthanthataround0–10Gyrago.ThistrendoftheGalacticSFRcanbeinterpretedalongwiththeinfallscenariooftheGalacticdiskformation:Gas-infalltotheGalacticdiskoccurredinthefirst∼5Gyr.ThusitisanimportantworktointerprettheirdatathroughtheinfallmodelandtoexaminewhetherthevariationofSFRisstillsignificantevenintheinfallscenario.Inthispaper,were-analyzethedatabyR00inthecontextoftheinfallmodel.Inthenextsection,webrieflyreviewthederivationoftheSFHintheGalacticdiskbyR00.Next,in§3,we–3–presenttheformulationoftheinfallmodel,basedonwhichwetestthedatabyR00statistically.In§4,weinterprettheobservedSFHbytwocomponents;underlyinginfall-modelcomponentandviolentlyvariablecomponent.Inthesamesection,possiblephysicalmechanismsforthevariationofstarformationarealsopresented.Finally,wesummarizetheco