Illuminating the Middle Ages of the Milky Way: the long history of the Galactic disc Jennifer Johnson Ohio State University
IlluminatingtheMiddleAgesoftheMilkyWay:thelong
historyoftheGalacticdisc
JenniferJohnsonOhioStateUniversity
ALongHistoryImagesfromtheGOODSfieldshowtheevolutionofdisksthroughtime
SpiralgalaxieshavehaddisksformanyGyrs–manyprocesseshaveaffectedtheirdevelopment
Maybeit’samazingthatwehavedisksatall.
Theeventfullifeofthedisk
• Starformationinthedisk• Chemicalevolutionofdisk
• Creationofthe“thickdisk”• Radialmigration&secularevolution
• Interactionwithsubstructure• Gasaccretion
AdvantagesofMilkyWay
• Observational– MilkyWaycanbeobservedinexquisitedetail– Varietyofprocessesofobserved– Fossilrecordinthestars
• Theoretical– Catchinguptothelevelofobservationaldetail
• MilkyWayonlyoneexampleofaspiralgalaxy• TalkwillfocusonthreepreliminaryresultsfromtheSEGUEsurveyplustwoupcomingprojects
But….
• WehaveobservedverysmallpartsoftheMilkyWay(soonwillchangetosmallpartsoftheMilkyWay)
• MilkyWayisnotneatandtidy• Limitednumberofsimulations–predictionsnotrobust– Salesetal.– Schoenrich&Binney– Loebmanetal– Birdetal
Theoriginofthethickdisk
• Accretionfromsatellite– (e.g.Abadietal.2003)
• Gascloudcollisionsinearlydisk– (e.g.Brooketal.2004)
• Heatedbysatellitepassage/merger– (e.g.Quinnetal.1993)
• Radialmixing– (e.g.Schoenrich&Binney2009)
TheSEGUEsurvey
• R~1800spectraof350,000starsintheMilkyWay
• Varietyofcategories:BHB,low‐metallicity,lateK‐Mgiants– Focushereonthe“G”dwarfsample
• ParametersfromtheSSPP(Leeetal.2008,Smolinskietal.2010)
• DR‐8nowavailable!
TheMetallicityDistributionFunction
• Schlesinger,Johnson,Morrison,Rockosi,etal.• Gdwarfsampleliesbetween1.5and5kpcaway• ~20,000stars• Propermotions&radialvelocityaswell• 0.48<g‐r<0.55,g<20–verycleancriteria• Weightspectroscopicresultsbyphotometricsampletocorrectforthelimitedfibersampling
• Correctionsformassfunction• TestedonGalacticModels
VerticalMetallicityGradientinDisk
SeealsoGilmoreetal.1995,AllendePrietoetal.2006,Ivezicetal.2008,etc
Littlemetallicitygradientbetween1.2kpcand3.0kpc
SimilartowhatisseeninNGC891(Rejkubaetal.2009)
Correlationsbetweenabundancesandkinematics
• Lee,Beers,An,Schoenrichetal.(2011)• Separationbetween“thick”and“thin”diskusing[α/Fe]values
• Vphivs.[Fe/H]• Naïveprediction:correlationdependsonage,andthereforenotinthickdiskbutmaybeinthindisk
• Distributionofeccentricities
RadialMixing&SatellitePerturbation
• Bird,Kazantzidis,&Weinberg(2011)• SPH+N‐bodysimulations
• Modelswithtwodifferentscaleheights
• Onesetisolated,onesetperturbedbyacosmologicallymotivatedsatelliteencounters
• Testedeffectofdiskheight,gasfractionandsatelliteperturbationonradialmixing
APOGEE• H‐bandsurveyofGalacticpopulations
• 100,000stars(80%redgiants)
• R~30,000• 15elements–includingC,N,O,Na,Mg,Ca,Mn,Fe,Co,Ni
• Targetedfrom2MASS• ComplimentsopticalsurveyssuchasHermes
Summaryof1stPart
• Bottomline:Combinationofextensivesurveysanddetailedtheoreticalmodelswillallowustofollowthedetailsofdiskevolution
Ages&CompositionsforRedGiants
• Epstein,Pinsonneault,Lai,Schoenrich,Chiappini,Rockosi,Bolte,….
• KASCandCoRoTteams
• Asteroseismologyyieldsmasses
• Asteroseismology+compositionyieldsmoreaccuratemasses
• Mass+compositionofredgiant=age
Pros&Cons
• Pro– Differentsystematicsthanisochrone‐basedmethods
– Redgiantsmoreluminouswithlargeramplitudeoscillations
• Con– OnlydatainCoRoTandKeplerfields– Scalingrelations