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!

SEGUEfields

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)

AccretedHalo?Disk?

SeealsoCarneyetal.1996

Avoidingdistanceissues

Correlationsbetweenabundancesandkinematics

•  Lee,Beers,An,Schoenrichetal.(2011)•  Separationbetween“thick”and“thin”diskusing[α/Fe]values

•  Vphivs.[Fe/H]•  Naïveprediction:correlationdependsonage,andthereforenotinthickdiskbutmaybeinthindisk

•  Distributionofeccentricities

Vphivs.[Fe/H]

SeealsoNordstrometal.2004,Ivezicetal.2008

Eccentricity

SeealsoWilsonetal.(2010),Dierickxetal.(2010)

RadialMixing&SatellitePerturbation

•  Bird,Kazantzidis,&Weinberg(2011)•  SPH+N‐bodysimulations

•  Modelswithtwodifferentscaleheights

•  Onesetisolated,onesetperturbedbyacosmologicallymotivatedsatelliteencounters

•  Testedeffectofdiskheight,gasfractionandsatelliteperturbationonradialmixing

Effectofperturbationonradialmixing

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

Whatisbetterthan1survey?

2Surveys!Spectroscopy+Asteroseismology

APOGEE+KeplerAPOGEE+CoRoT

Ages&CompositionsforRedGiants

•  Epstein,Pinsonneault,Lai,Schoenrich,Chiappini,Rockosi,Bolte,….

•  KASCandCoRoTteams

•  Asteroseismologyyieldsmasses

•  Asteroseismology+compositionyieldsmoreaccuratemasses

•  Mass+compositionofredgiant=age

Massofredgiant=ageofredgiant

Gaietal.2010

Mosseretal.2011,Kallingeretal.2010,Metcalfeetal.2010

LookingAcrosstheGalaxy

Pros&Cons

•  Pro– Differentsystematicsthanisochrone‐basedmethods

– Redgiantsmoreluminouswithlargeramplitudeoscillations

•  Con– OnlydatainCoRoTandKeplerfields– Scalingrelations

Outstandingquestions

•  Spreadinage‐metallicityrelation– Mixingtimescale

•  Ageasafunctionofvertical&horizontalposition–  Inside‐outformation

•  Metallicitygradientsasafunctionofage– Radialmixing

•  Abundanceratiosasafunctionofage– Timescalesfornucleosynthesis