Milky Way chemo-dynamics in the era of Gaia

0.8 MB·

by  I. Minchev

AstronomischeNachrichten,7January2016
Milky Way chemo-dynamics in the era of Gaia
I.Minchev1,⋆,C.Chiappini1,andM.Martig2
1 Leibniz-Institutfu¨rAstrophysikPotsdam(AIP),AnderSternwarte16,D-14482,Potsdam,Germany
2 Max-Planck-Institutfu¨rAstronomie,Ko¨nigstuhl17,D-69117Heidelberg,Germany
6 ReceivedXXXX,acceptedXXXX
1 PublishedonlineXXXX
0
2
Key words Galaxy: abundances – Galaxy: disc – Galaxy: evolution – Galaxy: formation – Galaxy: kinematics and

n dynamics
a
J ThemaingoalofGalacticArchaeologyisunderstandingtheformationandevolutionofthebasicGalacticcomponents.
Thisrequiressophisticatedchemo-dynamicalmodeling,wherediscasymmetries(e.g.,perturbationsfromthecentralbar,
6
spirals arms, and infalling satellites) and non-equilibrium processes are taken into account self-consistently. Here we

] discuss thecurrent statusof Galacticchemo-dynamical modeling and focus on a recent hybrid technique, which helps
A circumventtraditionalproblemswithchemicalenrichmentandstarformationencounteredinfullyself-consistentcosmo-
G logicalsimulations.Weshowthatthismodelcanaccountforanumberofchemo-kinematicrelationsintheMilkyWay.In
addition,wedemonstratethat(1)ourmodelmatcheswelltheobservedage-[α/Fe]andage-[Fe/H]relationsand(2)that
.
h thescatterintheage-[Fe/H] relationcannot besimplyexplained byblurring(starsonapo- and pericentersvisitingthe
p solarvicinity)butsignificantradialmigration(starsbornelsewherebutendingupatthesolarvicinitytodaybecauseofa
-
o changeinguidingradius)isneeded.Weemphasizetheimportanceofaccuratestellarages,suchasthoseobtainedthrough
r asteroseismology by the CoRoT and Kepler missions, for breaking the degeneracy among different Galactic evolution
t scenarios.
s
a
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1
v 1 Introduction (e.g.,Tisseraetal.2012),itisstillachallengeforcosmolog-
3
icalsimulationstomatchthepropertiesoftheMW(e.g.,the
1
3 Important information concerning the dominant mecha- typicalmetallicitiesofthedifferentcomponents–Tisseraet
1 nismsresponsiblefortheformationoftheMilkyWay(MW) al.2012).Additionally,thefractionoflow-metallicitystars
0 is encoded in the chemistry and kinematics of its stars. A areoftenoverestimated(Caluraetal.2012)andreproducing
1. number of Galactic surveys are currently being conducted thepositionofthin-andthick-discstarsinthe[α/Fe]-[Fe/H]
0 with the aim of obtaining chemical and kinematical infor- plane has proved challenging (Brook et al. 2012; Gibson
6 mation for a vast number of stars, e.g., RAVE (Steinmetz etal. 2013).While such issues couldbe dueto unresolved
1
eta.2006),SEGUE(Yannyeta.2009),APOGEE(Majew- metal mixing (Wiersma et al. 2009), none of the above-
:
v skietal.2010),HERMES(Freemanetal.2010),Gaia-ESO mentionedsimulationsreproducessimultaneouslythemass,
Xi (Gilmore et al. 2012),Gaia (Perrymanet al. 2001),LAM- themorphology,andthestarformationhistory(SFH)ofthe
OST(Zhaoetal.2006)and4MOST(deJongetal.2012). MW.
r
a Tobeabletointerpretthelargeamountsofforthcoming
dataweneedgalaxyformationmodelstailoredtotheMW.
1.1 SummaryofMWchemo-dynamicalevolution
Producingdisc-dominatedgalaxieshastraditionallybeena
modelingtechniques
challengeforcosmologicalsimulations(e.g.,Navarroetal.
1991;Abadietal.2003).Althoughanincreaseinresolution
Amajorconsiderationinadiscchemo-dynamicalmodelis
andimprovedmodelingofstarformationandfeedbackhave
taking into account the effect of radial migration, i.e., the
resultedinMW-massgalaxieswithreducedbulgefractions
factthatstarsendupawayfromtheirbirthplaces(see,e.g.,
(Guedesetal.2011;Martigetal.2012),typicallythesesim-
Sellwood and Binney 2002; Roskar et al. 2008, Minchev
ulationsdo notinclude chemicalevolution.Galaxy forma-
and Famaey 2010). Below we quickly summarize models
tionsimulationsincludingsometreatmentofchemicalevo-
whichincluderadialmigration.
lutionhavebeenperformedbyseveralgroups(e.g.,Scanna-
1. Semi-analytical models tuned to fit the local metal-
piecoetal.2005;Fewetal.2012).
licity distribution,velocitydispersion,and chemicalgradi-
However,while the results are encouragingand global
ents,etc.,today(e.g.,SchoenrichandBinney2009;Kubryk,
observed relations are reproduced, such as the metal-
Prantzos, and Athanssoula 2014) or Extended distribution
licity trends between the different galactic components
functions(SandersandBinney2015):
⋆ Correspondingauthor:[email protected] −Easytovaryparameters
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2 Minchevetal.:MilkyWaychemo-dynamics
− Provide good description of the disc chemo-kinematic To match the MW in terms of dynamics, at the end of
statetoday the simulation we downscale the disc radius by a factor
−TypicallynotconcernedwiththeMilkyWaypasthistory of 1.67 and adjust the rotational velocity at the solar ra-
−Timeandspatialvariationsofmigrationefficiencydueto dius to be 220 km/s, which affects the mass of each par-
dynamicsresultingfromnon-axisymmetricdiscstructureis ticle according to the relation GM ∼ v2r, where G is the
hardtotakeintoaccount. gravitationalconstant.Thisplacesthe bar’scorotationres-
2. Fully self-consistent cosmologicalsimulations (e.g., onance (CR) and 2:1 outer Lindblad resonance (OLR) at
Kawata and Gibson 2003; Scannapieco et al. 2005; ∼ 4.7 and ∼ 7.5 kpc, respectively, consistent with a num-
KobayashiandNakasato2011;Brooketal.2012): berofstudies(e.g.,Dehnen2000,Minchev,Nordhaus,and
−Dynamicsself-consistentinacosmologicalcontext Quillen 2007, Minchev et al. 2010). At the same time the
−Canlearnaboutdiscformationandevolution discscale-length,measuredfromparticlesofallagesinthe
−Notmuchcontroloverfinalchemo-kinematicstate range3<r<15kpc,becomes∼3kpc,incloseagreement
−ProblemswithSFHandchemicalenrichmentduetoun- withexpectationsintheMW.
knownsubgridphysics
The chemical evolution model we use here is for the
−Muchlargercomputationaltimesneededifchemicalen-
thin disc only. The idea behind this is to test, once radial
richmentincluded.
mixingand mergerperturbationsare taken into account,if
3.Hybridtechniqueusingsimulationinacosmological
wecanexplaintheobservationsofboththinandthickdiscs
context + a classical (semi-analytical) chemical evolution
without the need of invoking a discrete thick-disc compo-
model(Minchev,Chiappini,andMartig2013,2014):
nent.Adetaileddescriptionofthechemicalmodelisgiven
− Avoids problems with SFH and chemical enrichmentin
inMCM13.
fullyself-consistentmodels
−Canlearnaboutdiscformationandevolution
−NoteasytogetMilkyWay-likefinalstates.
2.1 Comparisontorecentobservationalresults
For the rest ofthe paperwe focuson the resultsof the
lattermodel.
The MCM13 model has been shown in previous works
to match simultaneously the following observational
2 Ourchemo-dynamical model
constraints,someofwhicharereproducedinFig.1:
ToproperlymodeltheMWitiscrucialtobeconsistentwith • The disc morphology, scale-length and rotation curve
someobservationalconstraintsatredshiftz = 0,forexam- today(seeMCM13);
ple, a flat rotationcurve,a smallbulge, a centralbarof an • The local age-velocitydispersion relation (Sharma et al.
intermediate size, gas to total disc mass ratio of ∼ 0.14 at 2014);
the solar vicinity, and local disc velocity dispersionsclose • The more centrally concentrated [α/Fe]-enhanced (old)
totheobservedones. disc (Bensby et al. 2011; Bovy et al. 2012; see Minchev,
It is clear that cosmological simulations would be the Chiappini,andMartig2014b,Fig.11);
natural framework for a state-of-the-art chemodynamical • The distribution of scale-heights for mono-abundance
study of the MW. Unfortunately, as described above, a subpopulations found in SEGUE G-dwarfs (MCM13,
number of star formation and chemical enrichment prob- Fig.13);
lemsstillexistinfullyself-consistentsimulations.Wehave, •Thereversaloftheradial[α/Fe]andmetallicitygradients
therefore,resortedtothenextbestthing–ahigh-resolution (e.g.,APOGEE-Andersetal.2014),whensampledistance
simulation in the cosmological context coupled with a from the disc mid-plane is increased (Minchev et al.
purechemicalevolutionmodel(see detaileddescriptionin 2014b).
Minchev,Chiappini,andMartig2013,hereafterMCM13). • The MDF for stellar samples at different distance from
The simulation we used is part of a suite of numerical thediscmidplane(Fig.1,a,b).
experiments first presented by Martig et al. (2012), where • The metallicity variation with vertical distance from the
the authors studied the evolution of 33 simulated galax- plane(Fig.1,c);
ies from z = 5 to z = 0 using the zoom-in technique de- •TheinversioninvelocitydispersionrelationinRAVEand
scribed by Martig et al. (2009).This techniqueconsists of SEGUE (Fig. 1, d, e; Minchev et al. 2014;Guiglion et al.
extracting merger and accretion histories (and geometry) 2015);
foragivenhaloinaΛ-CDMcosmologicalsimulation,and •The[α/Fe]-[Fe/H]plane(currentFig.1,f;Ramirezetal.
thenre-simulatingthesehistoriesatmuchhigherresolution 2013);
(150 pc spatial, and 104−5 M⊙ mass resolution).The inter- • The flaring of mono-abundance populations (assuming
estedreaderisreferredtoMartigetal.(2012)formorein- similarity to mono-age populations) found by Bovy et al.
formationonthesimulationmethod. (2015) and predicted earlier by Minchev et al. (2015),
Originally, our galaxy has a rotational velocity at the whereModel1inthelatterpaperpresentsthesamegalaxy
solar radius of 210 km/s and a scale-length of ∼ 5 kpc. astheoneusedfortheMCM13chemodynamicalmodel.
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asnaheaderwillbeprovidedbythepublisher 3
Adibekyan et al. (2012) SEGUE, DR9, Brauer et al. (2015) Schlesinger et al. (2012)
0.1<|z| kpc 0.5<|z|<3 kpc MCM13 Model
7.9<r<8.1 kpc 6<r<10 kpc
MCM13 Model
= 0 dex
= 0.2 dex
(a) (b) 6<r<10 kpc (c)
RAVE Giants, SN>65 MCM13 model
7<r<9 kpc –1.0[Fe/H]
|z|<0.6 kpc –0.8 dex Kinematical selection of simulated data as in
–0.45 high-resolution spectroscopic surveys (e.g.,
–0.3 Bensby et al. 2003)
–0.17
–0.04 Ramírez et al. (2013)
Thick disk
Thin disk
MCM13 Model
Thick disk
(d) (e) (f)
Thin disk
Fig.1 Comparisonbetween the prediction of the MCM13 modeland observations.Panels (a) and (b): The red his-
tograms show data from Adibekyan et al. (2012) and Bauer et al. (in preparation); the black and blue curves show the
model with and without convolvederror, respectively. The metallicity peak shifts to lower [Fe/H] for both the data and
model,whendistancefromthediscplaneincreases.Panel(c):Metallicityvariationwithdistancefromthediscplanefor
SEGUEG-dwarfdata(red).Panels(d)and(e):Variationsofverticalvelocitydispersionwith[Mg/Fe]forRAVEgiants
(d)andforthemodel(e).Panel(f):Comparisontothehigh-resolutiondatabyRamirezetal.(2013).Ashiftinthemodel
[O/Fe]of0.05dex(withintheuncertainty)hasbeenapplied.Panels(a),(b),(c),(f)arefromMCM13andpanels(d)and
(e)arefromMinchevetal.(2014a).
2.2 Theage-[α/Fe]relation majorityofstarsinthesolarvicinityatthefinalsimulation
time have been born close to the solar radius. The second
We nowcomparetheage-[α/Fe]andAMRrelationsinthe row,leftpanelindicatesthatthescatterinthemodelAMR
model to those resulting from the data by Haywood et al. is related to stars born inside and outside the solar radius.
(2013). These authors published ages for single stars with Most stars are containedin the three highestlevels(black,
known[Fe/H]and[α/Fe]in theimmediatesolarneighbor- blue,andpurple),i.e.,scatter isnotaslargeasitwouldbe
hood.TheirsampleisbasedontheHARPSGTOobserva- naivelyinferredfromthisparticularplot.
tions of 1111 stars, described by Adibekyan et al. (2012).
The top-right panel of Fig. 2 shows a comparison be-
Theoriginalsamplehadtobeseverelyreducedto363stars. tweentheage-[α/Fe]relationbyHaywoodetal.(2013)and
This down-selection was based on an absolute magnitude theage-[Mg/Fe]relationresultingfromourmodel.Asmall
cutatM < 4.75andonasomewhatlessreproduciblese-
V volumesamplearoundthesimulatedsolarradiuswasused,
lection of stars with ”a well defined probabilityfunction”.
asindicated,similarlytothe data.We expandedthemodel
Haywood et al. noted that their absolute age scale could
agerangeby10%,inordertomatchthedatarange.Wealso
be off by 1 to 1.5 Gyr, while relative ages would have un- shifted the model by 0.03 dex to lower [α/Fe] values. Un-
certaintiesof1Gyr.Theirdefinitionof[α/Fe]includesthe certainties of δ[Mg/H]= 0.05 and δ[Fe/H]= 0.1 dex were
meanofMg,Si,andTiabundances.
implementedinthemodel.Thisresultsinanuncertaintyof
Thetop-leftpanelofFig.2showsthemodelage-[O/Fe] 0.11 dex in [Mg/Fe]. We also implemented age errors of
relation for stars in the solar vicinity at the final time, but ∼ 1 Gyr, similar to the relative age uncertainty quoted by
with different birth radii (from MCM13, Fig. 7). It is im- Haywood et al. It can be seen that the modelmean (green
portantto realizethat,asseeninthebottom-leftpanel,the curve) provides a very good match to the data, while the
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4 Minchevetal.:MilkyWaychemo-dynamics
Haywood et al. (2013)
MCM13 model
]
e Mean model
F
e] g/
F M
/
O [
[ ],
e
F
/
α
[
δ [Fe/H] = 0.1 dex
δ [Mg/H] = 0.05 dex
S δ age = 1 Gyr
H] tella
Fe/ r de H]
[ n /
s e
it F
y [
Spatial selection in
model to match the data:
7.9 < r < 8.1 kpc
|z| < 0.05 kpc
]
H
/
e
F
[
Blurring only
Δ r < 1 kpc
g,final
Fig.2 ComparisonbetweentheHaywoodetal.(2013)dataandMCM13model.Leftcolumn:Themodelage-[O/Fe]
relationforstarsarrivingtothesolarvicinityfromdifferentradii(top),theage-[Fe/H]relation(middle),anddistributions
of birth and final guidingradii (bottom)of stars endingup in the solar vicinity,indicated by the green strip. The dotted
redandsolidblueverticallinesinthebottomleftpanelindicatethebar’scorotationandinnerLindbladresonance.Right
column: The age-[α/Fe] relation (top), where [Mg/Fe] from the modelis used; the age-[Fe/H]relation (AMR) shows a
smallerscatterinthemodelthaninthedata(middle);andtheAMRbutwithmigratorsremovedfromthemodel(bottom).
This figure shows that the MCM13 modelis consistent with the observedrelationsand suggests that blurringonly(i.e.,
lackofmigration)isinsufficienttoexplaintoobservedAMR.
scatter in the model is somewhat larger for this particular Finally,thebottom-rightpanelofFig.2showsagainthe
plot. modelAMR,butwithmigratorsremovedbyexcludingstars
whichhavechangedguidingradiisincetheirbirthbymore
Thesecondrow,rightpanelofFig.2 showsthe AMR, than1kpc.Thisassumptionisveryconservativeinthatthe
whereitcanbeseenthatthescatterissmallerinthemodel cutstillleavesstarswhichhavemigratedby1kpc.Ourse-
than in the data (contraryto what was stated by Haywood lection preserves stars with hot kinematics in the sample,
etal.2013).Notethatthedataselectioncontains625stars, e.g.,blurring(starsonapo-andpericentersvisitingtheso-
orabout2timesmorethantheobservations.Theobviously lar vicinity) still contaminates the local metallicity distri-
muchsmallerscatterinthemodelAMRisthenclearlyreal bution.Wetestedthisbymakingsurethattheage-velocity
and not a product of lower number statistics compared to dispersionrelationresultingfromtheoriginalmodelselec-
thedata.
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tion(middlerightpanelofFig.2) andthesamplewiththe References
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1 Dynamically,thisisagoodassumption,sincethestellardynamicsis
collisionless.
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