Strontium and Barium In Early-Type Galaxies

0.19 MB·English

by  Charlie Conroy

ACCEPTEDFORPUBLICATIONINAPJLETTERS
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
STRONTIUMANDBARIUMINEARLY-TYPEGALAXIES
CHARLIECONROY1,PIETERG.VANDOKKUM2,ANDGENEVIEVEJ.GRAVES3,4
acceptedforpublicationinApJLetters
ABSTRACT
Thedetailedabundancepatternsofthestarswithingalaxiesprovideauniquewindowintothehistoryofstar
formation(SF)atearlytimes. Twowidelyused‘chronometers’includetheαandiron-peakelements,which
3 are created on short and long timescales, respectively. These two clocks have been widely used to estimate
1 SFtimescalesfrommoderate-resolutionspectraofearly-typegalaxies.Elementsformedvias-processneutron
0 captures(e.g., Sr andBa) comprisea third typeof chronometer,as the site of the mains-processis believed
2 tobeintermediateandlow-massasymptoticgiantbranchstars. The[α/Ba]ratioinparticularshouldprovide
a powerful new constraint on the SF histories of galaxies, in part because it is insensitive to the uncertain
n
distribution of Type Ia SNe detonationtimes and the overallIa rate. Here we presentnew measurementsof
a
J theabundanceofSrandBainnearbyearly-typegalaxiesbyapplyingstellarpopulationsynthesistoolstohigh
S/N optical spectra. We find a strong anti-correlation between [Mg/Fe] and [Ba/Fe], and a strong positive
8
correlation between [Mg/Ba] and galaxy velocity dispersion. These trends are consistent with the idea that

more massive galaxies formed their stars on shorter timescales compared to less massive galaxies, and rule
]
O outseveral other proposedexplanationsfor the observedsuper-solar [Mg/Fe] valuesin massive galaxies. In
contrast,[Sr/Fe]∼0,withnostrongvariationacrossthesample.ItisdifficulttointerprettheSrtrendswithout
C
detailedchemicalevolutionmodelsowingtothemultiplicityofproposednucleosyntheticsitesforSr.
.
h Subjectheadings:galaxies:stellarcontent—galaxies:abundances—galaxies:ellipticalandlenticular,cD
p
-
o
1. INTRODUCTION troncaptures(Burbidgeetal.1957;Bussoetal.1999;Herwig
r
t While it is clear that the majority of massive early-type 2005).Theprecisemass-dependentyieldsofBaarepresently
s
notwell-known,butthereis hopethatprogresson thistopic
a galaxies formed the bulk of their stars at z>1, the precise
[ nature and duration of star formation in these galaxies has can be made in the near-term (in contrast to the delay time
distributionforIa’s,whichappearstobegovernedlargelyby
proven difficult to constrain. The difficulty arises from the
2 the initial conditions of stellar binarity). The prospects for
fact that the main sequence turnoff point varies little with
v measuring Ba in the integrated light spectra of galaxies are
ageatlatetimes,implyingthattheintegratedspectralenergy
1 distribution varies little with age at late times. For old stel- favorableowingto the strongtransitionsof BaII in the blue
2
spectralregion.
3 lar systems such as early-type galaxies, it has become com-
Sr is another neutron capture element with strong transi-
0 mon to instead rely on elemental abundance ratios to probe
tions in the blue. Like Ba, Sr is predominantly produced
. thestarformationhistoriesatearlytimes(e.g.,Wortheyetal.
1 by the s-process, at least in the solar system. These two el-
1992; Thomasetal. 2005). The classic example is the ratio
0 of α to iron-peak elements, [α/Fe]5. The α elements form ements probe two of the three s-process peaks, with Sr be-
3 longingto the first(alongwith Y andZr), andBa belonging
mostly in massive stars, which evolve on short timescales,
1 to the second (along with La, Ce, Pr, and Nd). While the
whileiron-peakelementsformmostlyinTypeIasupernovae
: nucleosynthetic origin of Ba is relatively secure, the same
v (SNe),whichoccuronlongertimescales. The[α/Fe]ratiois
i thus a sensitive probe of SF on ∼108- 9 yr timescales (e.g., cannot be said for the elements in the first s-process peak
X (Couchetal. 1974; Woosley&Hoffman 1992; Raiterietal.
Tinsley1979;Thomasetal.1998).
r Oneoftheprincipaldifficultiesinusing[α/Fe]tomeasure 1993;Snedenetal.2008).Indeed,currentchemicalevolution
a modelsoftheGalaxyareunabletoreproducetheobservedbe-
aSFtimescaleistheunknownTypeIaSNedelaytimedistri-
haviorofSr-Y-Zratlowmetallicitywithoutappealingtoex-
bution. Otherdifficultiesincludethepossibilitythattheover-
oticand/oradhocnucleosyntheticsites(e.g.,Travaglioetal.
all Ia rate varies in some systematic way with galaxy prop-
2004; Qian&Wasserburg 2008). Perhaps the [Sr/Ba] ratios
erties, the possibility of selective mass-loss such that Fe is
inmetal-richmassivegalaxieswillprovidesomeinsightinto
preferentiallylostfromthesystem,andpotentialvariationin
thisproblem.
the initial mass function (Wortheyetal. 1992; Thomasetal.
In this Letter we employ our new stellar population syn-
1999;Trageretal.2000). Apromisingalternativechronome-
thesis(SPS)modeltointerprethighqualityopticalspectraof
terisBa,whichisbelievedtoformpredominatelywithinthe
early-typegalaxiesinordertoconstraintheabundancesofSr
envelopesofasymptoticgiantbranchstarsvias-processneu-
andBa. InSections2and3wedescribethemodelanddata,
1DepartmentofAstronomy&Astrophysics,UniversityofCalifornia, andinSection4wepresentourresults.
SantaCruz,CA,USA
2DepartmentofAstrophysicalSciences,YaleUniversity, NewHaven, 2. MODEL
CT,USA The SPS model used herein was developed in
3Department of Astronomy, University of California, Berkeley, CA,
Conroy&vanDokkum (2012a, CvD12). The model
USA
4DepartmentofAstrophysicalSciences, PrincetonUniversity, Prince- adopts standard SPS techniques, including constructing
ton,NJ,USA librariesof isochronesand stellar spectra. Empiricalspectra
5[X/Fe]≡log(X/Fe)- log(X/Fe)⊙ formthecoreofthemodel.Theempiricalstarsareofapprox-
2 CONROY,VANDOKKUM,&GRAVES
0.0 0.0
0.10
−0.1
0.05
2 3
5 3
−0.5 45 0.00 49 −0.2
a a
B B
−0.05
e −0.3
g
n −0.10
ha −1.0 −0.4
% c 0.1
SDSS stacks 0.4
LRIS data
0.0
−1.5 [Ba/Fe]=+0.3 Ba6142 Sr4076 00..02
−0.1
[Sr/Fe] =+0.3

−2.0 −0.2
−0.2
0.40 0.45 0.50 0.55 0.60 2.5 3.0 3.5 4.0 4.5 2.5 3.0 3.5 4.0 4.5
l (m m) Mg b Mg b
Figure1. Changeinthespectrumduetoa0.3dexenhancementinthe[Sr/Fe] Figure2. Spectral indices forthestackedSDSSearly-type galaxyspectra
and[Ba/Fe]abundanceratios.Themodelisanintegratedlightspectrumwith andtheLRISspectraof35individual galaxies. Indexunitsareequivalent
anageof13Gyr,solarmetallicity, andaKroupa(2001)IMF.Thespectra widthsinÅ.TheLRISdatadonotcovertheBa6142spectralfeature. The
havebeensmoothedwithavelocitydispersionofσ=350kms- 1,whichis MgbLickindexissensitivetoMg,whiletheotherfeaturesaresensitiveto
thesamesmoothingscaleappliedtotheSDSSdata. Thedottedlinerepre- BaandSrandaredefinedinServenetal.(2005).
sentsthesensitivitylimitforasignal-to-noiseratioofS/N=103,whichisthe spectraareonlyuseddifferentially,severaluncertaintiesasso-
typicalS/NfortheSDSSstacksinthiswavelengthrange.
ciated with abundance analysis at high resolution (including
theadoptedmicroturbulentvelocity,treatmentofconvection,
imately solar metallicity and have solar abundance patterns.
non-LTEeffects,andhyperfinesplittingofcertainlevels)are
We computed a large grid of model stellar atmospheres and
mitigatedtosomeextentherein. Moreover,Bergemannetal.
spectra in order to construct response functions (i.e., the
relativechangeinthespectrumofastarduetoachangeinthe (2012) has demonstrated that the strong Sr II line at 4077Å
abundanceofasingleelement). Themodelatmospheresand suffersfrom negligible non-LTEcorrectionsat [Fe/H]≈0.0.
Ofcoursewhenworkingatlowresolutiononeismoredepen-
spectra were computed with the ATLAS model atmosphere
andspectrumsynthesispackage(Kurucz1970,1993),ported denton the fidelity of the modelthan the case where equiv-
to Linux by Sbordoneetal. (2004). Specifically, we use alentwidthsofindividualunsaturatedlinesaredirectlymea-
the ATLAS12 code, computing new atmospheres for each suredfromthedata.
changeinabundancepattern.ThelinelistwasprovidedbyR. Asan exampleof whatcanbe measuredfromlow resolu-
Kurucz6, includinglinelistsforTiOandH O,amongstother tion spectra, in Figure 1 we show the change in the model
2
molecules. The spectral response functions were applied to spectrumduetoa0.3dexincreaseinthe[Sr/Fe]and[Ba/Fe]
the empirical stellar spectra in order to create models with abundance ratios (holding [Fe/H] fixed to the solar value).
arbitraryabundancepatterns. Themodelallowsforarbitrary The reference model is for an age of 13 Gyr, with a Galac-
variation in the initial mass function (IMF) and spans ages tic(Kroupa2001)IMF, andforsolarmetallicity. Themodel
from 3- 13.5 Gyr. See CvD12 for further details regarding wasconvolvedwithavelocitydispersionof350kms- 1. No-
themodel. ticethatevenatthislowresolution,atleasttwoSrIIlinesand
We follow Conroy&vanDokkum (2012b) in fitting the twoBaII linesproducea changein the spectrumof>0.3%
modeltodata. Initspresentformthemodelcontains27free per0.3dexchangeinabundance. Aswewilldemonstratein
parameters, including the redshift and velocity dispersion, a Section4,theexistenceofmultiple,well-separatedSrandBa
two-partpower-lawIMF,twopopulationages,fournuisance linesallows us to test the robustnessof our results in a rela-
parameters,andtheabundancesofC,N,Na,Mg,Si, Ca, Ti, tivelymodel-independentfashion.
V, Cr, Mn, Fe, Co, Ni, Sr, Y, and Ba, and O,Ne,S are var-
ied in lock-step. These parameters are fit to the data via a 3. DATA
Markov Chain Monte Carlo fitting technique. The data and InthisLetterweconsidertwosamplesofearly-typegalax-
modelsaresplitintofourwavelengthintervals(definedinthe ies. The first sample of 34 early-type galaxies (and the nu-
following section) and, within each interval the spectra are clearbulgeofM31)waspresentedinvanDokkum&Conroy
normalizedbyahigh-orderpolynomial(withdegreenwhere (2012). The sample was selected from the SAURON sur-
n≡(λ - λ )/100Å; see Conroy&vanDokkum 2012b, vey of nearby galaxies (Baconetal. 2001). High S/N spec-
max min
fordetails).Wehavemaskedthespectralregionssurrounding tra of these objects were obtained with the Low Resolution
theHα,[NII],[SII],Hβ,[OIII],and[NI]emissionlines. Imaging Spectrometer (LRIS) on the Keck I telescope. The
The effectiveresolutionof the data variesfrom R≈400- wavelengthcoveragespans the range 3500Å- 10,400Å with
1200owingtointrinsicDopplerbroadening.Bothbecauseof a gap at 5600Å- 7100Å.Four wavelength intervals were de-
thislowresolutionandthefactthatinourmodelthesynthetic fined,0.4µm- 0.46µm,0.46µm- 0.55µm,0.80µm- 0.89µm,
and 0.96µm- 1.02µm for fitting models to data. The wave-
6kurucz.harvard.edu/ length choices reflect the limitations of the model and data
StrontiumandBariumInEarly-TypeGalaxies 3
wavelength coverage and regions of severe telluric correc-
tions.TheS/Nat5000Å variesfrom≈150Å- 1to≈450Å- 1. 1.10
SpectrawereextractedwithinR<R /8,withaweightingin-
e
tendedtomimicacircularaperture,whereR istheeffective 1.05
e
radius.ThesedatahavealreadybeencomparedtotheCvD12
modelsin Conroy&vanDokkum(2012b, see the Appendix ux 1.00
ofthatpaperforacomparisonofthebest-fitmodelsanddata fl
for each of the 35 LRIS galaxies). In that earlier work, we 0.95
didnotconsidervariationinSrandBa,andonlybrieflycom-
mented on the abundances of the other elements considered
0.90
inthefit. Wefoundthattheages,[Fe/H]and[Mg/Fe]values
1.0
derived with our full spectrum fitting technique agreed with
conventionalLickindex-basedtechniques. %) 0.5
The second source of data is provided by the Sloan Dig- al ( 0.0
u
ital Sky Survey (SDSS; Yorketal. 2000). Following the sid −0.5
methodology outlined in Gravesetal. (2009), we have se- re −1.0 [[BBaa//FFee]]==−+00..40
lected a sample of galaxieswithin a narrowredshift interval
of0.02- 0.06,withnodetectedemissioninHαnorin[OII], 0.44 0.45 0.46 0.47 0.48 0.49 0.50
l (m m)
andwithconcentrated,deVaucouleurs-likelightprofiles.The
SDSSobtainsspectrawithfibersthathave3”diameters. The
typicalS/NofSDSSspectrainoursampleismodest,oforder 1.10
20 Å- 1, and so we have chosento stack the spectra in seven
binsofvelocitydispersionwithmeandispersionsineachbin 1.05
of88,112,138,167,203,246,and300kms- 1. Forthesmall-
estsigmabintheSDSSfibersamplestheinner0.8Re, while flux 1.00
for the most massive bin the fiber samples the inner 0.4R .
e
We have verified that our results do not change if we select 0.95
galaxieswithineachbinsuchthatthefibersamplesthesame
fraction of Re. Each spectrum was continuum-normalized 0.90
and convolved to an effective dispersion of 350 kms- 1 be- 0.6
fore stacking, and each spectrum contributed equally to the %) 0.4
0.2
dsrataantcagk,e.sfofTurorhmewar≈evseu5lel0tni0nggtÅh-S1i/nNttoerov≈falt1sh8ew00estraÅec-kd1ee.dfinAsepsdewcftoirtrahtathhtee5S0LD0R0SIÅSS esidual ( −−00..20
stacks,0.4µm- 0.48µm,0.48µm- 0.58µm,0.58µm- 0.64µm, r −−00..64
and0.80µm- 0.88µmforfittingmodelstodata.Thesestacked 0.57 0.58 0.59 0.60 0.61 0.62
datawillbethefocusoffutureworkaimedatmeasuringthe l (m m)
detailed abundance patterns as a function of galaxy proper-
ties. Inthisworkwelimitourattentiontotheneutroncapture
elementsSrandBa.
Figure3. Comparisonbetweenmodelsanddataintwowavelengthregions.
As a first look at the data, in Figure 2 we show spectral Upperpanelsshowthecontinuumnormalizedspectraandlowerpanelsshow
indices for both the SDSS stacked spectra and the individ- residualsinpercent. Thedata(blacklines)areforthestackedSDSSearly-
ualLRISgalaxies. TheMgbindexissensitivetoMg,while typegalaxyspectruminthedispersionbincenteredonσ=250kms- 1.Two
modelsarecompared,oneinwhichthe[Ba/Fe]ratioisfixedtothesolarvalue
theotherindicesaresensitivetoBaandSrlines(withcentral
(redline),andamodelinwhichthe[Ba/Fe]valueisincludedinthefit(with
wavelengthsindicatedbytheindexname). TheBrandSrin- aresultingbest-fitvalueof- 0.4;blueline). ThegreybanddenotestheS/N,
dicesaredefinedinServenetal.(2005),whiletheMgbindex andthedottedlinesmarkthestrongestBaIIlinesinthiswavelengthrange.
isdefinedinWortheyetal.(1994,noeffortwasmadetoplace Thethinlineisthe[Ba/Fe]responsefunction(cf.Figure1).AnysingleBaII
featureonitsownisnotparticularlycompelling,giventhemagnitudeofthe
the latter on the Lick indexscale). For thisfigure, the LRIS
residualselsewhereinthespectrum,butthecoincidenceofanimprovement
data were convolved to a common dispersion of 350 kms- 1 inχ2 aroundallfourBaIIlinesisstrongevidence infavorofasub-solar
inordertoaffordadirectcomparisonwiththeSDSSstacked [Ba/Fe]abundanceratiointhisparticularspectrum.Moreover,thedifference
inχ2 betweenthetwomodelsis1400,signalingstrongpreferenceforthe
spectra. The LRISdata do notcoverthe 6142Å spectralre- modmelinwith[Ba/Fe]=- 0.4.
gion. Overall the agreement between the LRIS and SDSS
samplesisencouraging,giventheverydifferentobservational
setupsandreductiontechniques. Itishoweverdifficulttoin- Beforeshowing the derivedabundancetrends, in Figure3
terpret spectral indices because an index is in generalsensi- weexplorethequalityofthefitaroundtheBaIIlinesforthe
tivenotonlytothefeatureofinterestbutalsotofeaturescon- stackedSDSSspectruminthesecondhighestσbin.Inthefig-
tributing to the sidebands (i.e., the index measures a feature urewe comparetwo modelfits, onein which [Ba/Fe]=+0.0
strength relative to a pseudocontinuum). This is especially andanotherinwhichtheBaabundanceisallowedtovaryin
true when the feature of interest is weak compared to other the fit (with a resulting best-fit value of [Ba/Fe]=- 0.4). We
features. Fullspectrumfittingsuchasthe kindperformedin alsomarkthelocationsofthestrongestBaIIlinesinthisspec-
thepresentworkdoesnotsufferfromthisdrawback. tralregion. Itisnoteworthythattheresidualsdecreaseinall
spectralregionssensitive to Ba. This is an importanttest of
4. RESULTS theresultstofollowbecauseitisalsoevidentfromthisfigure
4 CONROY,VANDOKKUM,&GRAVES
0.2
0.2 SDSS stacks
LRIS data
[Sr/Fe]
−0.0
0.0
e]
F
e] −0.2 a/
F B
Ba/ e], [ −0.2
[ F
−0.4 Sr/ [Ba/Fe]
[
−0.4
−0.6
fiducial
blue only
no smoothing
−0.8 fixed IMF
0.3 −0.6

0.05 0.10 0.15 0.20 0.25 0.30
[Mg/Fe]
0.2
Figure5. Explorationofsystematiceffectsonthe[Sr/Fe]and[Ba/Fe]abun-
0.1 dancetrends. Thefiducialmodeliscomparedtomodelsinwhichonlythe
blue(λ<5800Å)spectralregionisfit,inwhichthereisnosmoothingofthe
e] stackedspectratoσ=350kms- 1,andinwhichtheIMFisfixedtotheGalac-
Sr/F −0.0 tsipcecfotrramf.orRcelasruilttys.aWrehislheotwhenseomnlyodfeolrvtahreiatsitoancskeinddSucDeScShaenagrleys-toyfptehegaolradxeyr
[ of0.1dex,thetrendsareclearlyrobusttothesemodelpermutations.
−0.1
consistentwiththestandardinterpretationof[Mg/Fe]probing
theSFtimescaleinthesegalaxies.Ashortertimescaleshould
−0.2
leadtohigher[Mg/Fe]andlower[Ba/Fe]values,asobserved.
Althoughnotshown,theSDSSdataalsodisplayastrongcor-
−0.3
relation between [Mg/Ba] and σ. The derived [Fe/H] val-
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 ues range from - 0.1 to 0.0 over the SDSS early-type sam-
[Mg/Fe] ple,andsothetrendsobservedinFigure4areunlikelytobe
due to metallicity-dependent yields. Other explanations for
the[Mg/Fe]- σrelationhavebeenproposed,includingavary-
Figure4. Derived[Ba/Fe]and[Sr/Fe]abundanceratiosasafunctionofthe
derived[Mg/Fe]abundanceratio. ResultsareshownbothforstackedSDSS ingIMF,varyingSNIarate,andselectivemass-lossofSNIa
early-typegalaxyspectraandfortheLRISspectraof35individualgalaxies. ejecta(Wortheyetal.1992;Thomasetal.1999;Trageretal.
FortheSDSSspectra,themeanvelocitydispersionincreasesmonotonically 2000).The[Mg/Ba]dataallowustoruleouttheexplanations
withincreasing[Mg/Fe].
thatare particularto the Ia’s, since the Ia’sinfluenceneither
thattherearenumerousplaceswheretheresidualsexceedthe Mg norBa, butit cannotdiscriminate betweena varyingSF
formalS/Nlimitsofthedata(demarcatedbythegreyband). timescaleandavaryinghigh-massIMFfortheobservedvari-
IftherewereonlyonestrongBalinethenitwouldhavebeen ationin[Mg/Fe]withσ.
difficultto arguethataregionofhighresidualrequirednon- The near constancy of [Sr/Fe] is puzzling. It has been
solar[Ba/Fe]values. Wereturntothispointlaterinthissec- suggested that Sr may have multiple nucleosynthetic ori-
tion. gins, including the r-process, both the weak and main s-
OurmainresultisshowninFigure4.Inthisfigureweshow processes, and perhapsevenhypernovae(Raiterietal. 1993;
thebest-fit[Ba/Fe]and[Sr/Fe]abundanceratiosasafunction Travaglioetal.2004;Qian&Wasserburg2008). As[Mg/Fe]
of [Mg/Fe]. We include both the stacked SDSS spectra and (and presumably the SF timescale) varies, then perhaps the
theLRISspectraofindividualgalaxies.Theformalstatistical main nucleosynthetic site varies in such a way as to keep a
errorsfromtheSDSSstacksareverysmallowingtothevery rough constancy in [Sr/Fe]. With detailed chemical evolu-
highS/Nofthespectra. ItisencouragingthattheLRISdata tion models constrained to match the observed [Mg/Fe] and
andSDSSstacksfollowthesamegeneraltrendsofdecreasing [Ba/Fe] trends, it may be possible to place interesting con-
[Ba/Fe] and constant[Sr/Fe] with increasing [Mg/Fe]. Such straintsontheorigin(s)ofSrinthesemassivegalaxies.
lowvaluesof[Ba/Fe]arenotuncommonamongstmetal-poor We turn now to several tests that have been performed to
halostars in the Galaxy(Snedenetal. 2008). Indeed,below assess the robustness of these results. In Figure 5 we show
[Fe/H]≈- 3essentiallyallhalostarshave[Ba/Fe]<0.0,with the same abundance trends as before, for only the SDSS
many stars having Ba abundances as low as [Ba/Fe]≈- 2.0 stackedspectra,andforavarietyofpermutationstothefidu-
(Françoisetal. 2007). Such stars are also α enhanced, and cial model. We consider a model in which the IMF is held
have [Sr/Ba]>0.0, again broadly consistent with the mean fixed to the Galactic (Kroupa2001) form, a modelin which
abundanceratiosofthemassiveearly-typegalaxies(although onlytheblue(λ<5800Å)spectralregionisfit,andamodel
withverydifferentoverallmetallicities). inwhichtheSDSSstacksarenotsmoothedtoacommonres-
Takenatfacevalue,the[Ba/Fe]versus[Mg/Fe]trendsare olutionofσ=350kms- 1. Thelatterisa particularlyimpor-
StrontiumandBariumInEarly-TypeGalaxies 5
tant test because much more information is available in the theAlfredP.SloanFoundation,theParticipatingInstitutions,
low dispersion bins, which effectively have higher spectral theNationalScienceFoundation,theU.S.DepartmentofEn-
resolution, compared to the high dispersion bins. Relatedly, ergy,theNationalAeronauticsandSpaceAdministration,the
whenthedataarenotbroadenedtoσ=350kms- 1,theasso- JapaneseMonbukagakusho,theMaxPlanckSociety,andthe
ciated response functions (Figure 1) become much stronger. Higher Education Funding Council for England. The SDSS
Thederivedabundanceratiosvarybetweenthesemodelper- Web Site is http://www.sdss.org/. The SDSS is managed by
mutationsby∼0.1dex,buttheoverallqualitativetrendsare the AstrophysicalResearch Consortiumfor the Participating
clearlyrobustto thesedetails. We havealsomaskedeachof Institutions. The Participating Institutions are the American
thetwostrongestBaIIlines(oneatatime)andrefitthemod- MuseumofNaturalHistory,AstrophysicalInstitutePotsdam,
els to the data. The resulting [Ba/Fe] versus[Mg/Fe] trends UniversityofBasel, Universityof Cambridge,Case Western
ReserveUniversity,UniversityofChicago,DrexelUniversity,
areunaltered,althoughinthecaseofmaskingtheBaII4555Å
linetheoverall[Ba/Fe]abundancesshiftlowerby≈0.2dex. Fermilab,theInstituteforAdvancedStudy,theJapanPartic-
ipation Group, Johns Hopkins University, the Joint Institute
This highlights that while trends appear to be robust in our
analysis,therearezero-pointuncertaintiesatthe0.1- 0.2dex forNuclearAstrophysics,theKavliInstituteforParticleAs-
trophysics and Cosmology, the Korean Scientist Group, the
level.
ChineseAcademyofSciences(LAMOST),LosAlamosNa-
Aconcernisthatthefeatureswearetryingtomeasureare
tional Laboratory, the Max-Planck-Institute for Astronomy
muchweakerthanothermetallines. Infact,asshowninFig-
(MPIA), the Max-Planck-Institute for Astrophysics (MPA),
ure3theexpectedBalinestrengthsaresimilartothetypical
NewMexicoStateUniversity,OhioStateUniversity,Univer-
systematicresidualinthespectraaftersubtractionofourbest-
sity of Pittsburgh, University of Portsmouth, Princeton Uni-
fitmodel!Arobusttestofourresultistorandomlyshufflethe
versity,theUnitedStatesNavalObservatory,andtheUniver-
centralwavelengthsofthestrongestBaIIlinesandrecompute
sityofWashington.
χ2 aroundthe shuffledlines. In practice, this is achievedby
alteringtheresponsefunctionforBa(seeFigure1)byassign-
ingrandomcentralwavelengthsintheinterval4000Å- 6000Å REFERENCES
while keeping the relative strengths and widths of the lines
thesame. We haveperformedthistest103 timesforeachof Bacon,R.etal.2001,MNRAS,326,23
the SDSS stacked spectra. As an example of the results for Bergemann,M.,Hansen,C.J.,Bautista,M.,&Ruchti,G.2012,A&A,546,
A90
thehighestσ bin,theresidualsdecreasearoundthestrongest
Burbidge,E.M.,Burbidge,G.R.,Fowler,W.A.,&Hoyle,F.1957,Reviews
shuffledBaII lineinonly0.1%oftheshuffledmodelswhen ofModernPhysics,29,547
using the best-fit [Ba/Fe] value shown in Figure 4. More- Busso,M.,Gallino,R.,&Wasserburg,G.J.1999,ARA&A,37,239
over,innoneoftheshuffledmodelsdotheresidualsdecrease Conroy,C.&vanDokkum,P.2012a,ApJ,747,69
Conroy,C.&vanDokkum,P.G.2012b,ApJ,760,71
in both of the two strongest shuffled BaII lines. In the four
highest σ bins, where the best-fit [Ba/Fe] ratios are <- 0.1, Couch,R.G.,Schmiedekamp,A.B.,&Arnett,W.D.1974,ApJ,190,95
François,P.etal.2007,A&A,476,935
the shuffled modelsare neverable to reduce the residualsin Graves,G.J.,Faber,S.M.,&Schiavon,R.P.2009,ApJ,693,486
allthreeofthestrongestBaIIlines. Itisthereforehighlyun- Herwig,F.2005,ARA&A,43,435
likelythatthederived[Ba/Fe]abundancesaretheresultofa Kroupa,P.2001,MNRAS,322,231
Kurucz,R.L.1970,SAOSpecialReport,309
chancealignmentbetweenthestrongBaII linesandspectral
—.1993,SYNTHEspectrumsynthesisprogramsandlinedata,ed.Kurucz,
regionswheretheresidualshappentobehigh.
R.L.
The elemental abundances derived herein should provide Qian,Y.-Z.&Wasserburg,G.J.2008,ApJ,687,272
novel constraints on the duration of star formation in early- Raiteri,C.M.,Gallino,R.,Busso,M.,Neuberger,D.,&Kaeppeler,F.1993,
type galaxies once chemical evolution models that include ApJ,419,207
Sbordone,L.,Bonifacio,P.,Castelli,F.,&Kurucz,R.L.2004,Memorie
yieldsfromthes-andr-processesareemployed.The[Mg/Ba]
dellaSocietaAstronomicaItalianaSupplementi,5,93
ratio is particularly promisingbecause the derived timescale
Serven,J.,Worthey,G.,&Briley,M.M.2005,ApJ,627,754
will not depend on the uncertain delay time distribution or Sneden,C.,Cowan,J.J.,&Gallino,R.2008,ARA&A,46,241
overallrateofTypeIaSNe,incontrasttotimescalesderived Thomas,D.,Greggio,L.,&Bender,R.1998,MNRAS,296,119
from the [Mg/Fe] ratio. In future work, we will present de- —.1999,MNRAS,302,537
tailed abundancepatternsfor≈17 elements, which promise Thomas,D.,Maraston,C.,Bender,R.,&MendesdeOliveira,C.2005,ApJ,
621,673
toprovidefurtherinsightintotheearlyformationhistoriesof Tinsley,B.M.1979,ApJ,229,1046
thesenowdormantgalaxies. Trager,S.C.,Faber,S.M.,Worthey,G.,&González,J.J.2000,AJ,120,
165
Travaglio,C.,Gallino,R.,Arnone,E.,Cowan,J.,Jordan,F.,&Sneden,C.
Some of the data presented herein were obtained at the 2004,ApJ,601,864
W.M. Keck Observatory, which is operated as a scientific vanDokkum,P.G.&Conroy,C.2012,ApJ,760,70
Woosley,S.E.&Hoffman,R.D.1992,ApJ,395,202
partnershipamongtheCaliforniaInstituteofTechnology,the
Worthey,G.,Faber,S.M.,&Gonzalez,J.J.1992,ApJ,398,69
University of California and the National Aeronautics and
Worthey,G.,Faber,S.M.,Gonzalez,J.J.,&Burstein,D.1994,ApJS,94,
Space Administration. The Observatory was made possible 687
bythegenerousfinancialsupportoftheW.M.KeckFounda- York,D.G.etal.2000,AJ,120,1579
tion.FundingfortheSDSSandSDSS-IIhasbeenprovidedby