The Radial Distribution of Star Formation in Galaxies at z~1 from the 3D-HST Survey

0.57 MB·English

by  Erica June Nelson

TOAPPEARINAPJLETTERS
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
.
THERADIALDISTRIBUTIONOFSTARFORMATIONINGALAXIESATZ∼1FROMTHE3D-HSTSURVEY
ERICAJUNENELSON1,PIETERG.VANDOKKUM1,IVELINAMOMCHEVA1,GABRIELBRAMMER2,BRITTLUNDGREN3,ROSALINDE.
SKELTON1,KATHERINEE.WHITAKER4,ELISABETEDACUNHA5,NATASCHAFÖRSTERSCHREIBER6,MARIJNFRANX7,MATTIA
FUMAGALLI7,MARISKAKRIEK8,IVOLABBE7,JOELLEJA1,SHANNONPATEL7,HANS-WALTERRIX5,KASPERB.SCHMIDT9,ARJEN
VANDERWEL5,STIJNWUYTS6
ToappearinApJLetters
ABSTRACT
3
1 Theassemblyofgalaxiescanbedescribedbythedistributionoftheirstarformationasafunctionofcosmic
0 time.ThankstotheWFC3grismonHSTitisnowpossibletomeasurethisbeyondthelocalUniverse.Herewe
2 presentthespatialdistributionofHαemissionforasampleof54stronglystar-forminggalaxiesatz∼1inthe
3D-HSTTreasurysurvey. BystackingtheHαemissionwefindthatstarformationoccurredinapproximately
n
a exponentialdistributionsat z∼1, with median Sérsic indexof n=1.0±0.2. The stacks are elongatedwith
J median axis ratios of b/a=0.58±0.09 in Hα consistent with (possibly thick) disks at random orientation
angles. Keck spectra obtained for a subset of eight of the galaxies show clear evidence for rotation, with
2
inclinationcorrectedvelocitiesof90to330km/s. Themoststraightforwardinterpretationofourresultsisthat

] star formationin stronglystar-forminggalaxiesatz∼1 generallyoccurredin disks. The disks appearto be
O "scaled-up"versionsofnearbyspiralgalaxies: theyhaveEW(Hα)∼100Å outtothesolarorbitandtheyhave
C starformationsurfacedensitiesabovethethresholdfordrivinggalacticscalewinds.
. Subjectheadings:galaxies: evolution—galaxies: formation—galaxies: high-redshift—galaxies: structure
h
—galaxies:kinematicsanddynamics—galaxies:starformation
p
-
o
1. INTRODUCTION pact dispersion-dominatedobjects (e.g., Genzel et al. 2008;
r
t Shapiro et al. 2008; Cresci et al. 2009; Förster Schreiber
s Galaxy formation is a complex process, involving star-
a bursts, mergers, and strong gas flows (e.g., Hopkins et al. etal. 2009;Lawetal. 2009;Jonesetal. 2010;Mancinietal.
[ 2006; Brookset al. 2009; Dekel et al. 2009). Furthermore, 2011).
stellarmigrationandsecularprocessescanchangethestruc- Some studies claim that there are trends in the structural
1
properties of Hα as a function of redshift implying that the
v tureofgalaxiesatlatetimes(e.g.,Roškaretal. 2008;Grand,
way galaxies assemble their stars varies fundamentally as a
0 Kawata, & Cropper2012). Therefore,evenif we couldper-
functionofcosmictime.Inparticular,Epinatetal.(2009)and
2 fectly locate and age date every star in the Milky Way, we
Kassinetal. (2012)suggestthatgalaxiesbecomecoolerand
3 still couldn’t say where and with what structural and kine-
morerotation-dominatedwithtime,z∼1- 0beingtheepoch
0 matic propertiesthose stars formed. The only way to estab-
of“disksettling”.Thisisinterestingbecausemostofthestars
. lishwhereagalaxy’sstarsformed,andhencehowitwasas-
1 inthedisksofgalaxiesliketheMilkyWaywereformedinthis
sembled, is to mapthe star formationwhile those stars were
0 epoch.
forming.
3 Itisnowpossibletoobtainhighspatialresolution(.1kpc)
ObtainingHα andstellarcontinuummapsofz≥1galax-
1 ies, with the ∼1kpc resolution necessary to put constraints informationonHα emissionatz∼1withahighStrehlratio,
:
v on the spatial distribution of star formation, is challenging owing to the near-IR slitless spectroscopic capabilities pro-
i and has so far only been possible using a combination of vided by the WFC3 camera on the Hubble Space Telescope
X
(HST).InNelsonetal. (2012)we useddatatakenaspartof
adaptiveopticsandintegralfieldunits(IFUs)on8–10mclass
r the3D-HSTsurveytobuildonpreviousground-basedstudies
telescopes. These studies painta complexpicture: they find
a
that star forming galaxies at z ∼ 2 are a mix of “puffy” bymappingthe Hα and stellar continuumwith highresolu-
tionforasampleof57galaxiesatz∼1andshowedthatstar
and often clumpy rotating disks, mergers, and more com-
formationbroadlyfollowsthe rest-frameopticallight, butis
slightlymoreextended. HerewestacktheHαmapsofthese
1AstronomyDepartment,YaleUniversity,NewHaven,CT06511,USA galaxies to construct high signal-to-noise ratio (S/N) radial
2European Southern Observatory, Alonson deCórdova 3107, Casilla profiles and measure structural parameters of stellar contin-
19001,Vitacura,Santiago,Chile uumemissionandstarformation. Theseunique,highspatial
3DepartmentofAstronomy,UniversityofWisconsin-Madison, Madi- resolutiondata from 3D-HST are combinedwith kinematics
son,WI53706,USA
4AstrophysicsScienceDivision,GoddardSpaceFlightCenter,Green- from the Near Infrared Spectrometer (NIRSPEC) on the W.
belt,MD20771,USA M.Kecktelescope(McLeanetal. 1998)WeassumeH0=70
He5idMelabxerPg,laGncekrmIannsytituteforAstronomy(MPIA),Königstuhl17,69117, kms- 1Mpc- 1,ΩM=0.3,andΩΛ=0.7.
6Max-Planck-InstitutfürextraterrestrischePhysik,Giessenbachstrasse,
D-85748Garching,Germany 2. 3D-HSTANDSAMPLESELECTION
7LeidenObservatory,LeidenUniversity,Leiden,TheNetherlands
The 3D-HSTsurvey, a 248-orbitTreasuryprogramon the
8Department of Astronomy, University of California, Berkeley, CA
HubbleSpaceTelescope,suppliesthe2Demissionlinemaps
94720,USA
9DepartmentofPhysics,UniversityofCalifornia, SantaBarbara, CA needed to measure the spatial distribution of star formation.
93106,USA TheWFC3G141grismprovidesspatiallyresolvedspectraof
2 Nelsonetal.
etal. 2007;Daddietal. 2007;Whitakeretal. 2012). These
galaxieswereselectedinHα notIR;whilethisselectionmay
biasthesample,itmaypossiblyalsohavetheadvantagethat
itaddslessuncertaintytointerpretingthespatialdistribution
ofstarformationduetodustextinction.
3. THESPATIALDISTRIBUTIONOFSTAR
FORMATION
3.1. StackedHαandStellarContinuumEmission
We stack the high spatial resolution Hα maps from 3D-
HST to create average Hα maps – increasing the S/N and
providingfor a reliable measurementof the structural prop-
erties of Hα to large radii for this sample at z∼1. We di-
vide the sample in three bins based on their Hα size10 in
order to homogenize the sample and investigate trends with
size.11 Themedianstellarmassesofthestacksarelog(M )=
∗
10.0,10.2,10.2.ThegalaxiesarenormalizedbytheirF140W
flux and centered according to their luminosity-weighted
F140Wimagecenters. Wemaskedthe[S II]λλ6716,6731Å
emissioninthefit. Atourspectralresolution,theFWHMofa
lineis∼100Å,meaningthatHαλ6563Å and[N II]λ6583Å
areunresolvedbutHα and[S II]areseparatedby∼3resolu-
tionelements.WerotatedtheimagesbasedontheirGALFIT-
FIG.1.— LocationofselectedgalaxiesintheSFR-Massplane.Sixsources derivedF140Wpositionanglestoalignthemalongtheirma-
withoutIRphotometryarenotshowninthefigure.Thehistogramshowsthe joraxes,summedthem,anddividedtheresultingstackbythe
positionoftheselectedgalaxiesrelativetotherestofthegalaxiesbydividing summedmasks. Finally,inordertocorrectfortheeffectsof
apowerlawfitoutofthedistribution.Thepurplelineshowsthefit,thelight
purpleregiondelineates±1σ. Theselectedgalaxiesappeartolielargelyon thePSF,thestacksweredeconvolvedbyaddingtheGALFIT-
theupperhalfofthe‘mainsequence’inSFR(IR+UV). derivedPSF-deconvolvedmodel to the residualsof the fit, a
methoddevelopedinSzomoruetal. (2010).
all sources in the field. The wavelength range of the G141 Figs.2 and 3 show the stacked, PSF-corrected Hα emis-
grism, 1.15µm<λ<1.65µm, covers the Hα emission line sion. The S/N in the stacks is high, and the emission is
for galaxiesin the redshiftrange 0.7<z<1.5. The survey clearly spatially extended. Furthermore, both the rest-frame
alsoprovidesbroad-bandnear-infraredimagingintheF140W R-band and Hα maps are clearly elongated. We quan-
filter, whichsamplesthe rest-frameR bandforz∼1. Using tified the structural properties of the stacks with GALFIT
the same camera underthe same conditionsto map the both (Peng et al. 2002) using empirical PSFs. For the contin-
Hα andcontinuumemissionallowsustocomparetheirdis- uumand Hα we find weightedmean axis ratiosof b/a(R)=
tributions directly, a crucial advantage of this strategy. The 0.55±0.07andb/a(Hα)=0.58±0.09respectively,consistent
mapshaveaspatialsamplingof∼0.5kpcatz∼1(0.′′06pix- withexpectationsforrandomlyorienteddiskswiththickness
els). The data presentedin this paperare basedon the ∼70 c/a=0.26,0.33. Thisthicknesscouldbeattributedtodisksbe-
pointings,roughlyhalfofthefulldataset,thatwereobtained ingintrinsicallythickatz∼1,heterogeneouspropertiesofthe
priortoJune2011seeNelsonetal. (2012). Thesurvey,data galaxiesbeingstacked,theeffectofbulges,aselectionbiased
reduction, and determinationof derivedquantities(e.g. red- towardface-ongalaxies,ornoise.
shiftsandmasses)aredescribedinBrammeretal. (2012)and 3.2. RadialProfilesareNearlyExponential
vanDokkumetal. (2011).
Fig.3showstheradialprofilesderivedfromthestacks. Ra-
The 3D-HST grism spectra have high spatial resolution
and low spectral resolution (R ∼ 130) meaning that emis- dialprofilesofthePSF-correctedstackswerecreatedbysum-
mingthefluxinellipticalradii.A30%correctionwasapplied
sionlinestructurereflectsalmostexclusivelyspatialstructure
(morphology) in contrast to data with high spectral resolu- toaccountfor[N II](see§4). Alluncertaintieswerederived
bybootstrapresamplingthestacks.
tionwherestructurereflectsvelocity(rotationordispersion).
The Hα emission ranges from exponential to much less
Emission line maps of galaxies are made by subtracting the
centrally concentrated than exponential. The stellar contin-
continuumemissionfromthetwo-dimensionalspectrum.The
uumemissioninallthestacksisnearlyexponential.
detailsoftheprocedurearedescribedin(Nelsonetal. 2012;
Theradialequivalentwidthprofiles(rightpanelsofFig.3)
Lundgrenetal. 2012;Schmidtetal. 2012submitted)
We selected galaxies with redshifts 0.8 < z < 1.3, total are flat to within a factor of two and EW(Hα)>100Å for
F140W AB magnitude < 21.9, and rest-frame EW(Hα)> all stacks within 1.5re. This suggests that the high global
EW(Hα)sseen in these galaxiesare notdrivenprimarilyby
100Å.Wealsolimitedthesampletogalaxieswitheffectively
centralstarburstsbutbystrongstarformationatallradii.
no contaminating flux from the spectra of other nearby ob-
The derived Sérsic indices (Fig.2) of the Hα and stellar
jects. Thepositionofthesampleinthe0.8<z<1.3starfor-
continuumprofilesaren(Hα)=1.2±0.6,0.9±0.1,n(Hα)<1
mationrate(SFR)–stellarmassplaneisshowninFig.1. Star
formationrateswere derivedfromthe IR+ UV luminosities
10ThesizesaremeasuredusinggrowthcurvesasdescribedinNelsonetal.
asdescribedinWhitakeretal. (2012). Theselectedgalaxies (2012)
liepredominantlyontheupperhalfofthe‘star-formingmain 11 The results are similar when the bins are defined according to the
sequence’(asshowninthehistogramofFig1)(e.g. Noeske F140Wsizes.
RadialDistributionofStarFormationatz∼1 3
Ha Line Emission N =10 N =35 N = 9
s stack stack stack
ck r(Ha )=1.2– 0.1 r(Ha )=3.7– 0.2 r(Ha )=6.3– 1.8
a e e e
t
S
d
e
v
ol
s
e
R
-
y
all
ti
a
Sp b/a=0.50– 0.05 n(Ha )=1.2– 0.6 b/a=0.60– 0.06 n(Ha )=0.9– 0.1 b/a=0.61– 0.13 n(Ha ) < 1.0
]
2
c
p
k1.00
/
r
y
/
⊙
M
[0.10
)
R
F
S
(
Σ
0.01
−10 −5 0 5 10 −10 −5 0 5 10 −10 −5 0 5 10
r [kpc] r [kpc] r [kpc]
FIG.2.—Stacksofthehighspatialresolution, PSF-corrected mapsofHα emission(top). StackingwasdonebasedonHα size; thenumberofgalaxies
includedineachsizebinislisted(Nstack)asisthemajoraxiseffectiveradius(inkpc)ofeachstackmeasuredbygalfit(re). Thestackshaveaweightedmean
Sérsicindexn(Hα)∼1andaxisratioofb/a(Hα)=0.58±0.09,consistentwithdisksatrandomorientationangles. Bottompanelsshowcorrespondingradial
profileswithdarkpurple–small,mediumpurple–mid-sized,lightpurple–large(Fig.3,§3.2).ThestackedHαemissionalwayshasSérsicindexn.1.
andn(F140W)=1.8±0.4,1.4±0.1,1.1±0.2withweighted Thecolorprofileisdeterminedfromthecombinationofthe
means of n(Hα)=1.0±0.4 and n(F140W)=1.4±0.2. As F140W stacks with ACS F814W stacks, which can be con-
shown in van Dokkum et al. (2010), structural parameters vertedinto rest-frameU- V color profiles. As shown in the
measured from stacks are close to the mean values for the leftpanelofFig.4,formostoftheradialextentofthestacks
individual galaxies going into the stacks. The upper limit thecolorisfairlyconstant,butitbecomesredderinsideara-
of n(Hα)<1 for the largest Hα stack reflects the fact that dius of 2kpc (see also Szomoru et al. 2012). To estimate
thederivedn(Hα)dependssomewhatonthedetailsofthefit roughly how color translates into missed star formation, we
(treatmentofsky,fittingregion)butalwaysn(Hα)<1. assumethatthestarformationthatisnotcapturedbyHα will
We find that the stacked spatial distribution of Hα for becapturedbytheIR(Kennicuttetal.2009).Usingphotome-
galaxiesinthissampleisexponentialorshallower. Allstacks tryfromSkeltonetal.(2013)inprep,inadditiontorest-frame
have n<2, implyinga bulgefractionof less than 20%(van U-VcolorsandIR-basedSFRsfromtheNEWFIRMMedium
Dokkum et al. 1998). The radial Hα profiles (Fig.3, left), BandSurvey(NMBS, Whitakeretal. 2012),empiricalrela-
seem to show a trend toward lower n(Hα) with increasing tionswerederivedforthetranslationofrest-frameU-Vcolors
size. However,thistrendisnotstatistically significantgiven intostarformationcorrections:
theerrorsinthederivedSérsicindices.Additionally,notethat
log(SFR(IR+Hα))- log(SFR(Hα))∼0.3×(U- V) (1)
themediangalaxywithEW(Hα)>100Åhasahighenough
star formation surface density to drive winds out to ∼ 1re The dust-corrected radial profiles are shown in the right
(Heckman2002). panel of Fig 4. The smallest galaxies appear to have star
formationwhichismarginallysteeperthanexponential. The
3.3. EffectsofDust
large galaxies have the largest radial gradients but their im-
A major uncertainty in the interpretationof the radial Hα plied star formation is still less steep than exponential. Al-
profile is the effect of differential dust extinction: if some thoughthisanalysisassignstheentireobservedcolorgradient
partsof the galaxiesare moreobscuredthan others(see e.g. toextinction,thecentersoftheradialcolorprofilescouldbe
Wuytsetal.2012),thederivedradialprofileofHα wouldnot redbecauseofdustorage. Importantly,theSérsicindicesof
reflect the radial profile of star formation. To assess the im- the star formation in these stacks based on the implied dust
portanceofthiseffect,weestimatetheextinctionasafunction correctionremain close to one – n=1.4,1.1,0.6for each of
of radius. We determinethe extinctionfromthe radialcolor thestacks, weightedmean=1.0±0.4So, evenwhenaccount-
profile. ingfordust,theaveragedstarformationinthesegalaxieshas
4 Nelsonetal.
FIG.3.— Thestacks ofHα (left) andrest-frame R-band(middle) emissionhave nearly exponential (orshallower) radial profiles. TheEW(Hα)profile
(discussedin§3.2)isshownintherightpanel. Thebottompanelsshowtheprofilesnormalizedbytheireffectiveradius. ThehorizontallineistheSFRsurface
densitycriterionfordrivinglarge-scaleoutflows(Heckman2002)AsinFig.2,thedark,medium,andlightcolorscorrespondtothesmall,medium,andlarge
stacksrespectivelyasshownatthetop.ThespatialdistributionofHαemissionisexponentialorshallower.
anearlyexponentialspatialdistribution. be the velocity differencebetween the geometricalcenter of
thegalaxyandthemaximumvelocity.Wecorrecttherotation
4. KINEMATICS
velocitiesforinclinationangleusingthe3D-HSTaxisratios.
TheflatteningofthestacksandtheexponentialHαprofiles Wecalculateonedimensionalvelocitydispersionsbyfittinga
suggest that the star formation occurs in rotating disks. To gaussiantotheHαemissioninthecentralrowofeachspec-
test this, we measured kinematics for a subset of this sam- trum. Wecalculatetheinstrumentalσ tobe∼80km/sbyfit-
ple using NIRSPEC on the Keck II telescope on April9-10, tingagaussiantotheskylines. Theintrinsicvelocitydisper-
2012. The samplecompriseseightgalaxieschosenfromthe sion is calculated by subtracting the instrumental dispersion
small (1), middle (5), and large (2) stack, which have sizes, fromthemeasureddispersioninquadrature.
masses, Sérsic indices, star formation rates, and axis ratios We find that all galaxies show velocity shear in their two
representativeofthesampleasawhole. dimensional spectra, with derived rotation velocities of 90–
We used the low dispersion mode of NIRSPEC with 0.5" 235km/suncorrectedand110–330km/s correctedfor incli-
seeing and a slit width of 0.7", giving a spectral resolution nation.12 Fig.5 shows the rotation curves for one example
ofσ∼80km/sintheJ band(comparedto∼500km/sinthe galaxy for each stack. These spectra are of the best quality
grism spectra). The slit was aligned alongthe major axis of butappeartoberepresentativeoftheeight. Thevelocitypro-
each galaxy and observations were conducted in a series of fileofthesmallestgalaxyinFig.3doesnotshowevidencefor
four900sexposures,ditheringalongtheslit. Thedatareduc- aturnover,whichmeansthemeasuredmaximumvelocityisa
tion followed standard proceduresfor long slit spectroscopy lowerlimitontherotationvelocityatlargeradii.Weconclude
(see, e.g., van Dokkum et al. 2004). We extracted kine- thatthekinematicsareconsistentwiththediskinterpretation
matic information from the two dimensional spectra by fit- of the structural properties of the Hα emission. Although
tingagaussiantotheHα and[NII]emissionsimultaneously different classification methods make it difficult to perform
ateachspatialpositionalongthe slit. The median[NII]/Hα
is 0.3. Assuming the velocity shear in the two dimensional 12Measuredvelocitydispersionsarealsorelativelyhigh,butaredifficult
spectra is due to rotation, we take the rotational velocity to tointerpretgiventhelowspatialresolutionoftheKeckspectra.
RadialDistributionofStarFormationatz∼1 5
2.5
2] 10.00
c
p
2.0 /k
r
y
r /
o ⊙
Col1.5 [M 1.00
V y
U- sit
n
d e
e1.0 D
v
eri ce 0.10
D a
f
r
u
0.5 S
R
F
S
0.01
0.0
0 2 4 6 8 10 12 0 2 4 6 8 10 12
r [kpc] r [kpc]
FIG.4.—Theleftpanelshowstheradialcolorprofilesforeachofthestackswithblackforthesmallstack,darkgrayforthemediumstack,andlightgrayfor
thelargestack.TherightpanelshowstheimplieddustcorrectedradialprofilesofstarformationsurfacedensitywiththecolorsasinFig.2and3.Errorbaron
therightpaneldenotesatypicaluncertainty.Dust-correctedradialprofilesofstarformationremainnearlyexponential.
a quantitative comparison, our results are qualitatively con- sion.(e.g.,vandenBosch2001;DeBuhr,Ma,&White2012;
sistent with the finding that a large fraction of star-forming Forbes,Krumholz,&Burkert2012)
galaxiesatz∼1appeartoberotating. (e.g.,Wisnioskietal. Wealsonotethesomewhatsurprisingpresenceofapopula-
2011;Epinatetal. 2012;Swinbanketal. 2012). tionoflarge,rapidly-rotatingdiskswithrelativelylowstellar
masses at z∼1. These galaxieshave mean r (Hα)=7.3kpc,
e
5. DISCUSSION v=240km/s, σ = 89km/s, and M∗ = 2.2×1010M⊙, mean-
ing they are disks 8Gyr ago, larger in size than the Milky
The central result of this paper is that the radial distribu- Way(Drimmel&Spergel2001)andthicker(Freeman1987),
tionofstackedHα emissioninz∼1galaxiesisclosetoex- with ∼1/3 of the stellar mass (McMillan2011)and similar
ponential out to ∼ 10 kpc. Combined with the axis ratios or higher maximum rotation velocities (Bovy, Hogg, & Rix
of the stacks and the kinematics of a subset of the sample, 2009). Both what these galaxies become in the local uni-
the most straightforwardinterpretationis that star formation verse and how such extended star forming disks were made
seemstypicallytooccurindisksatz∼1atleastforgalaxies inan epochofhighdisk turbulenceareopenquestions. The
with high EW(Hα). The factor of ∼10 increase in the star smalldisksontheotherendofthesizedistributionarealsoof
formation rates of galaxies from z=0 to z=1 (e.g., Damen interest. With median re(Hα)=1.9kpc, re(R)=2.1kpc, M∗ =
et al. 2009; Fumagalli et al. 2012) is apparently driven by 8.9×109M⊙,andstackedn(Hα)=1.2,theymorecloselyre-
increasedstar formationactivity in disks rather than a much
semble compactversionsof disky star formersthan merger-
greaterprevalenceof merger-drivencentralstarbursts– con-
drivenstarformationinspheroids(seealsovanderWeletal.
sistentwithotherstudies(e.g.,Rodighieroetal. 2011;Wuyts
(2011).)
etal. 2011,2012).
There are several caveats. First, our sample is not com-
Thisresultraisesanumberofquestions.First,itisnotclear
pleteinmassorinSFR(UV+IR).Infuturepapers(usingthe
howthesegalaxiesarerelatedtoz∼0galaxies. Anaverage
full3D-HSTsurvey)wewillstudythedistributionofstarfor-
galaxy in this sample has a distribution of stars with a Sér-
mationasa functionoftheseparameters. Second,thedistri-
sic index of n=1.4. If this average galaxy formsstars with
butionofstarformationinthestacksisnotnecessarilyrepre-
n=1.0,itwillhavealowerSérsicindexatlatertimes. Either
sentativeofindividualgalaxies.Inindividualgalaxiesthestar
we arewitnessing thebuildupof onlythe latestof late-type
formationis clumpy(e.g. Genzel et al. 2008, 2011; Förster
galaxies;thesegalaxiessubstantiallychangewheretheirstar
Schreiber et al. 2009; Nelson et al. 2012, Fig.1) and it is
formationisoccurringbetweenz∼1andz∼0;ortheirstars,
difficulttoquantifythestructure. Stackingtheseclumpyob-
onceformed, mustmigrateinto a differentconfiguration. In
jectscouldbehazardous,butitmayalsoprovidemoreinsight
otherwords, if theseare theancestorsoftypicalz∼0 spiral
thancan begleanedfromindividualgalaxies: assumingthat
galaxies,theirbulgesneedtobebuiltinsomewaybesidesthe
theclumpsaretransientand“lightup”apartoftheunderly-
starformationweareseeing. Thiscouldbeaccomplishedby
ing gas disk for a short time (as in e.g. Wuyts et al. 2012),
mergersor secular evolution (e.g., Bournaud, Elmegreen, &
our stacking technique effectively produces a time-averaged
Martig2009;Ellison etal. 2011). Theseobservationscould
mapofthestarformationinthegalaxies. Finally,dustatten-
beunderstoodinthecontextofthefollowingpicture:atz>1,
uationremainsakeyuncertainty,bothintheselectionandin
disks are gas-rich and turbulent. As redshift decreases, the
theinterpretationoftheprofiles. Thiscanbeaddressedwith
gasfractiondecreases,andabarinstabilitygrows,increasing
mapsof the IR emission at the fullALMA resolution, or by
thecentralconcentrationandcentralverticalvelocitydisper-
6 Nelsonetal.
FIG.5.—Shownherearetherotationcurves(middlerow)derivedfromtheNIRSPECspectra(insetsincorners),oneexamplefromeachstackwiththeimplied
rotationvelocities(vcorr–correctedandvuncorr–uncorrectedforinclination)andvelocitydispersion(σ)listedinkm/s.Thetoprowshowsthecorresponding
rest-frameR-bandimagesandthebottomrowshowsafalsecolorimagewiththeHα emissioninredandstellarcontinuuminblue.Intheserows,ellipsesmark
theR-bandandHα effectiveradii(re(R),re(Hα))respectivelyandgrayarrowshaveascaledlengthcorrespondingtotheprojectedspecificangularmomentum.
measuring(spatially-resolved)Balmerdecrements. acknowledged.
Wethanktherefereeforathoroughreportwhichimproved .
the paper. Support from grant HST GO-12177 is gratefully
REFERENCES
Bournaud,F.,Elmegreen,B.G.,&Martig,M.2009,ApJ,707,1 Ellison,S.L.,Patton,D.R.,Nair,P.,Simard,L.,Mendel,J.T.,McConnachie,
Bovy,J.,Hogg,D.W.,&Rix,H.-W.2009,ApJ,704,1704 A. W., & Scudder, J. M. 2011, in IAU Symposium, Vol. 277, IAU
Brammer,G.B.,etal.2012,eprintarXiv:1206.1867,200,13 Symposium,ed.C.Carignan,F.Combes,&K.C.Freeman,178–181
Brooks,A.M.,Governato,F.,Quinn,T.,Brook,C.B.,&Wadsley,J.2009, Epinat,B.,etal.2009,AstronomyandAstrophysics,504,789
TheAstrophysicalJournal,694,396 Epinat,B.,etal.2012,AstronomyandAstrophysics,539,A92
Forbes,J.,Krumholz,M.,&Burkert,A.2012,ApJ,754,48
Cresci,G.,etal.2009,TheAstrophysicalJournal,697,115
FörsterSchreiber,N.M.,etal.2009,ApJ,706,1364
Daddi,E.,etal.2007,ApJ,670,156
Förster Schreiber, N. M., Shapley, A. E., Erb, D. K., Genzel, R., Steidel,
Damen, M., Labbé, I., Franx, M., van Dokkum, P. G., Taylor, E. N., & C.C.,Bouché,N.,Cresci,G.,&Davies,R.2011,ApJ,731,65
Gawiser,E.J.2009,ApJ,690,937 Freeman,K.C.1987,ARA&A,25,603
DeBuhr,J.,Ma,C.-P.,&White,S.D.M.2012,MNRAS,426,938 Fumagalli,M.,etal.2012,eprintarXiv:1206.1867,757,L22
Dekel,A.,Sari,R.,&Ceverino,D.2009,ApJ,703,785 Genzel,R.,etal.2008,ApJ,687,59
Drimmel,R.,&Spergel,D.N.2001,TheAstrophysicalJournal,556,181 Genzel,R.,etal.2011,ApJ,733,101
RadialDistributionofStarFormationatz∼1 7
Grand,R.J.J.,Kawata, D.,&Cropper, M.2012,eprint arXiv:1206.1867, Rodighiero,G.,etal.2011,ApJ,739,L40
421,1529 Roškar,R.,Debattista,V.P.,Quinn,T.R.,Stinson,G.S.,&Wadsley,J.2008,
Heckman,T.M.2002,ExtragalacticGasatLowRedshift,254,292 TheAstrophysicalJournal,684,L79
Hopkins, P. F., Hernquist, L.,Cox, T. J., Di Matteo, T., Robertson, B., & Shapiro,K.L.,etal.2008,ApJ,682,231
Springel,V.2006,ApJS,163,1 Swinbank,M.,Smail,I.,Sobral,D.,Theuns,T.,Best,P.,&Geach,J.2012,
Jones,T.A.,Swinbank,A.M.,Ellis,R.S.,Richard,J.,&Stark,D.P.2010, arXiv:1209:1395
MNRAS,404,1247 Szomoru,D.,etal.2010,eprintarXiv:1206.1867,714,L244
Kassin,S.A.,Weiner,B.,Faber,S.M.,etal.2012,ApJ,758,106 Szomoru,D.,Franx,M.,vanDokkum,P.G.,Trenti,M.,Illingworth,G.D.,
Labbe,I.,&Oesch,P.2012,arXiv:1208:4363
Kennicutt,R.C.,etal.2009,TheAstrophysicalJournal,703,1672
van den Bosch, F. C. 2001, Monthly Notices of the Royal Astronomical
Law,D.R.,Steidel, C.C.,Erb,D.K.,Larkin, J.E.,Pettini, M.,Shapley,
Society,327,1334
A.E.,&Wright,S.A.2009,ApJ,697,2057
vanderWel,A.,etal.2011,TheAstrophysicalJournal,742,111
Lundgren,B.F.,etal.2012,ApJ,760,49
vanDokkum,P.G.,Franx,M.,Kelson,D.D.,Illingworth,G.D.,Fisher,D.,
Mancini,C.,etal.2011,TheAstrophysicalJournal,743,86
&Fabricant,D.1998,TheAstrophysicalJournal,500,714
McLean, I. S., et al. 1998, in Society of Photo-Optical Instrumentation vanDokkum,P.G.,etal.2004,TheAstrophysicalJournal,611,703
Engineers(SPIE)ConferenceSeries,Vol.3354,SocietyofPhoto-Optical vanDokkum,P.G.,etal.2010,ApJ,709,1018
Instrumentation Engineers(SPIE)Conference Series,ed.A.M.Fowler, vanDokkum,P.G.,etal.2011,ApJ,743,L15
566–578 Whitaker,K.E.,vanDokkum,P.G.,Brammer,G.,&Franx,M.2012,ApJ,
McMillan,P.J.2011,MNRAS,414,2446 754,L29
Nelson,E.J.,etal.2012,ApJ,747,L28 Wisnioski, E., et al. 2011, Monthly Notices of the Royal Astronomical
Noeske,K.G.,etal.2007,ApJ,660,L43 Society,417,2601
Peng,C.Y.,Ho,L.C.,Impey,C.D.,&Rix,H.-W.2002,TheAstronomical Wuyts,S.,etal.2012,ApJ,753,114
Journal,124,266 Wuyts,S.,etal.2011,ApJ,742,96