Star-formation rates from young-star counts and the structure of the ISM across the NGC346/N66 complex in the SMC

2.7 MB·English

by  S. Hony

Mon.Not.R.Astron.Soc.000,1–16(XXX) PrintedJanuary16,2015 (MNLATEXstylefilev2.2)
Star-formation rates from young-star counts and the structure of the
ISM across the NGC 346/N66 complex in the SMC(cid:63)
S. Hony1†, D. A. Gouliermis1,2, F. Galliano3, M. Galametz4, D. Cormier1, C.-H. R. Chen5,
S. Dib6,7, A. Hughes8,9,2, R. S. Klessen1,10,11, J. Roman-Duval12, L. Smith13, J.-P. Bernard8,9,
C. Bot14, L. Carlson15, K. Gordon12,16, R. Indebetouw15,17, V. Lebouteiller3, M.-Y. Lee3,
S. C. Madden3, M. Meixner12,18, J. Oliveira19, M. Rubio20, M. Sauvage3, R. Wu21
5
1 1Universita¨tHeidelberg,Zentrumfu¨rAstronomie,Institutfu¨rTheoretischeAstrophysik,Albert-Ueberle-Str.2,69120Heidelberg,Germany2MaxPlanckIn-
0 stituteforAstronomy,Ko¨nigstuhl17,69117Heidelberg,Germany3LaboratoireAIM,CEA/IRFU/Serviced’Astrophysique,Universite´ParisDiderot,Bat.709,
2 91191Gif-sur-Yvette,France4EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,85748Garching-bei-Mu¨nchen,Germany5Max-Planck-Institutfu¨r
n Radioastronomie,AufdemHu¨gel69,53121Bonn,Germany6NielsBohrInternationalAcademy,NielsBohrInstitute,Blegdamsvej17,2100,Copenhagen,
a Denmark7CentreforStarandPlanetFormation,UniversityofCopenhagen,ØsterVoldgade5-7,1350,Copenhagen,Denmark8CNRS,IRAP,9Av.Colonel
J Roche,BP44346,31028ToulouseCedex4,France9Universite´deToulouse,UPS-OMP,IRAP,31028ToulouseCedex4,France10DepartmentofAstronomy
andAstrophysics,UniversityofCalifornia,1156HighStreet,SantaCruz,CA95064,USA11KavliInstituteforParticleAstrophysicsandCosmology,Stanford
5
University,SLACNationalAcceleratorLaboratory,MenloPark,CA94025,USA12SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore,
1
MD21218,USA13SpaceTelescopeScienceInstituteandEuropeanSpaceAgency,3700SanMartinDrive,Baltimore,MD21218,USA14Observatoire

astronomiquedeStrasbourg,Universite´ deStrasbourg,CNRS,UMR7550,11ruedel’Universite´,67000Strasbourg,France15DepartmentofAstronomy,
]
A UniversityofVirginia,POBox3818,Charlottesville,VA22903,USA16SterrenkundigObservatorium,UniversiteitGent,Gent,Belgium17NationalRadio
AstronomicalObservatory,Charlottesville,VA22904,USA18TheJohnsHopkinsUniversity,DepartmentofPhysicsandAstronomy,366BloombergCenter,
G
3400N.CharlesStreet,Baltimore,MD21218,USA19SchoolofPhysical&GeographicalSciences,Lennard-JonesLaboratories,KeeleUniversity,Stafford-
. shireST55BG,UK20DepartamentodeAstronomia,UniversidaddeChile,Casilla36-D,Santiago,Chile21DepartmentofAstronomy,GraduateSchoolof
h
Science,TheUniversityofTokyo,Bunkyo-ku,Tokyo113-0033,Japan
p
-
o
r Accepted2015January15.Received2015January14;inoriginalform2014October30
t
s
a
[ ABSTRACT

Therateatwhichinterstellargasisconvertedintostars,anditsdependenceonenvironment,
1
isoneofthepillarsonwhichourunderstandingofthevisibleUniverseisbuild.Wepresenta
v
comparisonofthesurfacedensityofyoungstars(Σ )anddustsurfacedensity(Σ )across
4 (cid:63) dust
3 NGC346(N66)in115independentpixelsof6×6pc2.WefindacorrelationbetweenΣ(cid:63)and
6 Σdust withaconsiderablescatter.Apowerlawfittothedatayieldsasteeprelationwithan
3 exponentof2.6±0.2.WeconvertΣ togassurfacedensity(Σ )andΣ tostarformation
dust gas (cid:63)
0 rate(SFR)surfacedensities(Σ ),usingsimpleassumptionsforthegas-to-dustmassratio
SFR
. andthedurationofstarformation.ThederivedtotalSFR(4±1·10−3 M yr−1)isconsistent
1 (cid:12)
0 with SFR estimated from the Hα emission integrated over the Hα nebula. On small scales
5 theΣSFR derivedusingHαsystematicallyunderestimatesthecount-basedΣSFR,byuptoa
1 factor of 10. This is due to ionizing photons escaping the area, where the stars are counted.
: Wefindthatindividual36pc2 pixelsfallsystematicallyaboveintegrateddisc-galaxiesinthe
v
Schmidt-Kennicuttdiagrambyonaverageafactorof∼7.TheNGC346averageSFRovera
i
X larger area (90pc radius) lies closer to the relation but remains high by a factor of ∼3. The
r fractionofthetotalmass(gasplusyoungstars)lockedinyoungstarsissystematicallyhigh
a (∼10percent)withinthecentral15pcandsystematicallyloweroutside(2percent),which
weinterpretasvariationsinstarformationefficiency.Theinner15pcisdominatedbyyoung
stars belonging to a centrally condensed cluster, while the outer parts are dominated by a
dispersedpopulation.Therefore,theobservedtrend couldreflectachangeofstarformation
efficiencybetweenclusteredandnon-clusteredstar-formation.
Keywords: ISM:individual:NGC346–ISM:Structure–stars:pre-main-sequence–galax-
ies:MagellanicClouds
(cid:63) basedonobservationsobtainedwithHST,Spitzer,HerschelandAPEX. † E-mail:[email protected]
(cid:13)c XXXRAS
2 S.Honyetal.
1 INTRODUCTION complex may have been subject to significant photo-dissociation
andusingamoleculargastracer(suchasCO)mayleadtoanin-
It is generally accepted that the local star-formation rate (SFR) completeviewofthetotalgassurfacedensities.
scaleswiththelocalgasdensityoftheinterstellarmedium(ISM). The object of choice is the star-forming complex related to
This wisdom is captured in the Schmidt-Kennicutt relation (SK- NGC 346, also often referred to as N66. NGC 346 is the most
relation,seeforareviewKennicutt&Evans2012)whichconnects prominent star-formation region in the Small Magellanic Cloud
the observed gas-surface density (Σgas) along various sight-lines (SMC). In order to assess the ΣSFR, an unbiased census of the
with the observed surface density of SFR (ΣSFR). Even though youngstellarpopulationisrequired.ThisisavailableforNGC346
therearemultipleincarnationsoftheSchmidt-Kennicuttdiagram, thanks to HST observations (Nota et al. 2006; Gouliermis et al.
depending on which tracers are used to measure both Σgas and 2006, see Sect. 3.1). We have access to sensitive IR to submm
ΣSFR,andwhethertoconsideronlythedense(molecular)gasor photometryfromSpitzer(PIGordon),Herschel(PIMeixner)and
includetheatomicphase,theemergingpictureisclear:thehigher APEX/Laboca (PI Hony) in order to derive reliable dust surface
theISMcolumndensity,thehigherthestar-formationrate.Akey densities.Theproximity(60kpc)oftheSMCincombinationwith
featureoftheSK-relationisitspowerlawbehaviouroveralarge the size ofthe star forming complex (∼100×100pc2) enables us
dynamic range of gas column-densities and star-formation rates. to‘zoomin’(∼6pclinearscale)andstudylocalvariationsatthe
Theslopeoftherelationdependsonwhetheronemeasuresonlythe angularresolutionoftheavailablesubmmobservations.
densegas(exponent∼1)oralsotheatomicgas(exponent∼1.4). Moreover, the entire complex encompasses a range of envi-
The physical interpretation of this interdependence, i.e. how ronments.IntermsoftheISM,theregionexhibitsawiderangeof
toderivetheefficiencywithwhichinterstellargasisconvertedinto surfacebrightnessofhotdust(24µm),polycyclicaromatichydro-
stars, is much less self-evident, especially on small spatial scales carbon (PAH; 8 µm) and molecular gas (CO(J=2-1), Rubio et al.
(<1kpc).Thisisduetothefollowing:i)The‘star-formationrate’ 2000). In terms of the stellar content, there are zones which are
measuresstarsthathavealreadyformed,whilethegascolumnden- clusterdominatedaswellasregionswheretheyoungstarsaremore
sity relates to (the capacity for) future star formation. On small dispersed (Gouliermis, Hony & Klessen 2014). This allows us to
scales, with low numbers of stars, one can no longer expect to examinehowtheenvironmentinfluencestherelationbetweenstar
measureatime-averagedefficiencyasisthecasewhenaveraging formationandtheavailableISMreservoir.Themainquestionswe
over a large number of star forming sites. ii) Dynamical simula- addressare:
tions (e.g. Kiseleva et al. 1998) and observations (e.g. Goodman
• How well do young stars and the densest ISM track each
2004) show that young stars may be displaced, by several pc per
other?
Myr, from the location where they were born, and thus removed
• DostarcountsyieldthesameΣ asindirecttracers?
from the reservoir of gas from which they formed iii) The large SFR
• Doestheefficiencyofstarformationvarybetweenclustered
scaleaveragesincludenon-star-formingpartsoftheISMandthe
andmoreextendedspatialdistributions?
relativeproportionofthis‘inactive’gaswillchangewhenzooming
in on smaller scales. iv) Many of the used SFR-tracers (e.g. Hα • What is the effect of averaging over different scales on the
derivedparametersinthepositionintheSK-diagram?
orfar-ultraviolet(FUV)emission)aresensitiveonlytohigh-mass
stars,whichrepresentonlyafractionofthestellarmassassembled
Additionally, we examine whether there may be an effect of the
inthestar-formationeventandtoderivethetotalSFRoneneedsto
reducedmetalabundanceoftheSMC(0.2solar,Haseretal.1998)
extrapolatethemass-functiontolowermasses.Sincethehigh-mass
onthestarformationefficiency.
slopeoftheinitialmassfunction(IMF)isverysteep–veryfewof
The paper is organised as follows. Previous work on deriv-
theformedstarsarehigh-mass–thecorrectionsarelargeandsen-
ing SFR from individual young sources is reviewed in Sect. 2.
sitivetotheassumedshapeoftheIMF.Someoralloftheseeffects
In Sect. 3 we present the catalogue of young stars and the IR to
areprobablythereasonwhythestudiesofstarformationratesin
submmdata-setsthatweuseforouranalysis.Wefurtherdescribe
nearby individual molecular clouds (Evans et al. 2009; Wu et al.
thedatatreatmenttoderiveindependentsurfacedensitymeasure-
2010;Heidermanetal.2010;Gutermuthetal.2011)findverydif- mentsin20×20arcsec2(6×6pc2)beams(Sect.3.4).Ourmethod-
ferentstarformationratesatagivengascolumndensitycompared
ologytoobtaingassurfacedensities,usingSEDfitting,isdescribed
toglobalaverages.
inSect.4.1and4.2.TheSFRderivedfromouryoungstarcounts
Theaimofthispaperistocircumventsomeofthesecompli- iscomparedtoindirectSFRtracersinSect.4.3.InSect.5wecom-
cationsbystudyingthecorrelationbetweennewlyformedstarsand parethederivedΣ andΣ .Possiblebiasesthatmayaffectthe
gas SFR
thegasreservoir(ΣSFR,andΣgas)inmanysmallbeams(∼6pc) observedvalueofΣSFR/ΣgasarediscussedinSect.6.Weconclude
whichcoveranentirestarformingcomplex.Weinvestigate,thus, inSect.7.
thesmallscalerelationbetweenyoungstarsandgasreservoirwhile
coveringalargerregion.Weusethemostdirectwayofdetermining
Σ ,i.e.bycountingthenewlyassembledstars.Atthesametime
SFR
weemploythe‘dustmethod’todeterminetheISMcolumndensity,
2 PREVIOUSWORK
i.e.usingtheinfrared(IR)tosubmillimetre(submm)dustcontin-
uumemissiontoderivedustsurfacedensities.Thismethodhasthe Most of the work done on comparing ISM column densities and
advantage of not being sensitive to the state of the gas (ionized, star-formation rates based on individual objects has concentrated
neutralormolecular)aslongasdustandgasarewellmixed.This onnearby(withinonekpcfromthesun)MilkyWaystar-forming
insensitivityisimportantbecausewewilluseacatalogueoflow- sites (Evans et al. 2009; Lada, Lombardi & Alves 2010; Heider-
andintermediate-masspre-mainsequence(PMS)starsplusupper- man et al. 2010; Wu et al. 2010; Gutermuth et al. 2011). These
mainsequence(UMS)stars,i.e.OandBtypestars,asthebasisfor studies have consistently shown that when one considers the im-
ourstarcounts.DuetotheevolutionarystageofthePMSstars–a mediate dense environment (on a few parcec scales) surrounding
fewMyraftertheonsetofstarformation–theparentalmolecular the newly formed stars, the derived SFR is higher than predicted
(cid:13)c XXXRAS,MNRAS000,1–16
RelationbetweengasandyoungstarsinNGC346 3
Figure1.Falsecolourcompositeimage(green:optical,red:infrared,blueHα)ofNGC346basedonHSTimaging(PINota,Notaetal.2006).Thecontours
ofthe,2arcsecresolution,densitymap(Σ(cid:63))oftheyoungstarsareshowninyellow.Thecontourlevelscorrespondto3,10and20starspc−2.Themaximum
valueofΣ(cid:63)is45starspc−2.
bythegalaxyaveragedSK-relation(byuptoafactorof54)fora parsec.TheirdatashowaverysharpdecreaseinΣ belowΣ
SFR gas
givenISMcolumndensity. ≈102 M pc−2 which is interpreted as resulting from a density
(cid:12)
thresholdforstar-formation.Note,thatsuchathreshold,ifitexists,
Evansetal.(2009);Lada,Lombardi&Alves(2010)present cannotbeauniversalthreshold.Thereareseveralobservationsof
thetotalstar-formationratesbasedonyoungstellarobject(YSO) specificregionswhosegassurfacedensitiesexceedthethreshold,
candidates, identified from their mid-IR excess using Spitzer and withmuchlessstarformationthanwouldbeexpected(e.g.Long-
comparethosetomolecularcloudmassesderivedusingextinction moreetal.2013;Rathborneetal.2014).Moreover,theexistence
maps for ∼10 nearby molecular clouds. These studies show that ofacolumndensity-thresholdforstarformationinaturbulentISM
there is a large variety of star formation efficiency (SFE) among isnotsupportedbytheoreticalmodels(Clark&Glover2014).
thoseclouds.Lada,Lombardi&Alves(2010)showthatsomeof
theobservedspreadinSFEdisappearswhenconsideringonlythe A different picture emerges from the analysis presented by
massofgasaboveanextinctionthresholdofA >0.8mag.They Gutermuthetal.(2011).Theseauthorspresentscatterdiagramsof
K
interpretthisasanindicationforandensitythresholdbelowwhich surface density of from YSOs versus Σ (from extinction) for
gas
star-formationdoesnotoccur.Asimilarconclusionisreachedby eightmolecularcloudswithin1kpcfromthesun.Theyfindasteep
Heiderman et al. (2010) who study the Σ versus Σ within powerlawcorrelation(withindicesbetween1.4and2.7,withan
SFR gas
thosecloudsusingYSOs.Thetypicalspatialscaleoverwhichthey outlierat3.8)withsignificantscatterandnoindicationforadensity
comparethesequantitiesisfromafewtenthsofaparsectoafew threshold.TheyarguethatthedataareconsistentwithaSFRwhich
(cid:13)c XXXRAS,MNRAS000,1–16
4 S.Honyetal.
Table1.Valuesandsymbolsusedthroughoutthispaper.
Quantity Symbol Value Comments/Refs
SMCdistance dSMC 60±3kpc Harries,Hilditch&Howarth(2003)
Detectedyoungstars Nstar 5150 Gouliermisetal.(2006),seeSect.3.1
Totalyoungstellarmass Mtot 2.2·104M(cid:12) Sabbietal.(2008),seeSect.3.1
Masspercataloguesource Mcata 4.3M(cid:12) =MtotNs−ta1r,seeSect.3.1
SFduration ∆tSFR 5·106yr Mokiemetal.(2006),seeSect.3.1
Gas-to-dustmassratio rgd 1250 Sect.4.2
DerivedQuantity
Youngstarsurfacedensity Σ(cid:63) fromstarcatalogue
Stellarmasssurf.dens. ΣM(cid:63) =Σ(cid:63)·Mcat
SFRsurf.dens. ΣSFR =ΣM(cid:63)∆t−SF1R
Dustsurf.dens. Σdust fromSEDfitting
Gassurf.dens. Σgas =Σdust·rgd
Stellarmassfraction fracM(cid:63) =ΣM(cid:63)(ΣM(cid:63)+Σgas)−1
aThismassisnotthemeanmassoftheHSTdetectedsourcesbutthemasseachsourcerepresentsaftercorrectingforcompleteness.Themeanmassofthe
youngstarsintheHSTcatalogueis∼2M(cid:12).
isproportionaltoΣ2 .Notethatthedifferentconclusionreached sities (dust emission versus extinction). This has the following
gas
bythelatterauthorsisbased,inpart,onthewaythesurfacedensity (dis-)advantages:
ofYSOsiscalculatedinthelowsurface-densityregimeastheyuse
• Richstellarcatalogue:HSThasprovidedaccuratephotometry
the nth-nearest-neighbour centred on the detected YSOs. We fur-
formorethan5000stellarsourcescoveringthepre-main-sequence
ther note that, given the steep dependence on Σ and the low
SFR
(Gouliermisetal.2006).
numbercounts,distinguishingbetweenathresholdandthispower
• Measuredmassfunctionandage:agesandthemassfunction
lawisdifficultandalmostamatterofsemantics.
havebeendeterminedfortheUMS+PMSstellarcatalogue(Sabbi
The work by Gutermuth et al. (2011) is the Galactic study etal.2008).Thismeanswecanrelyontheobservedmassfunction,
that can be compared most directly to the NGC 346 analysis we which covers a significant (0.8−40M ) part of the entire mass
(cid:12)
presenthere,becausetheirΣgasmapscoverseveraltensofparsecs range,andtheobserveddurationofthestar-formationeventtode-
aroundthemolecularcloudsandeachcloudharboursasignificant rivetheSFR.TheYSOsstudiesassumeatypicalageandmassfor
number(>100)ofYSOs.Comparedtotheworkwepresenthere, a YSO, both of which are uncertain (e.g. Preibisch 2012), which
these previous studies have the advantage of proximity and as a couldintroducealargesystematicuncertainty.
result,theyareabletoexaminethestar-formationwithinindividual • Largecoverage:theIRandsubmmobservationsthatweuse
molecularcloudsonspatialscalesof<1pcwhilethebestangular tomeasuredustcolumndensitiesareverysensitiveandmostcover
resolutionwecanattainintheSMCintheISMtracersis∼6pc. theentireSMC(Sect.3.2).Asaresult,wehaveaccesstoanun-
Recent Spitzer surveys of the Large and Small Magellanic biasedprobeofthedustsurfacedensityouttolargedistancesand
Clouds(LMCandSMC)madeitpossibletouseresolvedmassive lowcolumndensities.Thisisimportanttoassessthebehaviourof
YSOstodeterminecurrentSFRsoutsideoftheGalaxy.Unlikethe theSFRatlowΣgas,wheresignificantdisagreementexistsinthe
consistentlyhigherSFRsofsmall-sizecloudsreportedintheafore- literature(seeabove).
mentionedGalacticstudies,Chenetal.(2010)showedthatcurrent
However, since we are using stars that are no longer embedded,
SFR determined from massive YSOs in individual giant molecu-
thesourcesinourcataloguearemoreevolved(<5Myr,Sect.3.1)
lar clouds in H II complexes in the LMC are distributed above
than YSOs (<2Myr) and have had more time to displace (or be
and below the SK relation, depending on the evolutionary states
displaced from) their parental gas reservoir. Moreover, since the
of the giant molecular cloud. The scatter in SFR diminishes and
wholeregionhastobemoreevolvedforittohostsomanyPMS
followtheSKrelationwhenpropertiesareaveragedovertheen-
stars,theradiationfromthehot,youngstarshashadmoretimeto
tireHIIcomplexesof150−200pc.Thisisinagreementwiththe
affectthesurroundingmedium.
findingsofKruijssen&Longmore(2014),whoattributethescatter
onsmallerscalestoincompletestatisticalsamplingofindependent
star-formingregions.Furthermore,thecommonlyusedSFRtrac-
erssuchasHαand24µmluminositieswouldunder-estimatethe 3 OBSERVATIONS
amountofstarformationinregionsnotactivelyproducingO-type
stars, such as the LMC’s largest molecular ridge and the Magel- 3.1 Youngstellarcatalogue
lanic Bridge. They show that using Hα or 24µm as SFR tracers
Imaging with the Hubble Space Telescope (HST) of star-forming
thesepointsareoutliersontheSK-relationwhiletheyfollowthe
regions in the Magellanic Clouds has provided access to excep-
SK-relationwhenusingSFRdeterminedfromYSOcounts(Inde-
tionally rich samples of newly-born stars down to the sub-solar
betouwetal.2008;Chenetal.2014).
regimeoverlargeareasofthesky.Ourcatalogueofyoungstarsin
The work presented here differs from these previous stud- NGC346,oneofthelargeststar-formingcomplexesintheSMC,
iesbythesourcesweusetotracethestarformation(PMS+UMS isamongtherichestMagellanicCloudsstellarensemblesevercol-
stars versus YSOs) and the way we derive the ISM column den- lectedwithHST.Threepointings(withsignificantoffsets),centred
(cid:13)c XXXRAS,MNRAS000,1–16
RelationbetweengasandyoungstarsinNGC346 5
onNGC346wereobservedwiththeWide-FieldChannel(WFC) massindirectlydetectedstars(M >0.8M )is1.2·104M ,i.e.
(cid:63) (cid:12) (cid:12)
of ACS covering an area of about 5×5arcmin2 (∼88×88pc2). 50percentofthetotal.Themainparametersusedinouranalysis
TheimageswereobtainedwithintheHSTProgramGO-10248(PI aresummarisedinTab.1.
Nota, Nota et al. 2006) in the filters F555W and F814W (equiv- Thetotalstarformationrate,averagedoverthelast5Myr,of
alenttostandardVandIbands).Detaileddescriptionsofthedata thecomplexcalculatedinthiswayis4±1·10−3 M yr−1,where
(cid:12)
reductionandphotometryaregiveninGouliermisetal.(2006).The theuncertaintytakesintoaccountpossiblestarformationoutsideof
photometryrevealsmorethan98000starsatvariousevolutionary thefield-of-viewofHST(20percent)andtheextrapolationofthe
stages,andits50%completenesslimitism (cid:39)27(Gouliermis observedmassfunctiontolowermasses(20percent)butnotthe
555
etal.2006). systematicuncertaintyduetheassumeddurationofthestarforma-
ThesampleofyoungstellarpopulationsinNGC346consists tionevent.ThisvalueisconsistentwiththeresultsofSimonetal.
ofthelow-masspre-main-sequence(PMS)stars,whichareidenti- (2007)whofindaSFR>3.2·10−3 M yr−1 overthelast1Myr
(cid:12)
fiedfromtheirfaint-redpositionsonthecolour–magnitudediagram usingSpitzeridentifiedYSOcandidates.
(CMD).SeeGouliermis(2012)forareviewonlow-massPMSstars Fortherestoftheanalysisweassumethatthemassfunction
in the Magellanic Clouds. This sample is complemented by the andthedurationofstarformationareinvariantacrossthestarform-
UMSstars(selectedwithcoloursm −m (cid:54) 0.0andmag- ingcomplex.Therearesomeindicationsofvariationsintheslope
555 814
nitudes12∼m555<∼17),includingmorethan30early-typestars ofthemassfunctioninNGC346(Sabbietal.2008;Cignonietal.
(seealsoMassey,Parker&Garmany1989;Mokiemetal.2006). 2011).Thegeneraltrendisasteepingoftheslopewithincreasing
ThemassesoftheselectedUMSstarsaresuchthattheywouldhave distance to the central cluster, i.e. a larger fraction of high-mass
evolvedoffthemainsequenceiftheywereolderthanthePMSpop- starstowardsthecentralcluster,consistentwithacertaindegreeof
ulation.Thus,thecatalogueweuseinthisanalysiscontains5150 masssegregationinthecluster.However,suchvariationshavelittle
young(PMSandUMS)starsinthemassrangebetween∼0.5M impactonourresultswhicharebasedonstarcounts,whilelumi-
(cid:12)
and∼40M .Thelocationoftheyoungstarswithinthecomplexis nositybasedSFRtracers(e.g.Hα)areverysensitivetothenum-
(cid:12)
indicatedinFig.1withyellowcontours.Thisimageshowsthevery ber of high-mass stars and thus the high-mass slope of the mass
prominentHαemissionassociatedwiththecentralclusterandthe function.Asanillustrationofthisinvarianceconsiderthestandard
presenceofseveralsecondaryclustersofyoungstarstotheNorth Kroupamassfunction(Kroupa2001)withahigh-masspowerlaw
whichcauselittleHαemission. slopeof-1.3.Whenwevarythisslopebetween-1.0and-2.0–the
Itishardtogetaccurateageandmassdeterminationsofthe observedrangeofslopesinNGC346(Sabbietal.2008)–thetotal
individualPMSstarsbasedontheirpositionintheCMD(Jeffries masspernumberofstarsabove0.8M (i.e.M )variesbyonly
(cid:12) cat
2012). An isochronal median age around ∼3Myr has been de- ∼20percent.Forthesamevariationsinslopethefractionofmass
fined for the ensemble of PMS stars in NGC 346 on the basis of locked in the high-mass stars – those powering the Hα emission
comparisonwithevolutionarymodels(Sabbietal.2008;Cignoni (M >8M )–variesby1500percent!
(cid:63) (cid:12)
etal.2011).Mokiemetal.(2006)havespectroscopicallystudied
thehigh-massstarstowardsNGC346.Theyfindagesbetween0
and5Myrforthehigh-massstarsthatbelongtothestar-formation 3.2 HerschelandSpitzerdata
complex.Thisagerangeisfullycompatiblewiththepositionand
We use the mid-IR to submm photometric maps that have been
spreadofthelow-massPMSstarsintheCMD.Sinceourobjective
obtainedwithSpitzer(SAGE-SMC:Gordonetal.2011)andHer-
istomeasuretheaverageSFRcountingallstarsthathaveformed
schel (HERITAGE: Meixner et al. 2013). Within these two large
duringthestar-formationevent,weuse5Myrasthestar-formation
programstheentireSMChasbeenobservedatIRAC(3.6,4.55.8
duration(∆t ).Thus,∆t referstothedurationoftheevent
SFR SFR and8.0µm),MIPS(24,70,and160µm),PACS(100and160µm)
nottotheagesoftheindividualstars.Therefore,thisapproachis
andSPIRE(250,350and500µm)wavelengths.Thesensitivityof
conceptuallydifferentfromtheYSO-countingstudies(e.g.Lada,
thesedatasetsissuchthatfortheenvironmentoftheprominentstar
Lombardi&Alves2010;Heidermanetal.2010)inwhichagesof
formingregionNGC346thephotometricuncertaintyisdominated
theindividualobjectshavebeenassumed.
bytheabsolutefluxcalibration(seealsoSect.3.4).Anoverviewof
WeconstructtheentiremassfunctionoftheUMS+PMScata-
NGC346insomekeyISMtracersisshowninFig.2.Inthefollow-
loguebyextrapolatingthemassfunctionformassesabove0.8M
(cid:12)
inganalysiswemainlyusetheIRAC,MIPS-24,MIPS-70,PACS-
derivedbySabbietal.(2008,Fig3bottompanel)fromthesame
100, PACS-160 and SPIRE-250 data, because these sufficiently
observational material. Their mass function is completeness cor-
coverthespectralenergydistribution(SED)toderiveaccuratedust
rected for crowding and photometric sensitivity but not for inter-
surface densities at optimal angular resolution (∼20arcsec). The
stellarextinction.Overthemeasuredrangethemassfunctionhas
SPIRE350µmand500µmdatahavepoorerangularresolution.
aslopeof-1.431.WeextrapolatetolowermassesusingaKroupa
Theonlyexceptionisthedustmassesderivedwithinlargecircular
massfunctionwithaslopeof-0.3between0.08and0.5M and
(cid:12) apertures(Tab.2)whichusestheSPIRE-350µmdatatoreplacethe
aslopeof0.7between0.01and0.08M (Kroupa2001).Theto-
(cid:12) Labocabecausethelatterisnotobservedoversuchalargearea.
tal stellar mass formed in the last 5Myr, obtained by integrating
thismassfunctionandmultiplyingbythesurveyarea(88×88pc2)
equals∼2.24·104M .Sincewehave5150starsinourcatalogue,
(cid:12)
3.3 Laboca870µmimagingdata
eachsourcerepresentsonaverageamassof4.3M (M ).The
(cid:12) cat
To constrain the emission by the coolest dust, we use Laboca
data at 870µm, instead of the SPIRE 350 data. This data set al-
1 Weusetheformalismofthemassfunction–log10(N(∆log10(m)))ver- lowsustoobtainmeasurementsat∼20arcsecresolutionwhichis
suslog10(m)–asgivenbyScalo(1986).InthisdefinitiontheSalpetermass unattainablewithHerschel/SPIRE350and500.Labocaisamulti-
functionhasaslopeof-1.35andtheKroupamassfunctionhasaslopeof channelbolometerarrayforcontinuumobservationsat870µmin-
-1.3above0.5M(cid:12) stalledontheAtacamaPathfinderEXperiment(APEX)telescope
(cid:13)c XXXRAS,MNRAS000,1–16
6 S.Honyetal.
Figure3.Laboca870µmmapofNGC346.Thecontoursshowthe36cm
Figure2.Falsecolourimageshowinganoverviewofthelargeenviron- radiocontinuumemissionfromSUMSS(Mauchetal.2003).The3and
mentofNGC346inkeyISMtracers.Weshow8µmemissionindicative 6cm radio continuum images (not shown) exhibit the same morphology
ofthelocationofthemostprominentPDRs(blue;IRAC4emissionminus (Dickeletal.2010).Thepossiblecorrespondingcontributionoffree-free
thestellarcontinuumestimatedfromIRAC2),thewarmerdusttracedby emissionat870µmhasbeenfiltered-outbythemedianfilteringappliedin
PACS100(green)andthecoolerdust(SPIRE250,red).Eachwaveband thedatareductionprocess.Thedashedcircleindicatestheapproximateex-
isshownatitsnativeresolution.Thebluecontoursshowthestellardensity tentoftheHαnebula.Weshowaclose-upofthebrightestregioninthe
(seeFig.1).Thegeneralobservedtrendisadecreasingdusttemperature inset,inwhichthe250µmdustemissioncontours,inperfectcorrespon-
withdistancetothemostrichcluster(yellowtoredcolourgradient)and dencewiththeLabocamap,areshown.
strongPDRsnear(butnotpeakingon)thestellarconcentrations.There-
gionwherebothdustsurfacedensity,includingLabocameasurements,and
stellardensitiesarewelldefinedisindicatedbythewhiteline(seeSect.3.4 mapisshowninFig.3.Withintheregionofinteresttherearethree
fordetails).
mainemissionfeatures.Thereisanemissionpeaktowardsthecen-
tralcluster,adirectindicationthatthissight-lineisdustrich.We
detectabrighteremissionpeaktowardsthenorth-westofthemain
in Chile. It has a field-of-view of 11.4arcmin and a PSF full-
clusterandaplumeof870µmemissionwhichfollowsthestring
width-at-half-max of 19.2±0.3arcsec. Observations of the NGC
ofstellarclusteringtowardsthenorth-west.Thisplumecoincides
346 region with Laboca were carried out in December 2008 and
withthebrightestCO(j=2-1)emissiondetectedintheregionRubio
April, May and September 2009 (Program ID: O-081.F-9329A-
etal.(2000).Duetothemedianfilteringthatisappliedtoremove
2008 PI: Hony). The mapping was done with a raster of spiral
theatmosphericandtelescopesignal,anyspatiallyveryextended
pattern in order to fully sample the final map (size of the region
mapped: 25×25arcmin2). Data are reduced with the BOlometer emissionhasbeenremovedaswell.Thisexplainswhywedonot
detectanyfree-freeemissionfromtheionizedgaswhichshouldbe
Array Analysis Software (BoA Schuller et al. 2010). The scans
presentinthisregion(seeFig.3)
are reduced individually. They are calibrated using the observa-
tionsofplanets(Mars,Neptune,Uranus,Venus)andsecondarycal-
ibrators(G10.62,G5.89,HLTAU,PKS0506.61,PMNJ0210-5101,
PMNJ0303-6211,PMNJ0450-8100,N2071IRandVY-CMa).We 3.4 Beammatcheddata
determinezenithopacitiesusingalinearcombinationoftheopac-
In our analysis, we use maps with a resolution of 20arcsec (6pc
itydeterminedusingsky-dipsandthatcomputedfromtheprecip-
at the distance of the SMC). All maps with intrinsic spatial res-
itablewatervapor2.Weremovedeadornoisychannelsaswellas
olutionsbetterthanthisresolution,includingthestellarcatalogue,
stationary points or data taken at fast scanning velocity or above
havebeenconvolvedtothethiseffectivespatialresolution.Laboca,
anaccelerationthreshold.Wesubtractthecorrelatednoise,apply
MIPS70andSPIRE250mapshavenotbeenconvolvedsincethey
stepsofmediannoiseremoval,baselinecorrectionanddespiking.
alreadyhaveanintrinsicspatialresolutionverycloseto20arcsec.
Thereducedscansarethencombinedintoafinalmap.Weisolate
WeusetheconvolutionkernelsavailablefromKarlGordonsweb
the pixels above a given signal-to-noise ratio to define a ‘source page4forconvolvingtotheSPIRE250beams.Forthestellarden-
model’thatissubtractedfromthetime-ordereddata.Wererunthe
sitymap–whichhasanexceedinglygoodintrinsicangularreso-
reduction pipeline and add the source model to the time-ordered
lutionwehavedirectlyconvolvedthe2arcsecstellardensitymap
data to obtain a new map. The process is iterated until the result
with the SPIRE 250 µm beam. After convolution, all maps have
mapconverges(5iterationswereappliedforNGC346)3.Thefinal
been re-projected on the same pixels scheme with pixels the size
2 Tabulated sky opacities are provided at http://www.
apex-telescope.org/bolometer/laboca/calibration 4 http://dirty.as.arizona.edu/˜kgordon/mips/conv_
3 MoredetailsonthetechniquesareprovidedinGalametzetal.(2013). psfs/conv_psfs.html
(cid:13)c XXXRAS,MNRAS000,1–16
RelationbetweengasandyoungstarsinNGC346 7
of20×20arcsec2 inordertohavespatiallyindependentmeasure-
mentsandtoallowpixel-by-pixelSEDextractionandcomparison
betweenthestellardensitiesandderiveddustsurfacedensities.We
use the program swarp from AstroMatic5 for calculating the re-
projected maps, as it is in our experience the most accurate tool
inconservingthemeansurfacebrightness.Thisisparticularlytrue
whenre-projectingtoapixelschemewithverydifferentpixelsizes,
whichisourcase6.
In order to create reliable random noise maps that represent
wellthenoiseintheoriginalmapsandtheeffectsofconvolution
andre-projectionweapplythefollowingprocedure:
• Theamplitudeofthenoiseineachmaphasbeenestimatedby
measuringthemedianabsolutedeviationfromthemedian(MAD).
This value is a more robust measure of the noise in a map than
thestandarddeviationinthepresenceofrealsourceemission.The
estimatedstandarddeviationequals1.4825*MAD.
• Wegenerateapurenoisemapattheoriginalresolutionusing
themeasuredstandarddeviation.
• We generate a projected noise map convolving and re-
projectingthisnoisemapintheexactsamemanneraswedothe
sciencemap.
• Wemeasuretheamplitudeofthenoiseintheprojectednoise
mapasthestandarddeviationofthefittedGaussianfunctiontothe
histogramofpixelvalues. Figure4.MapsoftheoutputparametersofthedustSEDfitting.Theregion
showncorrespondstothewhitecontourinFig.2.Forreferenceweshow
Theseoutputnoiselevelshavebeencomparedtotheformal thesurfacedensitymapofyoungstarsincontours(seeFig.1).Weshow
calibrationuncertainties(expressedaspercentagesoftheobserved theluminosityofthedustemissionrangingfrom150to1300L(cid:12)(a),the
surface brightness levels). For all maps except the Laboca map, averageradiationfieldasexperiencedbythedustrangingbetween6and
theabsolutecalibrationisdominantthroughouttheregionofinter- 45G0(b),dustsurfacedensityrangingfrom0.05to0.3M(cid:12) pc−2(c)and
est.InthecaseoftheLabocamap,weretainforeachpixelinthe themassofPAHsrelativetothetotaldustmassnormalisedtotheGalactic
projectednoisemapthelargervalueofthefluxcalibrationuncer- valuerangingfrom0to0.3(d).
taintyandtherandomnoisevalue.Theseuncertaintiesshouldnot
be added (quadratically) because the Laboca flux calibration un-
certaintyreflectsthescatterinthemeasuredcalibratorfluxesandis shape of the SED (in particular, both its peak wavelength and its
thus,inessence,alsoarandomnoiseterm. width)constraintheparametersofthispowerlaw.
PixelsoutsidetheareasurveyedbyHSTandpixelswherethe Foragivenstarlightintensity,U,thedustemissivityisderived
Laboca measurement is less than 2 σ are masked. There are 115 fromtheZubko,Dwek&Arendt(2004,dustfamily:BARE-GR-S)
valid, independent 20×20arcsec2 pixels (covering about ∼4000 model,designedtofitthediffuseGalacticemission.However,we
pc2)inthefollowingpixel-by-pixelanalysis.Theiroutlineisshown replacegraphitebyamorphouscarbon(Zubkoetal.1996,ACAR).
inFig.2.Allmapsattheotherwavelengthshavesignal-to-noisera- Indeed,Gallianoetal.(2011a)showedthatthestandardGalactic
tioslargerthan2–inmostcasesmuchlarger–inthesepixels. grainpropertiesappliedtoSpitzerandHerschelobservationsofthe
LMC lead to violation of elemental abundance constraints, while
themodifiedcomposition(the‘ACmodel’)givesrealisticgas-to-
dustmassratios.Insummary,wemaketheassumptionthatgrains
4 DUST,GASANDSFRSURFACEDENSITIES intheSMCwillhavesimilaropticalpropertiesasintheLMC.
Thechoiceofasetofgrainemissivitiesistaintedbysystem-
4.1 SEDfitting
aticuncertainties.AsdemonstratedbyGallianoetal.(2011b),the
In order to derive dust surface densities, we fit the IR to submm derived dust mass change by a factor of 2 to 3 depending on the
dustcontinuumSEDforeachindependentpixelintheregion.The opticalproperties.However,itisalsoshownthatthechangeises-
dustemissionfittingisperformedwiththeGallianoetal.(2011a) sentiallyasimplescalingofthedustmass,anddonotsignificantly
SED model. This model accounts for variations of dust excita- affect the trends. Besides, if the origin of the difference in emis-
tion rates along the line-of-sight. Following the prescription of sivitybetweentheMilkyWayandtheLMCisanevolutionaryef-
Dale et al. (2001), it assumes that the distribution of dust mass, fect,linkedtometallicity(cf.PlanckCollaborationetal.2011),it
M ,isparametrizedbythestarlightintensity,U,asapowerlaw: is likely that using LMC emissivities puts us closer to the reality
d
dM /dU ∝U−α,betweenU andU +∆U.Theparameter thansimplyassumingGalacticproperties.
d min min
U isthestellarfluxreceivedbythedustandisnormalisedtothe The fit of the observed SED, with the previously described
solarneighbourhoodvalue(Mathis,Mezger&Panagia1983).The dustmodel,providesconstraintson:thedustmass,M ;theparam-
d
eters of the power law distribution of starlight intensities, U ,
min
∆U andα,whichwesummarisebyquotingtheaveragestarlight
5 http://www.astromatic.net intensity,(cid:104)U(cid:105);andthePAHmassfraction,normalisedtotheGalac-
6 TheIDL/astrolibtoolhastrom,oftenusedforthesamepurpose,performs ticvalue,fPAH.Inordertoestimatethefinaluncertaintiesonthese
poorlyinthiscase. derived dust parameters, we perform Monte-Carlo iterations, fol-
(cid:13)c XXXRAS,MNRAS000,1–16
8 S.Honyetal.
of1021[cm−2(Kkms−1)−1](Bolatto,Wolfire&Leroy2013).If
Table2.Totaldustmass,gasconsumptiontime-scaleandintegratedSFE
asafunctionoftheareaconsideredintheanalysis.Thefirstrowcorre- weuseanXCO of1022 [cm−2 (Kkms−1)−1]wefindamolecu-
spondstotheareaincludedinthepixel-by-pixelanalysis.Aradiusof52pc largasmassof7.5·105 M(cid:12).SuchanelevatedXCO maybemore
isequivalenttotheareacoveredbytheHSTobservations.The90pcradius appropriate given the harsh radiation field that is prevalent in the
matchestheprominentSPIRE250and350µmwavebandemission,while region,whichmayinduceenhancedphoto-dissociationoftheCO
thelargestradiusprobesthefullextentoftheHαnebulaaroundNGC346. molecules.Thus,wederiveatotalgasmassintherangeof2.7·106
Thegasconsumptiontime-scale(τgas)isdefinedastheMgas/SFRand to 3.4·106 M . The dust mass derived using the dust continuum
(cid:12)
the integrated star-formation efficiency (SFE10Myr) is calculated as the photometryinthisapertureandapplyingthesameSEDfittingrou-
percentageofgasthatwillhavebeenconvertedintostarsafter10Myr,as-
tineweuseforthepixel-by-pixelanalysisyields2.7·103M .The
sumingthattheSFRremainsconstant.Weuse10Myrasthelimitingage, (cid:12)
derivedr rangesfrom∼1000to∼1250.Inthefollowingweusea
whensupernovaemightdisrupttheISMandhaltthestarformation. gd
fiducialvalueforr of1250,whichismuchhigherthantheGalac-
gd
ticvalue(100,Draine&Li2007)duetothelowermetallicityofthe
area/radius Mdust τgas SFE10Myr SMC.Ourvalueforrgd of1250islowerthanthetotalSMCinte-
[M(cid:12)] [108yr] [%] gratedvalueof1740fromGordonetal.(2014).Theglobalvalue
includesasignificantamountofatomicgasatlowcolumndensity
mask 498±18% 1.7 5.8
intheoutskirtsoftheSMCwhichisnotdetectedindustemission.
52pc 900±19% 2.5 4.0
Thisdrivesuptheglobalr .
90pc 1616±19% 4.5 2.2 gd
145pc 2741±22% 7.6 1.3 Given that the derivation of rgd is susceptible to significant
systematicuncertaintiesinX forthegassurfacedensitiesand
CO
dust optical properties for the dust surface densities, we will dis-
cusstheimplicationsofafactoroftwovariationsintheabsolute
lowingthemethoddescribedinGallianoetal.(2011a),takinginto value of r on our results. Note that, part of the purpose of this
gd
accountthecorrelationbetweencalibrationuncertainties.Theme- paperistodeterminehowwelltheISMandstarsfolloweachother
dianuncertaintiesonthederivedparameters(σx/x)are6percent onsmallspatialscales.Thispartoftheanalysisdoesnotdepend
forLdust,and19percentforΣdustand(cid:104)U(cid:105).Themedianstandard onthechosenabsolutevalueforrgd.ThecomparisonofSFRand
deviationonfPAHis0.02. star-formationefficiencieswithvaluesderivedforotherregionsis
Maps of the best fit output parameters are shown in Fig. 4. howeversensitivetothechosenvalueandwewilldiscusstheinflu-
OneofthekeyresultsfromtheSEDfittingwhichisevidentfrom enceofourchoiceontheresultswefind.
these maps is the clear distinction between the parameters which Wefurtherassumer tobeinvariantfortheentireregion.In
gd
tracetheheatingofthedust(panelsaandb)andthesurfaceden- principle,itispossiblethatr couldvarysignificantly,inpartic-
gd
sityofdust(panelc).Clearlythedustluminosityintheveryactive ular in the vicinity of high-mass star-formation sites, where dust
starformingregionNGC346(panela)tracescloselytheradiation destruction can become important due to supernovae explosions.
field. This is in stark contrast to other (more usual) observations Reidetal.(2006)havestudiedthesupernovaeremnantswithinthe
ofmorediffuseregionswheretheincidentradiationfield(anddust largeenvironmentofNGC346.Theseauthorsshowthattheregion
temperature)doesnotvarymuchandLdusttracesthedustsurface we study here appearslargely unaffected by the impact of super-
density.PaneldshowsthatthePAHabundanceclosetothemain novae. The closest remnant they identified (SNR B0057-724, see
cluster is so low that their emission is undetectable in the broad- also Fig. 2) lies to the North-East of the main cluster, outside of
bandphotometry.ThislowabundanceofPAHisinperfectagree- theregionofinterest.Theobservationaldataargueagainststrong
mentwiththefindingsofSauvage,Vigroux&Thuan(1990)who variationsofr becausetheyshowthattheratioofdustheating
gd
deducedanincreaseddifferentialdepletionofPAHstowardsNGC comparedtogasheatingisveryconstantthroughoutthewholere-
346basedonIRASphotometry.SimilarpaucitiesofPAHstowards gion(seeSect.6.2).
otherhigh-massstarformingregionintheMagellanicCloudshave
beenreported(e.g.Sandstrometal.2010;Honyetal.2010).
4.3 Starformationratesurfacedensities
4.2 Conversionofdustsurfacedensitytogassurfacedensity The star-formation-rate map has been derived using the observed
numberofyoungstarsalongtheline-of-sightineachpixelofthe
We assume that gas and dust are well mixed along each line-of- beam-matchedstellardensitymap.Itiscalculatedas(seealsoTa-
sightinwhichcasethegassurfacedensity(Σgas)isproportional ble1):
tothedustsurfacedensity(Σ )andtheproportionalityfactoris
dust
N M N ·4.3
thegas-to-dustmassratio(rgd).ThedustmassesderivedbySED SFR= ∆(cid:63)t cat = 5(cid:63)·106 [M(cid:12)yr−1] (1)
fittingmethodsvarysignificantlyandsystematically(e.g.Galliano SFR
etal.2011b;Gordonetal.2014)dependingontheappliedmethod Thestar-formationsurfacedensitymap(Σ )isobtainedbydi-
SFR
and assumed dust optical properties. It is therefore important to vidingthismapbytheareaofapixelinkpc2.
use a value of r consistent with the used SED fitting method- InFig.5,wecomparethederivedΣ withselected,indirect
gd SFR
ology, i.e. the value of r that gives the correct gas mass given star-formationratetracers.Notethatweonlycomparewiththose
gd
thederiveddustmass.Wehavecalibratedther usingtheavail- tracers(Σ –calibrationfromHaoetal.(2011)foraKroupaIMF;
gd Hα
able gas tracers on a large scale. Within a 500arcsec radius cen- Σ –calibrationfromRiekeetal.(2009)andΣ –calibration
24µm TIR
tredonNGC346wefindanatomicgasmassof2.7·106M using fromMurphyetal.2011)whichareappropriatefortheveryrecent
(cid:12)
the Parkes+ATCA HI map (Kim et al. 2003) and a spin temper- epochofstar-formation(<5Myr)andwedonotconsiderFUVbe-
ature of 60K (Bernard et al. 2008). The molecular gas mass de- causeitscalibrationassumesaconstantSFRover>10−100Myr.
terminedfromCO(J=1-0)withinthesameapertureis7.4·104M Ascanbeseen,theΣ fromstarcountsisgenerallyhigherthan
(cid:12) SFR
whenusingaCO-to-H columndensityconversionfactor(X ) those obtained from Hα or dust emission. Moreover, the differ-
2 CO
(cid:13)c XXXRAS,MNRAS000,1–16
RelationbetweengasandyoungstarsinNGC346 9
Figure 5. Comparison of derived pixel-by-pixel ΣSFR using different
methods.Thex-axisshowstheΣSFR usingtheyoung-starsurfaceden-
sity. In blue we show ΣSFR calculated following Hao et al. (2011, for Figure6.DerivedtotalSFRfortheNGC346complexbasedonthecom-
theKroupaIMF)usingthecontinuumsubtractedHαmapformMCELS bined emission of Hα and 24 µm as function of the radius (centred on
(Smith,Leiton&Pizarro2000)thatwaskindlyprovidedbyS.Points.The 14.776◦;-72.1735◦)overwhichthefluxisintegrated.Wefollowthepre-
redcrossesareΣSFRestimatesusingthetotalinfrared(TIR)(3−1100µm) scriptionfromCalzettietal.(2007,Eqn.5).TheSFRisexpressedinterms
surfacebrightnessfollowingHaoetal.(2011)andtheblacksymbolsshow ofthefractionofthetotalSFRderivedfromstarcounts.TheHαand24µm
theestimatedΣSFRbasedonSpitzer/MIPS24µm(Riekeetal.2009). emissionaremuchmoreextendedthanthedistributionoftheyoungstars.
As can be seen, the SFR derived within the radius of the HST coverage
(∼50pc)isabouthalfofthetotalSFR.Onlywhenweintegrateoverthe
completenebula(∼500arcsec),wheretheHαemissionreachesthediffuse
ences are largest where the stellar density (i.e. Σ ) is highest.
SFR backgroundleveldowederivesimilarSFRvalues.
Towardsthedensestclusterthedifferencebetweencountingstars
andusingHαisafactorof10,whilethedusttracersyieldalocal
Σ lowerbyafactorof50.Thedusttracersyieldmuchlower
SFR
values(byafactorof∼5)thanbothHαduetothereduceddust percent.Whenweintegrateouttoaradiusof500arcsec(145pc),
abundanceintheSMC. wheretheHαemissionfromtheNGC346complexbecomesindis-
TheoriginoftheweakdustemissioninNGC346isnotthe tinguishablefromthegeneralSMCemission,werecover∼90per
sameastheweakemissionintheGould-beltstar-formationregions cent. In this calculation we assume that outside the field-of-view
reportedbyVutisalchavakul&Evans(2013).Theseauthorshave oftheHSTobservations,thestarformationisinsignificant.Thisis
compared the SFR one would derive from Spitzer/MIPS 24 µm corroboratedbytheresultsofSewiłoetal.(2013)whoidentified
measurementsorTIRwithSFRderivedfromcountingYSOs.They YSOcandidatesbasedonIRphotometryoftheentireSMC.Ofthe
find that the IR tracers underpredict the true star formation rate sourcesfromtheirhighly-reliableYSO-candidatelistwhichareas-
bytwoordersofmagnitude,becausetheseregionslackhigh-mass sociated with NGC 346, less than 20 per cent (4/23) are located
starsduetoincompletesamplingoftheinitialmassfunction.NGC outsideofthefield-of-viewofHST.Averysimilarresultisfound
346 is a very rich environment and does not have such a lack of whenusingthespectroscopicallyconfirmedearly-typestarsfrom
high-massstars. Bonanosetal.(2010).Ofthe43O-typestarsfromthiscatalogue
ThedifferencebetweenyoungstarcountsandtheHαderived withina100pcradiusaroundNGC346only∼15percent(6)are
values is due to the fact that the individual massive young stars notwithinthefield-of-viewofHST.
inNGC346produceHα(anddust)emissionoveramuchlarger We conclude that the total SFR as measured from the (pre-
surfacethanthesizeofthepixelinwhichtheyarecounted,signif- dominantlylow-mass)youngstarcountsisconsistentwiththeSFR
icantlylargerthanthefullHSTfield-of-view.Note,thatthiseffect measuredfromHαemissiontowithin∼10percent.However,the
willcausethetightrelationbetweenSFR(derivedfromHα)and absorptionofthehardradiationfromthehotstarsandsubsequent
thegascolumndensitytobreakdownonsmall(<∼100pc)scales, re-emissionaseitherrecombinationlinesordustcontinuumoccurs
similar to what is observed in M33 (Onodera et al. 2010). This far–atdistances>100pc,inthecaseofNGC346–fromtheac-
break-downoccursevenintheabsenceofdriftingofthestarsfrom tual location of these stars. See also Relan˜o et al. (2012) and Li
theirbirthenvironmentorspreadinducedbyevolutionaryeffects, et al. (2013) for excellent discussions on how radiation escaping
simplybecausethelocalHα(ordust)emissionnolongermeasures thelocalenvironmentaffectsHαanddustemissionasSFRtrac-
thelocallyproducedUV-radiationfield.Figure6demonstrateshow ersonsmallspatialscales.Asaconsequence,theseindirecttracers
large this distance is. We show the fraction of the SFR which is which depend on absorption and re-emission, cannot be used for
recoveredwhenintegratingtheextinctioncorrectedHαemission ourpresentpurpose,whichistorelatethestellardensitieswiththe
(basedonthecombinationofHαand24µmemission)inapertures ISMonsmallerscaleswithinthestarformingcomplex.Notethat
ofincreasingradius.WithintheHSTfield-of-viewwerecover50 mostextra-galacticstudiesthatuseHαtoderivedSFRareprobing
(cid:13)c XXXRAS,MNRAS000,1–16
10 S.Honyetal.
which is currently, actively forming stars, while the galaxy aver-
agedvaluesincludealsoregionswhicharenotpresentlyforming
stars. Compared to the study from Heiderman et al. (2010), our
approachhastheadvantageofusingthedustinemissiontomea-
surethecolumndensitiesandthatwehaveaccesstodustemission
coveringalargeareasurroundingthestarformingcomplex.This
allowsustocalculateaveragedvaluesoverrelevantspatialscales,
takingintoaccountalsopartsoftheISMwhicharelessvigorously
formingstars.
The thick blue stars correspond to averages over the entire
masked area, the entire HST field-of-view, the prominent SPIRE
250 µm emission and the Hα nebula (top-right to bottom left,
respectively, see also Tab. 2). The mean value lies progressively
closer to the SK-relation as we increase the area over which we
calculatetheaverage.Itshouldbenotedthatthesameshiftingto
bottom left must occur also to the values from Heiderman et al.
(2010)iftheyweretointegratetolargerdistancefromtheiryoung
stars(tolowercolumndensities),astheconditionsthatprevailin
their low-mass star forming regions are probably more represen-
Figure7.Scatterplotofsurfacedensityofyoungstarsversussurfaceden- tativeforthespiralgalaxiesatlowΣSFR ratherthanthoseonthe
sityofdustperpixelof20×20arcsec2 (6×6pc2).Thedottedlineshows dividinglinebetweenthemostactivespiralsandstarburstgalaxies.
the‘standard’relationfromKennicutt&Evans(2012,Fig.11a).Thevalues ThethirdstarfromthetopinFig.7–highlightedbyacircle–indi-
fromHeidermanetal.(2010),basedonYSOcountsandISMcolumndensi- catestheaverageovertheareawheresignificantcolddustemission
tiesinnearbyindividualmolecularclumps,arealsoshown.Thevaluesfor isdetectedinexcesstothegeneralSMCemission.Thisisprobably
NGC346(blackpluses)liesystematicallyabovethe‘standard’line.The thebestestimateoftheaverageSFRandISMcolumndensityfor
thickbluestarsindicatewhereNGC346islocatedwhenweaverageover
thecomplexinitsentirety.ThisvalueisabovetheSK-relationby
allpixelsinthemaskedarea,andsubsequentlyoverapertureswithradiiof
aboutafactorofthreealbeitroughlycompatiblewiththeaverage
52,90and145pc(fromtop-righttobottomleft).Theseradiicorrespondto
relationwhentakingintoaccountitsscatterof0.3dex.Thisfactoris
theapproximatesizeoftheHSTsurveyedarea,theprominentfar-IRemis-
consistentwiththepredictionfromKruijssen&Longmore(2014)
sionnebulaandthesizeoftheHαnebula,respectively.Thepowerlaw
exponentoftheeffectofaveraging,i.etheslopeofthearrowinthefigure, forthesizeoftheregion∼300pcdiameteroverwhichwedothe
is2.9. comparison.WealsoindicateinFig.7byhowmuchtheNGC346
values would be displaced (left-to-right) relative to the data from
Heidermanetal.(2010)andtheSKlineasaresultofafactoroftwo
much larger regions (>∼kpc) than the scale on which the indirect variationsinr .Ifr were2500asopposedto1250,theaverage
gd gd
tracersarereconciledwiththedirectmeasurements(∼150pc)and valuesofNGC346wouldfallontheSKline.Suchahighvaluefor
arethusunaffectedbythiseffect. r wouldimplythathalfofthetotalgas(morethan80percentof
gd
themoleculargas)withina500arcsec(145pc)radiusaroundNGC
346ismolecularwithoutcorrespondingCOemission,asaresultof
5 COMPARISONOFGASANDSTARS photo-dissociationoftheCOmolecule.Suchanlargereservoirof
hiddenmoleculargas,notrelatedtoCO,seemsdifficulttoreconcile
In the following section, we compare the derived SFR and ISM withtheemissionofC+whichtracesthephoto-dissociation.Israel
columndensities.Wefirstconcentrateontheglobalbehaviourand &Maloney(2011)showthatthedistributionofthe[CII]158µm
howNGC346comparestoothermeasurementsbeforelookingin lineemissioncloselyresemblestheCOdistribution.
moredetailatthetendencieswithinthecomplex.
5.2 Thephysicaloriginofvariationsinstellarmassfraction
5.1 Globalcomparison
On the spatial scale that we probe here (∼6×6pc2) we measure
WeshowinFig.7thestellarsurfacedensityversusthedustsurface significantscatterintherelationbetweenΣ andΣ (Fig.7).
gas M(cid:63)
density for each pixel. These are the basic observational quanti- Suchscatteronsmallscalesisexpected,duetothestochasticna-
ties which do not depend on assumptions about Mcat, ∆tSFR or tureofthelocalstar-formationprocess,aswellasrelativemotions
rgd.UsingthevaluesinTable1weconverttheseintoΣSFR and betweentheformedstarsandthereservoirfromwhichtheyformed
Σgas(rightandtopaxes).TheindividualpixelswithinNGC346lie (Kruijssen&Longmore2014).Theobservedscatterismoreeasily
abovethefiducialSK-relationforΣSFRversusΣHI+H2(Kennicutt assessedinFig.8,whereweconcentrateonthevaluesderivedfor
&Evans2012,Fig.11a)byonaverageafactorof7.Forreference, NGC346.Thisfigurealsoshowsthederivedpowerlawfortheen-
wealsoshowthepointsforindividualclumpsinnearbyMilkyWay tireensembleofsight-lines.WeusetheIDLroutineMPFITEXY7
starformingregionsfromHeidermanetal.(2010),whichalsolie (Williams, Bureau & Cappellari 2010) which uses MPFIT Mark-
consistently above the SK-relation. These authors use SFR from wardt(2009)todetermineastraight-linefittothedata(inlog-log
low-massYSOsandcolumndensitiesfromthedensegasmeasured space) with uncertainties in both variables. The best fit value for
fromextinctionmapsonatypicalscaleofafewparsec.Thefact theexponentis2.6±0.2,i.e.therelationismuchsteeperthatthe
thatallthesesmall-scalemeasurementslieabovethegalaxyaver-
aged values is expected (Kruijssen & Longmore 2014). The off-
setisduetotheselectionbiasedintroducedbystudyingaregion 7 http://purl.org/mike/mpfitexy
(cid:13)c XXXRAS,MNRAS000,1–16