Metallicity gradients in local field star-forming galaxies: Insights on inflows, outflows, and the coevolution of gas, stars and metals

3.3 MB·English

by  I-Ting Ho

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed13January2015 (MNLATEXstylefilev2.2)
Metallicity gradients in local field star-forming galaxies: Insights on
inflows, outflows, and the coevolution of gas, stars and metals
5
1 I-Ting Ho1, Rolf-Peter Kudritzki1,2, Lisa J. Kewley1,3, H. Jabran Zahid4,
0
2 Michael A. Dopita3,5, Fabio Bresolin1, and David S. N. Rupke6

n 1InstituteforAstronomy,UniversityofHawaii,2680WoodlawnDrive,Honolulu,HI96822,USA
a 2UniversityObservatoryMunich,Scheinerstr.1,D-81679Munich,Germany
J 3ResearchSchoolofAstronomyandAstrophysics,AustralianNationalUniversity,CotterRoad,WestonACT2611,Australia
2 4Harvard-SmithsonianCenterforAstrophysics,60GardenStreetMS-20,Cambridge,MA02138,USA
1 5AstronomyDepartment,KingAbdulazizUniversity,P.O.Box80203,Jeddah,SaudiArabia
6DepartmentofPhysics,RhodesCollege,Memphis,TN38112,USA

]
A
G Accepted2015January10.Received2015January9;inoriginalform2014October23
.
h
p
ABSTRACT
-
o
r We present metallicity gradientsin 49 local field star-forminggalaxies. We derivegas-
t phase oxygen abundances using two widely adopted metallicity calibrations based on the
s
a [OIII]/Hβ, [NII]/Hα and [NII]/[OII] line ratios. The two derived metallicity gradients are
[ usually in good agreement within ±0.14 dex R2−51 (R25 is the B-band iso-photoal radius),
but the metallicity gradients can differ significantly when the ionisation parameters change
1
v systematically with radius. We investigate the metallicity gradients as a function of stellar
8 mass (8 < log(M∗/M⊙) < 11) and absolute B-band luminosity (−16 > MB > −22).
6 Whenthemetallicitygradientsareexpressedindexkpc−1,weshowthatgalaxieswithlower
6 mass and luminosity, on average, have steeper metallicity gradients. When the metallicity
2 gradientsareexpressedindexR−1,wefindnocorrelationbetweenthemetallicitygradients,
25
0 and stellar mass and luminosity.We providea local benchmarkmetallicity gradientof field
.
1 star-forminggalaxiesusefulforcomparisonwithstudiesathighredshifts.Weinvestigatethe
0 originofthelocalbenchmarkgradientusingsimplechemicalevolutionmodelsandobserved
5 gas and stellar surface density profiles in nearby field spiral galaxies. Our models suggest
1 thatthelocalbenchmarkgradientisadirectresultofthecoevolutionofgasandstellar disk
: under virtually closed-box chemical evolution when the stellar-to-gas mass ratio becomes
v
i high(≫0.3).Thesemodelsimplylowcurrentmassaccretionrates(.0.3×SFR),andlow
X massoutflowrates(.3×SFR)inlocalfieldstar-forminggalaxies.
r
a Keywords:
1 INTRODUCTION Rupkeetal.2008;Kewleyetal.2010;Rupkeetal.2010a,b).The
relationsbetween metallicityandother fundamental propertiesof
The content of heavy elements in a galaxy is one of the key galaxiescanplacetightconstraintsontheprocessesgoverningthe
properties for understanding its formation and evolutionary his- evolutionofgalaxies.
tory. Thegas-phase oxygen abundance intheinterstellar medium The correlation between global metallicity and stellar mass
(ISM) of a galaxy (or “metallicities”), defined as the number in star-forming galaxies, i.e. the mass-metallicity relation, is one
ratio of oxygen to hydrogen atom and commonly expressed as of the fundamental relations for measuring the chemical evolu-
12+log(O/H),isregulatedbyvariousprocessesduringtheevo- tionofgalaxies(Lequeuxetal.1979;Tremontietal.2004).Whilst
lutionary history of a galaxy. Whilethe oxygen is predominately the mass-metallicity relation was first established locally, mod-
synthesisedinhigh-massstars(>8M⊙)andsubsequentlyreleased ern spectroscopic surveys have enabled precise measurements of
totheISMbystellarwindsandsupernova explosion, theoxygen metallicity out to high redshifts on large numbers of galaxies
intheISMcould alsobeexpelled tothecircumgalactic medium, (e.g.,Savaglioetal.2005;Erbetal.2006;Zahidetal.2011,2013,
or potentially become gravitationallyunbound, via feedback pro- 2014b;Wuytsetal.2014;Steideletal.2014;Sandersetal.2014)
cesses,e.g.galactic-scaleoutflows.Gasinflowstriggeredbymerg- andlaidthefoundationforsubsequentinvestigationintothephys-
ersandinflowsofpristinegasfromtheintergalacticmediumcould ical origin of the relation. Various physical processes including
also dilute the metallicity of a galaxy (e.g. Kewleyetal. 2006a; metalenrichedoutflows(e.g.,Larson1974;Tremontietal.2004),
2 Ho et al.
accretionofmetal-freegas(e.g.Dalcantonetal.2004),andvaria- B-band luminosity or higher total mass have shallower metallic-
tionintheinitialmassfunction(Ko¨ppenetal.2007)orstarforma- ity gradients (Vila-Costas&Edmunds 1992; Garnettetal. 1997);
tionefficiency(e.g.,Brooksetal.2007;Caluraetal.2009)haveall however, such behaviour is not pronounced in some measure-
been proposed tobe responsible for shaping themass-metallicity ments(e.g.,vanZeeetal.1998;Prantzos&Boissier2000).Some
relation.Themass-metallicityrelationmayalsohaveanadditional studies find that non-barred galaxies show a statistically signif-
dependency on star formation rate (SFR; e.g., Mannuccietal. icant correlation between metallicity gradient and Hubble Type,
2010;Lara-Lo´pezetal.2010;Yatesetal.2012).Spatiallyresolved whereearliertypeshaveshallowermetallicitygradientsthanlater
studies have shown that the mass-metallicity relation also holds types(Vila-Costas&Edmunds1992;Oey&Kennicutt1993),but
onsmallerscalesforindividualstar-formingregionswithingalax- considerable scatter exists in other measurements (Zaritskyetal.
ies (Rosales-Ortegaetal. 2012). Recent work suggests that the 1994).Moststudiesfindnocorrelationwhenmetallicitygradients
mass-metallicity relation could be a direct result of some more arenormalisedtosomescale-length(i.e.,R25,thediskscale-length
fundamental relationsbetweenmetallicity,stellarandgascontent R ,ortheeffectiveradiusR 2).Thecontradictoryresultsofsome
d e
(Zahidetal.2014c;Ascasibaretal.2014). earlierstudies might be due tothesmall samplesizes andincon-
The spatial distribution of metals in a disk galaxy can also sistentmethodologiesofmeasuringmetallicitygradients.Applying
providecriticalinsightintoitsmassassemblyhistory.Diskgalax- differentmetallicitydiagnosticscanintroduceconsiderablesystem-
ies in the local Universe universally exhibit negative metallic- aticerrors(Kewley&Ellison2008).
ity gradients, i.e., the centre of a galaxy has a higher metallicity Advances in instrumentation such as multi-slit spectroscopy
thantheoutskirts(e.g.,Zaritskyetal.1994;Moustakasetal.2010; and wide-field integral field spectroscopy (IFS) is in the pro-
Rupkeetal.2010b;Sa´nchezetal.2014,andreferencestherein).In cess of revolutionising statistical studies of metallicity gradients
severalcases,wheremeasurementsarepossibleouttoverylarger in the local Universe (e.g., Sa´nchezetal. 2014). Large on-going
radii(2×R251),themetallicitygradientsflattenintheouterdisks, and future large IFS surveys include the Calar Alto Legacy In-
suggestinginner-to-outertransportationofmetalsviamechanisms tegral Field Area Survey (CALIFA; Sa´nchezetal. 2012a), the
such as galactic fountains (e.g., Werketal. 2011; Bresolinetal. Sydney-AAOMulti-objectIntegralfieldspectrograph(SAMI)Sur-
2012; Kudritzkietal. 2014; Sa´nchezetal. 2014). Extreme exam- vey(Croometal.2012;Bryantetal.2014;Allenetal.2014),the
plesofmetalmixingoccursininteractinggalaxies,wherethenon MappingNearbyGalaxiesatApachePointObservatory(MaNGA)
axis-symmetric potential induces radial inflows of gas. Both ob- Survey,theHectorSurvey(Lawrenceetal.2012;Bland-Hawthorn
servations and simulations confirm that mergers of disk galaxies 2014),andmanyothers.TheseIFSsurveysarenotonlyextremely
presentshallowermetallicitygradientsthanisolateddiskgalaxies efficientincollectinglargenumbersofspectrasimultaneouslyand
duetoeffectivegasmixing(e.g.,Kewleyetal.2010;Rupkeetal. seamlessly across anentire galaxy, but alsohave desirable wave-
2008,2010a,b;Torreyetal.2012;Richetal.2012). lengthcoveragetocapturemultiplekeyemissionlinesforderiving
Sophisticated modelling of the evolution of metallicity gra- metallicity. Such features pose a unique opportunity to eliminate
dients in disk galaxies has shed light on the formation, gas ac- systematicerrorsusingstatisticalapproaches.
cretion, and star formation history of the disks. Whilethedetails Inthispaper,westudythemetallicitygradientsinasampleof
varyfrommodeltomodel,typicalassumptionsofinside-outdisk 49localfieldstar-forminggalaxies.Wederivemetallicitygradients
growth,noradialmatterexchange,andclosed-boxchemicalevolu- using different abundance calibrations and discuss potential sys-
tioncansuccessfullyreproducethecurrentgradientsinlocalgalax- tematiceffectsinducedbythecalibrationsadopted.Weinvestigate
ies (e.g., Chiappinietal. 2001; Fuetal. 2009). However, some whether metallicitygradients infieldstar-forminggalaxiescorre-
models predict that metallicity gradients steepen with time (e.g., latewiththeirphysicalproperties.Weshowthatthereisacommon
Chiappinietal.1997,2001;seealsoMottetal.2013whoincluded metallicitygradientinlocalfieldstar-forminggalaxiesandwepro-
radialinflow), whileotherspredict theopposite(e.g.,Mollaetal. vide some benchmark values. Finally, we adopt simple chemical
1997; Prantzos&Boissier 2000; Fuetal. 2009; Pilkingtonetal. evolutionmodelstoexplaintheformationofthecommonmetallic-
2012). Testing the model predictions using observations of high itygradient.
redshiftgalaxiesarechallenging(e.g.,Yuanetal.2011;Jonesetal. Thepaperisstructuredasfollows.Wedescribeoursamples,
2010,2013, andreferencestherein).Inaddition,systematiceffects observationsanddatareductioninSection2,andourdataanalysis
frominsufficientresolutionand/orbinningofthedataunfortunately inSection3.InSection4,wedetailourmethodology ofderiving
can seriously the reliability of metallicity gradients measured at themetallicity, ionisation parameter, and metallicitygradients. In
highredshifts(Yuanetal.2013;Mastetal.2014). Section 5, we present our measurements of metallicity gradients,
Statistical studies of metallicity gradients in the local Uni- discuss thesystematiceffects, and compare themetallicitygradi-
verse provide an alternative approach to constrain the theoreti- entswithstellarmassesandabsoluteB-bandmagnitudes.Wepro-
calsimulations. Althoughthemeasurements aretime-consuming, videabenchmarkmetallicitygradientinSection6andinvestigate
sample sizes of few tens of galaxies have been achieved in the theoriginofthebenchmarkgradientinSection7usingthesimple
past using long-slit spectroscopy. These samples gave intriguing chemicalevolutionmodels.Finally,asummaryandconclusionsare
(and sometimes contradictory) correlations between metallicity giveninSection8.Throughoutthispaper,weassumethestandard
gradients and physical properties of the disk galaxies. For exam- Λ cold dark matter cosmology with H0 = 70 km s−1 Mpc−1,
ple, barred galaxies tend to exhibit shallower metallicity gradi- ΩM =0.3andΩΛ =0.7.
ents than non-barred galaxies even when galaxy sizes are taken
into account (e.g., Vila-Costas&Edmunds 1992; Zaritskyetal.
1994),butsuchdiscrepancyisinsignificantinsomerecentstudies
(Sa´nchezetal.2014).Forunbarredgalaxies,galaxieswithhigher
2 Theeffectiveradiusistheradiusatwhichtheintegratedfluxishalfofthe
totalone.Comparingtothediskscale-lengthfortheclassicalexponential
1 Radiusofthe25thmagnitude/arcsec2 isophoteinB-band. profile,Re=1.67835Rd.
Metallicitygradientsin localfieldstar-forminggalaxies 3
2 SAMPLES massderivedbytheMPA/JHUgroupasareferencefortargetse-
lection(Kauffmannetal. 2003a; Salimetal.2007);thefinal stel-
The 49 galaxies studied in this work are drawn from various
lar masses adopted in this work are derived separately and con-
sources,includingliteraturedata,publicdataandourtargetedob-
sistently for all our samples. As a result of this selection, one of
servations. Wedescribe thefour samplesinthenext foursubsec-
theWiFeSgalaxiespresented hasasubstantially largerfinal stel-
tions. The focus of this paper is to investigate field star-forming
lar mass (log(M∗/M⊙) ∼ 10.2) due to the incorrect apertures
galaxies,andthereforeweselectonlyfieldgalaxiesthatarenotun-
adopted by MPA/JHU. We remove galaxies with AGN from our
dergoingmajormergersandnotinclosepairs;noneofourgalaxies
mothersampleusingtheopticallineratios[NII]/Hαand[OIII]/Hβ
havemassivecompanions(i.e.stellarmasshigherthanone-thirdof
(Kewleyetal. 2006b). We further constrained the mother sample
themaingalaxies)within70kpcinprojectionand1000kms−1in
tohavelowinclinationdisksandspatialextentcomparabletothe
line-of-sightvelocityseparation.
WideFieldSpectrograph(seeSection2.2.1).Wealsovisuallycon-
firmedthatthesegalaxiesarenotundergoingmajormergeranddo
not have massive companions. From the mother sample, we then
2.1 CALIFAgalaxiesinDataRelease1
selected our final targets based on observability and instrumental
We use the publicly available IFS data from the first data re- sensitivity.Intotal,weobserved19galaxies,10ofwhichyieldre-
lease(DR1) oftheCALIFAsurvey. Afulldescriptionofthesur- liablemetallicitygradientsandarepresentedinthispaper.
veydesign,includingdetailsoftargetselectionanddatareduction
scheme,canbefoundinSa´nchezetal.(2012a).Specificdetailsre- 2.0
latedtotheDR1canbefoundinHusemannetal.(2013,seealso
Walcheretal.2014).Belowwebrieflysummarisetheinformation
relevanttothiswork.
1.5
The CALIFA DR1 contains reduced IFS data of 100 local
galaxies(0.005<z<0.03)coveredbytheSloanDigitalSkySur-
vey(SDSS;Yorketal.2000;Abazajianetal.2009).Inthisstudy,
x
we focus only on spiral galaxies that are not undergoing a major de 1.0
n
merger,andnotinclosepairs.Extremelyedge-onsystemswithin- i
2
clinationanglelargerthan70◦areexcludedfromouranalysissince 3N
O
de-projectingradialdistanceisuncertain.Systemswithoutenough
sufficiently high signal-to-noise spaxels (S/N > 3) to measure SS 0.5
D
emissionlinefluxesarealsoexcluded.Intotal,21CALIFAgalax- S
iesareanalysed.
The released CALIFA datacubes, as processed by the CAL- 0.0
IFA automatic data reduction pipeline, have a spatial size of ∼
y =x
74′′ × 64′′ on a rectangular 1′′ grid. The point spread func-
y =x ±0.1
tion, as measured from field stars in the datacubes, has a me-
dianfull-widthmeasuredathalf-maximum(FWHM)of3.7′′.Ev- −0.5
−0.5 0.0 0.5 1.0 1.5 2.0
ery CALIFA galaxy is observed using the fibre bundle integral
WiFeScentral3”O3N2index
field unit (IFU) PPak, and two different setups with the Pots-
damMulti-ApertureSpectrophotometer(PMAS;Rothetal.2005;
Kelzetal. 2006). The low-resolution (V500) and high-resolution
Figure1.ComparisonoftheobservedO3N2index(Equation1)between
(V1200) setup cover wavelength ranges of ∼ 3745–7500 A˚ and
thosefromtheWiFeSandtheSDSSdata.Eachdotcorrespondstooneof
∼3650–4840A˚,respectively.TheV500reduceddatacubeshavea the10WiFeSgalaxiesthatwerealsoobservedbytheSDSSspectroscopic
FWHMspectralresolutionof6.0A˚ (R∼850)andaspectralchan- survey. Toderive the O3N2 index from the WiFeS data, we extract line
nel widthof 2.0A˚. TheV1200 reduced datacubes haveaFWHM fluxesin3′′aperturesatthelocationsoftheSDSSfibres.TheO3N2index
spectral resolution of 2.3A˚ (R ∼ 1650) and a spectral channel derivedfromthetwodatasetsareconsistentwithinapproximately0.1dex.
widthof0.7A˚.
2.2.1 Observationanddatareduction
2.2 WiFeSgalaxies
Weobserved our low-mass galaxies using the WiFeSon the 2.3-
TheCALIFAsamplewasselectedbasedontheangulariso-photal mtelescopeatSidingSpringObservatory inDecember 2012and
diameter of the galaxies (45′′ < D25 < 80′′). Therefore, the April 2013. WiFeS is a dual beam, image-slicing IFU consisting
CALIFA sample is inevitably biased towards galaxies of higher of 25 slitlets. Each slitlet is 38′′ long and 1′′ wide, yielding a
mass (& 109 M⊙). To probe metallicity gradients in galaxies of 25′′×38′′ fieldofview.Forathoroughdescriptionoftheinstru-
lowermassinastatisticallysignificantway,weconductedsupple- ment,seeDopitaetal.(2007)andDopitaetal.(2010).Eachgalaxy
mental IFS observations to specifically target low mass systems was observed with the blue and red arms simultaneously using
(i.e., log(M∗/M⊙) = 8–9). We first selected a mother sample theB3000andR7000gratings,respectively.Allgalaxieswereob-
oflow-massgalaxiesfromtheSDSSDataRelease7value-added servedwithasingleWiFeSpointingexceptforJ031752.75-071804
catalogconstructedbytheMPA/JHUgroup3.Weusedthestellar whereweadopted atwo-point mosaic.Thetypicalexposuretime
is∼1−2hourspergalaxyunderseeingconditionsof1.5−2.5′′.
We reduce the data using the custom-built data reduction
3 http://www.mpa-garching.mpg.de/SDSS/DR7/ pipeline PYWIFES (Childressetal. 2014). The final reduced data
4 Ho et al.
consistoftwodatacubeson1′′×1′′ spatialgridsforeachgalaxy. spaxelofCALIFAandWiFeSgalaxiesbyspectralfitting.Weuse
Thebluecubecovers∼3500–5700A˚ withaFWHMvelocityreso- an earlier version of the spectral fitting tool LZIFU described in
lutionof∼100kms−1atHβ(∼1.7A˚ orR∼3000)andaspec- Hoet al. (inpreparation; see alsoHoetal. 2014). Thefittingap-
tralchannelwidthof∼0.8A˚.Theredcubecovers∼5500–7000A˚ proachesforthetwosamplesareverysimilar,thoughsomedetails
withaFWHMvelocityresolutionof∼40kms−1atHα(∼0.9A˚ arefinetunedtoaccommodatethedifferencesinspectralcoverage
orR∼7000)andaspectralchannelwidthof∼0.4A˚. andresolutionbetweenthetwodatasets.Below,wefirstelaborate
TocompareourreducedWiFeSdatacubeswiththeSDSSfibre ourmethodfortheCALIFAgalaxiesbeforedescribingthedifferent
spectra,wepresentinFigure1theemissionlineratios,theO3N2 treatmentsfortheWiFeSgalaxies.
indexes(seebelow;Equation1),derivedfromthetwodatasets.For Priorto fittingtheCALIFAgalaxies, wefirst correct for the
theSDSSdata,weadoptthelinefluxesfromtheMPA/JHUvalue- known spatial misalignment between the V500 and V1200 dat-
addedcatalog;fortheWiFeSdata,weextractlinefluxeswithinthe acubes (Husemannetal. 2013). For each CALIFAgalaxy, were-
correspondingfibreaperturesfromtheemissionlinemaps(seebe- alignthetwocubes bycorrelatingcontinuum imagesconstructed
low).Figure1demonstratesthattheO3N2indexesfromtheWiFeS separately from the two cubes using a wavelength range covered
andSDSSdataareconsistentwithinapproximately0.1dex. by both settings(4240–4620A˚). Typical misalignments are ∼ 1′′
to2′′ whileinseveralextremecases∼ 3′′ to5′′.Wethenrescale
theV1200datatomatchtheV500datausingscalefactorsdeter-
2.3 GalaxiesfromSa´nchezetal.(2012b) minedfromthemedianfluxdensityratiosbetweenanoverlapped
spectralcoverageof4000A˚ and4500A˚. Slightdifferencesinflux
Tofurtherincreaseoursamplesize,weanalyse9fieldgalaxiespre-
levelsmaybearesultofimperfectcalibrationandanyresidualsys-
sentedinSa´nchezetal.(2012b,hereafterS12)withlowinclination
angles(<50◦). tematicerrors.
Wethenperformsimplestellarpopulation(SSP)synthesisto
S12 studied ∼ 2600 HII regions in 38 nearby galaxies se-
remove the underlying stellar continuum before fitting emission
lected from the PINGS survey (Rosales-Ortegaetal. 2010) and
lines.Tomodelthestellarcontinuum,weadoptthepenalisedpixel-
Ma´rmol-Queralto´ etal.(2011).All38galaxieswereobservedwith
PPak and PMAS, which deliver spectral coverages similar to the fitting routine (PPXF; Cappellari&Emsellem 2004) and employ
CALIFAdataof∼ 3700−6900A˚.S12appliedasemi-automatic theempiricalMIUSCAT4SSPmodelsof13ages5and4metallic-
ities6(Vazdekisetal.2010)whileassumingaSalpeterinitialmass
procedure, HIIEXPLORER,to search for HII regions in IFU data
function(IMF;Salpeter1955).Badpixels,skylinesandthevicin-
under the assumptions that HII regions are peaky and isolated
ity(±15A˚) of emission lines are masked prior to fittingthecon-
line-emitting structures with typical physical sizes of few hun-
tinuummodels.Aftersubtractingthecontinuum modelsfromthe
dred parsecs (more details in S12). A very similar spectral fit-
data,wefurtherremovelowfrequencyfluctuationsintheresiduals
tingapproach(toSection3.1)wasappliedonsyntheticspectraof
byfittingfourthorderb-splinemodels.Wenotethatthemajorgoal
HII regions to decouple the underlying stellar contribution from
ofperformingSSPsynthesisfittingistocorrectforstellarBalmer
lineemissions.Thefinalpublicfluxcatalogscontainsevenstrong
absorption; wedo not derive stellar age and metallicity from our
lines, [OII] λλ3726,3729, Hβ, [OIII] λ5007, [OI] λ6300, Hα,
SSPfits.
[NII]λ6583,and[SII]λλ6716,6731.
Subsequent to removing the continuum in the V500
All the 9 galaxies analysed in this study have multiple
and V1200 datacubes separately, we model the strong emis-
bright HII regions measured in all the strong lines including
[OII] λλ3726,3729, which allows reliable constraints simultane- sion lines ([OII] λλ3726,3729, Hβ, [OIII] λλ4959,5007,
ouslyonmetallicityandionisationparameterswithdifferentdiag- [NII]λλ6548,6583,Hα,and[SII]λλ6716,6731)assimplegaus-
sians.Weperformaboundedvaluenonlinearleast-squaresfitusing
nostics.
theLevenberg-MarquardtmethodimplementedinIDL(Markwardt
2009,MPFIT).Weconstrain(1)alllinestohavethesamevelocity
2.4 GalaxiesfromRupkeetal.(2010b) and velocity dispersion, (2) the ratios [NII] λ6583/[NII] λ6548
and[OIII]λ5007/[OIII]λ4959totheirtheoreticalvaluesgivenby
Rupkeetal.(2010b,hereafterR10)measuredmetallicitygradients quantummechanics,(3)thevelocitytobebetween+600 kms−1
ininteractingandnon-interactinggalaxies.Theyshowthat,onav- and −600 km s−1 to the systemic velocities as measured from
erage, interacting systems present shallower metallicity gradients
SDSS,and(4)thevelocitydispersiontobebetween50and1000
than non-interacting systems. Their control sample comprises 11 kms−1.
non-interacting local galaxies and they measure their metallicity
Figure 2 shows a typical spectrum and spectral fit. Thisfig-
gradientsusingpublishedemissionlinedatafromHIIregions.Two urebestillustratestheaboveprocedureofdecouplingstellarcon-
of these control sample galaxies overlaps with the S12 sample.
tribution in each spaxel from emission lines originated predomi-
We include their measurements for the remaining 9 galaxies in
nantlyfromHII regions.Figure3showstwoemissionlinemaps,
our analysis. Their methods of correcting for extinction and de-
SDSScompositeimage,extinctionmap,velocityfieldmap,anda
riving metallicityareexactly the same as our work and therefore
keydiagnosticsline-ratiomapoftheCALIFAgalaxyNGC7321to
we include their measurements of metallicitygradients without a
demonstratethefinalproductsfromouranalysisdescribedabove.
re-analysisofthedata.
ForfittingtheWiFeSgalaxies,theaboveprocedureisadopted
withsomeminormodificationstoaccommodatethedifferentspec-
3 ANALYSIS
3.1 Emissionlinemaps 4 http://miles.iac.es/pages/ssp-models.php
5 0.06,0.10,0.16,0.25,0.40,0.63,1.00,1.58,2.51,3.98,6.31,10.00,and
Toplaceconstraintsonmetallicityandionisationparameterusing 15.85Gyr
emissionlinediagnostics,wemeasureemissionlinefluxesineach 6 [M/H]=-0.71,-0.40,0.00,and0.22
Metallicitygradientsin localfieldstar-forminggalaxies 5
0.5
−1Å) 0.4
−2m
−1s c 0.3
g
er
−160 0.2 V500
x (1 VV510200 0continuum model
Flu 0.1 V1200 continuum model
0.0
4000 4500 5000 5500 6000 6500 7000
[OII] (V1200) Hbeta + [OIII] (V500) Halpha + [NII] (V500) [SII] (V500)
0.3 0.20 1.0 0.15
−2−1mÅ) 0.2 0.15 0.8 0.10
c 0.6
−1s 0.10
g 0.1 0.4 0.05
er
−160 0.05 0.2
ux (1 0.0 0.00 0.0 0.00
Fl
−0.1 −0.05 −0.2 −0.05
0.10 0.03 0.015 0.010
0.05 0.02 0.010 0.005
0.01 0.005
0.00 0.000 0.000
0.00 −0.005
−0.05 −0.01 −0.010 −0.005
−0.10 −0.02 −0.015 −0.010
3740 3760 3780 4900 5000 5100 6600 6640 6680 6780 6810
Wavelength Å
Figure2.AnexampleofthespectralfittingapproachappliedontheCALIFAdata(seeSection3.1fordetails).Thegreyandblackthicklinesintheupper
panelindicatethewavelengthrangeswherethedata(black:V500data;grey:V1200data)areadoptedtoconstrainthecontinuummodels(red:V500;pink:
V1200).Badchannels,thevicinityofstrongemissionlinesandskylinesareexcludedfromthefit.Thefourmiddlepanelsshowemissionlines(red)fittothe
continuumsubtractedspectra(black).Alllinesarefitsimultaneouslyandsharethesamevelocityandvelocitydispersion.Thebottomfourpanelsshowthe
residuals,andthebluedashedlinesindicatethe±1σnoiselevels.
tral coverages and resolutions. No correction for misalignment is a given measurement (i.e., a spaxel for the CALIFA and WiFeS
requiredfortheWiFeSdatacubessincetheblueandreddatawere samples or an HII region for the S12 samples), we assume the
observedsimultaneously.SSPsynthesisfittingisperformedsimul- classical extinction law by Cardellietal. (1989) with Rv=3.1
taneouslyonbothcubestotakefulladvantageofthe4000A˚ break, and Hα/Hβ = 2.86 under the case-B recombination of Te =
animportantageindicator,capturedonlyinthebluedata.Fitting 10,000Kandne=100cm−3(Osterbrock&Ferland2006).This
the red side separately would in principle increase the degenera- prescriptionisconsistentwiththatadoptedinR10.
cies in SSPsynthesis fitting. We first down-grade the red data to
the same spectral resolution as the blue data (R ∼ 3000), mask
3.3 Otherphysicalquantities
out noisy parts of the spectra due to poor CCD sensitivities, and
mergethetwodatacubestoformamasterdatacubewhichcovers 3.3.1 Stellarmass(M∗)
∼ 3700–6950A˚. We then use PPXF and theoretical SSP models,
assumingPadovaisochrones,of18ages7and3metallicities8from Weuse the LE PHARE9 code developed by Arnouts, S.& Ilbert,
Gonza´lezDelgadoetal.(2005)todeterminethecontributionfrom O.toestimatethegalacticstellarmass.LEPHAREcomparespho-
differentstellarpopulationsandthedegreeofdustextinction.The tometry measurements with stellar population synthesis models,
resultsarethenusedtoreconstructcontinuaoftheblueandreddata based on a χ2 template-fitting procedure, to determine mass-to-
light ratios, which are then used to estimate the stellar mass of
attheirnativespectralresolution.Thesamelinefittingapproachis
thenappliedtothecontinuumsubtractedcubesandyieldsemission galaxies. The stellar templates of Bruzual&Charlot (2003) and
linemaps. a Chabrier IMF (Chabrier 2003) are used to synthesise magni-
tudes. The 27 models span three metallicities and seven expo-
nentially decreasing star formation models (SFR ∝ e−t/τ) with
3.2 Extinctioncorrection τ =0.1,0.3,1,2,3,5,10,15and30Gyr.Weapplythedustatten-
uationlawfromCalzettietal.(2000)allowingE(B−V)tovary
We use a consistent method for all the galaxies to correct the from0to0.6andstellarpopulationagesrangingfrom0to13Gyr.
wavelength-dependent extinction caused by dust attenuation. For Photometric measurements are collected from various
sources.Forallthe21CALIFAgalaxies,10WiFeSgalaxies,and
5/9S12galaxies,weadoptvaluesfromtheSDSSDR7photometry
7 0.004, 0.006, 0.008, 0.011, 0.016, 0.022, 0.032, 0.045, 0.063, 0.089,
0.126,0.178,0.251,0.355,0.501,0.708,1.000,and1.413Gyr.
8 Z=0.004,0.008,and0.019. 9 http://www.cfht.hawaii.edu/∼arnouts/LEPHARE/lephare.html
6 Ho et al.
NGC7321 Halpha [OII]3726,3729 SDSS composite

60
ec]
s
c
et [ar 40
s
off
c.
e
D 20

0
0 20 40 60
R.A. offset [arcsec]
E(B−V) Velocity field O3N2

−0.2 0.0 0.2 0.4 0.6 7000 7125 7250 7375 −0.4 0.0 0.4 0.8
[km/s]
Figure3.Anexampleof2DmapsfromourspectralanalysisdescribedinSection3.1.ThefirstrowshowstheHαmap,[OII]λλ3726,3729map,andSDSS
3-colourimageofNGC7321,oneoftheCALIFAgalaxies.ThesecondrowshowstheE(B-V),velocityfield,andO3N2maps.Thebrightforegroundstarin
theSDSSimageismaskedoutinalltheothermaps.
catalog(Abazajianetal.2009)and2MASSextendedsourcecata- 3.3.2 Inclinationangle
log(Skrutskieetal.2006).ThePetrosianmagnitudesofSDSSu,g,
InclinationanglesofCALIFAgalaxiesareestimatedusingthecon-
r,i,andz-bandarecorrectedfortheforegroundGalactic-extinction
versionprovided byPadilla&Strauss(2008). Theeffectsof dust
(Schlegeletal.1998).The2MASSJ,H,andK magnitudesmea-
s extinctionandreddeningweretakenintoaccountintheiranalysis.
sured from fit extrapolation are adopted to approximate the total
Anestimateofinclinationangleforspiralgalaxyisinferredfrom
magnitudes.Allthesegalaxieshavethe5-bandSDSSphotometry
the measured axis ratio (b/a) and r-band absolute magnitudes.
andthemajority(28/36)alsohavethe3-band2MASSphotometry.
AxisratiosestimatedbytheCALIFAteamfromSDSSr-bandim-
FortherestofthefourS12galaxiesandthenineR10galaxies,we
agesareadopted. InclinationanglesofWiFeSgalaxiesaredrawn
collectavailableU,B,V,andthe3-band2MASSphotometricmea-
fromHyperleda(Patureletal.2003),whichassumestheclassical
surements from NASA/IPAC Extragalactic Database (NED). All
Hubbleformula(Hubble1926).InclinationanglesofS12galaxies
thesegalaxieshavethe3-band2MASSphotometry.TheBandV-
aretakendirectlyfromtable1inS12,whichalsoreferstovalues
bandphotometryareavailableforallgalaxies,andtheU-bandpho-
from Hyperleda. Inclination angles of R10 galaxies are taken di-
tometryisavailableforabouthalfofthesegalaxies(6/13).These
rectly from table 2 in R10, which is a compilation from various
optical photometric measurements are also corrected foreground
references.
Galactic-extinction.Whenreasonableuncertaintiesofthemeasure-
mentsareunavailable,weassume0.1dexfortheopticalbandsand
0.05dexfortheinfraredbands.
3.3.3 Sizeanddistance
We note that the uncertainties in stellar mass are typically WecompilethesizesanddistancesofoursamplesfromNEDand
dominated by systematic errors. Different stellar mass estimators Hyperleda.R25fromHyperledaisadoptedthroughoutthepaperto
employdifferentalgorithmsandstellarlibraries,buttheestimated quantifysizesof the galaxies.Redshift independent distances are
M∗ typically agrees within ∼ 0.3 dex, marking the degree of availableforallR10galaxiesinTable2ofR10.Redshiftindepen-
systematic errors in the measurements (e.g., Droryetal. 2004; dentdistancesformostS12galaxies(7/10)areadoptedfromNED.
Conroyetal.2009). Theseredshiftindependentdistances(d .30Mpc)aremeasured
Metallicitygradientsin localfieldstar-forminggalaxies 7
from theTully-Fisher relation, tipof thered-giant brach method, isanalternativemetallicitydiagnosticthathasbeencalibratedby
planetary nebulae, type-II supernovae, or Cepheid variables. For Kewley&Dopita (2002, hereafter KD02) using theoretical pho-
CALIFA,WiFeS,andtherestofthethreeS12galaxies,weadopt toionisation models. We adopt the parametrisation for q = 2×
Hubbledistancesinferredfromtheirredshifts. 107cms−1 inKD02toderivemetallicity.TheN2O2indexisin-
sensitivetovariationofionisationparameterbyvirtueofthesim-
ilarionisingpotential of N+ andO+.Despitetheinsensitivityto
ionisationparameter, N2O2isnot oftenused inlocal studiespri-
4 DERIVATIONOFMETALLICITIESAND
marily because some spectrographs are not sensitive enough at
METALLICITYGRADIENTS ∼ 3700A˚ to observe [OII] λλ3726,3729. The N2O2 calibration
In this section, we describe our methodology for using emission depends more on the assumed extinction law and extinction esti-
lineratiostoderivemetallicities,metallicitygradientsandionisa- matethanO3N2duetothelargerwavelengthseparationbetween
tionparameters. [NII]λ6583and[OII]λλ3726,3729.Wenotethatthevariationin
theN/OratiowithO/Hcould be thelargest uncertainty affecting
these strong line diagnostics (Henryetal. 2000). The strength of
4.1 Metallicity the[OII]λλ3726,3729linesarestronglyaffectedbytheelectron
temperature,governedpredominatelybytheO/Hratioandionisa-
The most direct way to determine an ISM metallicity is by first tionparameter,while[OIII]λ5007ismostlysensitivetotheioni-
measuring electron temperatures (Te) with temperature sensitive sationparameter.
line ratios, e.g., [OIII] λ4363 to [OIII] λ5007, and then convert A long standing problem in chemical studies has been that
emissionmeasurestometallicityaftercorrectingforunseenstages different metallicity calibrations do not return consistent metal-
of ionisation. Since[OIII]λ4363 istypicallyunavailable or only licity measurements. Kewley&Ellison (2008) applied 10 differ-
detected in very limited (i.e. the hottest) regions in IFU surveys, ent metallicity calibrations on the same SDSS dataset and found
measuringmetallicityusuallyreliesonempiricalortheoreticalcal- thatdifferentcalibrationsyielddifferentmass-metallicityrelations.
ibrations(oracombinationofboth)basedonstrongemissionlines, Boththeslopesandtheinterceptsofthemass-metallicityrelation
suchasthoseavailableinoursamples.Varioussuchcalibrationsare aresignificantlydifferentfromcalibrationtocalibration.Byallow-
availableintheliteraturesandarewidelyadoptedtoderivemetal- ing the mass-metallicity relations derived with different calibra-
licity(e.g.,Kewley&Dopita2002;Pettini&Pagel2004). tionstobeconvertedtothesamebases,Kewley&Ellison(2008)
Wederivemetallicitieswithtwodifferentcalibrationselabo- derived empirical conversions between different calibrations. In
ratedbelow.Throughoutthepaper,weexpressthemetallicityisin this paper, metallicitiesderived using the O3N2 method are sub-
termsofthenumberratioas12+log(O/H). sequently converted to the KD02 scale using the conversion by
Kewley&Ellison(2008).
Wenotethat,whenderivingmetallicitiesusingacertaindiag-
4.1.1 O3N2index/PP04 nostic,weonlyusedatawithS/N>3onallthelinesassociatedto
thatparticulardiagnostic.Thesamerulealsoappliestothederiva-
TheO3N2index,definedas
tionoftheionisationparameter(seeSection4.3).
[OIII]λ5007/Hβ
O3N2≡log , (1)
[NII]λ6583/Hα
4.2 Theimportanceofnon-thermalexcitationanddiffuse
is a widely used metallicity diagnostic in the literature. An em- ionisedgas
pirical calibration isprovided by Pettini&Pagel (2004, hereafter
PP04). The popularity of this diagnostics arises for two reasons. It is important to understand that all the above metallicity diag-
Firstly, the four lines involved are usually easily measured in lo- nostics are calibrated empirically or theoretically using HII re-
calgalaxiesouttolargeradiiusingmoderninstruments.Secondly, gions. This means, by applying the diagnostics, one implicitly
becauseoftheminimumwavelengthdifferencesbetweenthetwo assumes that that all the nebular emission originates from pho-
pairs of lines, the O3N2 index is virtually free from systematics toionisation and heating caused by the extreme ultraviolet pho-
causedbytheassumptionofanextinctionlaworreddeninguncer- tons emitted by O and B-type stars. If other ionisation sources
tainties. Nevertheless, the calibrationprovided by Pettini&Pagel are present, metallicity measurements would be contaminated.
(2004)iscalibratedempiricallywithalinearfittometallicitiesof Other ionisation sources such as active galactic nuclei (AGNs)
137extragalacticHIIregions(131withTe-basedmetallicityand6 can have localised or even global effects on line ratios. Interstel-
withdetailed photoionisation models). Variationof ionisationpa- lar shocks originating from AGN outflows, supernovae, or stel-
rameter,q,isnotconsideredinthecalibration.Neglectofthispa- lar winds are also sources of non-thermal radiation and can af-
rametermaycauseserioussystematicerrorsintheresults.Anup- fect emission line ratios. To remove measurements affected by
datedcalibrationbasedonmanymoreTe-basedmetallicitiesofHII non-thermal radiation from our subsequent analyses, we use line
regions (309) is provided by Marinoetal. (2013). Thisnew cali- ratio diagnostics commonly adopted to distinguish normal from
brationpresentsasignificantlyshallowerslopebetweentheO3N2 active galaxies (i.e., [OIII] λ5007/Hβ v.s. [NII] λ6583/Hα or
indexandmetallicitythanthePP04calibration. BPTdiagram; Baldwinetal.1981;Veilleux&Osterbrock1987).
We adopted the empirically separation line derived from SDSS
byKauffmannetal.(2003b,hereafterK03;seealsoKewleyetal.
4.1.2 N2O2index/KD02 2001 and Kewleyetal. 2006b) to exclude data contaminated by
non-thermalexcitation.
TheN2O2index,definedas Emission from the diffuse ionise gas (DIG; also known as
[NII]λ6583 the warm ionised medium) can be a non-negligible component
N2O2≡log[OII]λλ3726,3729, (2) in IFU measurements. The DIG is hot (∼ 104 K) and tenuous
8 Ho et al.
ofHIIregionsineachgalaxyasasimplefunctionof
00..00
1.4 CHII = 1− f(fH0α), (4)
1.2
00..22
1.0 wheref(Hα)istheHαflux.Combiningequation3andequation4,
a wefitsimultaneouslyZ′andf0,andshowthebestfitasthegreen
SII]/H 00..44 00..68CHII creuarsvoenianblFyigwureell,4.buWtethteyptihceaollryetfiicnadl ebqausiastioofnt4hidsefsucnricbteiosntahlefdoarmta
[
is unclear. The best fit provides a guide for imposing a criterion
00..66 0.4 on the Hα flux to reject spaxels below a characteristic covering
0.2 fractionof HII regions. Weexcludeallspaxelsbelow anHαflux
00..88 valueatwhichthecorrespondingCHII is0.8onthebestfitcurve.
110033 110044 110055 Althoughthiscriterionseemsarbitrarilystrict,changingthechar-
Ha flux (arbitrary) acteristiccoveringfractiontozerohasonlyaminorimpactonthe
metallicitygradients of individual galaxies, and none of our con-
clusionschange.
Figure4.AnExampleofdeterminingspaxelsheavilycontaminatedbythe
ThereasonthatrejectingDIGdominatedspaxelshaslimited
diffuseionisedgas.DetailsaredescribedinSection4.2.Theblackpoints
effectisthatthecoveringfractioncutdoesnotremovemanyspax-
areallstar-formingspaxelswith>3σdetectionsonthe[SII]λλ6716,6731
elsthathavenotalreadybeenrejectedbytheS/Ncutsformetal-
andHαlines.Theblueandredsquaresarethosespaxelswith> 3σde-
tections onalllines associated withtheO3N2index, i.e.,[OIII]λ5007, licitydiagnosticsandtheBPTcriterion(K03)fornon-thermalex-
[NII]λ6583, Hα, and Hβ. The green curve indicates the best fit to the citation.ThestrictestcriteriainrejectingthedataaretheS/Ncuts
blackpointsusingequations3and4,whichweadopttodeterminethecrit- ontheweak linessuch asHβ and[NII]λλ6548,6583. Sincethe
icalHαfluxabovewhichthecoveringfractionofHIIregionexceeds80%. DIGhasintrinsicallylowsurfacebrightness,withthecurrentdepth
Data below the critical Hα flux (red) are excluded from deriving O3N2 inCALIFAandWiFeSsamples,most spaxelssatisfyingmultiple
metallicity. S/N > 3 criteriain lineemissions arethose with low DIG cov-
eringfractions.WedemonstratethisinFigure4.Theblueandred
pointsarethespaxelssatisfyingtheS/N>3criteriaintheO3N2
gas(∼ 10−1 cm−3)permeatingtheinterstellarspaceandextend- diagnosticandthecriterionontheBPT(K03).Theredpointsare
thosefurtherrejectedbytheDIGcriterion.
ing more than 1 kpc above the disk plane (see Mathis 2000 and
WenotethatrejectingdatadominatedbytheDIGisonlyre-
Haffneretal.2009forreviews).TheDIGgenerallypresentsmuch
quired for spaxel-to-spaxel analysis, i.e. the CALIFA and WiFeS
lowerHαsurfacebrightnessthantypicalHIIregionsanddisplays
samples. For the S12 galaxies, the fluxes were measured on ex-
distinctly different line ratios from HII regions. The primary ex-
citationsourcesarethoughttobepredominatelyionisingphotons tractedspectraofHIIregions(i.e.,binningspaxelsinthevicinity
ofbrightandcompactHαknots).ForR10,thefluxmeasurements
escaping and traveling kilo-parcsec distances from O and B-type
stars. Measurements in an IFU spaxel can contain both emission wereperformedbyplacinglong-slitsonHIIregions.DIGcontam-
inationisexpectedtobenegligibleinthesetwocases.
fromunderlying HII regionsandfromtheDIGalong theline-of-
In principle, after the fractional contribution is determined,
sight.
oneshouldbeabletosubtractthecontributionfromtheDIGand
To quantify the fractional contributions from the DIG and
HII regions in a given spaxel, we adopt a similar approach as derive the fluxes from underlying HII regions (as in Blancetal.
Blancetal. (2009). Spaxels dominated by the DIG are subse- 2009). However, line ratiosinvolving [OII] λλ3726,3729 for the
DIGarenotwellconstrainedandcouldproducelargescatter(e.g.,
quentlyremovedfromouranalyses.Weconstrainthecontribution
fromtheDIGwiththeobservedtheobserved[SII]/Hαratio [OII] λλ3726,3729/Hα; Mierkiewiczetal. 2006). Even for the
well-measuredratios,suchas[SII]/Hαand[NII]/Hα,considerable
[HSαII] = Z′ CHII([HSαII])HII +CDIG([HSαII])DIG , (3) cscoarrtteecrtiinognsanadncdorinredluactieonpobteetnwteiaelns[ySsItIe]m/[aNtiIcI]ecrorourldsc(Momapdlsiecnateettahle.
(cid:20) (cid:21)
2006). In this work, we adopt the simplest approach of rejecting
where[SII]denotesthetotalfluxof[SII]λ6716and[SII]λ6731. inappropriate data. Comprehensive theoretical modelling and ob-
The terms CHII and CDIG represent fractions of emission lines servational constraints of DIG are crucial for extracting more in-
originatedfromHIIregionsandtheDIG,respectively.Thesumof formationfromdeeperIFUdata.
CHII andCDIG isunity.Z′ denotesmetallicityofthegalaxynor-
malisedtothatoftheMilkyWay,i.e.Z′ = Z/Z .Following
MW
Blancetal.(2009),weadopt the value of ([SII]/Hα)HII as 0.11 4.3 Ionisationparameter
and([SII]/Hα)DIG as0.34.Thesevaluesaresupportedbyobser-
vationsofHIIregionsandtheDIGinourMilkyWay(Madsenetal. Theionisationparameterisquantifiedastheionisingphoton flux
2006). throughaunitareadividedbythelocalnumberdensityofhydro-
Figure4showsatypical[SII]/HαversusHαfluxplotofthe gen atoms. The ionisation parameter can be measured by taking
CALIFA galaxy NGC6497. All the data points (spaxels) are sig- the ratio of high ionisation to low ionisation species of the same
nificantlydetected(>3σ)in[SII]andHα.SpaxelswithhighHα atom. Using the strong lines available in this study, we measure
fluxes have low (high) [SII]/Hα, consistent with the low (high) the ionisation parameter using [OIII] λ5007/[OII] λλ3726,3729
lineratiosofHIIregions(DIG).HIIregionsaregenerallylocated (hereafter, [OIII]/[OII]). A theoretical calibration was first pre-
onspiralarmsandDIGisgenerallylocatedininter-armregions. sentedinKD02andamoreuser-friendlyparametrisationisgiven
SimilartoBlancetal.(2009),wemodelthecoveringfractions in Kobulnicky&Kewley (2004, hereafter KK04). The latter is
Metallicitygradientsin localfieldstar-forminggalaxies 9
deproj. radius [kpc] deproj. radius [kpc] deproj. radius [kpc]
0 5 10 15 0 5 10 15 0 5 10 15
+log(O/H) O3N2 899...505 CALIFA:NGC4210 +log(O/H) N2O2 899...505 log(q) [cm/s] 777788......246802
2 2 7.0
1 8.0 1 8.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
deproj. radius[R ] deproj. radius [R ] deproj. radius [R
25 25 25
deproj. radius [kpc] deproj. radius [kpc] deproj. radius [kpc]
0 5 10 15 0 5 10 15 0 5 10 15
+log(O/H) O3N2 899...505 CALIFA:UGC03253 +log(O/H) N2O2 899...505 log(q) [cm/s] 777788......246802
2 2 7.0
1 8.0 1 8.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
deproj. radius[R ] deproj. radius [R ] deproj. radius [R
25 25 25
Figure5.Leftandmiddlepanels:Metallicity gradientsofindividual CALIFAgalaxies measuredusingthetwodifferentabundancediagnostics (seeSec-
tion4.1).Thestraightlinesindicatethebestfits,andthedashedlinesindicate±1σerrors.Theerrorsoftheintercepts andtheslopesareestimatedfrom
bootstrapping(seeSection4.4).Rightpanels:ionisationparameterasafunctionofradius.Theionisationparametersarederivedusingthe[OIII]/[OII]diag-
nostic(KK04;seeSection4.3).EachpointintheseplotscorrespondstooneIFUspaxelwithsignificant(>3σ)detectionsonalltheemissionlinesassociated
withthediagnostics.Spaxelscontaminatedbynon-thermalexcitationordominatedbyDIGemissionarerejected(seeSection4.2).Theverticaldashedlines
correspondtotheradialcutoffwithinwhichthedataarenotconsideredinconstrainingthediskmetallicitygradients.
deproj. radius [kpc] deproj. radius [kpc] deproj. radius [kpc]
0 5 10 15 20 0 5 10 15 20 0 5 10 15 20
+log(O/H) O3N2 899...505 S12:NGC3184 +log(O/H) N2O2 899...505 log(q) [cm/s] 777788......246802
2 2 7.0
1 8.0 1 8.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
deproj. radius[R ] deproj. radius [R ] deproj. radius [R
25 25 25
deproj. radius [kpc] deproj. radius [kpc] deproj. radius [kpc]
0 5 10 15 20 0 5 10 15 20 0 5 10 15 20
+log(O/H) O3N2 899...505 S12:UGC06410 +log(O/H) N2O2 899...505 log(q) [cm/s] 777788......246802
2 2 7.0
1 8.0 1 8.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
deproj. radius[R ] deproj. radius [R ] deproj. radius [R
25 25 25
Figure6.SameasFigure5,butfortheS12galaxies.EachpointcorrespondstooneHIIregionextracted fromtheIFUdata(seeSection2.3andS12for
details).
adoptedinthispaper.AsemphasisedinKD02,inadditiontoion- bustandmoresensitivetoboththeSSPmodelsandthealgorithms
isationparameter,the[OIII]/[OII]ratioalsostrongly depends on adopted (e.g.,CidFernandesetal.2014, and referencestherein).
metallicitythatneedstobeknownapriori.Weadoptthemetallic- TheWiFeSgalaxiesdonotsufferfromthiscontaminationbecause
ityfromtheN2O2calibrationtoderivetheionisationparameterfor theselow-masssystemsaretypicallyemissiondominatedevenat
eachspectrum. thecentres.TheS12andR10galaxiesalsodonotsufferfromthis
artefactbecause theemissionlinesaredirectlymeasuredtowards
HIIregions.
4.4 Measuringmetallicitygradients
To derive metallicity gradients, we first convert the flux ratios to To estimate the errors in the metallicity gradients, we
metallicities.Forbulge-dominated galaxiesorthosewithobvious adopt a bootstrapping approach similar to Kewleyetal. (2010),
barstructuresintheCALIFAsample,wedonotincludedatawithin Rupkeetal. (2010b) and Richetal. (2012). We randomly draw
(0.1–0.2)×R25.Atthecentresofthesegenerallyhigh-masssys- fromthemeasurementsthesamenumberofdatapointsbutwithre-
tems, the optical spectrum isdominated by the stellar continuum placement,andperformanunweightedleast-squareslinearfitwith
withstrongBalmerabsorptionoriginatingfromanoldstellarpopu- thedrawndata.Ineachgalaxy,thisprocessisrepeated1,000times
lation.CorrectingforBalmerabsorptionsintheseregionsislessro- andeachfitresultisrecorded.Themedianandstandarddeviation
10 Ho etal.
oftheslopesandinterceptsareconsideredasthebestestimatesof 5.2 Theeffectofionisationparameter
themetallicitygradient.
IntheleftpanelofFigure8,wecomparethemetallicitiesderived
usingtheO3N2andN2O2calibrations.Eachpointrepresentsone
measurement,i.e.,onespaxelfromtheCALIFAgalaxiesoroneHII
regionfromtheS12galaxies.Wecolour-codethepointsbylog(q)
derived usingthe [OIII]/[OII](KK04) calibration. Theleftpanel
of Figure8clearly demonstrates that superficially thetwometal-
5 RESULT
licitiesagreereasonablywell;57%/83%ofthemetallicitiesagree
within±0.05/0.1dex.Similarcomparisonswerealsocarriedout
5.1 Metallicitygradientsofindividualgalaxies
byRupkeetal.(2010b)wherecomparablescatteringalsoexists.
InFigure5andFigure6,wepresent4metallicitygradientsmea- In the left panel of Figure 8, the scattering around the one-
suredintwoCALIFAandtwoS12galaxies,respectively.Thetwo to-one line is not random, but correlates with ionisation param-
differentmetallicitesareshowninthefirsttwopanels,andthelast eter. Spaxels or slits with high ionisation parameter have higher
panelsshowradialprofilesoftheionisationparameter.Inthefirst N2O2 metallicitiesthan O3N2 metallicities.Those withlow ion-
twopanels,straightlinesindicatethebestfitsofthegradients,and isationparameterhavelowerN2O2metallicitythanO3N2metal-
dashed lines indicate 1σ errors as propagated using analytic ex- licities.IntherightpanelofFigure8,weshowthedifferencebe-
pressions with bootstrapped errors. The rest of the CALIFA and tween metallicities derived with the O3N2 and N2O2 ratios ver-
S12galaxiesarepresentedinFiguresA1andA2inAppendixA. sus ionisation parameter. The differences between the two diag-
The metallicity gradients and the radial profiles of the ioni- nostics can be up to 0.2 – 0.4 dex at extreme values of ionisa-
sation parameter of our 10 WiFeS galaxies are also presented in tion parameter (log(q) < 7.0 cm s−1 or log(q) > 8.2 cm s−1
FigureA3.ThemetallicityofthemajorityofWiFeSgalaxieshave ).Atlog(q) & (.)7.3cms−1,theO3N2diagnosticgiveshigher
metallicities12+log(O/H)<8.4(inKD02scale),wherethecon- (lower) metallicity than the N2O2 diagnostic. Clearly, ionisation
versionfromO3N2toN2O2isdifficulttodetermine,andtherefore parameteristhecauseofthediscrepancy. Asweemphasisedear-
weareonlyabletoderivetheirN2O2metallicitygradients.These lier,O3N2iscalibratedempiricallywithouttakenintoaccountthe
galaxiesareexcludedfromtherestofthecomparisonsbetweentwo changeofionisationparameter.AtheoreticalcalibrationofO3N2
metallicitycalibrations,butweincludethemlaterwhilecomparing will be presented in Kewley et al. (in preparation) and will rec-
metallicitygradientsofdifferentgalaxies(Section5.4). oncile this discrepancy withnew stellar population synthesis and
In Figure 7, we compare metallicity gradients derived photoionisationmodels.
from O3N2 and from N2O2 for the CALIFA and S12 galax-
ies. All the galaxies exhibit negative metallicity gradients in
both calibrations, and 33%/73% of the galaxies agrees within 5.3 Discrepanciesamongmetallicitygradients
±0.05/±0.14 dex R−1. These values can be considered as the
25 We now return to discuss the cause of the discrepancies in the
levelofresidualsystematicsinthemetallicitycalibrations.Notice-
metallicitygradientsshowninFigure7.
ablythereareseveraloutlierswellbelowtheone-to-onelinewhich
Figure 5 (and Figure 6) show metallicity gradients for two
welabelinFigure7.Theseobjectscouldprovideinsightintothe
CALIFA(twoS12galaxies)thatarelabeledasoutliersinFigure7.
cause of the disagreement between the two calibrations. We fur-
ThesegalaxieshavesteeperO3N2thanN2O2metallicitygradients
therinvestigatethecauseofthisdiscrepancyinthefollowingtwo
because,atlargeradii,O3N2metallicitiesaresystematicallylower
subsections.
thanN2O2metallicities.AsshowninFigure8,lowerO3N2than
N2O2 metallicities naturally arises when log(q) & 7.3 cm s−1.
IndeedinthethirdpanelsofFigure5andFigure6,thesegalaxies
0.2 typically have log(q) & 7.3 cm s−1 at large radii. These galax-
−1 R]25 0.0 ies all show indications of smooth rising of ionisation parameter
x fromtheircentrestooutskirts,implyingacontinuousradialchange
e
d oftheirpropertiesoftheionisingradiation.Thehigherionisation
nt [ −0.2 parameterscouldbecausedbythemoreactivestarformationac-
e
adi tivitieswithdifferent distributionsof molecular gas (Dopitaetal.
y gr −0.4 UGC06410 2014).
cit Inextremecases,thedifferencesinmetallicitygradientmea-
etalli −0.6 NGC3184 sured with N2O2 and O3N2 can be up to a ∼ 0.4 dex R2−51
2 m UGC03253 +/− 0x.0=5y (e.g., NGC3184 in Figure 6). Similar findings are also reported
N −0.8 +/− 0.14 in Rupkeetal. (2010b). These results have important implica-
3 CALIFA
O S12 tions for metallicity gradient studies at high redshift, where typ-
−1.0 NGC4210
ically only [NII] λλ6548,6583 and Hα are available (in some
−1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2
N2O2 metallicity gradient [dex R−1 ] cases also [OIII] λλ4959,5007 and Hβ, e.g., Crescietal. 2010;
25
Yuanetal. 2011; Jonesetal. 2010, 2013; Queyreletal. 2012;
Swinbanketal. 2012). While all the diagnostics using these four
Figure7.Left:Comparisonbetweenthemetallicitygradients(dexR−251) lines,i.e.[NII]λ6583/HαandO3N2,aresensitivetothechangeof
derivedusingtheO3N2andtheN2O2diagnostics (seeSection4.1).The
ionisationparameter,quantitativeinterpretationofmetallicitygra-
metallicitygradientsofthemajorityofthegalaxies(33%/73%)agreewithin
±0.05/±0.14dexR−1.Themetallicitygradientsofthefouroutliersla- dients should bear in mind the potential impact of ionisation pa-
25 rametergradientsingalaxies.
beledareshowninFigure5andFigure6.
ForgalaxiesnotlabeledinFigure7,wedonotfindobvious