Spatially-resolved NUV-r color of local star-forming galaxies and clues for quenching

5.9 MB·

by  Zhizheng Pan

DRAFTVERSIONJANUARY22,2016
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
THESPATIALLY-RESOLVEDNUV- RCOLOROFLOCALSTAR-FORMINGGALAXIESANDCLUESFOR
QUENCHING
ZHIZHENGPAN1,XIANZHONGZHENG1,WEIPENGLIN2,3,JINRONGLI4,5,JINGWANG6,LULUFAN7,XUKONG4,5
DraftversionJanuary22,2016
ABSTRACT
6 Usingasampleof∼6,000localface-onstar-forminggalaxies(SFGs),weexaminethecorrelationsbetween
1 theNUV- rcolorsbothinsideandoutsidethehalf-lightradius,stellarmassM andSérsicindexninorderto
∗
0
understandhow the quenchingof star formationis linked to galaxystructure. For these less dust-attenuated
2 galaxies,NUV- risfoundtobelinearlycorrelatedwithD 4000,supportingthatNUV- risagoodphotometric
n
n indicatorofstellarage(orspecificstarformationrate).Wefindthat:(1)AtM∗<1010.2M⊙,thecentralNUV- r
a isonaverageonly∼0.25magredderthantheouterNUV- r. Theintrinsicvaluewouldbeevensmallerafter
J accounting for dust correction. However, the central NUV- r becomes systematically much redder than the

1 outerNUV- rformoremassivegalaxiesatM∗>1010.2M⊙. (2)ThecentralNUV- rshowsnodependenceon
2 SérsicindexnatM∗<1010.2M⊙,whileabovethismassgalaxieswithahigherntendtoberedderinthecentral
] NUV- rcolor. TheseresultssuggestthatgalaxieswithM∗<1010.2M⊙ exhibitsimilarstarformationactivity
A fromtheinnerR<R50regiontotheR>R50region.Incontrast,aconsiderablefractionoftheM∗>1010.2M⊙
galaxies,especiallythosewithahighn,haveharboredarelativelyinactivebulgecomponent.
G
Subjectheadings:galaxies:evolution–galaxies:starformation
.
h
p
- 1. INTRODUCTION formation shuts down in galaxies, much works explore the
o dependence of quiescence, or the quenched fraction upon a
The issue of how star formation gets shutdown in galax-
r specificvariable,whilekeepingothervariablesfixed(Penget
t ies (quenching) remains less understood to date. From the
s al. 2010; Bell et al. 2012; Cheung et al. 2012; Quadri et al.
a theoreticallyperspective,severalquenchingmechanismshave
2012;Woo etal.2013;Kovacˇ etal. 2014;Blucketal. 2014;
[ been proposed, including the active galactic nucleus (AGN)
feedback (Croton et al. 2006; Hopkins et al. 2006), halo Langetal.2014;Omandetal.2014;Knobeletal.2015;Woo
1 etal. 2015). Forinstance, Cheunget al. (2012)find thatthe
shock-heating(Dekel&Birnboim2006;Cattaneoetal.2006),
v U- B color of a galaxy is mostly tightly linked to a dense
morphological quenching (Martig et al. 2009) and the en-
3 galaxy inner structure. Similar results are reported by Lang
vironmental effects (Gunn & Gott 1972; Toomre& Toomre
0 et al. (2014) and Bluck et al. (2014), who further show that
1972;Mooreetal.1996;Boselli&Gavazzi2006;Weinmann
5 quenchedfractionismosttightlylinkedtobulgemass. Given
etal.2009).However,observationallyitisextremelydifficult
5 thetightcorrelationbetweenbulgemassandblackholemass,
to identify the dominantworking mechanism. For instance,
0 theauthorssuggestthatAGNfeedbackismostfavored.
massivegalaxiespreferentiallyresideindenseregions(Liet
1. al.2006),.i.e.,thehaloquenchingandenvironmentaleffects Note that the above methodology strongly relies on the
0 arepossiblybothatwork.Inthemeanwhile,theyhaveahigh tightnessofthemeasuredquiescence–variablerelation. This
6 probability of simultaneously hosting an AGN and a bulge can be misleading in some cases. As shown in observations
1 andsimulations,afractionofthequenchedgalaxieswillun-
component (Kauffmann et al. 2003a; Heckman et al. 2004;
: dergo dry merging (van Dokkum 2005; Naab et al. 2007;
v Schawinskiet al. 2010), making it difficult to isolate the ef-
Bundyetal.2009;Kormendyetal.2009;Taletal.2009;Hop-
i fectofAGNfeedbackfromthatofacentralbulge.Suchcom-
X kinsetal.2010),throughwhichthepropertiesofgalaxiesare
plexitieshavegreatlyhamperedourknowledgeofthedetailed
reshaped and the clues of quenching are smeared out. As a
r quenchingpicture.
a consequence, some properties of the dry merging remnants
Inordertoidentifythedominantmechanismbywhichstar
maybewellcorrelatedwithquiescence,howevertheyarenot
directlypointingto the quenchingprocesses. An exampleis
[email protected], [email protected], [email protected], thephenomenologyof"massquenching"(Pengetal. 2010),
[email protected]
thatgalaxieswithhighstellarmass(M )tendtohaveahigh
1Purple Mountain Observatory, Chinese Academy of Sciences, 2 ∗
quenchedfraction. There have been evidencesshowing that
West-BeijingRoad,Nanjing210008,China
2School of Astronomy and Space Science, Sun Yat-Sen University, the properties of giant quenched galaxies (such as structure
Guangzhou,510275,China andFaber–Jacksonrelation)are morereasonablyinterpreted
3Key laboratory for research in galaxies and cosmology, Shanghai inadrymergingframework(Kormendyetal.2009;Bernardi
Astronomical Observatory, Chinese Academy of Science, 80 Nandan
etal.2011a,b;Kormendy&Bender2013).
Road,Shanghai,200030,China
4Center of Astrophysics, University of Science and Technology of Drymergingisobviouslyirrelevanttoquenching.Toavoid
China,JinzhaiRoad96,Hefei230026,China suchaconfusion,onecouldalternativelysearchfortheclues
5Key Laboratory for Research in Galaxies and Cosmology, USTC, at the epoch when a galaxy is not fully quenchedyet. Such
CAS,China attempts have been carried out by studying the green valley
6CSIRO Astronomy & Space Science, Australia Telescope National
galaxies(Mendezetal.2011;Panetal.2013;Schawinskiet
Facility,POBox76,Epping,NSW1710,Australia
7Institute of Space Sciences, Shandong University, Weihai, 264209, al.2014;Panetal.2014).Tomakeastepfurther,itwillbeex-
China itingtoprobetheonsetofquenchinginastar-forminggalaxy
2 Panetal.
(SFG).However,thisisverychallengingsinceitisdifficultto the GALEX DR6 frames and downloaded their UV+opical
determinewhichSFGisgoingtoquench. Apracticableway images. Forthe FUV andUV bands, onlythosewith anex-
would be studying the star formation properties of individ- posure time greater than 1000 seconds are finally used. We
ualSFGstosearchfortheearlyquenchingsignals. Ideally,a registereveryimagetotheframegeometryoftheNUVband.
directinvestigationofthespecificstarformationrate(sSFR) The images of each galaxiesare then sheared into 3.′0×3.′0
distribution of a SFG will be helpful in diagnosing whether stamps. Since the SDSS image has a higher resolution than
quenchinghas occurredin that galaxy. Such studies require theUVimage,wedegradetheformerbyconvolvingaNUV
adatasetwhichcontains2-dimensionalinformationofindi- PSF kernel so that the UV and optical images are finally
vidualgalaxies, for example, the integralfield spectroscopic spatially matched. The PSF kernel is generated by stacking
(IFS) data. The IFS sample obtained in the early projects thestar imagesin thatGALEX frame. For thePSF-matched
(such as SAURON (Bacon et al. 2001) and DiskMass (Ber- galaxies,wethenmeasuretheirmagnitudesover5apertures,
shadyetal.2010))isprovedtobepowerfulinstudyinggalac- withr=[1.′′5, 3.′′0, 6.′′0, 9.′′0, 12.′′0]byrunningthe software
ticphysics.However,theexistedIFSsampleisstillrelatively SExtractor(Bertin&Arnouts1996). SExtractoralso
small. To collect a larger IFS sample, several large projects measuresthe total magnitudeof a galaxy(mag ). Finally,
auto
suchasMaNGA(Bundyetal.2015),CALIFA(Sánchezetal. we correct the magnitudes for galactic extinction using the
2012)and SAMI (Croomet al. 2012)are launchedrecently. galacticdustmapofSchlegel,Finkbeiner&Davis(1998).
Exploiting the early data products of these projects, some Somepubliclyavailabledataarealsoutilized.Thestructure
studieshavebeguntostudythe2-dimensionalstarformation parametercatalogusedinthefollowingisfromSimardetal.
historiesofalargelocalgalaxysample(Pérezetal.2013;Cid (2011). Simard et al. (2011)performa bulge+diskstructure
Fernandesetal.2013;GonzálezDelgadoetal.2014;Lietal. decomposition to the SDSS spectroscopic galaxies with the
2015;GonzálezDelgadoetal.2015). software GIM2D. The global galaxy Sérsic index n used in
To resolve the sSFR of a large galaxy sample, one can this work is from the r-band modeling, through which n is
alternatively use the existed multi-band imaging data. The allowed to vary from 0.5 to 8. Both stellar mass (M ) and
∗
2-dimensional properties of galaxies can also be studied by the4000Å breakstrength(D 4000)arefromtheMPA/JHU
n
utilizing spatially-resolved multi-band photometry (de Jong database8. M isderivedfollowingKauffmannetal.(2003b),
∗
1996;Kongetal.2000;Muñoz-Mateosetal.2007;Suhetal. withatypicaluncertaintyof∆logM =0.07dex.
∗
2010;Linetal.2013).Inthispaper,weusetheGALEX(Mar-
tinetal.2005)andSDSS(Yorketal.2000)datatoresolvethe 3. SAMPLESELECTION
starformationpropertiesinside/outsidethehalflightradiusof
Our UV-optical matched catalog contains 222,065 galax-
alargelocalSFGsample. Throughoutthispaper,weassume ies. To ensure that the measured central NUV- r color is
a concordanceΛCDM cosmology with Ωm =0.3, ΩΛ =0.7, notsignificantly affectedby the relativelypoorresolutionof
H =70kms- 1Mpc- 1,andaKroupa(2001)IMF.
0 the GALEX images, we limit the sample galaxies to have
2. METHODANDDATAUSED R50∼6.′′0,whichisapproximately2timesthePSFsize.With
this size limit, we can simply take the NUV- r measured in-
This work makes use of the UV–optical multi-band data, sidetheR=6.′′0apertureasaproxyofNUV- r .Inthefol-
twhhesicehdaartea,dirtawisnsftirlolmchtahleleSnDgiSnSg taonddiGreAcLtlEyXmesuasrvuerey.oWf tihthe lowing,NUV- rcentralreferstoNUV- rR=6.′′0ifRn<oRt50specifically
stated. Thedetailedsampleselectioncriteriaareasfollows:
sSFR distributionofgalaxies. Tobe simplified, in thiswork
wewilluseNUV- rcolorastherepresentativeofsSFR.Since (1)z=[0.01,0.1],wherez is the SDSS spectroscopicred-
shift. 88,465outofthe222,065galaxiesarewithinthisred-
UVluminosityL isanSFRindicator(e.g.,Kennicutt1998),
NUV- r is thus UaVproxy of SFR/L, where L is the r-band shiftrange.
luminosity. In this sense, NUV- rris a luminrosity weighted (2)minor- majoraxisratiob/a>0.5.NUV- rcolorisvery
sensitive to dust attenuation. Removing the edge on objects
sSFR.Salimetal.(2005)firstestablishthatthestarformation
can greatly alleviate the dust reddening effect on the color
historyofagalaxycanbewellconstrainedonthebasisofits
NUV- r color. Inarecentpaper,Salim (2014)furthershows measurement. Besides dust effect, the central region of an
that NUV- r is more tightly correlated with sSFR than u- r edge-ongalaxyisa superpositionofitsdisk andbulgecom-
andg- r,especiallyinthelowsSFRregime. ponent,makingitdifficulttoderiveitstruecentralcolor.Thus
we removethe edge on galaxies from furtheranalysis. This
The GALEX imaging has a relatively low resolution (The
GALEX NUV image has a resolution of 1 pixel=1.′′5 and a criterionkeeps59,177outofthe88,465onesthatpass(1).
(3) 4.′′0<R <8.′′0, to ensure that the r=6.′′0 aperture en-
pointspreadfunction(PSF)withfullwidthathalf-maximum 50
(FWHM)=5.′′3), which hampers a high-resolution study for closes ∼ 50% of the total flux of a galaxy. This criterion
is quite strict and only 9,391 out of the 59,177 galaxies are
a large SFG sample. To investigate a large sample, in this
work we only resolve the NUV- r color of galaxies in two passed.
(4)Werestrictthegalaxiestobebrighterthan23.0magin
regions, .i.e, a central region and an outer region. We refer
thecentralregiontobeR ≈R ,whereR istheradius theNUV band,to ensurethe accuracyof thecolormeasure-
central 50 50 ment.Infact,asshownbellow,ourselectedSFGsmostlyhave
enclosing50%oftheSDSSr-bandpetrosianflux. Bydoing
m <20.0mag. Ofthe9,391galaxies,8,980werekept.
this, one can simply perform aperture photometry to obtain NUV
NUV- r for a galaxy sample which with a similar R (5)stellarmassM∗>109.0M⊙.
central 50 Criteriaof(1)- (5)finallyyieldasampleof8,200galaxies.
angularsize.
We note that this sample is not volume-completed and our
BasedontheUV–opticalrawimagingdata,wehavegener-
conclusionsdonotrely on the selection completenessin the
atedamulti-bandphotometriccatalogfollowingthepipeline
volume.
of Wang et al. (2010). Here we give a brief review of the
dataprocessing.Asthefirststep,wecross-matchedtheSDSS
DataRelease8spectroscopicsample(Aiharaetal.2011)with 8http://www.mpa-garching.mpg.de/SDSS/DR7
Spatially-resolvedNUV- rcolorofstar-forminggalaxies 3
FIG.1.—a):theNUV- uversusu- zcolor–colordiagram. Itisclearthatgalaxiescanbewellseparatedintostar-formingandquiescentonesinthetwocolor
diagram.However,inthestar-formingsequence,aconsiderablefractionofthegalaxies(denotedinpinksymbols)havearelativelyredu- zcolorsimilartothe
quiescentones.Theblue-dashedlineenclosesaregionofu- z>2.8andNUV- u<1.5.ThearrowindicatestheshiftduetodustforanE(B–V)=0.1,assuming
adustattenuationlawofCardelli,Clayton&Mathis(1989).b):Theu- zversusM∗diagram.ItisclearthatmanydustySFGswillbemisclassifiedasquiescent
galaxieswhenusingthisdiagram.c)TheNUV- rversusM∗diagram.OurfinalSFGselectioncriterionisshownintheblueline.
FIG.2.—Left: thecorrelationbetweenNUV- rr=3.′′0 andtheDn4000indicesmeasuredfromtheSDSSr=1.′′5fiberspectra. Face-onSFGsareshownin
blacksymbols,whiletheedge-ononesareshowninredsymbols.Thelow-rightpanelshowsthecontourplotofthescattereddatapoints.Right:thecorrelation
betweenu- rr=3.′′0andDn4000.ItisclearthatNUV- risasuperiorphotometricindicatorofDn4000thanu- r.
Theobservedcolorsofgalaxiesaremoreorlessreddened imately equivalent to the color–color selection and will not
by their dust contents. To separate the dust-reddened SFGs miss the dusty SFGs. The final SFG sample contains 6,324
from truely passive galaxies, a color–color diagram is com- galaxies.
monlyadoptedintheliterature(Williamsetal.2009;Bellet Althoughthe good correlationbetween NUV- r and sSFR
al.2012;Changetal.2015). Inpanela)ofFigure1,weplot has been established in Salim (2014), at this point it is still
theselected8,200galaxiesontheNUV- uversusu- zcolor– necessary to investigate to what extend NUV- r can trace
colordiagram. Onecanfindthatasignificantfractionofthe sSFR for our sample. We explore the relation between
galaxieshaveu- zcolorsthatareasredasthoseofthepassive NUV- r and the 4000 Å break strength (D 4000). D 4000
n n
galaxies,whereaslie onthestar-formingsequence,i.e., they isingoodcorrelationwithsSFRandusuallytakenasamean
arestillSFGs. Inpanelb),itisclearmanyopticallyredSFGs stellar age indicator (Brinchmannet al. 2004). More impor-
willbemisclassifiedaspassivegalaxieswhenusingtheopti- tantly, D 4000 is dust-free. In Figure 2, we compare the
n
calcolor–massdiagram.Notethatmostofthesegalaxieshave NUV- r measuredin a centralr=3.′′0aperturewith D 4000
n
aloggra(Mm∗./MOn⊙)t>hi1s0d.4i.agPraanmel,co)nsehocwanstsheeeNthUatVt-hrevoeprstiucsalMly∗rdeid- 9im. agWineg.usFeoNrUcoVm- prar=r3is.′′o0n,duweetaolstoheplroetsothluetiSoFnGosfthGaAtLwEitXh
SFGsstilllieonthestarformingsequence.OurfinalSFGse- b/a<0.5 on this diagram. As shown in the figure, NUV- r
lectioncriterionisdefinedasNUV- r=0.6×logM –2.5,which
∗
isshowninthebluelineofpanelc). Thiscriterionisapprox-
9Dn4000ismeasuredfromtheSDSSr=1.′′5fiberspectra.
4 Panetal.
FIG.3.—a):theredshiftdistributionoftheSFGsample.TheSFGsampleisdividedinto7subsamplesaccordingtostellarmassandeachsubsampleiscoded
withaspecificcolor. b):theR50andR90distributionsofSFGs,showninthesolidlineandthedotedline,respectively. BothR50andR90andshowninangular
sizebutnotinphysicalsize. c):the fcentraldistributionofSFGs,where fcentralwasdefinedas fcentral=Fluxr=6.′′0/Fluxtotal. TheGALEXNUVandSDSSr-band
isshowninblueandredhistograms,respectively..
and D 4000 form a linear correlation. After excluding the
n
edge on galaxies, the tightness of this correlation is signifi-
cantlyimproved. Intherightpanel,we showthecorrelation
betweenu- r and D 4000. It is clear thatu- r is less sensi-
n
tivetoD 4000whenD 4000>1.6,becauseopticalcolorsare
n n
insensitivetolowlevelsSFR.
Note that in Figure 2, NUV- r and D 4000 are measured
n
over differentapertures. If D 4000 exhibits a non-zerogra-
n
dient within the r = 3.′′0 aperture, then it will bring some
scatters into this relation. Li et al. (2015) show that cen-
trallyquenchedgalaxiesusuallyexhibitsignificantlynegative
D 4000 gradient over R∼R , where R is the effective ra-
n e e
dius. Incontrast,centrallystar-formingonesdonothaveob-
viousgradient. One can find thatthe correlationseemsshift
rightward at D 4000>1.6 than the outward extrapolation of
n
the D 4000<1.5data points, which is expected accordingto
n
the findings of Li et al. (2015). Once NUV- r and Dn4000 FIG.4.—Topleft: themagnitudeuncertaintyasafunctionofmagnitude
aretakenfromthesameregion,thecorrelationbetweenthem in the GALEX NUV band. Black symbols denote those measured in the
shouldbeevenbetterthanthatshowninpanela). Ingeneral, centralr=6.′′0aperture,whileredsymbolsdenotethoseofthetotalgalaxy.
Figure 2 supports that for the face-on objects, NUV- r is a Topright: similartotopleft,butintheSDSSr- band. Bottom: theNUV- r
uncertaintydistribution.
goodphotometricsSFR(orstellarage)indicator.
In the next section we will carry out an analysis on
Inpanela)ofFigure3,weshowtheredshiftdistributionof
NUV- r and NUV- r . Before doing this it is impor-
the6,324SFG sample. Ascanbeseen, forgalaxiesoffixed central outer
tant to assess the accuracy of our color measurement. Fig-
M , only those within a certain redshift interval are finally
∗
ure4presentstheoutputmagnitudeuncertaintymeasuredby
selected. This is due to the combination of the underlying
SExtractor for our SFGs. Measurements of the r=6.′′0
M –size relation and criterion (3). Panel b) shows the R
∗ 50 aperture and the total galaxies are shown in black and red
and R distributions for our sample. Both R and R are
90 50 90 symbols,respectively.Ingeneral,fainterobjectshavegreater
showninangularsizebutnotinphysicalsize. Weexpectthe
SFGstohaveaR distributionwhichpeaksatR ∼6.′′0,so magnitude uncertainties. One can see that most SFGs have
thatther=6.′′0ap50erturecanwellcorrespondstot5h0ehalflight magauto <20.0 mag in the NUV band, which is ∼3.0 mag
brighterthantheNUVlimitingmagnitude. Thecoloruncer-
radiusofthissample.Frompanelb)onefindsthatmostofthe
SFGshaveanangularsizeofR ∼5.′′0. Paneld)showsthe tainty distributions are shown in the bottom panel. In this
50 panel one can see that the uncertainties of the NUV- r
centralfluxfraction(f = flux /flux )distribution. central
central central total and NUV- r are ∼ 0.08 mag and 0.04 mag, respectively.
Herethetotalfluxisconvertedfrommag . Onecanseethat total
thecentralfluxfractionpeaksat f ∼au0to.55inther- band, Assuming that the color uncertainty of NUV- router is of
central a similar order to that of NUV- r , the uncertainty of
which is slightly higherthanour expectation. This is dueto central
the color discrepancy between the two probed sub-regions,
the relatively low R distribution of this sample. However,
50 σ[(NUV- r )–(NUV- r )],is∼0.15mag.
the f distribution is quite narrow, which mainly spans central outer
central
from0.4 to 0.6. Thusin the followinganalysisone can still
5. RESULT
treatR roughlyasR .
central 50 5.1. ThedependenceofNUV- rcentralandNUV- routeronM∗
Panel a) of Figure 5 explores the relation between
4. THEUNCERTAINTYOFNUV- R NUV- r and M . The SFGs are divided into 6 mass
outer ∗
Spatially-resolvedNUV- rcolorofstar-forminggalaxies 5
FIG.5.—a): NUV- routerasafunctionofM∗. Galaxiesarebinnedin6massbins,asshownindifferentcolors. Theopenlargesymbolsdenotethemedian
NUV- routerinthatmassbin. ThebluelineshowsourSFGselectioncriterion. b): similartoa),butshowninNUV- rcentral. ThemedianvalueofNUV- rcentral
isshowninlargesolidsymbols. Forcomparison,wealsoplotthemedianvalueofNUV- router. c)the(NUV- rcentral)–(NUV- router)distributionsofthe6mass
bins.Ineachmassbin,thegalaxynumberismarked.
bins, with a bin size of ∆ logM =0.4 dex. As one can see, stronglyindicatesanoldbulgestellar population. Ourresult
∗
NUV- r becomes redder when increasing M . Panel b) isbroadlyconsistentwithNelsonetal.(2015a),whofindthat
outer ∗
shows the (NUV- r )–M relation. For comparison, we the stacked sSFR profile of the z ∼1 SFGs is quite flat at
central ∗
overplot the (NUV- router)–M∗ relation as well. It is clear log(M∗/M⊙)<10.0,whereasitexhibitaremarkablenegative
that NUV- r is only slightly redder than NUV- r at gradient for more massive galaxies (see their Figure 12). If
central outer
log(M∗/M⊙)<10.2. Above log(M∗/M⊙)=10.2, the discrep- thepresenceofoldbulgecomponentsinmassiveSFGsiscon-
ancy between the two colors becomes larger towards higher firmed,itnaturallyexplainstheflatteningoftheSFR–M rela-
∗
M . tionofSFGsseenatthemassiveendtowardslowz(Whitaker
∗
Panel c) shows the (NUV- r )–(NUV- r ) distribu- et al. 2014; Schreiber et al. 2015; Lee et al. 2015; Gavazzi
central outer
tions. Note thatwe donotapplyanyinternalextinctioncor- etal.2015). Ourfindingalsoprovidesawindowtointerpret
rection to both NUV- r and NUV- r . If there were theintrinsicscatterofthestarformationmainsequence(MS)
central outer
nodust, (NUV- r )–(NUV- r ) isa directtracerofthe of local SFGs. Recently, Guo et al. (2013) and Guo et al.
central outer
sSFRdiscrepancyofthetworegions.However,SFGscontain (2015)findthattheintrinsicscatterofthelow-zMSincreases
dustanddustattenuationhasbeenshowntobeastrongfunc- with M , whichis notreportedin othersimilar studies(e.g.,
∗
tion of M (Reddy et al. 2010; Whitaker et al. 2012, 2014). Whitaker et al. 2012). In the context of this work, Guo et
∗
For this work, understanding the dust maps of galaxies will al’sfindingsseemtobemorereasonablesincemanymassive
behelpfulininterpretingthepresentedresults. GonzálezDel- SFGshavebeenpartlyquenched(seeFigure5).
gadoetal. (2015)explorethe2-dimensionalpropertiesof∼
5.2. ThedependenceofNUV- r andNUV- r upon
300galaxiesusingthe CALIFAIFSdata. Theyfindthatthe central outer
galaxystructure
stackedA profileshowsasignificantnegativegradientinthe
V
innermost regions of galaxies. Beyond R∼0.5R , the A Recent processes show that quiescence is well correlated
50 V
profile becomes flat. Nelson et al. (2015b)use the 3D-HST with the presence of a dense galaxy inner structure (Bell et
grism data to investigate the dust maps of SFGs at z∼1.4. al. 2012; Cheung et al. 2012; Fang et al. 2013; Woo et al.
They find that low mass SFGs have little dust attenuations 2015). Given this, it will be interesting to explore whether
at all radii. Above log(M∗/M⊙)∼10.0, the dust attenuation bulgySFGshavealreadyshownsomesignsofearlyquench-
hasasignificantnegativegradientintheinnerR<1.0kpcre- ing. Galaxy structure can be parameterizedby the bulge-to-
gion.BeyondR∼1kpc,dustattenuationdoesnotvarymuch. total ratio (B/T), or Sérsic index n. The petrosian concen-
ThefindingsofGonzálezDelgadoetal.(2015)andNelsonet tration index, parameterized by C =R /R , is also preva-
90 50
al. (2015b) indicate that in the galaxy scale, the dust atten- lentlyadoptedintheearlySDSSstudies(Stratevaetal.2001;
uation is not a strong function of radius. A similar conclu- Kauffmann et al. 2003b; Brinchmann et al. 2004; Li et al.
sion is also reachedby Iglesias-Páramoet al. (2013). Given 2006). In the present work we do not attempt to investigate
this, NUV- r might be only slightly dust-reddenedthan thedependenceofourmeasuredNUV- rcoloronallthemen-
central
NUV- r . tionedstructuralparameters. We haveassessed which struc-
outer
An interesting feature presented in panel c) is that ture parameter is best tracing the early quenching of SFGs.
(NUV- r )–(NUV- r ) all peaks at ∼0.25 mag for the Assuming that the quenching processes of massive galaxies
central outer
log(M∗/M⊙)<10.2 SFGs. After accounting for dust correc- start from their central regions, we select a massive SFGs
tion, the intrinsic color discrepancy should be even smaller. sample(whichwithlog(M∗/M⊙)>10.5)toinvestigatethede-
Thisindicatesthatthe centralregionsoflow mass SFGs are pendence of their inner stellar age indicator (.i.e., the cen-
still actively forming new stars. Above log(M∗/M⊙)=10.2, tral Dn4000) upon B/T, n and C. We found that the cen-
the peaks of the distributions shift redward and show a de- tral D 4000 is most strongly dependent on n (Pan et al. in
n
pendence on M . Since we have excluded the edge on prep).Thusinthissectionweonlyexplorethedependenceof
∗
SFGs to minimize dust effects on colors, a red centralcolor NUV- ruponSérsicindexn.
In Figure 6 and Figure 7, we show how NUV- r and
central
6 Panetal.
FIG.6.— Toppanels: theNUV- router distributions. Fromlefttoright, weshow theresults forthelog(M∗/M⊙)=[9.0,9.4], log(M∗/M⊙)=[9.4,9.8] and
log(M∗/M⊙)=[9.8,10.2]subsamples,respectively. Galaxieswithindifferentnbinsareshownindifferentcolors. Thenumberofgalaxiesarealsomarkedin
eachpanel.Middlepanels:theNUV- rcentraldistributions.Bottompanels:thecolordiscrepancydistributions.
FIG.7.—SimilartoFigure6,butfortheSFGswithlog(M∗/M⊙)>10.2
NUV- r , as well as (NUV- r )–(NUV- r ) depend a high n indicates a large B/T. However, in a bulge+disk
outer central outer
on n. Figure 6 shows the results for the log(M∗/M⊙)<10.2 system,thebest-fitnwillincreaseeitherbyplacingmorelight
SFGs. Galaxiesarebinnedaccordington.Inthemassregime to the bulge, or by growing the disk extent at a fixed bulge
of log(M∗/M⊙)<10.2, one can see that NUV- router is some- size(seeLangetal.(2014)foradetaileddiscussion). Inthe
whatbluerforthehighngalaxies. Inthemiddlepanels,itis formercase,ahighnindicatesalargebulge,whileinthelatter
interestingthatNUV- r seemslargelyindependentofn. it indicates a very extendeddisk component. To distinguish
central
Figure7showstheresultsofthelog(M∗/M⊙)>10.2galaxies. between these two cases, we have investigated the R50–M∗
As can be seen, NUV- r is not strongly dependenton n. andR –M relationsofthe SFGs. The resultsareshownin
outer 90 ∗
However, NUV- rcentral shows a strong dependence on n, in Figure 8. We find that at log(M∗/M⊙)<10.2, those galaxies
thesensethatthehighngalaxieshavereddercentralcolors. withn>2.0tendtohavelargerR butsimilarR comparedto
90 50
Understandingthemeaningofnisimportantininterpreting thosewithn<2.0,i.e.,thelowmass,highnSFGstendtohave
the results presented in Figure 6 and Figure 7. The Sérsic averyextendeddiskcomponent(atleastforoursample).For
indexndescribesthelight/massprofileofagalaxy.Normally SFGswithlog(M∗/M⊙)>10.2,wefindthatthen>2.0galaxies
Spatially-resolvedNUV- rcolorofstar-forminggalaxies 7
FIG.8.—Left: theR50–M∗relation. SFGswithn<2.0andn>2.0areshowninblackandgreensmallsymbols,respectively. Themedianvaluesofthen<2.0
andn>2.0subsamplesareshowninredandpinklargesymbols,respectively.Right:theR90–M∗relation.
tendtohavesmallerR butsimilarR thanthosewithn<2.0.
50 90
This supports that massive high n galaxies tend to contain a
prominentbulgecomponent.
Wangetal.(2011)findthataHIrichgalaxytendtohavea
largerandbluerouterdiskthanthemean. AccordingtoFig-
ure8andWangetal.(2011),itisnotsurprisingtoseethatthe
low mass, high n galaxies have relatively bluer NUV- r .
outer
In a recent work, Erfanianfar et al. (2016) also find that the
low mass, highn SFGs have higherSFR thanthe mean(see
theirFigure9),forwhichtheyhavegavenoexplanation. At
the massive end, they also founda dependenceof SFR on n
similar to ours. Whitaker et al. (2015) also suggest that the
massive, high n SFGs give rise to the flattened slope of the
star formation main sequence at z<1.0. In this work, our
spatially-resolvedNUV- rcolorenablesadirectinterpretation
of the connectionsbetween star formationand n in different
massregimes.
6. DISCUSSION
6.1. IstheredNUV- rcentralduetoheavilydustobscuration?
The NUV- r color is a good proxy of sSFR. However, all
colorindicesaremoreorlessaffectedbythemetallicityand
dustofthatgalaxy.Itisthusnecessarytoassesstheimpactsof
metallicityanddustonourresults. InFigure5,wedrawthat FIG.9.—Thecolordiscrepancydistributionsforlog(M∗/M⊙)>10.5SFGs.
nearly all galaxies have their NUV- r and NUV- r Fromtoptobottom,wedividethesampleinto3nbins.Galaxieswithdiffer-
bellow 5.0. Over this range, NUV- rciesntrmalainly determionuetedr entM∗areshownindifferentcolors.
bysSFR(orstellarage),ratherthanmetallicity(Kavirajetal. low log(M∗/M⊙)=10.2. Previous works support that the
2007a,b). Thusmetallicity should has a minor effecton our dust attenuation of SFGs (traced by LIR/LUV) has signifi-
results. However,dustattenuationisknowntobemoreseri- cantlyincreasedfromlog(M∗/M⊙)=9.0tolog(M∗/M⊙)=10.2
(Whitakeretal.2012;Guoetal.2013;Whitakeretal.2014).
oustowardsthemassiveend. AretheresultsofFigure5and
However, one draw from Figure 5 that the (NUV- r )–
Figure7causedbyheavilydustattenuationofmassiveSFGs? central
(NUV- r ) distributions all peak around ∼ 0.25 and with
To access the role of dust on our results, we explore the outer
(NUV- r )–(NUV- r ) distributions as a function of a similar scatter for the log(M∗/M⊙)<10.2SFGs, indicating
central outer that the (NUV- r )–(NUV- r ) distribution is insensi-
M∗ within the range of log(M∗/M⊙)=[10.5,12.0]. The re- central outer
tivetothedustamountofSFGs. Toconclude,wesuggestthat
sults are presented in three fixed n bins, as shown in Fig-
theredNUV- r colorsofmassiveSFGsareowingtoold
ure 9. It can be seen that when keeping n fixed, the color central
bulgestellar populations,althoughthedusteffectcannotbe
discrepancy distribution shows a weak dependence on M .
∗
fullyruledout.FuturestudiesbasedonlargeIFSsampleswill
However, when comparing the results between different n
shedmorelightonthistopic.
bins, we find that these distributions show a clear depen-
dence on n, in the sense that high n SFGs tend to have red
(NUV- r )–(NUV- r ) distributions. The insensitivity 6.2. bulgegrowthandstarformationquenching
central outer
of(NUV- r )–(NUV- r )onM isnotexpectedifdust AGNfeedbackhaslongbeenservedasacandidatemech-
central outer ∗
producestheresultsofFigure5. anismofceasingstarformationinmodels(e.g.,Crotonetal.
An additional supporting evidence comes from the uni- 2006;Hopkinsetal.2006;Somervilleetal.2008). Observa-
formityofthe(NUV- r )–(NUV- r )distributionsbel- tionally, there have been growing evidences supporting this
central outer
8 Panetal.
Recently,Abramsonetal.(2014)suggestthatakeypartof
"massquenching"isthebuildupofaninactivebulgecompo-
nent. ThisissupportedbyFigure7 ofthiswork. Giventhat
many massive, bulgy SFGs have been partly quenched, it is
importanttoinvestigatethecontributionofthe"bulgygalax-
ies"tothewholeSFGpopulation. Figure10showsthefrac-
tionsof"bulgySFGs"(withn>2.5)asafunctionofM . For
∗
oursample, one can see that the this fractionis around10%
at M∗ <1010.2M⊙, while it rapidly increases to 60%– 70%
atM ∼1011.0M . A similar trendcanbe foundin Fisher&
∗ ⊙
Drory(2011)andErfanianfaretal.(2016).
Forcomparison,weoverplottheresultsoftwoSFGssam-
ples drawn from z=[0.01,0.05]and z=[0.05,0.1]. We use
similar SFG selection criteria but do not apply any R cut
50
to the comparison samples. It can be seen that the results
ofthetwocomparisonsamplesarewellconsistentwitheach
other. One can also see that the fractions derived from the
SFGs sample of this work are obviously lower than those
of the comparison samples, especially at the mass range of
log(M∗/M⊙)=[10.0,11.0].WehavecheckedtheR50 distribu-
tions of SFGs and confirmed that at fixed M and z, SFGs
∗
in the highestn quartile have a smaller R distribution than
50
FIG.10.—ThefractionofbulgySFGs(withn>2.5)asafunctionofM∗. themean,especiallyforthemassiveones,i.e.,ourR cutis
DatapointsarecalculatedintheM∗binsizeof∆M∗=0.2dex. Theredand 50
bluearefromthecomparisonsampleswithin2differentredshiftbins. The biasedagainsthighnSFGs. Accountingforthesampleselec-
redandbluedashed-vertical linesdenotethemasslimitofthecomparison tion bias, the intrinsic (NUV- rcentral)–(NUV- router) distribu-
samples. tionsshouldbeevenredderthanthose presentedin Figure5
atthemassiveend.
scenario (Schawinski et al. 2006; Page et al. 2012; Fabian
2012;Maiolinoetal.2012;Bargeretal. 2015). Recentpro- 7. SUMMARY
gresses have confirmed the tight correlation between quies- Aiming to resolve the sSFR of individual galaxies and to
cence and the presence of a dense galaxy inner structure, better understand how quenching process at its early phase,
whatever parameterized by n (Bell et al. 2004; Driver et al. we measure the NUV- r colors both inside and outside the
2006;Belletal.2012;Cheungetal.2012),bytheinner1kpc half-light radius for a sample of 6,324 face-on local SFGs.
stellar mass density (Cheung et al. 2012; Fang et al. 2013; AverytightlinearcorrelationisfoundbetweenNUV- r and
Woo et al. 2015) or by bulge mass (Bluck et al. 2014;Lang D 4000, supportingthat NUV- r is a good photometricstel-
n
etal.2014). Giventhetightcorrelationsbetweenblackhole lar age (orspecific star formationrate) indicator, at least for
mass, bulge mass and n, this observation supports the AGN theseface-onSFGs. Weinvestigatehowthesetwocolorsde-
feedbackparadigmataqualitativelevel. Figure7showsthat pend on stellar mass and galaxy structure. Our findings are
massive,highnSFGsindeedhaverelativelyredNUV- rcentral thefollowing:
distributions, supporting that the star formation of bulge is 1.BellowM ∼1010.2M ,thecentralNUV- risonaverage
∗ ⊙
atleast partlycontrolledby thebulge-relatedprocess. How- only∼0.25magredderthantheouterNUV- r. Theintrinsic
ever, this trend does not hold for the less massive galaxies
value would be even smaller after accounting for dust cor-
(seeFigure6). Figure6thussuggeststhatthequenchingpic- rection; aboveM ∼1010.2M , the centralNUV- r becomes
∗ ⊙
tureoflowmassSFGsisnotthesameasthatofthemassive systematically much redder than the outer NUV- r for more
ones. As already established, low mass galaxies are mainly
massivegalaxies.
quenched through environmental effects (Peng et al. 2010).
2.BellowM ∼1010.2M ,thecentralNUV- rshowsnode-
In a dense environment, gas stripping will cause quenching ∗ ⊙
pendenceonSérsicindexn.However,abovethismass,galax-
firstongalacticoutskirts,especiallyforthelowmasssystems
ies with a higher n tend to be redder in the central NUV- r
(Boselli&Gavazzi2014). The"outside-in"quenchmodeof
color. Our analysis suggests that the red central color pre-
low mass galaxieshas beenrecentlyconfirmedby Pan etal.
sentedin massive galaxiesis owingto theirold bulgestellar
(2015).
population.
Martigetal.(2009)proposethebuildupofacentralbulge
Our findings thus suggest that the central regions of less
is sufficient to stabilize the surrounding gas disk from col-
massiveSFGsarecomparablyactivetotheouterregions. In
lapsingandformingnewstars,whichcancausequenchingin
contrast,thebulgeofaportionofmassivegalaxies,especially
a SFG withoutexpelling/depletingits gas. In a recentwork,
forthosewithahighn,hasbeenquenched. Thepresenceof
Guo et al. (2015)find that the high B/T SFGs have a larger
oldbulgesinmassivegalaxiesnaturallyexplainstheflattened
intrinsic scatter in their disk sSFR, based on which they ar-
slope of the SFR–M relation seen at the massive end. Our
gued that the presence of a central bulge has a dramatically ∗
findings also provide a direct interpretation of the intrinsic
impact on the galaxy scale star formation. In this work, we
scatterofthestarformationmainsequence.
showthattheNUV- r ofmassiveSFGsstronglydepends
central
onn,whereasNUV- r doesnot(seeFigure 7). Thisindi-
outer
catesthatthediskstarformationofaSFGislessaffectedby We thank the anonymous referee for insightful com-
its inner structure. The "morphologicalquenching"scenario ments and a careful reading of the manuscript. This
isthusnotsupportedbyourdata. work was supported by the NSFC projects (Grant Nos.
Spatially-resolvedNUV- rcolorofstar-forminggalaxies 9
11473053, 11121062, 11233005, U1331201, U1331110, Academy of Sciences (Grant No. XDB09000000), the Spe-
11225315,1320101002,11433005,and 11421303),the Na- cialized Research Fund for the Doctoral Program of Higher
tional Key Basic Research Program of China (Grant No. Education(SRFDP, No. 20123402110037),andtheChinese
2015CB857001), the “Strategic Priority Research Program National973 FundamentalScience Programs(973program)
the Emergence of Cosmological Structures” of the Chinese (2013CB834900,2015CB857004).
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