Planck Early Results: The Planck View of Nearby Galaxies

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by  Planck Collaboration

Astronomy&Astrophysicsmanuscriptno.planck˙nearby˙galaxies˙aa6 (cid:13)c ESO2011
January12,2011
Planck Early Results: The Planck View of Nearby Galaxies
PlanckCollaboration:P.A.R.Ade67,N.Aghanim44,M.Arnaud54,M.Ashdown52,72,J.Aumont44,C.Baccigalupi65,A.Balbi25,
A.J.Banday70,6,59,R.B.Barreiro49,J.G.Bartlett3,50,E.Battaner74,K.Benabed45,A.Benoˆıt45,J.-P.Bernard70,6,M.Bersanelli23,38,R.Bhatia30,
J.J.Bock50,7,A.Bonaldi34,J.R.Bond5,J.Borrill58,68,F.R.Bouchet45,M.Bucher3,C.Burigana37,P.Cabella25,J.-F.Cardoso55,3,45,
A.Catalano3,53,L.Cayo´n16,A.Challinor73,52,8,A.Chamballu42,R.-R.Chary43,L.-YChiang46,P.R.Christensen62,26,D.L.Clements42⋆,
S.Colombi45,F.Couchot57,A.Coulais53,B.P.Crill50,63,F.Cuttaia37,L.Danese65,R.D.Davies51,R.J.Davis51,P.deBernardis22,G.de
Gasperis25,A.deRosa37,G.deZotti34,65,J.Delabrouille3,J.-M.Delouis45,F.-X.De´sert40,C.Dickinson51,H.Dole44,S.Donzelli38,47,O.Dore´50,7,
U.Do¨rl59,M.Douspis44,X.Dupac29,G.Efstathiou73,T.A.Enßlin59,F.Finelli37,O.Forni70,6,M.Frailis36,E.Franceschi37,S.Galeotta36,
K.Ganga3,43,M.Giard70,6,G.Giardino30,Y.Giraud-He´raud3,J.Gonza´lez-Nuevo65,K.M.Go´rski50,76,S.Gratton52,73,A.Gregorio24,
A.Gruppuso37,F.K.Hansen47,D.Harrison73,52,G.Helou7,S.Henrot-Versille´57,D.Herranz49,S.R.Hildebrandt7,56,48,E.Hivon45,M.Hobson72,
1
W.A.Holmes50,W.Hovest59,R.J.Hoyland48,K.M.Huffenberger75,A.H.Jaffe42,W.C.Jones15,M.Juvela14,E.Keiha¨nen14,R.Keskitalo50,14,
1
0 T.S.Kisner58,R.Kneissl28,4,L.Knox18,H.Kurki-Suonio14,32,G.Lagache44,A.La¨hteenma¨ki1,32,J.-M.Lamarre53,A.Lasenby72,52,
2 R.J.Laureijs30,C.R.Lawrence50,S.Leach65,R.Leonardi29,30,19,M.Linden-Vørnle10,M.Lo´pez-Caniego49,P.M.Lubin19,J.F.Mac´ıas-Pe´rez56,
C.J.MacTavish52,S.Madden54,B.Maffei51,D.Maino23,38,N.Mandolesi37,R.Mann66,M.Maris36,E.Mart´ınez-Gonza´lez49,S.Masi22,
n
S.Matarrese21,F.Matthai59,P.Mazzotta25,A.Melchiorri22,L.Mendes29,A.Mennella23,36,M.-A.Miville-Descheˆnes44,5,A.Moneti45,
a
J L.Montier70,6,G.Morgante37,D.Mortlock42,D.Munshi67,73,A.Murphy61,P.Naselsky62,26,P.Natoli25,2,37,C.B.Netterfield12,
H.U.Nørgaard-Nielsen10,F.Noviello44,D.Novikov42,I.Novikov62,S.Osborne69,F.Pajot44,B.Partridge31,F.Pasian36,G.Patanchon3,
1
1 M.Peel51,O.Perdereau57,L.Perotto56,F.Perrotta65,F.Piacentini22,M.Piat3,S.Plaszczynski57,E.Pointecouteau70,6,G.Polenta2,35,
N.Ponthieu44,T.Poutanen32,14,1,G.Pre´zeau7,50,S.Prunet45,J.-L.Puget44,W.T.Reach71,R.Rebolo48,27,M.Reinecke59,C.Renault56,

] S.Ricciardi37,T.Riller59,I.Ristorcelli70,6,G.Rocha50,7,C.Rosset3,M.Rowan-Robinson42,J.A.Rubin˜o-Mart´ın48,27,B.Rusholme43,
O M.Sandri37,G.Savini64,D.Scott13,M.D.Seiffert50,7,P.Shellard8,G.F.Smoot17,58,3,J.-L.Starck54,9,F.Stivoli39,V.Stolyarov72,R.Sudiwala67,
C J.-F.Sygnet45,J.A.Tauber30,L.Terenzi37,L.Toffolatti11,M.Tomasi23,38,J.-P.Torre44,M.Tristram57,J.Tuovinen60,M.Tu¨rler41,G.Umana33,
. L.Valenziano37,J.Varis60,P.Vielva49,F.Villa37,N.Vittorio25,L.A.Wade50,B.D.Wandelt45,20,D.Yvon9,A.Zacchei36,andA.Zonca19
h
p (Affiliationscanbefoundafterthereferences)
-
o
r
t
s ABSTRACT
a
[ Theall-skycoverageofthePlanckEarlyReleaseCompactSourceCatalogue(ERCSC)providesanunsurpassedsurveyofgalaxiesatsubmillimetre

(submm)wavelengths,representingamajorimprovementinthenumbersofgalaxiesdetected,aswellastherangeoffar-IR/submmwavelengths
1
overwhichtheyhavebeenobserved.Weherepresentthefirstresultsonthepropertiesofnearbygalaxiesusingthesedata.WematchtheERCSC
v
cataloguetoIRAS-detectedgalaxiesintheImperialIRASFaintSourceRedshiftCatalogue(IIFSCz),sothatwecanmeasurethespectralenergy
5
distributions(SEDs)oftheseobjectsfrom60to850µm.Thisproducesalistof1717galaxieswithreliableassociationsbetweenPlanckandIRAS,
4
fromwhichweselectasubsetof468forSEDstudies,namelythosewithstrongdetectionsinthethreehighestfrequencyPlanckbandsandno
0
evidenceofcirruscontamination.TheSEDsarefittedusingparametricdustmodelstodeterminetherangeofdusttemperaturesandemissivities.
2
Wefindevidence for colderdust thanhaspreviously beenfound inexternal galaxies,withT < 20K.Suchcoldtemperatures arefoundusing
.
1 boththestandardsingletemperaturedustmodelwithvariableemissivityβ,oratwodusttemperaturemodelwithβfixedat2.Wealsocompare
0 ourresultstostudiesofdistantsubmmgalaxies(SMGs)whichhavebeenclaimedtocontaincooler dustthantheirlocalcounterparts. Wefind
1 thatincludingoursampleof468galaxiessignificantlyreducesthedistinctionbetweenthetwopopulations.FitstoSEDsofselectedobjectsusing
1 moresophisticatedtemplatesderivedfromradiativetransfermodelsconfirmthepresenceofthecolderdustfoundthroughparametricfitting.We
: thusconcludethatcold(T <20K)dustisasignificantandlargelyunexploredcomponentofmanynearbygalaxies.
v
i Keywords.Infrared:galaxies–Submillimetre:galaxies–Galaxies:ISM
X
r
a
1. Introduction tion(SED)ofdustemission,combinedwithlimitedwavelength
coverage,meansthatIRASwasinsensitivetodustbelowatem-
Dustisanimportantconstituentoftheinterstellarmedium(ISM)
peratureof∼ 30K.ObservationsofdustinourownGalaxyby
of galaxies. Whilst some properties of dust in our own and
the FIRAS instrumenton COBE (Reach et al. 1995)foundev-
very nearby galaxies can be studied through its absorption of
idence for dust at several differenttemperatures. This included
starlight, it was the IRAS satellite that first allowed dust emis-
a widespread component at 16–21K, and another at 10–14K
sionto be directlyobservedin largesamplesofexternalgalax-
associated with molecular clouds in the inner Galaxy. A third
ies (e.g., Devereux & Young 1990). The all-sky IRAS survey
widespreadcoldercomponent,at4–7K,waslateridentifiedwith
at 12, 25, 60 and 100µm in wavelength provided much new
theCosmicInfraredBackground(CIB;Pugetetal.1996;Fixsen
information on the properties of dust and how this relates to
et al. 1998).None of these componentswould be detectable in
other aspects of galaxies and galaxy evolution. However, the
externalgalaxiesbyIRAS.COBE-DIRBEobservationsalsode-
strong temperature dependenceof the spectral energy distribu-
tected56externalgalaxies,findinganaveragedusttemperature
of27.6K(Odenwaldetal.1996).
⋆ Correspondingauthor:D.L.Clements,[email protected]
1
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
Observations at longer far-infrared (FIR) or submillimetre comparisonoftheERCSCwithexistingdatafromground-based
(submm)wavelengthsfromtheground(e.g.,Dunneetal.2000), observatories.Section4presentstheresultsoffittingparametric
from space (e.g., Dale et al. 2005) or in combination (e.g., models to the dust SEDs of ERCSC galaxies, while Section 5
Willmeretal.2009),haveprovidedhintsthatcoolerdustplays discusses the results of physical template fitting. Finally, we
asignificantroleinnearbygalaxies.ObservationswithHerschel draw conclusions in Section 6. Throughout this paper we as-
ofpre-selectedobjects(e.g.,Bosellietal.2010)orofrelatively sume a concordance cosmology, with H = 70kms−1Mpc−1,
0
small fields (e.g., H-ATLAS Eales et al. 2010, covering up to Ω =0.7andΩ =0.3.
Λ M
550deg2) provide valuable data at 250 to 500µm, which con-
strain the long wavelength dust properties for specific popula-
tions. However, the availability of the Planck1 Early Release 2. Planck ObservationsofNearbyGalaxies
Compact Source Catalogue (ERCSC) provides a long wave-
2.1.ThePlanckMission
length counterpart to IRAS, allowing us an unbiased view of
theFIR-to-submmSEDsofa large,unbiasedsampleofnearby Planck (Tauberet al. 2010;Planck Collaboration2011a)is the
(z<0.25)galaxies.Wearenow,forthefirsttime,abletoexam- thirdgenerationspacemissiontomeasuretheanisotropyofthe
inetheroleofcolddustforawiderangeofobjectsinthelocal cosmic microwave background (CMB). It observes the sky in
Universe. ninefrequencybandscovering30–857GHzwith highsensitiv-
The discovery of the CIB (Puget et al. 1996; Fixsen et al. ity and angular resolution from 31′ to 5′. The Low Frequency
1998) has added to the importance of our understanding of InstrumentLFI; (Mandolesiet al. 2010;Bersanelli et al. 2010;
dust in galaxies. The CIB demonstrates that roughly 50% of Mennellaetal.2011)coversthe28.5,44.1,and70.3GHzbands,
all energy generated in the history of the Universe was ab- withamplifierscooledto20K.TheHighFrequencyInstrument
sorbed by dust and reprocessed into the FIR/submm (Gispert (HFI;Lamarreetal.2010;PlanckHFI CoreTeam2011a)cov-
etal. 2000).Deep surveysat850µmwith SCUBA (e.g.,Smail ers the 100, 143, 217, 353, 545, and 857 GHz bands, with
etal.1997;Hughesetal.1998;Ealesetal.2000;Coppinetal. bolometerscooled to 0.1K. Polarization is measured in all but
2006) and at nearby wavelengths with other instruments (e.g., the highest two bands (Leahy et al. 2010; Rosset et al. 2010).
MAMBO, AzTEC and LABOCA) have revealed much higher A combinationof radiative coolingand three mechanicalcool-
number counts than would be predicted by a non-evolvingex- ers producesthe temperaturesneededfor the detectorsand op-
trapolation of the local population. There must thus be very tics(PlanckCollaboration2011b).Twodataprocessingcenters
rapidevolutionoftheFIR/submmgalaxypopulation,something (DPCs) checkandcalibratethedata andmakemapsof thesky
confirmed by observations with ISO, (e.g., Dole et al. 2001) (Planck HFI Core Team 2011b; Zacchei et al. 2011). Planck’s
Spitzer(e.g.,Frayeretal.2006;Be´therminetal.2010;Clements sensitivity,angularresolution,andfrequencycoveragemakeita
et al. 2010a) and BLAST (Devlin et al. 2009). Herschel ob- powerfulinstrumentforGalacticandextragalacticastrophysics
servations have now confirmed this rapid evolution through a as well as cosmology. Early astrophysics results are given in
combination of number count (Clements et al. 2010c; Oliver PlanckCollaboration,2011h–z.
et al. 2010) and luminosity function (Dye et al. 2010) studies.
However,detailedinterpretationoftheseresultsishamperedby
2.2.ThePlanckEarlyReleaseCompactSourceCatalogue
ourpoorknowledgeofgalaxySEDsinthe100–1000µmrange.
Thisisdemonstrated,forexample,bytheapparentseparationin ThePlanckERCSC(PlanckCollaboration2011c)providespo-
thetemperature-luminosityplaneoflocalIRAS-selectedgalaxies sitionsandfluxdensitiesofcompactsourcesfoundineachofthe
andhighredshiftSCUBA-selectedSMGs(e.g.,Clementsetal. nine Planck frequency maps. The flux densities are calculated
2010b). The origin of this separation is unclear. It might rep- using aperture photometry, with careful modelling of Planck’s
resent a genuine change in dust temperature with redshift, and elliptical beams. The colour corrections for sources with spec-
selectionbiasesmaybepartlyinvolved,butitcouldalsoreflect tral index α = −0.5 (using the conventionSν ∝ να) are 1.017,
ourignoranceofthefullFIR/submmSEDoflocalgalaxies.By 1.021and 1.030,respectively,forthe 28.5,44.1,and 70.3GHz
properlyestablishingazeroredshiftbaselineforthedustSEDs LFI channels. Flux densities taken from the ERCSC should be
oftypicalgalaxies,thePlanckERCSCwillallowtheoriginsof dividedby the appropriatecolourcorrectionto givethe correct
theCIBandthenatureofthegalaxiesthatcontributetoittobe flux values for an assumed narrow band measurement at the
muchbetterdetermined. centralfrequency.Forfrequenciesfrom28.5to143GHz,com-
The central goals of this paper are thus twofold: to exam- pactsourceshavebeendetectedusinga versionofthe “Powell
inethepropertiesofalargesampleoflocal(z < 0.25)galaxies Snakes”techniques(Carvalhoetal.2009);fordetailsseePlanck
toestablish therangeofdusttemperaturesandotherproperties Collaboration (2011c). In the four higher frequency channels,
found locally; and thus to set the local baseline against which sources were located using the SExtractor package (Bertin &
higher redshift studies, and especially studies of the SMGs re- Arnouts1996).Sourcesdetectedinoneormoreofthefrequency
sponsiblefortheCIB,canbecompared. mapswerethenputthroughafurthersetofsecondaryselection
criteria; these are discussed in detail in Planck Collaboration
The rest of this paper is organisedas follows. In Section 2
(2011c). The primary criterion utilized was a Monte Carlo as-
we give details of Planck’s observations of local galaxies and
sessment designed to ensure that ≥90% of the sources in the
the ERCSC. We also discuss the results of matching ERCSC
cataloguehaveafluxaccuracyofatleast30%.
galaxiestosourcesobservedbyIRAS.InSection3wepresenta
1 Planck (http://www.esa.int/Planck) is a project of the European 2.3.MatchingTheERCSCtoIRASData
SpaceAgency(ESA)withinstrumentsprovidedbytwoscientificcon-
To understand the FIR Spectral SEDs we need a combination
sortia funded by ESA member states (in particular the lead countries
FranceandItaly),withcontributionsfromNASA(USA)andtelescope of data at long wavelengths, provided by the Planck ERCSC,
reflectorsprovidedbyacollaborationbetweenESAandascientificcon- and data near the peak of a typical galaxy dust SED at about
sortiumledandfundedbyDenmark. 100µm. The best source for the latter information is the IRAS
2
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
testthevalidityoftheassociationwiththeIRASsource.Thisin-
cludedinspectionoftheSkySurveypostagestampsprovidedin
NED.Associationswereacceptedasrealiftheassociatedgalaxy
had a blue (g or B) magnitude brighter than 16. The surface-
densityofsuchgalaxiesleadstotheprobabilityofachanceas-
sociationbeing∼3%.ForsourceswheretherewasbothanIRAS
anda 2MASSassociation,thislimitwasrelaxedto B = 17(or
K ∼13).Ofthe88ERCSC-IIFSCzassociationswithpositional
offsets 3–5′ (and with spectroscopic or photometric redshifts)
72 were associated with bright galaxies, two were associated
withasecondFSCsourcehavingcirrus-likecoloursandarepre-
sumed to be cirrus, and the remaining14 are classified as pos-
sible galaxy associations (these are excluded from the reliable
galaxycatalogueusedhereforfurtheranalysis).
ThesecondcategoryofERCSC-IIFSCz associationswhich
wescrutinisedindetailwerethoseforwhichthereisnoredshift
in the IIFSCz. There were 165 of these and the NED associa-
tionssuggestthat38arebrightgalaxies,sevenarecirrus,twoare
brightplanetarynebulae,andtheremaining118areclassifiedas
possible galaxyassociations (andexcludedhere). We were left
with1717reliableERCSC-IIFSCzgalaxyassociationsofwhich
Fig.1. Histogram of offsets between ERCSC and IIFSCz posi- 337areflaggedasextendedintheERCSC. 1597ofthese1717
objectshavespectroscopicredshifts.
tions.
all-sky FIR survey, and the most recent analysis of the IRAS 2.4.ERCSCSourcesnotassociatedwithIIFSCzSources
Faint SourceCatalogue (FSC) is providedby Wang & Rowan-
Fig.2showstheskydistributionofERCSCsourcesat|b|>20◦,
Robinson (2009) in the Imperial IRAS FSC redshift survey
with sourcesflagged as extendedin the ERCSC shownas blue
(IIFSCz). This was constructed using IRAS FSC sources, all
filledhexagons,andpoint-sourcesshowninblack.Associations
of which are at |b| > 20◦, with IRAS colours used to exclude
withtheIIFSCzareshownasredcircles.Theextendedsources
stars and cirrus sources (with S(100)/S(60) > 8). The NASA
not associated with IIFSCz sources have a strikingly clustered
Extragalactic Database (NED) was then used to find spectro-
distribution,whichmatchestheareasofourGalaxywithstrong
scopic redshifts for the resulting FSC source list, and to asso-
cirrusemission,asevidencedbyIRAS100µmmapsandbythe
ciatethesourceswithSDSS(whereavailable;Yorketal.2000)
ERCSCcirrusflag(values> 0.25).Wepresumethemajorityof
and2MASS(Skrutskieetal.2006)galaxiestofindphotometry
thesearecirrussourcesandnotextragalactic.
at0.36–2.2µm.Thisphotometrywasthenusedtoestimatepho-
To test this further, we looked for NED associations with
tometricredshiftsforsourceswithoutspectroscopicredshifts.
all444extendedERCSCsourceslackingIIFSCzassociationsat
ThestartingpointformatchingtheERCSCtotheIIFSCzis
|b| > 60◦. Only12 werefoundto haveassociationswith bright
the 9042 sources detected at 857GHz by Planck. This is then
restricted to the 5773 sourcesat |b| > 20◦ for which there will (b,g < 16)galaxies.Extrapolatingto|b|= 20–60◦,weestimate
that a further ∼ 50 of these extended non-FSC sources will be
beFSCdata.AssociationsofERCSCsourceswithIIFSCzwere
looked for using a search radius of 5′. The histogram of posi- bright galaxies. The remainder of the 3431 extended non-FSC
sourcesat|b|>20◦arepresumedtobeGalacticcirrus.
tionaloffsets is shown in Fig. 1. The bulk of associations have
offsetswithin2′.Evenat5′thereisnosteepincreaseinthenum- A ridge of non-FSC point sources can be seen in Fig. 2
berofassociationswhichwouldbeindicativeofalargefraction at b ∼ 70◦, l ∼ 120–230◦. These correspond to one of the
of chance associations. On the basis of source surface-density, IRAScoveragegaps.WeexaminedNEDassociationsforall482
thechanceofarandomassociationwithanIIFSCzsourcewithin ERCSC point-sources not associated with IIFSCz sources. 32
3′is1.6%,andwithin5′itis4.5%.Atotalof1966associations were found to be associated with Local Group galaxies (M31,
were found within 5′. There were 106 cases where an ERCSC SMCandWLM,with28intheLMC),123arebrightgalaxies,
source picked up an association with more than one IIFSCz 27areassociatedwithIRASFSCorPSCGalacticcirrussources,
source.Weexaminedthesecasescarefullytoensurethatonlya and10arebrightstarsorplanetarynebulae.Mostofthebright
singleassociationwasaccepted.Generallythenearerassociation galaxies lie in the IRAS coveragegaps. The remaining289 are
was preferred. Where the offsets of the two associations were classifiedaspossiblegalaxyassociations(andexcludedhere).
comparable,the brighter IRAS source was preferred.There are To summarise, we have found a net total of 1884 definite
20caseswheretwobrightgalaxieslessthan5′aparthavegener- associations with galaxies. These constitute an ERCSC galaxy
atedasingleERCSCsource,forwhichtheremaybeasignificant catalogue.A further419sourcesare notassociated with bright
contributionfrombothgalaxiestotheERCSCflux.Thesecon- galaxies,buttherearegroundsforthinkingtheycouldbeextra-
fused sources would benefit from additional observations with galactic sources. Some have IIFSCz associations, but there are
ground-basedsubmminstrumentstodeterminethecontribution toomanypossiblefaintopticalornear-IRgalaxycounterpartsto
ofeachcomponenttothesubmmemissiondetectedbyPlanck. beconfidentwhichmightbeassociatedwiththeERCSCsource.
The remaining ERCSC-IIFSCz associations were further Someofthese419sourcesarealmostcertainlyfaintergalaxies,
scrutinised as follows. Firstly, for sources with positional off- althoughmanycouldturnouttobecirrus.Improvedsubmmor
sets between the two catalogues in the range 3–5′ all NED as- FIR positions are needed, either via Herschel or ground-based
sociationswithin 5′ ofthe ERCSC positionswere examinedto telescopes,toidentifythesesourcesreliably.
3
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
Fig.2.SkyplotofERCSCsourcesingalacticcoordinates.BlackfilledhexagonsareERCSCpoint-sourcesandbluefilledhexagons
areERCSC sourcesflaggedasextended.Red hexagonsaresourcesassociatedwith IIFSCz IRASFSC galaxies,afterscrutinising
suspect categorieswith NED (and excludingsome, as described in the text). Green hexagonsare ERCSC sources notassociated
withIIFSCz,butassociatedwithbrightgalaxiesinNED(onlyfor|b|>60◦forextendedsources).
Followingthisidentificationanalysiswerestrictourselvesto are thosemostlikelyto cause problemsforpointsourcedetec-
those galaxies with reliable IIFSCz associations and with de- tionintheERCSC.WemeasurethecirrusRMSina3×3array
tections in the 857 and 545GHz bands at significance of 5σ of points, separated by 0.1◦ and centred on the position of the
or greater, as well as detections in the 353GHz band of 3σ or ERCSC source. Since cirrus emission is likely to have cooler
greater.Thisamountstoatotalsamplesizeof595galaxies. FIR-to-submmcoloursthantheintegratedemissionofanexter-
nalgalaxy,wethenlookforanycorrelationbetweencirrusRMS
andthe60µm-to-857GHzcolour.WeplotthisrelationinFig.3.
2.5.CirrusContamination
Our analysis of the non-IIFSCz-identified ERCSC sources in As can be seem from Fig. 3, there appears to be a correla-
Section 2.4 led us to the conclusion that sources which are tion between colour and cirrus RMS for the sources classified
extended in the ERCSC are a result of cirrus structure in our asextendedintheERCSC.WeconcludethattheERCSCfluxes
own Galaxy, or at best are a combination of cirrus structure forthesesourcesarepartiallycontaminatedwithcirrusemission
with flux from a galaxy. Of the 595 reliably detected IIFSCz- fromourownGalaxy.We thusexcludethese127sourcesfrom
identified ERCSC sources, 127 are listed as extended in the further analysis. Of the remaining 468 non-extended ERCSC
ERCSC. We test these objectsfor the possibility of cirruscon- sources,fewerthan10lieintheregionofthiscorrelation.These
taminationbyexaminingthe amplitudeof the localcirrusfluc- sourcesare retainedforthe followinganalysis,butanyconclu-
tuationsinthe100µmcirrusmapsofSchlegeletal.(1998).We sionsthatcomesolelyfromthesespecificsourceswillbetreated
adopt this approach since regions of greatest cirrus fluctuation withcaution.
4
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
FIR/submm colours than were found for the SLUGS objects,
suggesting that the galaxies detected in the ERCSC contain
coolerdustthanwasdetectedinthemajorityofSLUGSsources.
3.2.COContamination
One factor that has complicated the interpretation of ground-
basedsubmmobservationsofgalaxieshasbeenthepresenceof
COemissionlineswithinthesubmmpassbandsthatmakeasig-
nificantcontributiontothecontinuumflux.Seaquistetal.(2004)
estimatedthattheCO(3-2)linecontributedanaverage25%of
the flux received in the SCUBA 850µm continuum passband
for galaxies observed in the SLUGS survey, with the range of
fluxcontributionsgoingfrom10–45%forthesubsetofSLUGS
galaxies for which CO(3-2) observations were available. The
SCUBA850µmfilterhasabandwidthof∼30GHz.ThePlanck
353filterissignificantlybroader,at∼90GHz,sothelinecontri-
Fig.3. 60µm to 857GHz (i.e., 350µm) colour plotted against butionwillbecorrespondinglysmallerat∼8%onaverage.This
thecirrusRMSat100µminIRAS.ERCSCsourcesclassifiedas contaminationfractioncanbecheckedusingobservedvaluesof
point-likeareshownasopendiamonds,whileextendedsources integratedCO3-2linefluxesforavarietyofobjectsfromBayet
areshownassoliddots.Notethattheextendedsourcesshowa et al. (2006) and matching them to continuum observations of
clearcorrelationbetweencolourandcirrusRMS,indicatingthat similarbeamsizetotheCOobservations.Wefindcontamination
thesesourcesarelikelytobecontaminatedbycirrusemission. fractionsof2%forArp220,6%forMrk231andaworstcaseex-
ampleinthecentralregionofNGC253,whereacontamination
of11%iscalculated.
Moregenerally,thisanalysishighlightsoneoftheissuesthat We can extend this analysis to other Planck bands using
mustbefacedwhenusingtheERCSCcatalogue.Anyonewish- NGC 253 as a worst case since, unlike mostothers, thisobject
ing to cross-matchPlanck sources, especially those detectedat has been observed over the full range of CO transitions acces-
high frequencies, with sources at other wavelengths, needs to sible fromtheground.We findthatthehigherfrequencybands
take greatcare in ensuringthat the ERCSC fluxes are not con- haveareducedlevelofcontaminationcomparedtothe353chan-
taminatedbycirrusemission. nel, with <1% at 857 and 6% at 545 (assuming a flat spectral
line energy distribution to estimate the contribution of the CO
5-4linethatisinaccessiblefromtheground).Morenormalob-
3. ComparisontoExistingSubmmData
jectsthanNGC253,notdominatedbyanongoingstarburst,will
3.1.GalaxiesdetectedwithSCUBA have an even smaller level of CO copntaminationthan this. At
lowerfrequencies,though,thecontaminationcanbecomemore
Thelargeststudiesofcooldustinexternalgalaxiestodatehave serious. CO 2-1 could contribute as much as 21% of the con-
beenassociatedwiththeSCUBALocalUniverseGalaxySurvey tinuum flux in the 217 band in the inner regions of NGC253.
(SLUGS)anditsextensions(Dunneetal.2000;Dunne&Eales In the 100 band for this objectthe CO line could contributeas
2001; Vlahakis et al. 2005; Clements et al. 2010b). These en- muchas75%ofthefluxofthethermalcontinuum.NGC253is
compass a total of about 250 objects that were observed with a worstcase scenario,so moretypicalsourceswouldof course
SCUBA.ThetargetswereselectedonthebasisofIRASflux B- suffermuchlesscontamination.However,veryfewgalaxiesare
band optical magnitudeor FIR luminosity.Most of the objects detectedbyPlancksolelyinthermalemissioninthisband,and
weredetectedonlyat850µm(i.e.,notalsoat450µm),allowing, thefewthataredetectedarebrightnearbyobjectswithsubstan-
with the IRAS data, only a single component(T,β) fit – where tial archival data that can allow a direct assessment of the CO
the SED is describedas Sν ∝ νβB(ν,T),with B(ν,T)beingthe contribution.
Planck function,andthe parametersT and βrepresentingtem-
OurconclusionfromthisanalysisisthattheCOcontribution
peratureanddustemissivityindex,respectively.Asmallfraction
tothecontinuumfluxislikelytobesmallerthanothersourcesof
of SLUGS galaxieswere also detected at 450µm,allowing for
uncertaintyforgenericERCSC-detectedgalaxiesexceptforthe
theexistenceofasecond,cooler,dustcomponenttobeassessed.
smallnumberwhicharedetectedinthe217band.Fluxexcesses
For these objects, and more recently for an ultra-luminous IR
detectedinthisbandalonemightthusresultfromCOemission
galaxy (ULIRG) sample, Dunne & Eales (2001) and Clements
ratherthanfromanyputativeverycolddustcomponent.
et al. (2010b)found some evidence for a colder dust contribu-
tion.
The presence of colder dust can be inferred from colour-
4. ParametricModelsofDustSEDs
colour diagrams when two submm flux densities are available.
We show the SLUGS sources and the ERCSC sources (after 4.1.FittingMethod
colour corrections to the Planck flux densities and a suitable
scaling has been applied to convert from Planck 857GHz flux GiventhePlanckandIRASfluxdatadescribedinSection2,with
densitytothe SCUBA 450µmband)in Fig.4.Ascanbeseen, appropriatecolourcorrectionsappliedtothePlanckfluxdensi-
the Planck galaxieslie on the same broadtrend as the SLUGS ties,wemodeltheunderlyingsignalinobservedfrequencyband
galaxies(withtheexceptionofasmallnumberofobjectsdom- νasoneormoregrey-bodysourceswithfluxdensity
inated by a non-thermal AGN component, such as 3C273 and
3C279).TheERCSCsources,though,extendthetrendtocooler G(ν;T,β)∝νβB (T), (1)
ν
5
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
0 ≤ β ≤ 3 and 3K ≤ T ≤ 100K with a uniformprior proba-
bilitybetweenthoselimits(detectionsofverylowtemperatures,
T <10K,areactuallydominatedbynon-thermalemission).We
adoptauniformprioronlnA,asitrangesovermanyordersof
i
magnitude for sources of widely varying absolute luminosities
anddistances.
The MCMC engine first creates a 15,000-sample Markov
chain, varying one parameter at a time, using this to find
an approximately-orthogonal linear combination of parame-
ters, with which a subsequent 100,000-sample chain is run.
Convergenceisassessedbyre-runningasmallnumberofchains
from a different starting point and checking for agreement to
muchbetterthanonestandarddeviationinallparameters.
WecalculateanapproximationtotheBayesianevidence,or
modellikelihood(Jaffe1996;Jaynes2003)inordertocompare
the two-temperature and one-temperature fits. The evidence is
calculatedastheaverageofthelikelihoodfunctionovertheprior
distribution; we approximate the likelihood as a multivariate
Fig.4. Colours for the ERCSC galaxies (black dots) compared
Gaussian function of the parameters centred at the maximum-
tothosefoundforSLUGSgalaxies(red;Dunne& Eales2001;
likelihoodMCMC sample with covariancegivenby the empir-
Vlahakisetal.2005)andULIRGs(blue;Clementsetal.2010b).
ical covariance of the samples (this approximation ignores the
Afluxcorrectionfactorof0.506hasbeenappliedtothePlanck
priorontheamplitudeoftheindividualgreybodies).
857GHz flux densities to extrapolate them to the SCUBA
InFigures5,6weshowsampleoutputfromourMCMCruns
450µm band. Only sources detected at > 3σ in the 353GHz
fordifferentobjectsandmodels,alongwiththemeasuredSEDs
(850µm) band and at > 5σ in the 857GHz band (the require-
andfits.ForobjectssuchasF01384-7515inFigure5,ifwein-
ment for inclusion in our analysis) are shown. The four points
stead performa two-temperaturefit, it prefersthe amplitudeof
abovethegeneraltrendin thelowerleftofthe diagramare the
thesecondtemperaturecomponenttobemanytensofordersof
well-knownnon-thermaldominatedsources3C279,0537−441,
magnitudebelowthefirst,andgivestemperaturevaluesconsis-
OJ+287and3C273.TheSLUGSpointwiththelowest60µmto
tent with the one-temperaturefit; this indicates, along with the
SCUBAfluxratioscorrespondstothegalaxyIC979;itisoffset
approximate evidence discussed above, that a one-temperature
from the general correlation for SLUGS and ERCSC galaxies,
modelisstronglypreferred.
andVlahakisetal.(2005)notethatitsIRASfluxdensitiesshould
betreatedwithcaution.
4.2.ResultsfromParametricFits
where B (T) is the Planck functionfor blackbodyflux density. 4.2.1. SingleComponentFits
ν
Wefitthedatad toone-componentmodelsoftheform
ν Figure7showstheT−βplaneforparametricfitstoall468reli-
ablyidentifiednon-extendedsourceswithintheERCSC-IIFSCz
d = AG[ν(1+z);T,β]+n , (2)
ν ν
crossmatchwhosefluxdensitiespassourS/N ratiocriteria for
ortotwo-componentmodelswithafixedβ=2grey-bodyexpo- inclusion, together with similar single temperature parametric
nent, fits from Dunne et al. (2000) and Clements et al. (2010b). As
canbeseen,thePlanckERCSCsourcesoverlapwiththeSLUGS
d = A G[ν(1+z);T ,2]+A G[ν(1+z);T ,2]+n . (3) galaxiesbutextendtocoolertemperaturesandflatter,i.e.,lower
ν 1 1 2 2 ν
β,SEDs.ThemedianparametersforthePlancksourcesareT =
Intheseequations,AorA isanoverallamplitudeforeachcom- 26.3K with temperatures ranging from 15 to 50K, and β=1.2
i
ponent, and the factor of (1+z) converts from rest-frame fre- compared to corresponding values from 104 SLUGS galaxies
quency to observed frequency for an object at redshift z. The Dunneetal.(2000)ofT =35Kandβ=1.3andofT =41Kand
noisecontributionisgivenbyn ,whichwemodelasaGaussian β=1.6for26ULIRGsClementsetal.(2010b).Thisconfirmsthe
ν
with variance σ2. For the Planck channels, the determination resultfromconsiderationofPlanck-IRAScoloursin Fig.4 that
ν
of the noise contribution is described in Planck Collaboration weareseeingcoolerdustintheERCSC-IIFSCzgalaxies.
(2011c).ForIRAS,thedetectionsareclassifiedintheIIFSCZof TherearetensourcescommontotheERCSC-IIFSCzcross-
Wang & Rowan-Robinson (2009) into (1) good detections, for matchedcatalogueandtheSLUGSstudies.ThefitsusingPlanck
which we take σ = 0.1d ; (2) marginal detections, for which data and using SLUGS data for all but two of these sources
ν ν
we take σ = 0.5d ; and (3) upper limits, for which we take are in good agreement. The two exceptions are NGC7541 and
ν ν
σ to be the reported upper limit, and d = 0. As mentioned NGC5676.Thelikelycauseofthedisagreementsinthesecases,
ν ν
above,weonlyconsidersourceswithdetectionsinthe857and isthepresenceofaclosecompanionIRASsourcetoNGC7541
545bandsatsignificanceofatleast5σorgreaterandinthe353 (NGC7537 3.1′ separation, and thus only ∼0.7 beam FWHM
bandofatleast3σ. away), so that the Planck flux density is likely over-estimated,
Thus, the parameters of our model are some subset of the andextendedIRASemissioninNGC5676,makingtheIRASFSC
A,T,β, dependingon which modelwe fit. We use a Bayesian fluxdensitiesusedinouranalysisunderestimates.
i i
Markov Chain Monte Carlo (MCMC) (e.g., Lewis & Bridle ThepositionofourgalaxiesintheLuminosity-Temperature
2002;Jaynes2003)techniquetoprobetheparameterspace;with planeis animportantquestionsince itrelatesto claimsof evo-
ourGaussiannoise,thisisequivalenttoanexplorationoftheχ2 lution in the dust propertiesof galaxies. It has previouslybeen
surface, albeit with a nonlinear parameterization.We require a suggestedthathighredshift,highluminositySMGshavelower
6
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
Fig.5.Leftpanel:SamplesfromtheSEDlikelihoodfunction(χ2)forF01384-7515andtheone-temperaturemodel.Thebottomrow
showstheone-dimensional(marginalized)posteriorfortheparameters(logA,β,T),andtheotherpanelsshowalltwo-dimensional
marginaldistributions.Rightpanel:thedatapointsforthisobject,alongwiththeone-temperaturemodelforthemaximumlikelihood
sample (yellowcurve;T = 30,K, β = 1.7)and the modeldeterminedby the mean of the samples of each parameter(redcurve;
T =(32±5)K,β=1.6±0.4wheretheerrorsarethevariances).Notethelogarithmicaxeswhichmakeinterpretationoftheerror
barsdifficult.
Fig.6.AsinFigure5,butforF00322-0840andthetwo-temperaturemodelwithfixedβ=2.Themaximum-likelihoodtemperatures
are20Kand49Kthemeansandvariancesare(20±0.8)Kand(44±8)K.
dusttemperaturesandhigherdustmassesthanmorenearbyob- localgalaxiesandtheSMGsisstartingtobefilledbythePlanck
jects Yang et al. (2007). Comparison of SMGs from Chapman objects.ThePlanckfluxesforoneofthesesourcesmayinclude
etal.(2005),Coppinetal.(2008)andKova´csetal.(2006)with somecontaminationfromcirrus,buttherestlieinareasofnor-
morelocalgalaxiesfromDunneetal.(2000)andlocalULIRGs mal to low Galactic cirrus noise and should thus be fully reli-
Clements et al. (2010b)confirms this effect. Claims have been able.ThelargeareacoverageofPlanckisparticularlyimportant
madethatsourcesselectedatlongerwavelengthsthanthe60µm in this as it allows us to probe generic L/ULIRG-class objects
typicalof IRASderivedsamples(e.g.,Symeonidisetal. 2009), (L > 1011L )ratherthanhavingtorelyonpre-selectedIRAS
FIR ⊙
Patelet al. (in prep))showless ofa separationbetweenthe lo- brightsourcesasinClementsetal. (2010b).Herschelobserva-
calsourcesandthehigherredshiftSMGs.Muchofthisworkis tions,whicharealsobeginningtoshowthegapbeingfilled,do
hamperedbythepoorsamplingofthedustSEDsoflocalobjects notcoverenoughareatoincludemanysuchL/ULRGobjectsin
at wavelengths between 100 and 850µm. Recent results from the local Universe Amblard et al. (2010). We find several cool
Herschel Amblard et al. (2010) and BLAST Dye et al. (2009) (T < 30K) ULIRGs that have very similar characteristics to
have begunto fill the gapbetween local IRAS galaxiesand the SMGs.Theissueoftheapparentdistinctionbetweenlocalgalax-
SMGs,suggestingthatourviewofdusttemperaturesinlocalob- ies and high-z SMG population thus seems to be approaching
jectsarebiasedtowarmertemperaturesthroughourdependence resolution.
onIRASfluxdensities.InFig.8weshowthepositionsofPlanck
While the temperaturedistributionof our objectsis consis-
galaxiesontheL−T plane.Ascanbeseen,thegapbetweenthe
tentwithwhathasbeenseenelsewhere,wefindthatsomeofour
7
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
Fig.7. Temperature-β correlation for ERCSC-IIFSCz matched Fig.8.TheTemperature-Luminosityplaneshowingavarietyof
galaxies(soliddots)togetherwithdatafromDunneetal.(2000) FIRpopulations.Opensquaresare SMGsfromChapmanetal.
and Clements et al. (2010b)for SCUBA observedsources(tri- (2005), open triangles are SMGs observed with SHARCII by
anglesandsquaresrespectively).Thefoursourceswithβ=0and Coppin et al. (2008) and Kova´cs et al. (2006) . + signs are
T < 10K at the left of the diagram are the non-thermaldomi- the SLUGS sources from Dunne et al. (2000), x are interme-
natedsources3C279,0537-441,OJ+287and3C273. diate redshift ULIRGs from Yang et al. (2007), ULIRGs from
Clementsetal.(2010b)areopencircles.Planck-ERCSC-IIFSCz
galaxies are shown as solid dots. As can be seen the previous
galaxieshaveβmorethan3σlessthan1.Onepossiblecausefor apparentdistinction between the local FIR populationsand the
this mightbe the 353 flux density beingaffected by Eddington SMGsisweakenedbysourcesfromthisworklyinginthesame
bias (see e.g., Teerikorpi (2004)),leading to ’flux-boosting’of regionastheSMGsandbyfillinginsomeofthegapbetweenthe
lowersignificancedetections,sinceweacceptthesefluxesdown populations.ThefouraberrantsourceswithT <15KandhighL
to 3σ. This is tested by repeating the fits using only the IRAS, in the bottom right of the plot are the non-thermal dominated
857and545fluxdensities.Whiletherearesmalldifferencesin sources3C279,0537-441,OJ+287and3C273.
fitstoindividualobjectsresultingformtheexclusionofthe353
flux densities, the general distribution remains the same, com-
pletewiththelowβsources.Wethusconcludethatthisisareal
effectandnotduetoEddingtonbiasoranyotherissuerelatedto
brightblazarsandarethusdominatedbynon-thermalemission,
the353fluxdensities,suchascontaminationbyemissionfrom
whileoneisasourcethatmightstillcontainsomecirruscontam-
theCO3-2molecularline.Whilesuchlowβvaluesarenotex-
ination.Wethusfindatleast13galaxieswhichappeartocontain
pected in simple modelsof dust, it is suggestive that the SEDs
verycolddust.Suchdusthaspreviouslybeenfoundinourown
canbebetterfitbyaparameterizationthatusesamixtureofdust
galaxy(e.g.Reach et al. 1995)buthas notbeenseen beforein
attwotemperatures,assuggestedbyDunne&Eales(2001)and
large scale extragalactic surveys. The small number of sources
whichis a goodfitto ourown galaxyasseen by COBE Reach
which have T lower than 7K are all dominated by nonther-
etal.(1995).Weinvestigatethisbyapplyingtwocomponentfits cold
malemission.Thedetailsofthetemperature-temperatureplotin
tothedustSEDs.
Fig. 9, with small scatter and the temperature of the hot com-
ponentbeinglargelyindependentofthatofthecoldcomponent
4.2.2. TwoTemperatureFits uptoT ∼ 18Kwilllikelyhaveimplicationsconcerningthe
cold
relationshipbetweenhotandcolddustcomponents.
Wecarryouttwotemperaturecomponentfitsonoursources,as-
sumingβ=2forbothcomponents,andpresentthetemperature-
temperature plot in Fig. 9. We also use the Baysian evidence 4.3.TheBroaderERCSC-IIFSCzSample
calculated during the fitting process (Jaffe 1996; Jaynes 2003)
todeterminehowmanyofoursourcesshowevidenceforatwo Theparametricfittingreportedhereonlyconcernsthe468non-
componentfitabovethatofthesinglecomponent(T,β)fit. We extendedERCSC-IIFSCz matched galaxieswhich are detected
find thatthe two componentfit is favoured in mostcases, with at 5σor greaterin the 857and 545bandsand at3σ or greater
425objectsgivingahigherevidenceforthismodelandonly43 at 353.This ensuresthat uncertaintiesin the fluxesdo notpre-
preferringthesinglecomponentfit.Onceagainwetestthepos- cludeagoodfittotheSED,butbringstheriskthatwemightbe
sibilitythatissueswiththelowersignificance353GHzfluxden- missingasignificantlydifferentsubclassofobjectintheremain-
sities might bias these fits by repeating the analysis with these ing1122.Wehavethusappliedourfittingmethodstothiswhole
fluxesexcluded.Whilethisincreasestheuncertaintiesinthefits, sample,regardlessofERCSCS/Nbeyondthebasicdetectionre-
aswiththe(T,β)fitswefindthattheexclusionofthe353GHz quirementofa5σorgreaterdetectionat857GHz.Whilethere
fluxesmakesnosystematicdifferencetothetemperaturesfound. arelargererrorsbarsonthefittedparameterswefindnoindica-
We find 17 galaxies fit by models containing a dust com- tionthatgalaxiesinthislargersamplehaveadifferentrangeof
ponentwith temperaturesas low as 10K. Fourof these are the dustpropertiestothosediscussedabove.
8
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
usetwofurthercirrustemplateswithφ=1and0.1,whichcor-
respond to significantly cooler dust than in the standard cirrus
template.Thestarting pointforthe presentanalysisisthe tem-
plate fit for each object given in the IIFSCz Catalogue Wang
& Rowan-Robinson (2009). This was done by fitting the op-
tical and near-IR fluxes with an optical galaxy or QSO tem-
plate. The IRAS data were then fitted with one of the original
fourRowan-Robinsonetal.(2008)templates.Thismodelisthen
comparedtothePlanckdata.Inalmostallcasesadditionalcom-
ponentsareneeded,sincetheIRAS-basedpredictionsunderesti-
matethesubmmfluxdensitiesprovidedbyPlanck.Moreweight
isgivento857and545fluxdensitiesinthisprocesssincethey
are generally at higher signal to noise and are thus less sub-
jecttoEddingtonbiaseffects.The353GHzand,wheredetected,
217GHzfluxdensitiesarealsosubjecttocontaminationbyCO
3-2and2-1emission,respectively,whichmayalsocontributeto
an excess flux in these bands beyond what might be expected
fromthecontinuumfit.
Fig.9. The Temperature-Temperature plane for two tempera-
5.1.ResultsfromTemplateFits
ture component fits for ERCSC-IIFSCz matched galaxies. to-
getherwithdatafromDunne&Eales(2001)andClementsetal. InFigure9weanalyzetheSEDsofthearchetypalnearbygalax-
(2010b)forSCUBAobservedsources(trianglesandsquaresre- ies,M51,M100,M2andArp220.M51andM100aremodelled
spectively). Only those ERCSC-IIFSCz sources where the two withtwocirrustemplateswithφ=1(solarneighbourhood)and5
temperature model is preferred and with a reliably determined (GalacticCentre),andwithamodestM82starburstcomponent.
colddustcomponenttemperature(i.e.,T/σ(T)>4)areplotted. M82itselfneedsanadditionalcomponentofcoolcirrus(φ=1)
The sourceswith the coldestTcold < 7K in this plot are domi- aswellastheM82templateofEfstathiouandRowan-Robinson.
natedbynon-thermalemission. FinallyArp220ismodelledextremelywelloverallinfraredand
submmwavelengthsbytheArp220templateusedbyEfstathiou
and Rowan-Robinson. The models for these galaxies by Silva
5. PhysicalModels:TemplateFitting et al. (1998) are also shown and perform well, especially for
M51andArp220.
The FIR and submm spectral energy distributions of galaxies Fig. 10 shows fits to galaxies with detections in 16 photo-
fromtheIRAS,ISOandSpitzer surveyshavebeensuccessfully metric bands: 5 optical bands (SDSS), 3 near infrared bands
modelled with a small number of templates (Rowan-Robinson (2MASS), 4 mid and FIR bands (IRAS) and 4 submm bands
(1992), Rowan-Robinson et al. (2005), Rowan-Robinson et al. (Planck).Thebluecurveisthesolarneighbourhoodcirrustem-
(2008)). However the submm data available in such studies is plate(φ= 1)andcontributessignificantlytotheSEDsof7out
quitelimitedandweexpecttogetamuchbetterunderstanding ofthe8galaxies.Dustgrainsinthiscomponentareintherange
ofcooldustingalaxieswiththedatafromPlanck.Alreadywith 15–20K,dependingongrainradiusandtype(Rowan-Robinson
Herschel,thereisevidenceforunexpectedquantitiesofcolddust 1992).Dustmassesareintherange107−3.108M .
⊙
insomegalaxiesRowan-Robinsonetal.(2010). Togetan overviewof the wholesample,Fig. 12showsthe
The Rowan-Robinson et al. (2010). study of the SEDs of distributionof545GHz(550µm)fluxdensityversusredshiftfor
Herschel-detected galaxies used SPIRE flux densities extend- galaxies well detected at 350–850µm, and with spectroscopic
ing from 250 to 500µm combined with pre-existing data from redshifts. The loci of galaxies with an Arp 220 template at lu-
SWIREatSpitzerandopticalwavelengths.Throughthecombi- minosity L = 1012 and 1013L are shown. With the restric-
IR ⊙
nationofPlanckdatawiththeIIFSCz,thegalaxysampleconsid- tionsto point-sourceswith gooddetectionsinthe three highest
eredhereincludesdatafromtheopticaltoIRASfluxes,andthen frequencyPlanckbands,andtogalaxieswithspectroscopicred-
thePlanckdatahasfluxdensitiesat350,550and850µm(857, shifts,anumberofULIRGsarefoundintheERCSCsurvey,but
545and353GHz).Someofourobjectsarealsodetectedat1.4 noHLIRGs,apartfromthequasar3C273.
mm(217GHz)whichisincludedinouranalysisifavailable.The Fig. 13 shows the colour-ratio S /S versus redshift.
857 545
rangeofwavelengthsavailablewiththeERCSC-IIFSCzsample Galaxies in the IRAS Bright Galaxy Sample are indicated as
is thus broader than that available through Herschel. This en- red dots. Sources for which T < 10K in the 2-temperature
2
ablesustoplacebetterconstraintsontheroleandimportanceof fits discussed above are indicated as blue dots. Sources with
colddustin these objects.Oursample isalso muchlargerthan log (S /S ) < 0.4 represent a new population of cooler
10 857 545
the 68 objects considered in Rowan-Robinsonet al. (2010), so submmsources.Wehavemodelledall17galaxieswithcool857
we can better determinethe varietyand overallstatistics of the GHz/545GHzcolours(log (S /S < 0.4),andgoodopti-
10 857 545
SEDsoflocalgalaxies. caldata(Fig.14).Almostallrequiretheverycoldcirrusmodel
ThetemplatesusedinfittingIRAS,ISOandSpitzerdataare withT =10–13K,φ=0.1(greencurvesinFig.14).Table1
dust
(1)acirrus(opticallythininterstellardust)modelcharacterised givesthepropertiesofthegalaxieswhoseSEDswehavemod-
by a radiation intensity φ = I(galaxy)/I(ISRF) = 5, where elledindetail.Thecolumnsare:ERCSCRAandDec,IRASFSC
I(ISRF)istheintensityoftheradiationfieldinthesolarneigh- name,separationbetweenERCSCandIIFSCzposition,spectro-
bourhood,(2)a normalM82-likestarburst, (3)a higheroptical scopic redshift, luminosityin cirrus φ = 5, φ = 1 andφ = 0.1
depth Arp220-like starburst, (4) an AGN dust torus. In fitting components, luminosity in M82 and Arp220 starburst compo-
the SEDs of Herschel galaxies, Rowan-Robinson et al. (2010) nents, optical luminosity, optical SED type, extinction, stellar
9
PlanckCollaboration:ThePlanckViewofNearbyGalaxies
Fig.10. Templatefits for the fourarchetypalnearbygalaxies,M51,M100,M82 andArp 220.Black curves:fits with Efstathiou
and Rowan-Robinson templates (black, separate components as dotted lines), blue curves: Silva et al. (1998) models. Planck
ERCSCdatashownasredfilledhexagrams.ISO-SWSmid-infraredspectroscopydataforM82andSpitzer-IRSdataforArp220
(Siebenmorgen&Kru¨gel2007)areshowninmagenta.
massanddustmass.Stellarmassanddustmassarecalculatedas foundinthe solarneighbourhood.We alsofindthatsomelocal
inRowan-Robinsonetal.(2008),buttheremaybesomemodel- galaxiesare both luminousand cool, with propertiessimilar to
dependenciesinthedustmassessincewehaveassumedthatthe those of the distant SMGs uncovered in deep submm surveys.
cold dust in these galaxies is similar to Galactic dust. We can Thissuggeststhatpreviousstudiesofdustinlocalgalaxieshave
summarizethe SED modellingshown in Figs 10,11 and14 as beenbiasedawayfromsuchluminouscoolobjects.Wealsofind
follows:(1)mostnearbygalaxiesshowevidencefordustattem- thatthedustSEDsinmostgalaxiesarebetterdescribedbypara-
peraturessimilartothatseeninthesolarneighbourhood(φ=1), metric modelscontainingtwo dustcomponents,one warm and
aswellasthewarmerdustfoundinIRAS,ISOandSpitzerstud- onecold,withthecoldcomponentreachingtemperaturesaslow
ies; (2) there is a new populationof coolsubmmgalaxieswith as10K.SomeobjectshaveSEDsdominatedbythiscoldmate-
evencoolerdust(φ = 0.1,T = 10–13K).Thiscoolerdustis rial.Theseconclusionsarebasedonbothparametricfitsandby
dust
likelytohaveamoreextendedspatialdistributionthengenerally detailedfittingofradiativetransferderivedphysicaltemplatesto
assumedforthegasanddustingalaxies. theSEDs.However,otherphysicalorparametricdescriptionsof
dust, for example where β varies with wavelength, might lead
todifferentresultsregardingthisverycoldcomponent.Thispa-
6. Conclusions perrepresentsthefirstexploitationofPlanckdataforthestudy
of a large sample of galaxies in the local Universe. As such it
Our studies of nearby galaxiesdetected by Planck have shown
indicates both the benefits and hazards of the ERCSC for this
evidenceforcolderdustthanhaspreviouslybeenfoundinlocal
galaxies.Thetemperatureofthisdust,however,issimilartothat
10