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Planck intermediate results. XXV. The Andromeda Galaxy as seen by Planck P. A. R. Ade, N. Aghanim, M. Arnaud, M. Ashdown, J. Aumont, C. Baccigalupi, A. J. Banday, R. B. Barreiro, N. Bartolo, E. Battaner, et al. To cite this version: P. A. R. Ade, N. Aghanim, M. Arnaud, M. Ashdown, J. Aumont, et al.. Planck intermediate results. XXV. The Andromeda Galaxy as seen by Planck. Astronomy and Astrophysics - A&A, 2015, 582, pp.A28. ￿10.1051/0004-6361/201424643￿. ￿cea-01383743￿ HAL Id: cea-01383743 https://hal-cea.archives-ouvertes.fr/cea-01383743 Submitted on 19 Oct 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A&A582,A28(2015) Astronomy DOI:10.1051/0004-6361/201424643 & (cid:13)c ESO2015 Astrophysics Planck intermediate results XXV. The Andromeda galaxy as seen by Planck PlanckCollaboration:P.A.R.Ade82,N.Aghanim55,M.Arnaud69,M.Ashdown65,6,J.Aumont55,C.Baccigalupi81, A.J.Banday90,10,R.B.Barreiro61,N.Bartolo28,62,E.Battaner91,92,R.Battye64,K.Benabed56,89,G.J.Bendo88, A.Benoit-Lévy22,56,89,J.-P.Bernard90,10,M.Bersanelli31,48,P.Bielewicz78,10,81,A.Bonaldi64,L.Bonavera61, J.R.Bond9,J.Borrill13,84,F.R.Bouchet56,83,C.Burigana47,29,49,R.C.Butler47,E.Calabrese87,J.-F.Cardoso70,1,56, A.Catalano71,68,A.Chamballu69,14,55,R.-R.Chary53,X.Chen53,H.C.Chiang25,7,P.R.Christensen79,34, D.L.Clements52,L.P.L.Colombo21,63,C.Combet71,F.Couchot67,A.Coulais68,B.P.Crill63,11,A.Curto61,6,65, F.Cuttaia47,L.Danese81,R.D.Davies64,R.J.Davis64,P.deBernardis30,A.deRosa47,G.deZotti44,81, J.Delabrouille1,C.Dickinson64,J.M.Diego61,H.Dole55,54,S.Donzelli48,O.Doré63,11,M.Douspis55,A.Ducout56,52, X.Dupac36,G.Efstathiou58,F.Elsner22,56,89,T.A.Enßlin75,H.K.Eriksen59,F.Finelli47,49,O.Forni90,10,M.Frailis46, A.A.Fraisse25,E.Franceschi47,A.Frejsel79,S.Galeotta46,K.Ganga1,M.Giard90,10,Y.Giraud-Héraud1, E.Gjerløw59,J.González-Nuevo18,61,K.M.Górski63,93,A.Gregorio32,46,51,A.Gruppuso47,F.K.Hansen59, D.Hanson76,63,9,D.L.Harrison58,65,S.Henrot-Versillé67,C.Hernández-Monteagudo12,75,D.Herranz61, S.R.Hildebrandt63,11,E.Hivon56,89,M.Hobson6,W.A.Holmes63,A.Hornstrup15,W.Hovest75, K.M.Huffenberger23,G.Hurier55,F.P.Israel86,A.H.Jaffe52,T.R.Jaffe90,10,W.C.Jones25,M.Juvela24, E.Keihänen24,R.Keskitalo13,T.S.Kisner73,R.Kneissl35,8,J.Knoche75,M.Kunz16,55,3,H.Kurki-Suonio24,42, G.Lagache5,55,A.Lähteenmäki2,42,J.-M.Lamarre68,A.Lasenby6,65,M.Lattanzi29,C.R.Lawrence63,R.Leonardi36, F.Levrier68,M.Liguori28,62,P.B.Lilje59,M.Linden-Vørnle15,M.López-Caniego36,61,P.M.Lubin26, J.F.Macías-Pérez71,S.Madden69,B.Maffei64,D.Maino31,48,N.Mandolesi47,29,M.Maris46,P.G.Martin9, E.Martínez-González61,S.Masi30,S.Matarrese28,62,39,P.Mazzotta33,L.Mendes36,A.Mennella31,48, M.Migliaccio58,65,M.-A.Miville-Deschênes55,9,A.Moneti56,L.Montier90,10,G.Morgante47,D.Mortlock52, D.Munshi82,J.A.Murphy77,P.Naselsky79,34,F.Nati25,P.Natoli29,4,47,H.U.Nørgaard-Nielsen15,F.Noviello64, D.Novikov74,I.Novikov79,74,C.A.Oxborrow15,L.Pagano30,50,F.Pajot55,R.Paladini53,D.Paoletti47,49, B.Partridge41,F.Pasian46,T.J.Pearson11,53,M.Peel64,(cid:63),O.Perdereau67,F.Perrotta81,V.Pettorino40,F.Piacentini30, M.Piat1,E.Pierpaoli21,D.Pietrobon63,S.Plaszczynski67,E.Pointecouteau90,10,G.Polenta4,45,L.Popa57, G.W.Pratt69,S.Prunet56,89,J.-L.Puget55,J.P.Rachen19,75,M.Reinecke75,M.Remazeilles64,55,1,C.Renault71, S.Ricciardi47,I.Ristorcelli90,10,G.Rocha63,11,C.Rosset1,M.Rossetti31,48,G.Roudier1,68,63, J.A.Rubiño-Martín60,17,B.Rusholme53,M.Sandri47,G.Savini80,D.Scott20,L.D.Spencer82,V.Stolyarov6,85,66, R.Sudiwala82,D.Sutton58,65,A.-S.Suur-Uski24,42,J.-F.Sygnet56,J.A.Tauber37,L.Terenzi38,47,L.Toffolatti18,61,47, M.Tomasi31,48,M.Tristram67,M.Tucci16,G.Umana43,L.Valenziano47,J.Valiviita24,42,B.VanTent72,P.Vielva61, F.Villa47,L.A.Wade63,B.D.Wandelt56,89,27,R.Watson64,I.K.Wehus63,D.Yvon14,A.Zacchei46,andA.Zonca26 (Affiliationscanbefoundafterthereferences) Received21July2014/Accepted5September2015 ABSTRACT TheAndromedagalaxy(M31)isoneofafewgalaxiesthathassufficientangularsizeontheskytoberesolvedbythePlancksatellite.Planck has detected M31 in all of its frequency bands, and has mapped out the dust emission with the High Frequency Instrument, clearly resolving multiplespiralarmsandsub-features.Weexaminethemorphologyofthislong-wavelengthdustemissionasseenbyPlanck,includingastudyof itsoutermostspiralarms,andinvestigatethedustheatingmechanismacrossM31.Wefindthatdustdominatingthelongerwavelengthemission (>∼0.3mm)isheatedbythediffusestellarpopulation(astracedby3.6µmemission),withthedustdominatingtheshorterwavelengthemission heatedbyamixoftheoldstellarpopulationandstar-formingregions(astracedby24µmemission).Wealsofitspectralenergydistributionsfor individual5(cid:48)pixelsandquantifythedustpropertiesacrossthegalaxy,takingintoaccountthesedifferentheatingmechanisms,findingthatthereis alineardecreaseintemperaturewithgalactocentricdistancefordustheatedbytheoldstellarpopulation,aswouldbeexpected,withtemperatures rangingfromaround22Kinthenucleusto14Koutsideofthe10kpcring.Finally,wemeasuretheintegratedspectrumofthewholegalaxy, whichwefindtobewell-fittedwithaglobaldusttemperatureof(18.2±1.0)Kwithaspectralindexof1.62±0.11(assumingasinglemodified blackbody),andasignificantamountoffree-freeemissionatintermediatefrequenciesof20–60GHz,whichcorrespondstoastarformationrate ofaround0.12M yr−1.Wefinda2.3σdetectionofthepresenceofspinningdustemission,witha30GHzamplitudeof0.7±0.3Jy,whichisin (cid:12) linewithexpectationsfromourGalaxy. Keywords.galaxies:individual:Messier31–galaxies:structure–galaxies:ISM–submillimeter:galaxies–radiocontinuum:galaxies (cid:63) Correspondingauthor:M.Peel,[email protected] ArticlepublishedbyEDPSciences A28,page1of23 A&A582,A28(2015) 1. Introduction than 160µm was primarily heated by star-forming regions (henceforthabbreviatedas“SFRdust”),whiledust-dominating The infrared (IR) and submillimetre (submm) wavelength do- emission at wavelengths over 250µm may be primarily heated mainisparticularlyusefulforunderstandingtheprocessesdriv- by the total stellar populations, including evolved stars in the ing star formation in various galactic environments, since dust discsandbulgesofthegalaxies(henceforthabbreviatedasinter- grainsre-emitinthisfrequencywindowtheenergythathasbeen stellarradiationfielddust,or“ISRFdust”.Planck observations absorbed from the UV-optical starlight. Our view of the global of the SED of M31 allow us to examine dust heating within innerandouterdiskstarformationandISMpropertiesofthespi- the galaxy at frequencies much lower than was possible with ralgalaxyinwhichweliveislimitedbyourpositioninsidethe Herschel. Once the dust heating sources are identified empiri- Galaxy,butournearestspiralneighbour,theAndromedagalaxy callyandtheSEDisseparatedintodifferentthermalcomponents (alsoknownasMessier31),offersthebestviewoftheenviron- based on the heating sources, it will be possible to more accu- mentaleffectswithinanentiregalaxy,particularlybecauseofits ratelymeasurethetemperatureofthecoldestdustwithinM31, largeangularextentonthesky. which is also critically important to properly estimate the dust M31hasbeenextensivelystudiedatIR/submmwavelengths mass. with data from the Infrared Astronomical Satellite (IRAS, Non-thermal emission from M31 can also be measured at Habingetal.1984;Walterbos&Schwering1987),theInfrared thelowestfrequenciescoveredbyPlanck.Synchrotronemission Space Observatory (ISO, Haas et al. 1998), the Spitzer Space fromM31wasdiscoveredintheearlydaysofradioastronomy Telescope (Barmby et al. 2006; Gordon et al. 2006; Tabatabaei (Brown&Hazard1950,1951).Ithassubsequentlybeenmapped & Berkhuijsen 2010), and, most recently, the Herschel Space at low frequencies (Beck et al. 1998; Berkhuijsen et al. 2003), Observatory (Fritz et al. 2012; Groves et al. 2012; Smith et al. and associated emission has even been detected at gamma-ray 2012; Ford et al. 2013; Draine et al. 2014; Viaene et al. 2014; frequencies(Abdoetal.2010).However,thesynchrotronemis- Kirketal.2015).ExceptfortheHerscheldata,theseIRobserva- sion has not been studied at higher frequency. Free-free emis- tionshavebeenrestrictedtoobservingthepeakofthedustemis- sion is also expected from M31. This emission may be used sioninthefarinfrared(FIR)aswellasmid-infrared(MIR)emis- to measure star formation rates in a way that is not affected by sionfrom>∼100Kdustandpolycyclicaromatichydrocarbons.In dustextinctionorreliantuponassumptionsaboutthedustheat- contrast,duetothelargeangularsizeofM31ithasbeenpartic- ing sources (e.g., see Murphy et al. 2011). Free-free emission ularly difficult to map the entire galaxy at wavelengths longer from M31 was first seen by Hoernes et al. (1998), as well as than 500µm, which is needed to constrain the Rayleigh-Jeans Berkhuijsen et al. (2003) and Tabatabaei et al. (2013). Planck side of the dust emission and the contribution of non-thermal dataprovidetheopportunitytocharacterizethehigh-frequency emission sources to the spectral energy distribution (SED). In radioemissionforthefirsttime. fact, the submm data for nearby spiral galaxies that have been Section2ofthispaperdescribesthePlanckdata,andSect.3 published(e.g.,Dunneetal.2000;Stevensetal.2005;Daleetal. theancillarydatathatweusehere.Wediscussthemorphology 2007)havehadlowsignal-to-noiselevels,havebeenbiasedto- ofthedustasseenbyPlanckinSect.4,thecolourratiosandthe wardsinfrared-brightsources, orhaveonlycoveredthe centres implicationstheyhaveonthedustheatingmechanisminSect.5, ofgalaxies. andthespectralenergydistributionson5(cid:48) scalesinSect.6and Data from Planck (Tauber et al. 2010)1, which range forthewholeofM31inSect.7.WeconcludeinSect.8. from28.4to857GHz(10.5to0.35mm)withangularresolution between31(cid:48) and5(cid:48),allowustoexaminetheRayleigh-Jeanstail ofthedustSEDandthetransitionintofree-freeandsynchrotron 2. Planck data emission at longer wavelengths. Moreover, since Planck ob- Planck (Tauberetal.2010;PlanckCollaborationI2011)isthe served the entire sky at high sensitivity, its High Frequency thirdgenerationspacemissiontomeasuretheanisotropyofthe Instrument (HFI, Lamarre et al. 2010) provides high signal-to- cosmic microwave background (CMB). It observes the sky in noise maps of M31 that extend to the outermost edges of the nine frequency bands covering 30–857GHz (10.5 to 0.35mm) galaxy. with high sensitivity and angular resolution from 31(cid:48) to 5(cid:48). Planck’s large-scale map of the region provides the oppor- The Low Frequency Instrument (LFI; Mandolesi et al. 2010; tunitytostudydustheatinginM31outtotheopticalradiusof Bersanelli et al. 2010; Mennella et al. 2011) covers the 30, 44, thegalaxy.PriorinvestigationswithIRASofdustheatinginour and70GHz(10.5,6.8,and4.3mm)bandswithamplifierscooled Galaxyandothershadproducedseeminglycontradictoryresults, to 20K. The High Frequency Instrument (HFI; Lamarre et al. with some studies indicating that dust seen at 60–100µm was 2010;PlanckHFICoreTeam2011a)coversthe100,143,217, heated primarily by star-forming regions (Devereux & Young 353,545,and857GHz(3.0,2.1,1.38,0.85,0.55,and0.35mm) 1990; Buat & Xu 1996) and others demonstrating that evolved bandswithbolometerscooledto0.1K.Polarizationismeasured stellarpopulationscouldpartiallycontributetoheatingthedust inallbutthehighesttwobands(Leahyetal.2010;Rossetetal. seenbyIRAS(e.g.,LonsdalePersson&Helou1987;Walterbos 2010). A combination of radiative cooling and three mechani- & Schwering 1987; Sauvage & Thuan 1992; Walterbos & cal coolers produces the temperatures needed for the detectors Greenawalt1996).MorerecentobservationswithHerschelofa andoptics(PlanckCollaborationII2011).Twodataprocessing number of galaxies, including M81, M83, NGC 2403 (Bendo centres (DPCs) check and calibrate the data and make maps of et al. 2010, 2012a) and M33 (Boquien et al. 2011), demon- the sky (Planck HFI Core Team 2011b; Zacchei et al. 2011). strated that dust-dominating emission at wavelengths shorter Planck’ssensitivity,angularresolution,andfrequencycoverage makeitapowerfulinstrumentforGalacticandextragalacticas- 1 Planck (http://www.esa.int/Planck) is a project of the trophysics as well as cosmology. Early astrophysics results are EuropeanSpaceAgency–ESA–withinstrumentsprovidedbytwosci- giveninPlanckCollaborationVIII–XXVI(2011),basedondata entificConsortiafundedbyESAmemberstates(inparticularthelead countries:FranceandItaly)withcontributionsfromNASA(USA),and taken between 13 August 2009 and 7 June 2010. Intermediate telescopereflectorsprovidedinacollaborationbetweenESAandasci- astrophysicsresultsarepresentedinaseriesofpapersbetween entificConsortiumledandfundedbyDenmark. themajordatareleases. A28,page2of23 PlanckCollaboration:AndromedaasseenbyPlanck CMB map to subtract the CMB from the Planck data, as from +43:00 a visual inspection this appears to be the cleanest map of the -4 .0 -3 .5 -4 .0 -3 .5 CMBinthisregionfromthefourmethods(seeFig.1).TheNILC log(T (K)) log(T (K)) +42:00 RJ RJ andSEVEMmapsareparticularlycontaminatedbyemissionfrom M31; we also use the NILC map to test the impact of residual +41:00 foregroundemissionbeingsubtractedoutofthemapsalongwith theCMB.Figure1showsthe217GHz(1.38mm)mappre-and +40:00 post-CMB subtraction, along with the four CMB maps of the 217 GHz (1.38 mm) 217 GHz (1.38 mm) M31region. No CMB subtraction CMB-subtracted The maps are converted from CMB temperature (T ) to +43:00 Rayleigh-Jeansbrightnesstemperature(TRJ)usingthesCtaMnBdard 000) +42:00 -4 0T0CMB0 (µK40 )0 -4 0T0CMB0 (µK40 )0 acloseoffiucsieentthseansomdeisncarlibfreedquinenPcileasncfkorCthoellabbaonrdast,ioannd(2(0w1h3e)n; fiwte- 2 J ting a spectral model to the data) colour corrections, depend- n ( o ing on the spectra of the emission. The 100 and 217GHz (3.0 nati +41:00 and1.38mm)PlanckbandsincludeCOemission.TheCOemis- ecli sionfromM31hasbeenmappedwithground-basedtelescopes D +40:00 (e.g., Koperetal.1991; Dame et al. 1993; Nieten et al. 2006), SMICA CMB map Commander CMB map but these do not include the full extent of M31 that is consid- +43:00 eredhere.TheCOemissionistooweaktobereliablydetectedin thefull-skyPlanckCOmaps(PlanckCollaborationXIII2014)3. -4 00 0 40 0 -4 00 0 40 0 T (µK) T (µK) WedonotcorrectfortheCOemissioninthemaps;insteadwe +42:00 CMB CMB omit the 217GHz (1.38mm) channel from the SED fitting in Sect.6,andwecomparethelevelofCOemissionexpectedfrom +41:00 theground-basedCOmapofNietenetal.(2006)(describedin Sect.3.1)withtheemissionattributabletoCOintheintegrated +40:00 PlanckmeasurementsofM31inSect.7. SEVEM CMB map NILC CMB map To assess the uncertainty in the Planck maps, we estimate the instrumental noise and cirrus contamination by measuring 00 :50 00: 40 00: 50 00 :40 the scatter of flux densities in an adjacent background region Right Ascension (J2000) of the Planck maps (see Sect. 6 for details). We conservatively Fig.1. Top left: M31 at 217GHz (1.38mm), with no CMB subtrac- assume calibration uncertainties of 10% for 857 and 545GHz tion. Top right: the SMICA CMB-subtracted 217GHz (1.38mm) map. (350and550µm)and3%forallotherPlanck frequencies(see Middle left: the SMICA map of the CMB in the same region. Middle PlanckCollaboration2013). right: Commander CMB map. Bottom left: SEVEM CMB map. Bottom For the integrated spectrum analysis in Sect. 7, the Planck right:NILCCMBmap. full-sky maps are used directly in HEALPix4 format (Górski et al. 2005). For the higher resolution analyses, however, we InthispaperweusePlanck datafromthe2013distribution use “postage stamp” 2D projected maps centred on M31. To of released products (Planck Collaboration I 2014), which can conserve the photometry of the data whilst repixelizing, we beaccessedviathePlanckLegacyArchiveinterface2,basedon use the Gnomdrizz package (Paradis et al. 2012a) to create thedataacquiredbyPlanckduringits“nominal”operationspe- the postage stamp maps from the HEALPix data; since the riod from 13 August 2009 to 27 November 2010. In order to HEALPixGnomviewfunctionusesnearest-neighbourinterpola- studyM31inthePlanckmaps,theCMBneedstobesubtracted. tion, it does not necessarily conserve the photometry, although CMBmaps inthe M31 regionare showninFig. 1.The CMB- wetestedthatthereisnosignificantdifferenceinthiscase.The subtracted Planck maps are presented in Fig. 2 at their native resulting postage stamp maps in equatorial coordinates are of resolution and their properties are summarized in Table 1. We size 250(cid:48) ×250(cid:48) with 0.5(cid:48) ×0.5(cid:48) pixels, centred on RA 10◦.68, use various combinations of the Planck maps throughout this Dec41◦.27(l=121◦.2,b=−21◦.6). paper,andallmapsareusedtoqualitativelystudytheemission Whenquantitativelyanalysingthedata,wefirstsmoothtoa atallPlanckfrequenciesinSect.4.Mapsat217GHzandhigher commonresolutionofeither5(cid:48) (at217GHzandabove/1.38mm frequencies(1.38mmandshorterwavelengths)areusedtoquan- orlower,wherethedatahaveanativeresolutionof4.(cid:48)39–4.(cid:48)87) titativelyinvestigatetheemissioninSects.5and6,andallfre- or 1◦ (at all frequencies) by convolving the map with a circu- quenciesareusedtomeasurethetotalemissioninSect.7. larGaussianbeamwithafull-widthathalf-maximum(FWHM) CMB subtraction is particularly important for M31, of θ = (cid:16)θ2 −θ2 (cid:17)1/2, where θ is the desired FWHM conv new old new given the similarities in angular size between M31 and the andθ isthecurrentFWHMofthemaps.Forsomeofthelater old anisotropies in the CMB. Additionally, there is an unfortu- analysis,wealsorepixelizethe5(cid:48) resolutiondatainto5(cid:48) pixels nately large (approximately290µK) positive CMB fluctuation (see Sect. 5), while the 1◦ data are analysed at their native at the southern end of M31, which can be clearly seen in the CMB map panels of Fig. 1. As part of the Planck 2013 3 There is no detection in the Planck Type 1 CO map (Planck delivery, CMB maps from four component separation tech- Collaboration XIII 2014); the morphology is not consistent with the niques were released, namely maps from the Commander, knownstructureintheType2map,andalthoughthereisadetectionin NILC,SEVEMandSMICAcomponentseparationmethods(Planck theType3mapandtheringmorphologyisvisible,thedetectiondoes Collaboration XII 2014). We specifically use the SMICA nothaveahighsignal-to-noiseratioandmaybecontaminatedbydust emission. 2 http://pla.esac.esa.int/pla/ 4 http://healpix.jpl.nasa.gov A28,page3of23 A&A582,A28(2015) +43:00 -3. 75 -3. 2 5 - 4 .0 -3 .5 -4 .0 -3 .5 log(T (K)) log(T (K)) log(T (K)) +42:00 RJ RJ RJ +41:00 +40:00 28.4 GHz (10.5 mm) 44.1 GHz (6.8 mm) 70.4 GHz (4.3 mm) +43:00 0) -4 . 25 -3. 75 -4. 25 -3. 7 5 -4 .0 -3 .5 0 log(T (K)) log(T (K)) log(T (K)) 0 +42:00 RJ RJ RJ 2 J ( n o ati +41:00 n cli e D +40:00 100 GHz (3.0 mm) 143 GHz (2.1 mm) 217 GHz (1.38 mm) +43:00 -4 .0 -3 .5 - 4 .0 -3 .5 -3 .0 - 4 .0-3 .5-3 . 0 log(T (K)) log(T (K)) log(T (K)) +42:00 RJ RJ RJ +41:00 +40:00 353 GHz (850 µm) 545 GHz (550 µm) 857 GHz (350 µm) 00 :50 00: 40 00: 50 00: 40 00: 50 00: 40 Right Ascension (J2000) Fig.2. Maps of M31 in total intensity from Planck (after CMB subtraction). Top to bottom, left to right: Planck 28.4, 44.1, and 70.4GHz; Planck 100, 143, and 217GHz; Planck 353, 545, and 857GHz. All plots have units of Kelvin (T ), have a 1◦ equatorial graticule overlaid, RJ are250(cid:48)×250(cid:48)with0.5(cid:48)×0.5(cid:48)pixelsandarecentredonRA10◦.68,Dec41◦.27,withnorthupandeasttotheleft. HEALPixN resolution.ThePlanckbeamsarenotsymmetric ameterof190.(cid:48)5±0.(cid:48)1andamajor-to-minorratioof3.09±0.14 side at the roughly 20% level (e.g., see Zacchei et al. 2011; Planck (deVaucouleursetal.1991).FollowingXu&Helou(1996),we HFI Core Team 2011b), and the ancillary data sets used (see assumethatithasaninclinationanglei=79◦andapositionan- Sect.3)willalsohavenon-Gaussianbeams;smoothingthedata gleof37◦withrespecttoNorth(bothinequatorialcoordinates). to a common resolution reduces the effect of the asymmetry. However, a residual low-level effect will still be present in this 3.1. The5(cid:48) dataset analysis, for example in terms of introducing some correlation betweenadjacent5(cid:48)pixels. WeusesixdatasetsinadditiontothePlanckdatainour5(cid:48) res- olution study. At high frequencies, we use IRAS, Spitzer, Herschel, and ISO data to trace the shorter wavelength dust 3. Ancillarydata  emission. At lower frequencies, we use ground-based H and Theancillarydatathatweuseinthispaperfallundertwocate- CO data sets to trace the gas within the galaxy. To match the gories.Forthehigherresolutionspatialanalysis,weneedobser- pixelizationandresolutionoftheseotherdatatothePlanckdata, vationswithresolutionequaltoorgreaterthanthePlanck high weregridthedataonto0.25(cid:48) pixelsinthesamecoordinatesys- frequencyresolutionof5(cid:48);thesearedescribedinSect.3.1.For temandskyregionasthePlanckdataandthensmooththemto theintegratedSED,wecanmakeuseoflarge-scalesurveydata acommonresolutionof5(cid:48).Furtherrepixelizationto5(cid:48) pixelsis witharesolutionof1◦ orhigher;wedescribethesedatasetsin thencarriedoutfortheanalysisinlatersections. Sect.3.2.AllofthedatasetsaresummarizedinTable1. To trace dust emission in the infrared, we use 24, 70, Wealsomakeuseofancillaryinformationaboutthedistance and160µmdataoriginallyacquiredbyGordonetal.(2006)us- andinclinationofM31.M31isatadistanceof(785±25)kpc ingtheMultibandImagingPhotometerforSpitzer(MIPS;Rieke (McConnachie et al. 2005), with an optical major isophotal di- etal.2004)ontheSpitzerSpaceTelescope(Werneretal.2004). A28,page4of23 PlanckCollaboration:AndromedaasseenbyPlanck Table1.Sourcesofthedatasetsusedinthispaper,aswellastheirfrequency,wavelength,resolution,calibrationuncertainty,andrmson5(cid:48)scales (fordatawith5(cid:48)resolutionorbetteronly;seelater). Source ν[GHz] λ[mm] Res. Unc.σ(5(cid:48),Jy) Analysis Reference Haslam 0.408 734 60.(cid:48) 10% ... L Haslametal.(1982) Dwingeloo 0.820 365 72.(cid:48) 10% ... L Berkhuijsen(1972) Reich 1.4 214 35.(cid:48) 10% ... L Reichetal.(2001) WMAP9-year 22.8 13 49.(cid:48) 3% ... L Bennettetal.(2013) Planck 28.4 10.5 32.(cid:48)24 3% ... L PlanckCollaborationII(2014) WMAP9-year 33.0 9.0 40.(cid:48) 3% ... L Bennettetal.(2013) WMAP9-year 40.7 7.4 31.(cid:48) 3% ... L Bennettetal.(2013) Planck 44.1 6.8 27.(cid:48)01 3% ... L PlanckCollaborationII(2014) WMAP9-year 60.7 4.9 21.(cid:48) 3% ... L Bennettetal.(2013) Planck 70.4 4.3 13.(cid:48)25 3% ... L PlanckCollaborationII(2014) WMAP9-year 93.5 3.2 13.(cid:48) 3% ... L Bennettetal.(2013) Planck 100 3.0 9.(cid:48)65 3% ... L PlanckCollaborationVI(2014) Planck 143 2.1 7.(cid:48)25 3% ... L PlanckCollaborationVI(2014) Planck 217 1.38 4.(cid:48)99 3% 0.015 LH PlanckCollaborationVI(2014) Planck 353 0.85 4.(cid:48)82 3% 0.05 LH PlanckCollaborationVI(2014) Planck 545 0.55 4.(cid:48)68 7% 0.13 LH PlanckCollaborationVI(2014) HerschelSPIREPLW 600 0.50 0.(cid:48)59 4% 0.09 H Fritzetal.(2012) Planck 857 0.35 4.(cid:48)33 7% 0.31 LH PlanckCollaborationVI(2014) HerschelSPIREPMW 857 0.35 0.(cid:48)40 4% 0.23 H Fritzetal.(2012) HerschelSPIREPSW 1200 0.25 0.(cid:48)29 4% 0.29 H Fritzetal.(2012) COBE-DIRBE 1249 0.240 40.(cid:48) 13% ... L Hauseretal.(1998) ISO 1763 0.175 1.(cid:48)3 10% 0.8 H Haasetal.(1998) SpitzerMIPSB3 1870 0.16 0.(cid:48)63 12% 0.53 H Gordonetal.(2006),Bendoetal.(2012b) HerschelPACS 1874 0.16 0.(cid:48)22 4% 2.4 H Fritzetal.(2012) COBE-DIRBE 2141 0.14 40.(cid:48) 13% ... L Hauseretal.(1998) COBE-DIRBE 2997 0.10 40.(cid:48) 13% ... L Hauseretal.(1998) HerschelPACS 2998 0.10 0.(cid:48)21 4% 2.5 H Fritzetal.(2012) IRAS(IRIS)B4 3000 0.10 4.(cid:48)3 13% 0.37 LH Miville-Deschênes&Lagache(2005) SpitzerMIPSB2 4280 0.07 0.(cid:48)3 10% 0.12 H Gordonetal.(2006),Bendoetal.(2012b) COBE-DIRBE 5000 0.06 40.(cid:48) 13% ... L Hauseretal.(1998) IRAS(IRIS)B3 5000 0.06 4.(cid:48)0 13% 0.12 LH Miville-Deschênes&Lagache(2005) IRAS(IRIS)B2 12000 0.025 3.(cid:48)8 13% 0.06 LH Miville-Deschênes&Lagache(2005) SpitzerMIPSB1 12490 0.024 0.(cid:48)1 4% 0.012 H Gordonetal.(2006),Bendoetal.(2012b) IRAS(IRIS)B1 25000 0.012 3.(cid:48)8 13% 0.04 LH Miville-Deschênes&Lagache(2005) SpitzerIRAC 83000 0.0036 1.(cid:48)(cid:48)7 3% ... H Barmbyetal.(2006),SpitzerScienceCenter(2012) DRAOH 1.4 ... 0.(cid:48)37 5% ... H Cheminetal.(2009) IRAMCO 115. ... 0.(cid:48)38 15% ... H Nietenetal.(2006) Notes.TheAnalysiscolumnindicateswhetherthedatasethasbeenusedinthe5(cid:48)high-resolution(H)and/or1◦low-resolution(L)analysis.The firstpartofthetabledescribesthecontinuumdatasets,andthesecondpartdescribesthespectrallinedatasets. The 24, 70, and 160µm data have point spread functions with sideofthemasktoassessthermsuncertaintyintheSpitzerdata, FWHM of 6(cid:48)(cid:48), 18(cid:48)(cid:48), and 38(cid:48)(cid:48), respectively. The data were pro- includingthermalnoise,residualcirrusandotherlarge-scalefea- cessedusingtheMIPSDataAnalysisTools(Gordonetal.2005) turesthatcontributetotheuncertaintyon5(cid:48)scales. as well as additional data processing steps described by Bendo Weadditionallyutilisethe3.6µmdataproducedbyBarmby et al. (2012b). The data processing includes the removal of the et al. (2006) using the Spitzer Infrared Array Camera (IRAC; effect of cosmic ray hits and drift in the background signal. Fazioetal.2004)totracetheolderstellarpopulationinthebulge The background is initially subtracted from the individual data ofM31;thesedatahavearesolutionof1(cid:48).(cid:48)7. framesbycharacterizingthevariationsinthebackgroundsignal We also include Herschel data from the Herschel as a function of time using data that fall outside of the galaxy. Exploitation of Local Galaxy Andromeda (HELGA) survey This removes large-scale background/foreground structure out- (Fritz et al. 2012). These data are at 500, 350, 250, 160 sidethegalaxy,includingthecosmicinfraredbackground(CIB) and100µm(600,857,1200,1874and2998GHz)andhaveres- andzodiacallight,butresidualforegroundcirrusstructuresand olutionsbetween0.(cid:48)21and0.(cid:48)59(Lutz2012;Valtchanov2014). compact sources remain in the data. Any residual background We re-align the data to match the Spitzer 24µm data using emissioninthefinalmapsismeasuredoutsidetheopticaldiscof three compact sources that are seen at all Herschel frequen- thegalaxyandsubtractedfromthedata.Foradditionaldetailson cies.TheHerscheldatahaveafluxcalibrationuncertaintyof4% thedatareductionprocess,seethedescriptionofthedatareduc- (HerschelSpaceObservatory2013;Bendoetal.2013).Forthe tionbyBendoetal.(2012b).PSFcharacteristicsandcalibration integrated SED, we smooth the Herschel data to 1◦, and then uncertaintiesaredescribedbyEngelbrachtetal.(2007),Gordon regrid these data onto an oversampled N = 16384 HEALPix side etal.(2007),andStansberryetal.(2007).Weusethedataout- map,whichwasthenresampleddowntoN =256. side A28,page5of23 A&A582,A28(2015) We also make use of several data sets for comparison pur- 4. Infraredmorphology poses, namely: the IRAS data at 12, 25, 60, and 100µm from Miville-Deschênes & Lagache (2005); the ISO 175µm data ThePlanckmapsofM31areshowninFig.2.At100GHzand from Haas et al. (1998, priv. comm.); the H emission map allhigherPlanckfrequencies(3.0mmandshorterwavelengths), from Chemin et al. (2009); and the 12CO J = 1→0 map the prominent 10kpc dust ring of Andromeda can clearly be of M31 from Nieten et al. (2006). These are described in seen – as expected based on previous infrared observations of AppendixA. M31–aswellasanumberofotherextendedfeatures. At high frequencies, where we have the highest signal-to- noise and highest spatial resolution, we can see features much 3.2. The1◦ dataset further out than the 10kpc ring. The top panels of Fig. 3 show thePlanck857GHz(350µm)maplabelledtoshowthelocations of the key features in the dust emission; the bottom-left panel WhenlookingattheintegratedspectrumofM31,wemakeuse shows the Herschel 857GHz (350µm) data, and the bottom- of a number of large-area, low-resolution surveys. These are  rightpanelofFig.3showstheH mapfromCheminetal.(2009) publicly available in HEALPix format. We convolve the data for comparison. A cut through the Planck 857 and 353GHz sets to a resolution of 1◦ to match the resolution of the low- (350 and 850µm) maps is shown in Fig. 4. To the south, a to- frequency data sets and perform the analysis directly on the tal of four spiral arm or ring structures can clearly be seen – HEALPixmaps. the 10kpc ring (“F8”), as well as a second structure just out- At low frequencies, we use the low-resolution radio maps side of the ring (“F9”) and the more distant 21kpc (“F10”) of the sky at 408MHz (73.4cm; Haslam et al. 1981, 1982), and 26kpc (“F11”) arms. These features have also been iden- 820MHz, (36.5cm; Berkhuijsen 1972) and 1.4GHz (21.4cm; tifiedbyFritzetal.(2012),andcanalsobeseeninthepaneldis- Reich1982;Reich&Reich1986;Reichetal.2001).Weassume playing the Herschel 857GHz data. We see a hint of the emis- that there is a 10% uncertainty in these maps, which includes sion at 31kpc (“F12”), which is also seen in H emission, but both the uncertainty in the flux density calibration (between 5 weareunabletoconfirmitasbeingpartofM31,becauseofthe and 10% depending on the survey) and also uncertainties aris- largeamountofsurroundingcirrusemissionfromourGalaxy.To ingfromthemorphologyofthesurroundingstructureinteracting thenorth,weseethreesetsofspiralarmstructures(“F4”,“F3”, with the aperture photometry technique we use. This choice of and“F2”)andawispofemissionatthenorthernmostendofthe uncertaintyhasbeenshowntobereasonableforGalacticanoma- galaxy(“F1”,confirmedbycross-checkingagainstHemission, lous microwave emission (AME) clouds (Planck Collaboration as shown in the top-right panel) before running into confusion Int. XV 2014). For the 408MHz (73.4cm) map, we also add fromGalacticemission.Thesefeaturescanbeseenmostclearly 3.8Jy to the uncertainty to take into account the baseline stria- at857and545GHz(350and550µm),butarealsoclearlyvisi- tions in the map, which are at the level of ±3K (Haslam et al. bleinthe353,217and143GHz(0.85,1.38and2.1mm)maps 1982;PlanckCollaborationInt.XV2014).Allofthemapshave afterCMBsubtraction.The10kpcringcanstillbeseenclearly beencalibratedonangularscalesofaround5◦,andconsequently at100GHz,butatthatfrequencythemoreextendedstructureis thedifferencebetweenthemainandfullbeams(thelatterinclud- notvisible.Wewillusetheterminologyof“ring”forthe10kpc ing sidelobes) needs to be taken into account when measuring ring,sinceithasbeenclearlydemonstratedtobeanear-complete the flux densities of more compact sources. This is significant ring(e.g.,seeHaasetal.1998),and“arm”forallotherstructures for the 1.4GHz (21.4cm) map, where the factor for objects on thatmaynotcompletelycirclethegalaxy. 1◦ scales is approximately 1.55. As M31 is on an intermedi- Anareaofparticularlystronglong-wavelengthemissioncan ate scale, we adopt an intermediate correction factor of 1.3 ± be clearly seen at the southern end of the 10kpc ring, down 0.1,andalsoincludetheuncertaintyinthiscorrectionfactorin to frequencies of 100GHz (3.0mm; see Fig. 2; in Fig. 3 it is the flux density uncertainty. We assume that the value and un- just to the right of “F8” and is also marked as “S6”). This certainty on the ratio for the 408MHz (73.4cm) and 820MHz has the highest contrast with the rest of M31 at frequencies (36.5cm) maps is small, and hence will be well within the ex- of 143 and 217GHz (2.1 and 1.38mm); there are also hints of istingcalibrationuncertainties,aspere.g.,PlanckCollaboration it down to 70.4GHz (4.3mm). This is probably what is seen Int. XV (2014). We also make use of the integrated flux densi- at the highest frequencies of the WMAP data and in Planck tiesfromhigher-resolutionsurveyscollatedbyBerkhuijsenetal. data by De Paolis et al. (2011, 2014); i.e., the asymmetrical (2003)forcomparisontothoseextractedfromthemaps. emission between the north and south parts of M31 that they Atintermediatefrequencies,inadditiontoPlanck-LFIdata, detect is caused by either varying dust properties across the weusethedeconvolvedandsymmetrized1◦-smoothedWMAP galaxyorCMBfluctuations,ratherthanbeingcausedbygalactic 9-yeardataat22.8,33.0,40.7,60.7,and93.5GHz(13,9.0,7.4, rotation. 4.9,and3.2mm; Bennettetal.2013)5.Whenfittingamodelto Within the 10kpc ring, several features are also visible. A these data, we apply colour corrections following the recipe in brightspotinsidetheGalacticringtothenorthismarkedas“F5” Bennettetal.(2013),andweconservativelyassumeacalibration in the top-right panel of Fig. 3. This region corresponds to a uncertaintyof3%. limbofanasymmetricspiralstructurewithinthe10kpcringthat At higher frequencies, we include the low-resolution hasalsobeenseeninthehigher-resolutionSpitzerdata(Gordon COBE-DIRBE data at 1249, 2141, and 2997GHz (240, 140, etal.2006).Planckdoesnotdetecttheemissionfromthecentral and 100µm; Hauser et al. 1998), in addition to the IRAS data nucleus,despitetheprominenceofthisregioninmapsofM31 described in Appendix A. We assume that these data have an athigherfrequencies.Thisimpliesthatthemajorityofthedust uncertaintyof13%.WealsousetheHerscheldataasdescribed in this region is at a higher dust temperature than average – above. an implication which will be explored in later sections. Planck does, however, detect a compact object to the right of the nu- 5 http://lambda.gsfc.nasa.gov/product/map/dr4/maps_ cleus(markedas“F7”)thathasalsobeenseenwithIRAS(Rice band_smth_r9_i_9yr_get.cfm 1993),ISO(Haasetal.1998),andSpitzer(Gordonetal.2006); A28,page6of23 PlanckCollaboration:AndromedaasseenbyPlanck +43:00 0) F1 F2 -l4o .g0(T-3 .(5K)) - 4 .l0og(-T3 .5 (K-)3) .0 00 +42:00 F3 RJ S1 RJ 2 F6 S2 J F4 ( B3 F5 S3 n F7 o ati +41:00 S5 S4 n cli F8 S6 e F9 D +40:00 Planck Planck Fig.3. Top-left: the 857GHz (350µm) Planck 857 GHz F10 857 GHz map of M31, highlighting the extension of (350 µm) F11 (350 µm) the dust out to large distances. Fields of in- F12 terest are labelled with “F,” and circled in +43:00 some cases. The position of the radio source B3 0035+413 is also shown (labeled B3). 000) +42:00 F1 F3F2 l o- 5g .0(Jy a-4r c.0sec- 2 ) 1 .0log(T2R .J0 (K)) Tscoapl-erigchhot:sethnet8o57hiGghHlzig(h3t5t0hµemb)rimghatpe,rweimthisa- 2 F6 sion. Circles labelled with “S” indicate the J F4 sources included in the Planck ERCSC (see ( B3 n Appendix B); two fields in the centre re- o ati +41:00 gion are also labelled with “F”. Bottom-left: n the857GHz(350µm)Herschelmapconvolved cli F8 tothesameresolutionasthePlanckdata,with e F9 the same fields of interest labelled as in the D +40:00 Herschel 857 GHz F10 HI Planck map above. Bottom-right: a map of (350 µm) F11 the H emission from M31 (Chemin et al. F12 2009)convolvedtothesameresolutionasthe 00: 50 00: 40 00: 50 00: 40 Planck 857GHz (350µm) map, with 5, 10, and15kpcringsoverlaidingreen,andthecut Right Ascension (J2000) Right Ascension (J2000) usedinFig.4inblue. 9 oftheSunyaev-ZeldovicheffectinthehaloofM31(e.g.,Taylor F4 etal.2003)unlesstheemissionisproperlytakenintoaccountor 8 F8 suitablymasked. )MB 7 F5 AtPlanck’slowestfrequencies,M31isclearlydetectedbut C e (T 6 is not resolved, because of the low resolution at these frequen- ur F3 cies. It is clearest at 28.4GHz (10.5mm), but can also be seen erat 5 F9 at44.1and70.4GHz(6.8and4.3mm)–althoughatthesefre- p m quencies CMB subtraction uncertainty starts to become impor- e 4 s t tant. The emission mechanism powering the source detections es 3 F1 atthesefrequencieswillbediscussedinthecontextoftheinte- n Bright 2 F10F11F12 gratTedheSrEeDarienaSlseocts.e7v.eralsourcesinthefieldneartoM31that 1 arepresentinthemapsshownhere,includingthedwarfgalaxy M110/NGC 205 (“F6”) and the blazar B30035+413 (“B3”), 0 -40 -30 -20 -10 0 10 20 30 40 additionally a number of components of M31 are included in Galactocentric distance (kpc) thePlanck EarlyReleaseCompactSourceCatalogue(ERCSC; Fig.4. Cut along the major axis of M31. Negative values are on Planck Collaboration VII 2011) and the Planck Catalogue of the higher declination side of the galaxy. The red line is 857GHz CompactSources(PCCS; PlanckCollaborationXXVIII2014). (350µm) and the blue line is 353GHz (850µm) rescaled by a factor WediscusstheseinAppendixB. of 1000. Various spiral arm structures, including the outermost rings at 20–30kpc, are clearly visible at both frequencies. Fields are num- 5. Colourratiosandimplicationsfordustheating beredasperFig.3. 5.1. Colourratios AnumberofrecentanalysesbasedonHerscheldatahaveused thelattersuggeststhatthisemissionislocatedwhereabarand FIR surface brightness ratios to examine dust heating sources thespiralarmstructuremeet. within nearby galaxies. Bendo et al. (2010) performed the first Outside of the ring, we see some cirrus dust emission from such analysis on M81; subsequent analyses were performed our own Galaxy, particularly at the higher Planck frequencies. on M33 by Boquien et al. (2011), on M83 and NGC2403 by This is largely present on the north side of M31, towards the Bendo et al. (2012a), and on a wider sample of galaxies by Galactic plane, but it can also be seen elsewhere, for example Bendo et al. (2015). These techniques have been shown to be thereisanarcofcirrusemissionabovetheoutermostringsatthe morerobustatidentifyingdustheatingsourcesthancomparing south end of M31. Depending on the temperature and spectral FIRsurfacebrightnessesdirectlytostar-formationtracers,since propertiesofthisemission,itmaypresentproblemsforstudies brightnessratiosnormalizeoutthedustcolumndensity. A28,page7of23 A&A582,A28(2015) +42:30 +42:00 l og-5(J.5y arc-s5e.0c-2 ) l- o5g.0(Jy a-4rc.5sec-2 ) -4l o.5g(Jy -a4r.c0sec-2 ) -4l o.5g(Jy- 4a.r0csec-3-2 .)5 l- o4g.0(Jy a-3rc.5sec-2 ) +41:30 +41:00 +40:30 Planck Planck Planck Herschel Planck 217 GHz (1.38 mm) 353 GHz (850 µm) 545 GHz (550 µm) 600 GHz (500 µm) 857 GHz (350 µm) +40:00 +42:30 00) +42:00 l- o4g.0(Jy a-3rc.5sec-2 ) l og(-J3y. 5arcse-3c.-02 ) l og(-J3y.5 ar-c3s.e0c-2 ) l og(-J3y.5 ar-c3s.e0c-2 ) l og(-J3y.5 ar-c3s.e0c-2 ) 0 2 n (J +41:30 o ati +41:00 n ecli +40:30 D Herschel Herschel ISO Herschel Spitzer 857 GHz (350 µm) 1200 GHz (250 µm) 1763 GHz (175 µm) 1870 GHz (160 µm) 1870 GHz (160 µm) +40:00 +42:30 +42:00 l og-(4J.y0 -a3r.c5s-e3c.-02 ) l o-g4(.J0y a-3rc.5sec-3-2 .)0 -5l o.0g(Jy a-4rc.0sec-2 ) -5l o.0g(-J4y. 5ar-c4s.e0c-2 ) l o-g5(.J5y -a5r.c0se-c4-.2 5) +41:30 +41:00 +40:30 Herschel IRAS Spitzer IRAS IRAS 3000 GHz (100 µm) 3000 GHz (100 µm) 4280 GHz (70 µm) 5000 GHz (60 µm) 12490 GHz (24 µm) +40:00 00:50 00:40 00:50 00:40 00:50 00:40 00:50 00:40 00:50 00:40 Right Ascension (J2000) Fig.5.Maskedandpixelized5(cid:48) datasetorderedinincreasingfrequency(decreasingwavelength).Themapsare3◦×3◦ insize.Thereisaclear changeofmorphologyasfrequencyincreases,withtheringsbeingmostprominentatthelowerfrequencies,andthenucleusbeinghighlightedat higherfrequencies. We studied the dust heating sources in M31 using surface in surface brightness. The bright feature at the southern end of brightness ratios measured in all Planck data at frequencies M31(source“S6”inFig.3)isclearlypresentatallfrequencies, of 217GHz and above (1.38mm and shorter wavelengths), as althoughitismorenotableatν < 1700GHz.Thereisachange wellasinHerschel,ISO,Spitzer,andIRASdata.Weconvolved of morphology apparent as frequency increases, with the rings all of the high-resolution data (marked with “H” in Table 1) beingmostprominentatthelowerfrequencies,andthenucleus to 5(cid:48) resolution using a Gaussian kernel. We then rebinned the beinghighlightedathigherfrequencies. datato5(cid:48)pixels;weselectedthissizebecauseitisthesamesize Figure 6 shows the surface brightness ratios between ad- as the beam, and so the signal in each pixel should be largely jacent frequency bands in the combined Planck and ancillary independent of the others. It is important to do this repixeliza- data set, where e.g., S /S denotes the colour ratio be- 545 353 tioninordertoavoidtheappearanceofartificialcorrelationsin tween545and353GHz(0.55and0.85mm).Theratiosoflower thedata.Wemaskedoutdatafromoutsidetheopticaldiscofthe frequency bands (i.e., the Planck bands) appear to smoothly galaxybyrequiringthepixelcentretobelessthan22kpcaway decrease with radius, although the data are somewhat noisy from the centre of M31 (which is equivalent to a 1◦.42 radius at S /S . The 10kpc ring is hardly noticable at all in the 545 353 alongthemajoraxis).Toavoidpixelsstronglyaffectedbyfore- lowerfrequencyratiomaps.Inthehigherfrequencyratiomaps groundorbackgroundnoise,weonlyuseddatafrompixelsthat (e.g., the S /S map), the ring is much more prominent, 4280 3000 had been detected in the 353GHz (850µm) image at 10 times andthecoloursalsoappearenhancedinacompactregionaround thermsuncertaintyinthedata(measuredina10×10pixelre- thenucleus. gionofskytothefarbottom-leftofthedatasetusedhere).We Theseresultsimplythatthedifferentfrequenciesaredetect- also removed pixels at the top-left corner of the images, where ing dust heated by different sources. At higher frequencies, the the data have been contaminated by bright cirrus structures in enhancedsurfacebrightnessratiosintheringandthenucleusin- theforeground.Finally,weremovedpixelsattheedgeoftheop- dicatethatthedustisbeingheatedbythestar-formingregionsin ticaldisc,whichprimarilysampleacombinationofbackground these structures. At lower frequencies, however, the ratios vary emissionandemissionfromthewingsofthebeamsforsources more smoothly with radius, and the ring does not appear to be in the dust ring. This leaves 126 independent data points. We as enhanced as in the data for the higher frequencies. This is look at colour ratios in adjacent bands of the same telescope consistent with dust heating being dominated by the total stel- to minimize the effect of different telescope beams on the re- larpopulation,includingstarsinthebulgeofthegalaxy,which sults; the exception to this is the highest frequency compari- should vary smoothly with radius (as has been suggested for son, where we have no alternative but to compare Spitzer with othergalaxiesbyBendoetal.2010,2012a). Herscheldata. The rebinned data are shown in Fig. 5. The 10kpc ring is 5.2. Determinationofdustheatingmechanism present at all frequencies, but emission from the centre of the galaxy becomes more prominent as the frequency increases. Tolinkthesurfacebrightnessratiostoheatingsources,wecom- At1800GHz(170µm),bothsourcesareapproximatelysimilar paredtheratiostotracersofthetotalstellarpopulationandstar A28,page8of23 PlanckCollaboration:AndromedaasseenbyPlanck formation. As a tracer of the total stellar population, we used +42:30 the Spitzer 3.6µm image, which is generally expected to be +42:00 3.5 4.0 0 .0 0.5 1 .0 dominatedbytheRayleigh-Jeanstailofthephotosphericemis- +41:30 sion from the total stellar population (Lu et al. 2003). While +41:00 the 3.6µm band may also include roughly 1000K dust emis- sionfromstar-formingregions(Mentuchetal.2009,2010),this +40:30 effect is usually only a major issue in late-type galaxies with +40:00 SP545/SP353 ESF/ETotal (SP545/SP353) relativelystrongstarformationcomparedtothetotalstellarsur- +42:30 face density. A high infrared-to-visible ratio or a dominant ac- +42:00 3.0 3.5 0 .0 0.5 1 .0 tive galactic nuclei (AGN) could also contaminate the 3.6µm emission; however, neither of these issues are present in M31. +41:30 We used the Spitzer 24µm image as a star-formation tracer, +41:00 as this has been shown to generally originate from SFR dust +40:30 (Calzettietal.2005,2007;Prescottetal.2007;Zhuetal.2008; +40:00 SP857/SP545 ESF/ETotal (SP857/SP545) Kennicuttetal.2009),althoughwecautionthatitispossiblefor +42:30 some 24µm emission to originate from dust heated by the dif- +42:00 2.5 3 .0 0 .0 0.5 1 .0 fuseinterstellarradiationfieldfromolderstars(e.g.,Li&Draine 2001; Kennicutt et al. 2009). The 24µm band also includes a +41:30 small amount of stellar emission; this was removed by multi- +41:00 plying the 3.6µm image by 0.032 and then subtracting it from +40:30 the24µmimage,asdescribedbyHelouetal.(2004).Whilewe +40:00 SH857/SH600 ESF/ETotal (SH857/SH600) coulduse24µmdatacombinedwithHαorultravioletemission to trace both obscured and unobscured star formation (as sug- +42:30 gestedby,e.g.,Leroyetal.2008andKennicuttetal.2009),the 00)+42:00 2.0 2.5 0 .0 0.5 1 .0 publicly-availabledatahaveproblemsthatmakethemdifficultto 0 n (J2+41:30 includeinouranalysis6.Bendoetal.(2015)havedemonstrated o thatusingthe24µmdataasastar-formationtracerforthisanal- ati+41:00 Declin+40:30 yansids2w4ilµlmyiestladr-rfeosrumltastitohnattararecesrim(toilawrittohiunsainbgouatc5o%m)p,ossoitgeivHeαn +40:00 SH1200/SH857 ESF/ETotal (SH1200/SH857) theissueswiththeultravioletandHα wewillusesolely24µm +42:30 emission as our nominal star-formation tracer. We also use the +42:00 1.0 2.0 0 .0 0.5 1 .0 combination of the 24µm and far-ultraviolet (FUV) data, us- ingtheforegroundstar-subtractedGALEXdatafromFordetal. +41:30 (2013)andViaeneetal.(2014),tocheckwhetherthishasasig- +41:00 nificant impact on our results. We return to the results of using +40:30 thisstar-formationtracerlaterinthissection. +40:00 SH1870/SH1200 ESF/ETotal (SH1870/SH1200) InFig.7,weshowthesurfacebrightnessratiosforthebinned +42:30 dlaattiaonassaarfeungcivtieonnionfTthaebl3e.62a.nOdn2ly4µthmedSata.S/Statisticrsatoiontshheowres- +42:00 0.4 0.8 0 .0 0.5 1 .0 a stronger correlation with the 24µm em4i2s8s0ion3t0h0a0n the 3.6µm +41:30 emission,whichimpliesthatonlythedustdominatingemission at ≥4280GHz (≤70µm) is heated by star-forming regions. All +41:00 the other ratios are more strongly correlated with 3.6µm emis- +40:30 sion, which demonstrates that the total stellar populations, and +40:00 SH3000/SH1870 ESF/ETotal (SH3000/SH1870) notsolelythestarformingregions,areheatingthedustobserved +42:30 below3000GHz(100µm). +42:00 0.2 0.4 0 .0 0.5 1 .0 As an additional assessment of the heating sources of the dust observed at these frequencies, we fit the S /S , +41:30 545 353 S /S ,S /S ,S ,S ,andS /S ra- 857 545 1200 857 1874/1200 3000/1870 4280 3000 +41:00 tiodatatothe3.6and24µmdatausingtheequation +40:30 (cid:32) (cid:33) +40:00 SS4280/SH3000 ESF/ETotal (SS4280/SH3000) ln Sν1 =αln(I +A I )+A . (1) 00:50 00:40 00:50 00:40 Sν2 SFR 1 stars 2 Right Ascension (J2000) Thisequation,derivedfromtheStefan-BoltzmannlawbyBendo Fig.6.Left:colourratioplotswith5(cid:48) pixels.The5(cid:48)pixelsareslightly et al. (2012a), relates the surface brightness ratios of the dust biggerthanthebeamsize,suchthatthesignaltheycontainislargelyin- withthedustheatingsources.TheratioS /S ontheleftside dependent.Thecolourratiosforthelowerfrequencyplotsdonottrace ν1 ν2 the local features; rather, they depend on the galactocentric distance. 6 The publicly-available Hα data, such as from the Virginia Tech Thehigherfrequencyplots–inparticularthehighestfrequencyone– Spectral-Line Survey, Dennison et al. (1998), contain artefacts from tracethelocalstructuremuchmore.Right:imagesoftherelativecontri- incompletely-subtracted foreground stars, as well as incompletely- butionofstarformationtoheatingthedusttracedbythespecifiedpair subtractedcontinuumemissionfromthebulgeofM31,andarethere- offrequencies.ThesemapswerederivedusingEq.(2)andtheparam- foreunsuitableforouranalysis.Thepublicly-availableultravioletdata etersinTable2.Toptobottom:S /S ,S /S ,S /S , P545 P353 P857 P545 H857 H600 containmultiplebrightforegroundstars,aswellasemissionfromolder S /S ,S /S ,S /S ,andS /S . H1200 H857 H1874 H1200 H3000 H1874 S4280 H3000 stellarpopulationswithinM31. A28,page9of23

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Smith, M. W. L., Eales, S. A., Gomez, H. L., et al. 2012, ApJ, 756, 40 . 38 Facoltà di Ingegneria, Università degli Studi e-Campus, via. Isimbardi 10
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