1971MNRAS.152..439C 2 -1 postulated quasisphericalhaloaboutourGalaxy oftotalemission to theinterpretationofbackgroundsurveys ourownGalaxy.Inparticularthe the regionwillbesurveyedlaterwithMills Crossasaseparateproject. of theedge-onspirals NGC 253and4945indicatethatifsuchafeature exists two Magellanicclouds.Thesurveydidnotinclude theMagellaniccloudssince edge-on systems(Mills 1959a, b,1964;Baldwin1967).Thepresent observations at 408MHz,wouldbe unambiguouslydetectableatextra-galactic distances in similar totheundetected9*9magnitude,Sdor typeagalaxyNGC300andthe than theaverageofotherdetectedgalaxies(Cameron 1971a).Inthisregarditis about thesegalaxiesit contributes lessthan10percentofthetotal emission at and theextentofradioemissionislessthanorequaltoopticalextent. presented inSection7. no nuclearcomponent.Itsratioofradiotooptical emissionissignificantlylower spirals exceptNGC55haveradioindicestypicalofmostdetectedspirals.The with extendedradiosourcesofintrinsicallystrongemissionP408>ioW type radiotelescopeoftheMolongloRadioObservatory(Cameron1971a).The Molonglo Crossissufficienttocontourthestructureofemission.Allthese For galaxiesNGC55,253,1566,3521,4945and5236theresolutionof Hz ster.Theyarethereforegroupedseparatelyas‘radiogalaxies’and six EandSOgalaxiesNGC1316,1399,4261,4486,47605128areassociated and SOradiogalaxiesincludedinasurveyof264NGCusingthecross- Sm oralgalaxyNGC55hasacomponentofemissionoveritsopticalextentbut Mon, Not,R,astr.Soc,(1971)152,439-460. 2-1 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The observedcomponentsofradioemissionfrom brightspiralsarerelevant This paperpresentsradiocontoursofthebrightestspiralsandstrongE For spiralgalaxiesP408islessthanioWHzster(e.g.Cameron1971b) THE STRUCTUREOFBRIGHTGALAXIESAT408MHz no evidencewasfoundforahalocomponentsimilartothatpostulatedour ment. Inthecaseofedge-onspiralsNGC253and4945,inparticular, 4945» 5236andthebrightESOgalaxiesNGC1316,1399,4261,4760, Galaxy. such asNGC300whoseemissionisbelowthesensitivitylimitofinstru- as wellobservationsofourownGalaxy,theMagellanicClouds,andspirals bright spiralsarecomparedwithobservationsinthenorthernhemisphere 4486 and5128observedwiththeMolongloCross.Theobservationsof Radio contoursarepresentedforthebrightspiralsNGC55,253,1566,3521, 21 1-1 (0* 5toi-o)xioWHz” ster (Communicated byB.Y.Mills) (Received 1971January11) I. INTRODUCTION M, J,Cameron SUMMARY 440 M.J. Cameron Vol* 152

408 MHz. These observations are discussed along with the Cambridge observations of M31 and NGC 4631 which lie outside the declination coverage of the Mills Cross.

2. STRUCTURE OF INDIVIDUAL SPIRALS (a) NGC 253 NGC 253 is the brightest southern spiral, with classifications SAB(s)c or af:S6 and uncorrected magnitude 5(o) = 8m*2. Its fundamental plane is inclined to the line of sight by only 120 making it an obvious candidate for the detection of a radio halo similar to that postulated for our own Galaxy. The observations of NGC 253 are presented in Fig. i(a) and (b) as a contour map with a 2'*86 beam and as an EW fan-beam scan with resolution of i'-43 in RA. The observations show a bright source of half width o'*8 centred upon the optical nucleus of the galaxy. The extended components of emission are seen to be aligned with the optical disc in the contour map superposed on the optical photograph (Plate I). The coordinates of the central radio source and the optical nucleus are given in the following table along with the radio source position angle and position angle of the optical disc. (All radio source position angles have been corrected for the effect of an elliptical beam using the method of Wild, 1970).

© Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1971MNRAS.152..439C No. 4,1971Thestructureofbrightgalaxiesat408MHz temperature emissionisintheimmediateregionofgalaxyafan-beamscan formed fromthe11pencilbeamsof2'-86resolutionisalsoshown(Fig.i(b)ii). i'-43 EWfan-beamscanacrossthesource.Todemonstratethatalllow /,0 /0 / / / fan-beam scanformedfromthe11pencilbeamsof multibeamingsystem.Thisscanis from theEWfan-beamscanasdescribedinMills&Glanfield (1965).Ithasbeencalculated used todetectthepresenceofsmallconfusingsources inthei43x*2EWfanbeam minor axisofNGC253.In eachcasethefulllineisobservedpencilbeamscan andthe scan above.i{c).Theradialdistributionacrosstheoptical discofNGC253constructed across NGC253.Thei*43x•2fanbeamisformed fromtheEWarmofMolonglo 420, 540,900,1200,1500,i8oo°i£.Thetotalintegrated fluxdensityofthesourceis scan, withthenuclearsource inserted.Theopticalextentsofthegalaxyalong major separately forbotheasternandwesternsides.Thetwo separatecomponentsofthedisc and minoraxesaremarked usingthevaluesofDxd=24*ox6'3fromdeVaucouleurs crosses arethepointscalculated fromthederivedradialdistributionEW fan beam emission areclearlyseen.i{d).Scansconstructedalong (a)themajoraxis,andip) distance 0/2*6 mpcthelinearextentof themajoraxisis17*5kpc. Cross. TheEWextentofthegalaxyismarkedbeneath thescan,{ii)A2*86x14'*3EW Fig. i(a).ContoursofNGC253.Thecontourlevels shownare60,120,180,240,300, 28 (1964). Thehalf-powerbeamwidth alongboththemajorandminoraxisis2'*88. Ata 16 •2f.u.Thedimensionsofthebeamare-86x2'*90. 1(6).(i)AnEWfan-beamscan © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The structureofthesourceineast-westdirectionisclearlyseenon hms hms Radio coordinates(1950-0)oo4506-i Optical coordinates(1950-0)oo45o6*5 RA o o -25 33'-851 -25° 33'34"48 S angle Position 441 1971MNRAS.152..439C , 201-1 m 442 M.J.CameronVol.152 (de Vaucouleurs1968). the galaxyare5*6kpcand16respectively,assumingadistanceof2-6mpc scans havebeenconstructedalongthemajorandminoraxes(Fig.i(d)).However, The linearextentsofcomponents(2)and(3)projectedalongthemajoraxis and of 19'.Thesourcemayberepresentedbythefollowingthreecomponents: Both scansdemonstratethattheemissionextendsoverfullopticalEWextent at thepencil-beamresolutionof2'-86components(2)and(3)arenotseparately the 2'*86pencil-beamobservations,majorandminoraxisscansincluding in Mills&Glanfield(1965).Totestifthisdiscdistributionisconsistentwith scan isusedtoderivetheradialdistributionacrossdisc(Fig.i(c))asdescribed distinguishable intheextendedemission.Forthisreasoni*43EWfan-beam with themajoraxisandaxialratioofabout2. nuclear sourceareconstructedandconvolvedtoaresolutionof2'*86.Thederived points areshownbythecrossesinFig.i(d). are strongerthanthoseinourGalaxywhilethehalocomponentisatleast5to covering onethirdofthefullextent.AsdiscussedinSection5disccomponents originating withintheopticaldisc.Thelargerofthesecoversfullextent of extentequaltothattheopticalnucleusandtwoextendedcomponents or l’Ox10WHz“steronthetotalemissionofahalosizecomparable minor axisprofilethereisnoevidenceforahaloextendingabovetheopticaldisc. been derivedbydeVaucouleurs(1964)andits nucleusdiscussedbyShobbrook 9*5 withaninclinationtothelineofsight 8°.Itsphysicalparametershave (b) NGC4945 The observationsplaceanupperlimitofabout10percentthefullemission across thediscconstructedfromEWscanisconsistentwithobserved consists ofabrightsource of30"halfwidthcentreduponthenucleus andan as SB(s)cd?buthasnotbeenclassifiedbyMorgan. Itslowgalacticlatitudeof & Shaver(1967)usinginfra-redphotography. ItisclassifiedbydeVaucouleurs disc, whilethesecondmaybeidentifiedwith‘mainbody’ofgalaxy extended componentaligned withtheopticaldisc.Thepositionof thecentral the corrected radiopositionangleand thepositionangleof theopticaldisc. source iscomparedwith theapproximatenucleuspositionintable alongwith 10 timesweakerthanthatpostulatedforourGalaxy. om © Royal Astronomical Society • Provided by theNASA Astrophysics Data System To considertheextentofemissionperpendiculartodiscgalaxy, (3) anextendedcomponentoffullEWextenti6'andtotalemission (2) anintermediatecomponentoffullEWextent,5'-7andtotalemission (1) Anuclearsourceof600pcextenttohalfpowerwithtotalemission The emissioncomponentsinNGC253maybeinterpretedasanuclearsource NGC 4945isanotherbrightalmostedge-on spiralofapparentmagnitude The observationsofNGC 4945(Fig.2(a)and(b)PlateII)show thatit Since theradioemissionisconfinedtoopticalextentandradialprofile = 13-3impliesahighopticalabsorptionofi *7. 20-11 20-1 20-1 4-2 xioWHzster“. 2*7x ioWHzster, 3 •6xioWHzster, 1971MNRAS.152..439C 0 20-1 20-1 1ms 21-1 1ms0 0 0 0 0 No. 4,1971ThestructureofbrightgalaxiesatMHz443 over thefullopticalEWextentof9'*8.AsforNGC253scanisusedto are presentinthe1'•43x42fan-beam,itisclearthatemissionextends and onthescanformedfrom11pencilbeams.Althoughtwoconfusingsources Fig. 2(b)showstheextendedemissioncomponentonbothEWfan-beam side maybeusedduetotheproximityofconfusingsourceeast.In calculate theradialdistributionshowninFig.2(c).Inthiscaseonlywestern NGC 4945theobservationsareadequatelydescribedbyatwocomponentmodel of (a)acentralnuclearsource550pcdiametertohalfpowerandtotalemission 9*0 xioWHzster,and(b)anextendedcomponentoftotalemission photographs. Iftheradiosourceisatcentreofnucleusthenoverall who showedthatthewesternsideofnucleuswasunobscuredoninfra-red to thehalf-brightnessradioextentof550pc. the galaxyitisidentifiedwiththirdcomponentinNGC253.Optically to theemissionofhaloabout10percenttotalgalaxy Again theemissionisconfinedtoopticalimage.Moreover,usingradial distance of4*0mpc,deVaucouleurs1968). and fgSi.ThegalaxywasdetectedbyMills & Glanfield(1965)butsincethat in thisgalaxy. or about2xioWHzster. be reproducedasshownbythecrossesinFig.2(d).Thisimpliesanupperlimit diameter oftheopticalnucleuswouldbe45"or800pc.Thisisalmostidentical time theincreasedsensitivityofMolonglo Crossallowsthestructureof (c) NGC5236 for NGC253.Thismayexplaintheabsenceofadistinctintermediatecomponent scans haveagainbeenconstructedalongthemajorandminoraxes(Fig.2(d)). source tobemapped.Thereisaneighbouring source ofo*6f.u.at133515-7, distribution derivedfromtheEWscanthesemajorandminoraxisprofilescan component andanextendedcomponent.Aclear changeinslopeofthelinescans io WHzster,withstrongeremissiontowardsthecentre(assuminga in theEWdirectionasindicatedbyhalf-power widthsof2'*7xi'*4inthe across thesourcedelineatesthesetwocomponents. Theradiosourceiselongated Scd galaxyNGC4945doesnothaveawell-developedmainbodyasisthecase EW andNSdirections. Thesuperpositionofthecontoursonoptical photo- graph indicatesthatthis elongationisconsistentwiththeoptical luminosity 10 minofarcextentat1334,—2937'*o. concentration ofmoreluminousmaterial isinsegments50to 90and220to270°. distribution sincethe major spiralarmsemergefromthenucleus at position angle 50andturnanticlockwise beforebreakingintomanysegments.Therefore the — 293o'n"noted,whichispresumablyunrelated totheopticalgalaxyof m © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The nuclearcomponenthasbeendiscussedbyShobbrook&Shaver(1967), 1ms0, To considertheextentofemissionperpendiculartogalacticdisc, 1ms0 NGC 5236isaface-onspiralwithJ5(o)=8 *2 andclassificationsSAB(s)c The pencil-beamobservations(Fig.3(a)and(b), PlateIII)indicateacentral Since theextendedcomponentofNGC4945coversfullEWextent Radio coordinates(1950*0)130233-!—49°n'$z"365 Optical coordinates(1950*0)130232“49°iz'-o42 RA 8angle Position 1971MNRAS.152..439C 20-1 201-1 444 M.J.CameronVol.152 follows: (a)acentralcomponentoftotalemission2-2xioWHzster the galaxyisface-onitnotpossibletodistinguishbetweenadiscdistribution of totalemission7•2xioWHz'stercoveringthegalaxyextent small extentidentifiedwiththegalaxynucleus,and(b)amoreextendedcomponent in theNEdirection(Fig.4andPlateIV).Theradiocontourssuperposedon face-on. Itisidentifiedwithasourceof0-65f.u.at408MHzwhichextended (d) NGC1566 and ahalodistributionforthissecondcomponent. optical features. optical photographindicatethatthispositionanglefollowsthemostluminous io'-5xio'-o or12-2kpcxn-6kpcat4-0mpc(deVaucouleurs1968).Since m © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The observationsareadequatelydescribedbyatwocomponentmodelas NGC 1566isanSAB(s)bcspiralofmagnitudeio-xwhichapproximately (a) 1971MNRAS.152..439C 20-1 20-1 to separate.Howeverthesourceisadequatelydescribedbyatwocomponent No. 4,1971Thestructureofbrightgalaxiesat408MHz model withacentralcomponentof6xioWHzsterandanextended 25 kpcx20kpcatthedistanceof12-ompc(deVaucouleurs1968). component of3xioWHzstercoveringthegalaxyextent7''25'*7or /, /, /, fan beamscanacrossNGC4945.TheEWextentof thegalaxyismarkedbeneath half powerbeamwidthsinRAandDecare2'•86x2'*95. 2(6).(¿)Thei'*43x4°-2EW distribution acrossNGC4945.Thewassmoothed tothedottedlineconstruct scan, (ii)The286x14'*3EWfanbeamscanacross NGC4945.Acomparisonwith axis ofNGC4945.Ineachcasethefulllineis observed pencilbeamscanandthe and minoraxesaremarked usingthevaluesofDxd=i6*6x27fromdeVaucouleurs scan, withthenuclearsource inserted.Theopticalextentsofthegalaxyalong major crosses arethepointscalculated fromthederivedradialdistributionEWfan-beam (i) demonstratesthepresenceofconfusiontoeast thecentralsource.2(c).Theradial is 18*3kpc. the scansofFig.z{d).z(d).Scansconstructedalong(a) themajoraxis,and(p)minor Fig. z(a).ContoursofNGC4945.Thecontourlevels shownare45,135,225,315,765, 2*9 and8respectively. Atadistance0/4*0mpcthelinearextentofmajor axis (1964). Thecorresponding half-power beamwidthsalongthemajorandminor axes are 1215, 1665,2115,2565°K.Thetotalintegratedfluxdensity ofthesourceis12-6f.u.The © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Due tothesmallangularextentofNGC1566componentsaredifficult 445 1971MNRAS.152..439C 0 0 with anextendedsourcepositionangle145comparedthe 446 M.J.CameronVol.152 angles correspondthereisadiscrepancybetweenthepositionofpeakdeflection angle oftheopticaldisc~i6o°(Fig.5andPlateV).Althoughposition 24 (Danver1942)discussedbyBurbidgeetal.(1964).Thegalaxyisidentified (e) NGC3521 Molonglo Crossofresolution 2'-86.3(ft).ContoursofNGC5236withcontourunit 70K. Fig. 3(a).Linescansacross NGC5236fromthe11pencilbeamsofmultibeamed The totalintegratedfluxdensity ofthesourceis6*2f.u. m © Royal Astronomical Society • Provided by theNASA Astrophysics Data System NGC 3521isa9-8spiralwithclassificationSAB(rs)bcand£84tiltedat 1971MNRAS.152..439C No. 4,1971Thestructureofbrightgalaxiesat408MHz at 408MHzandtheopticalposition(Glanfield &Cameron1967;Whiteoak 5 GHzpositionofWhiteoak (1970). 1970). Moreover,thereisadiscrepancybetween the408MHzpositionand Optical Position o /7 perpendicular tothisdirection. width ofthesourcealongitsmajoraxisatpositionangle26isz'-2.Itunresolved density ofthesourceiso•65f.u.Thebeam286x304inRA8.half-power Fig. 4.ContoursofNGC1566withcontourunitio°Kabove50°K.Theintegratedflux © Royal Astronomical Society • Provided by theNASA Astrophysics Data System density ofthesourceis2‘6f.u.Thebeamshapemarked is2'•86inRAby3'declina- tion. Fig. $.ContoursofNGC3521withcontourunit4o°Kfrom 55°K.Theintegratedflux h ms0 hms Whiteoak (1970) n 03 i5*5+ooh'-i Glanfield &Cameron(1967) no315+oo°14'-3 Right ascension(1950) Right ascension(1950) 447 1971MNRAS.152..439C 448 M.J.CameronVol.152 Radio Position Either theemissionisallassociatedwithgalaxyoritacombinationof the opticaldisc. source unassociatedwiththegalaxyandacomponentofemissionassociated i'*6 fromthepeakradiodeflection.Thusemissionisdifficulttointerpret. in rightascensionismarked, 6(6).ContoursofÍVGC55obtainedfromseveralobservations © Royal Astronomical Society • Provided by theNASA Astrophysics Data System of thesource.Thebeamis circular atz'•86extent.Contourunitis15°Kfrom20 °K.The Fig. 6(a).Linescansacross theSB(s)mgalaxyNGC55.Theopticalextentof galaxy total integrated fluxdensityofthesource is1*3f.u. The opticalnucleusiscentrallylocatedintheNWextensionofsource, The emissioncanbeseparatedintoanunresolvedsourceandaNWextension. hms hms hms Peak deflection,408MHzn03i9*5+oo°12'58" Whiteoak, 5GHzn0315*+oo°13'*9 Radio Centroid,408MHzii03i8*4+oo°13'25" (a) I! 5hms OC^I^OO 00l3 1971MNRAS.152..439C m 20-1 20-1 19-1 (Í) NGC55 of galaxiesincludingNGC4631andtheLargeSmallMagellanicClouds correspond to2•8kpc.Incomparisonthediscradiusis7(b)ANWextension discussed inSection4. (de Vaucouleurs1961),withmagnitudeB(o)=8*i.Thisplacesitinagroup aligned withtheopticaldiscoftotalemission2*6xioWHzster. its totalemissionwouldbe6-3xioWHzsterandseparation has noapparentnuclearcomponentalthoughthepeakdeflectionisatbrightest No. 4,1971Thestructureofbrightgalaxiesat408MHz with thediscP408=8-4XioWHzsterplacingitamongweakest perpendicular totheplaneofedge-ongalaxy. optical properties. region. Thereisnoevidenceofemissionoutsidetheopticalextentindirection On thisseparationthesourcehastwocomponents:(a)anunresolved (g) NGC300 NGC 55isthussimilartogalaxy4321(Pooley1969b)inbothradioand detected galaxies(Cameron1971b). i'*6 SEofthenucleus.IfthissourceisatdistanceNGC3521(6-ompc) the easternextremityofgalaxyonrecordshowninFig.7.Thismay is difficulttodetect.Thisprimarilyaconfusionlimitduediscretesources (de Vaucouleurs&Page1962)areshowninFig.7.Thelargeopticalextentof contributing tothebackground.Forinstance,thereispossibleemissionnear I9'*9xi4'*i forthegalaxyresultsinextendedlow-brightnessemissionwhich ascension. Thegalaxyisundetected althoughtherearesourcesnearbyoutsidethe optical extent. Fig. 7.Linescansacross the SdspiralNGC300,showingopticalextent inright © Royal Astronomical Society • Provided by theNASA Astrophysics Data System m NGC 55isinterpretedasaMagellanictypesystem,SB(s)moral,seenedgewise The radioemission(Fig.6andPlateVI)extendsovertheopticalimagebut The extentofthediscis30*2minarcor23kpcatdistance2*6mpc. Thus theobservationsindicateasinglecomponentofemissioncoextensive Line scansacrossthelargeSdspiralNGC300of7W=9*at1-9mpc C hms 00 5lOO53O 449 1971MNRAS.152..439C 18-1 20-1 19-1 -1 10-1 198-1 19-1 20-1 each havingatotalemissionof1xioWHzster,(b)discorouterspiral they willbeobservedasaseparateproject. of theMillsCrosswhileLMCandSMCcoversuchalargeareaskythat be augmentedbytheobservationsofM31,NGC4631(Pooley1969a,b)and 450 M.J.CameronVol.152 with theopticaldiameterof30kpc)totalemission5xioWHzster 4x ioWHzster,and(c)ahalocomponentofdiameter65kpc(compared by Pooley(1969a).Itsemissioncanbedescribedthreecomponents,(a)A the opticalextentwhichareassumedtobeunassociatedwithgalaxy.Their the LMCandSMC. arm componentbetween8and15kpcfromthenucleuswithtotalemission nuclear componentwithtwonon-thermalregionsof200pcand1kpcdiameter LMC andSMC.BothM31NGC4631areoutsidethedeclinationcoverage (1963) isprobablyablendofsources(1)and(2)seeninFig.7. The weaksource7'ofarcwestthegalaxyobservedbyMathewson&Rome ster. Thereareanumberofsourcesintheneighbourhoodgalaxyoutside coordinates areasfollows: of thegalaxywhichcorrespondstoanupperlimitonP4083-ox10WHz correspond tothe85*5MHzradiation10'ofarceastgalaxyreportedby (e.g. Large,Mathewson&Haslam1959). absolute emissionof3 •3x1oand5*0x10WHzsterrespectively for with theopticalobject and noevidenceforacomponentofgreater extent. The no counterpartatthecentreofM31greaterthan1/20powerSagittariusA Mills (1955)althoughthereisnoidentificationwithasignificantopticalfeature. observations ofbothare consistentwithasingledisccomponent valuesof with anupperlimitof5xioWHzster on anextendedorhalocomponent. the galaxylessthan700pcthickwasobserved at408and1407MHz,withno difference isthatSagittariusA,thestrongcentralsourceof10pcdiameter,has diameter anda520pcnon-thermalregionasdescribedinSection5.Asecond observations ofMathewson &Healey(1963)showedradiationroughly coextensive to theLMCandNGC55.Non-thermalemission originatingintheplaneof of 5lessthanthediscemissionourGalaxyquotedbyBaldwin(1967). (Pooley &Kenderdine1967).Finally,thetotaldiscemissionofM31isafactor of adisccomponentradius5kpcwithtotal emission5XioWHzster 56 kpcfortheLMCand69SMC (Gascoigne1969).The408MHz emission originatingfromoutsidethedisc.The emissionat408MHzconsists It isclassified(G.andA.deVaucouleurs1963) asSB(s)dorSB(s)mbeingsimilar © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Accordingly, anupperlimitof0*85f.u.isplaceduponthetotalemission M31 isanSA(s)borgk-kS6spiralatadistanceof650kpcdiscussedindetail Discussion ofthestructureinsixspiralspresentobservationsmay In comparisonourGalacticnucleushasanextendedthermalsourceof170pc NGC 4631isanedge-onMagellanic-typegalaxy observedbyPooley(1969b). The MagellanicCloudsaretypeamagellanic irregularswithdistancesof oohms6ho hmso (3) 4948*-“37°57'3";(2)oo5Ói29,-3805'40". (1) oo5i4i-2,-3802'37";(2)si32•2,“37°56'31"; 3. STRUCTUREINOTHERGALAXIES 1971MNRAS.152..439C emission components.ItisclearthatthegalaxiesNGC4631,55,LMCand with thediscussionofCameron(1970a,Section4.2). absolute emissionalongwiththeirclassifications,radioindexandnumberof No. 4,1971Thestructureofbrightgalaxiesat408MHz451 low ratiosofradiotoopticalemission.IncomparisonthebrightspiralsNGC253, with theopticalobject.ThehighradioindicesforallexceptNGC4631indicate of thetypeagalaxies(includingNGC300)andgk-kspiralM31isconsistent emission componentsandclassificationsScdtoSbafk.Thelow SMC havesimilarradioandopticalproperties.AllareMagellanicSB(s)mor IB(s)m typesandMorgantypeawithweakabsoluteradioemissioncoextensive !566, 3521,4945,5236andM31,discussedinthenextsection,haveanumberof of variableemissivity.InthecaseNGC253 anintermediatecomponentalso exists whichcanbeidentifiedopticallywiththe mainbodyofthegalaxy. coextensive withtheopticalobjects.Inaddition eachhasanuclearcomponent Sbc toScdallhaveextendedradiocomponentsofthesameordertotalemission NGC 4945 NGC 4631 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System NGC 300Sd Galaxy Table IliststheelevengalaxiesofSections2and3inorderdecreasing The sixspirals(NGC253,1566,3521,4945,5236andM31)intherange (a) Acomplexnuclearregionconsistingofa10 pcnon-thermalsourceSagit- In ourGalaxythefollowingcomponentsarerecognized: (b) adisccomponentconsistingofbandemission extending6o°inlongi- 5. COMPARISONOFBRIGHTSPIRALSINCLUDINGOURGALAXY LMC 0 SMC M31 5236 3521 1566 tarius A;abackgroundthermalregionabout1 or 170pcdiameter(Downes tude and3°v5inlatitude betweenhalf-brightnesscontours.Thelongitu- (Cameron 1968)andalargenon-thermalregion of~520pcdiameter; & Maxwell1966);aringofsmall(10-20pc) thermalsourcesat200pc steps indirectionsassumed byMillstobetangentialthespiralarms dinal distribution(Mills 1959)isveryirregularandshowspronounced This suggeststhatinthe regioni?>4kpc,thebulkof‘disc’component is confinedtothespiral armsofradialhalf-brightnesswidth~500 pc. 253 55 de Vaucouleurs She Sc Sc SBcd? SB(s)d Sb Sbc SB(s)m SB(s)m IB(s)m or SB(s)m type 4. DISCUSSIONOFMAGELLANICTYPES Galaxies inorderofabsoluteemission Morgan emissionindexNo.of fg 13602-52 afrSy 500i*71 gk-k 5203*83 f 8902*12 af: 10402-33 al 844*4i aS4 <30>2*8— al 55-9i al 334*6i 18--1 type xioWHzsteri?408components — 19102-02 or al Table I Absolute Radio 960 3-02 1971MNRAS.152..439C 4945 Notes : M31 Our Galaxy 5336 contours. Forthedisccomponentoverallextentsaregiven.TheopticaldiameterquotedisI defined byG.andA.deVaucouleurs(1964). those forthediscandhaloarefromBaldwin(1967). 253 -18 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System (1) EmissioninWHzster“"(xio). (2) Thediametersquotedforthenuclearandhalocomponentsarewidthstohalf-powe (3) ThetotalemissionquotedforthenucleusofourGalaxyisfromMills&Glanfield(1965)whil 0 452 M.J.CameronVol.152 nature ofthecentralcomponent. of thecomponentsand,incaseNGC3521,duetouncertainty The twospiralsNGC1566and3521areexcludedduetotheincompleteseparation Vaucouleurs 1958;HubbleAtlas,p. 25). the nuclearbulgesin NGC 5746(deVaucouleurs1958)and 4565(de component istherefore elongatedalongthegalacticplaneasseenoptically for thermal source.Lequeux (1967)givesdimensionsof3°xifrom theouter from thebackgroundandpartialabsorption duetothesmallerextended 4945 sincetheyhavenon-thermalspectraand linearextentssimilartothoseof spirals. Incomparingthenuclearcomponentof our Galaxyitistheregionof520pc contours ofthiscomponent usingtheobservationsofMills(1956).The nuclear component aredifficulttodetermineduethe problemsofseparatingthesource optical nuclei.InourGalaxy,however,thetotal fluxdensityandextentofthis diameter whichisidentifiedwiththenuclearcomponents ofNGC253and 21-1 (c) apostulatedhalooftotalemission(0*5to1-o)xioWHzster The componentsofthegalaxies,includingourown,arecomparedinTableII. This tableallowsadirectcomparisonbetween ourGalaxyandtheotherfour Emission Diameter 21-1 Although theradialdistributionacrossplaneofGalaxyisuncertain it isclearthatthedischasstrongeremissiontowardscentre.Itstotal emission is~o*2xioWHzsterwithanequivalentthickness with totalextentofthehalf-emissivitycontour22kpcx14along of 750pc(Baldwin1967).Thedisccomponentisweakbutstilldistinguish- emission regions(Davies&Hazard1962);and able intheanticentreregionwhereitislargelyresolvedintoirregular to thesolardistanceof10kpc. and perpendiculartotheplaneusingvaluesofMills(1959a,b)adjusted (1 *0 (1*0 900 265 220 3 Nucleus Comparison ofourGalaxywithNGC253,4945,5236andM31 1000) pc 600 550 200) 250 Emission Diameter 1020 (355 (420 72O 200 40 Disc Table II 16 to30 17*4 kpc 5*6) 16) 18 12 500-1000 Emission Diameter < 100 < 190 ~$oo p Halo kpc 22 65 (D inkpc) diameter Visible galaxy 32 i? 30 18 12 700-120 emissior Total 1920 1040 940 540 1971MNRAS.152..439C 2-1 -1 18-1 Virgo A(NGC4486),thecentralcomponent of CentaurusA(NGC5128),and P408 >ioWHzster . the radiosourcesassociatedwithNGC1399, NGC 4261and4760.These and M31. with M31.Thisisconsistentthediscussion ofCameron(1970)whereitis (a) FornaxA(NGC1316) six objectsareintrinsically strongradioemitterswithabsolute emission concluded thatourGalaxyistypefgorgplacing itintermediatebetweenNGC253 ponents inM31,NGC253and4945coverarangeof1000:1thesourcesare other hand,thepostulatedhaloandweaknuclear sourcesuggestacomparison for individualcomponentsanevolutionarysequenceissuggestedrelatingthe with themoremassivecentrallycondensedspirals. was foundforsuchafeatureinanyotherdetectedgalaxy.Moreover,theextent the singlecomponentinMagellanicsgivenTableI,anditsdistribution show arangeof>500:1forthefivegalaxiesTableII.Incase of 10:i.Incomparisontheseparatenuclear,mainbodyandhalocomponents non-Magellanic spiralsrangesfrom250to2600xioWHzsterorarange arms butthereisnoemissionfromthelargeopticalmainbodyofintegrated disc componentthereisasmallervariation.Itsstrengthcomparabletothatof is similartoNGC253.Inbothcasestheradiationconcentratedintoacentral of comparableextents~700pccoveringtheopticalnucleus. source isweakandviceversa.Eventhoughtheemissionofnuclearcom- nuclear, mainbodyandhalocomponents.Whereaispresentthenuclear weak nuclearsourceinthetotalemission.Againhalomaybeassociatedonly that thestrengthofthiscomponentisrelatedtostellartypeproviding body ’radiocomponentofextentsimilartotheopticalmainbody.Henceitappears spectral typegk-k.Incomparisontheaf:spiralNGC253hasadistinct‘main component althoughweakeremissionisdetectedacrossthefulldisc.On in bothourGalaxyandM31thecontributionfromhalocompensatesfor it isdifficulttogeneralizeaboutthepropertiesofhalocomponent.However, its integratedspectraltypeisstillindicativeofearly(typeF)stars. luminosity ofthecentralregions.Thisisconsistentwithdependenceradio cases. IntheextremecaseofM31radiationfollowsyoungerouterspiral appears tobedirectlyrelatedthedistributionofpopulationIobjectsinboth No. 4,1971Thestructureofbrightgalaxiesat408MHz453 of thiscomponentinM31isfarlargerthanthatpostulatedforourGalaxy.Hence in thetypefgalaxieswhichshowclosestcorrelationbetweenradioandoptical index onMorgantypediscussedinCameron(1971aSection4.2b).Forinstance magnitude (Cameron1971a,Fig.i(b))theopticalmainbodyiswellformedbut © Royal Astronomical Society • Provided by theNASA Astrophysics Data System As farasthedistributionofradiationacrossitsdiscisconcerned,ourGalaxy Radio contoursandlineprofilesarepresented forFornaxA(NGC1316) In Cameron(1971b)itisshownthattheradioluminosityofdetected Contours of thelargecomplexradio sourceassociatedwith theSOgalaxy Since therangeoftotalemissionindetectedspiralsislessthan In M31andourGalaxythereisevidenceforaradiohaloalthoughno 7. RADIOGALAXIES 6. DISCUSSION 454 M. jf. Cameron Vol. 152

(a)

Right ascension (1950)

Fig. 8. (ci) Contours of NGC 1316 (Fornax A). The contour unit is 104°K from The integrated flux density of the source is 259/.m. (b) Line scans across Fornax A. The extent of the optical SO galaxy is marked in R.A. The scans are 1 ' *43 apart in declination.

© Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1971MNRAS.152..439C 0 form oflinescansacrossthesource. which frequencyitisthepredominantfeatureofdistribution.Itwasnot frequency results(Maltby&Moffet1962;LequeuxBiraud, (b) VirgoA(NGC4486) components andshowanunresolvedsourcecentredupontheopticalimage. the radioextentwouldbelessthanopticalextent.Afluxdensityofo•5f.u.for less thantheopticaldiameterof4'-o.Hence,consideringthissmallsourcealone, in emissionunresolvedaneast-westdirection.Incomparison,theeastern arc-like dispositionaboutthegalaxy.Thecurvedwesternedgehasasharpdrop This sourceissuperposedonthebridgeofemissionbetweentwocomponents. NGC 1316aregiveninFig.8(a),while8(b)presentsthesamedata extent inasouth-westdirection.Thisisgreaterthantheprevioushigh Neville 1968).The408MHzobservationsshowstructuraldetailwithgreatest detected at1407MHzbytheCambridgegroup(Macdonald,Kenderdine& diameter of3'-8.ThishalowasfirstobservedbyMills(1953)at101MHz angle of34(Fig.9).Itsoverallextenti6'x12'isfarlargerthantheoptical ratio ofradiotoopticalemissionforsuchagalaxy. this galaxywouldimplyaradioindexof3-5whichrepresenttypical baseline andlowfluxdensitythiswidthisdifficulttomeasurebutconsiderably structure. Theintegratedfluxdensityofthetotalsourceat408MHzis259f.u. extremity tapersoffrathergently.Bothcomponentsshowconsiderablefine Both easternandwesterncomponentshavecomplexdoublestructurean No. 4,1971Thestructureofbrightgalaxiesat408MHz455 Le Rouxi960).Theintegratedfluxdensityofthefullsourceat408MHzis519f.u. density ofthesourceiso•5f.u.andhasahalfwidth

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