1973Apj. . .183 . .411A the Astrophysical Journal, 183:411-439, 1973 July 15 © 1973. the American Astronomical Society. All

1973ApJ. . .183 . .411A _1 1- -1 -1 -1 The AstrophysicalJournal,183:411-439,1973July15 Subject headings:Galaxies—Hnregionsredshiftssupernovae acting andareattheclose(10Mpc)distance. same distance.ThatdistanceisindicatedtobethecloserofNGC7320(about10Mpc). (z =800kms)andthehigh-redshiftNGC7318AB57006700s“areat which appearintheQuintet. close togetherinthesky.Photographslatershowedsomeofthesegalaxiestobevery and theevidenceforluminoushydrogenemissionbetweenNGC7319both7318 of z=6700kms",andverymuchtoofaint(about6mag)forasystemredshift800s. apparent magnitudeatmaximumbrightnessistoofaintby1.5magforagalaxywithredshift The supernovainNGC7319appearstobeTypeIbutispeculiarsomerespects.Itsestimated Quintet. (2)Light-curveandspectraldataonasupernovathatappearedinNGC7319theyear NGC 7320,however,seemtoindicatethat,onbalance,allthemembersofQuintetareinter- The longtailonNGC7320,theevidenceforradiointeractionbetween7319and7318, as Stephan’sQuintet. best-known groupsofpeculiargalaxiesintheskyandtodayaregenerallyreferredto distorted andoneofthemtobedouble.Theseobjectseventuallybecamethe puzzle in1961,however,whenE.M.andG.R. Burbidge(1961a)firstreportedthat and distributionofhydrogenemissionregionsinthedifferentredshiftsystemswhichcomprise in NGC7320byAllen(1970).computedthat inorderforthegalaxytoavoid ideas ofdynamicsclustersorgroups. the lower-surface-brightnessmemberofgroup, NGC7320,hadaredshiftofonly dynamically unstableandthereforeshort-lived.Thegroupbecameanevengreater well asredshiftsinotherclosesystemsofgalaxies,toarguethatsuchwere 1971. (3)Locationandstructureofluminousregionsthefaintestrecordablesurfacebrightness distance (~10Mpc).This agreedwiththedistanceestimatedtointrinsically large the apriorichanceof(i)tobelessthan1in1500and (ii)tofalloutsidetheconventional (ii) NGC7320isliterallyexplodingawayfrom the othermembers.Theyestimated sumian, inapioneeringpaper,usedtheavailableredshiftsStephan’sQuintet,as Sandage) andrangedfrom5700to6700kms(seefig.1).Thenin1958Ambart- © 1973.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. having anexcessivemass ofneutralhydrogenitwouldhavetobefairly close-byin Quintet couldmeanonlyoneoftwothings:(i)NGC 7320isaforegroundgalaxy,or of nearly6000kmsbetweenthehigh-and low-redshift membersofStephan’s on InstabilityofSystems(BurbidgeandBurbidge 1961/?),thatthedifferenceinredshift 800 kms.Theyconcludedinthatpaper,andlater attheSantaBarbaraConference Sb spiral,NGC7331,which wasprojectedonly30'awayontheskyto northeast. The evidencefromtheHnregionspointstoconclusionthatlow-redshiftNGC7320 In 1877M.E.StephanobservedwiththeMarseilletelescopefourgalaxiesthatwere Observations inStephan’sQuintetarereportedthefollowingcategories:(1)Location,size, © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Hale Observatories,CarnegieInstitutionofWashington,CaliforniaInstituteTechnology The nextobservationsforthcomingwereradio oftheneutralhydrogen Redshifts offourthememberswerereportedin1956(Humason,Mayall,and STEPHAN’S QUINTETOFINTERACTINGGALAXIES Received 1972December27 I. INTRODUCTION HAlton Arp ABSTRACT 411 1973ApJ. . .183 . .411A 1 412 to eachredshift. redshift ofeachmember.Correctingtheseredshiftstothegalacticcenterwilladd+277kms“ tional evidencestronglyindicatestheconclusionthatNGC7320,asitsmorphological to NGC7331.Forthepurposesofthispaperitwillbeacceptedthat7331and type (theauthorwouldestimateSd)suggests,isadwarfcompaniongalaxybelonging NGC 7320hadalmostexactlythesameredshiftas7331.Therefore,observa- At theSantaBarbaraconference,vandenBergh(1961)hadalreadypointedoutthat close toNGC7320.Arp(1972a,c)pointedoutthattherewereanumberofcases whether theyrepresentagroupofbackgroundgalaxiesaccidentallyprojectedvery NGC 7320arecompanionsatthesamedistance. redshift galaxieswereejectedfromlargergalaxies. In1970ArpproposedthatStephan’s low-redshift galaxies,thatthehighredshiftswereapparentlyduetosomeintrinsic where high-redshiftgalaxieshadbeendemonstratedtobephysicallyassociatedwith his hypothesis(Arp1972a,b,d). concentrated intheregionbetweenNGC7331and Stephan’sQuintetassupportfor of Stephan’sQuintetareatthesamedistanceasNGC7331and7320,or redshift memberswerebackgroundgalaxies. nonvelocity cause,andthatevidenceindicated,in general,thatsmall,peculiarhigh- redshift? background galaxiesat a distancegivenbytheconventionalinterpretation oftheir redshift membersareassociatedwithNGC7320 andNGC7331orwhethertheyare established? Arethereanyindependentcriteria thatwilldecidewhetherthehigh- using criteriaotherthanredshift,canthedistance tothehigh-redshiftmembersbe relation bymerelyarguing,withoutanynewobservational evidence,thatthehigh- Quintet hadbeenejectedfromNGC7331and cited theevidenceofradiosources Fig. 1.—SchematicdiagramofStephan’sQuintetshowingtheNGCdesignationandmeasured © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The centralquestionthendevolvesintooneofwhetherthehigh-redshiftmembers The crucialquestionsconcerningStephan’sQuintet arenowthefollowing:By Later in1970Tammannattemptedtoavoidthe violationoftheredshift-distance HALTON ARP Vol. 183 1973ApJ. . .183 . .411A -1 -1 1 -1 -1 -1 in thephotograph. figure 4thatHnregionsinboththehigh-andlow-redshift systemsregisterverywell, 098-04 behindanRG2filter.Inotherwords,thisplate recordsbroad-bandH«(roughly check wasmadeinorder todeterminewhethertherewasanyhydrogen-« emission from 6300Â,thecutoffoffilter,to6950 cutoffoftheplate).Itisseenfrom corrector lens,takenwiththe200-inch.Thephotographic plateisEastmanKodak and theemissionpatchesinbothNGC7320 NGC7318canberecognizedeasily The printinfigure4isfromadirectphotograph (non-imagetube)behindthef/3.67 designated A1throughA24andthehigh-redshift regionsdesignatedB1throughB18. exposure IIIa-Jplate,tofallononeofasetthinspiralarmsthatsweepfromNGC presumably allinthe5700-6700kmsrange.(Theonehigh-redshiftHnregion figure 3,takenwiththe6670Âfilter,showsconspicuousemissionknotsinNGC7318 possible bythisproceduretoidentify24Hnregionsatthelowredshiftvalueand believe thatallthehigh-redshiftregionsareactually apartofthehigh-redshift that appearstooccurinthebodyofNGC7320actuallycanbeseen,onlong but noneinthelow-redshiftgalaxyNGC7320.TheHnregions7318are where. PresumablyalltheseHnregionsareatabout800kmsredshift.Conversely, carefully inablinkcomparatorwiththenarrow-bandinterference-filterplates.Itwas filter transmittingapproximately6850to7150Â.Thelatterplateswerecompared filters. Exposuresofaround40minregisteredapproximatelylimitingskydensity. NGC 7318pair.) shows theentirecentralbodyofNGC7320dottedwithHnregions,butnoneelse- revealed bythetwoseparateinterferencefilters.Figure2,takenwith6585Âfilter, Image-tube platestotheredofbothhydrogen-«redshiftsystemsweretakenthrougha Therefore, itshalf-transmissionintensitywasforhydrogen-aemissionredshifted ference filterwasalsoabout100Âwide,buttunedto6670peaktransmission. detuning totheblueinf/3.67beamof200-inchprimefocusgivesanaccep- 7318 downacrossthenorthendofNGC7320. Therefore, thereisnoreasonnotto 7151 electrostatic-focusimagetubewasusedtophotographthroughtheseinterference between 1900and6500kms“. tance ofhydrogen-«emissionfrom-1500to+2100kmsredshift.Theotherinter- ferent hydrogen-«interferencefilterswereused.Thefirsthaditspeaktransmission In ordertoidentifytheHnregionsunequivocallyineachredshiftsystem,twodif- other rangesfrom5700to6700kmsasrepresentedbytheremainingfourgalaxies. at 6585Âandwasabout80widehalf-peaktransmissionintensity.Allowing15 Stephan’s Quintet.OneisatK=800kms,asrepresentedbyNGC7320.The 18 Hiiregionsatthehighredshift. order tostudytheoutermost,low-surface-brightnessregionsofgalaxiesin NGC 7331.Theobservationsareofthreekinds:(1)interference-filterphotographs H iiregions;(2)spectraofnand(3)long-exposuredirectphotographsin mostly withthe200-inchatPalomar,onobjectsinvolvedinStephan’sQuintetand Quintet. No. 2,1973 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem All theHiiregionsdetectedareidentifiedinfigure 4withthelow-redshiftregions Since the6670interference filtercutsoffbeyondabout6720Àinthered, aspecial It isimpressivehowclearlytheemissionregionsintworedshiftsystemsare In ordertogetdeepexposuresthatwouldregisterevenfaintHnregions,theITT As farasthegalaxiesareconcerned,thereessentiallytworedshiftsystemsin The observationsarepresentedandanalyzedinthefollowingpaper. The followingpaperreportsobservationsmadeoveraperiodofseveralyears, II. THEHIIREGIONSINNGC7320AND7318 a) TheTwoRedshiftSystems STEPHAN’S QUINTET 413 1973ApJ. . .183 . .411A 1 _1 1 7 -1 414 present intheQuintetareawhichwasappreciablymoreredshiftedthanabout The galaxyNGC7317hastheappearanceofaspheroidalorEgalaxy;therefore,it mately 6700Â).OnlyoneadditionalHnregionwasfound.Itisaveryinterestingone, and the103a-E-fRG2plates(theemulsionsensitivitycutsoffatapproxi- the 098-04platewascarefullycomparedwithboth6670interferencefilter from itsneighborNGC7318,whichhasroughlythesameredshift. well-developed, barredspiralwithconspicuousarms.Yetthereisonlyone would notbesurprisingifitcontainednoHnregions.NGC7319,however,isa diameter (oftheorderof1").ItisdesignatedClinfigure4. s", itliesintheextendedarmofNGC7319.Itisquitefaintandextremelysmall however, becauseinadditiontopresumablybeingredshiftedgreaterthan6500km red wavelengthsensitivity(ortohydrogen-«redshifted18,000kms).Therefore 6500 kms".The098-04+RG2plateshowninfigure4reachestoabout6950Â investigation. ItisthatessentiallyonlyNGC7320and7318haveHiiregions. small, suspectedHnregioninthisgalaxyeventhoughlargeregionsemergerichly patches. TheHnregionswerenotvisibletotheeyeinfindingfieldof200-inch effort wasmadetoobtainspectroscopicobservationsofatleastafewtheemission telescope, sotheslitofspectrographwasorientedeast-westwithnucleus extend acrosstheemissionregionsB4andB3.Twoimage-tubespectraweretaken, NGC 7318Binthewestendofslit,andoffset23"southfromnucleus 7318 B.Theslitwas2"wideand76"long.Itcalculatedthattheshouldthen one of30-minexposureand40min,withthesameoffsetguide H« isrecordedinthespectrogramfromtwosharpknotsseparatedbyabout5". It isseenthatB3,thewesternmostoftwoHnregions,hasonaverageabout that theknotswereseparatingspatiallyataboutarateof1kpcper10yr,whichwould 90 kmssmallerredshift.Ifweinterpretthisasavelocitydifferential,itwouldmean strong. Relative tothecontinuumfromknot,emissionlines,Hainparticular,arevery star. Thelatter(Q2272)isreproducedinfigure5.Figure5showsthatindeedstrong be therightorderofmagnitudeforagalaxythatishavingitsrotationalsymmetry In summaryofthissectionthen,wecanpointtothefirstsurprisingresult In ordertostudyinmoredetailthenatureofsometheseHnregions,aspecial © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Table 1liststheredshiftsmeasuredforlineswhichwereidentifiedinspectrum. 1 Hß B45804 Ha...... B45623 5657 Avg. (B4-B3)=90±25 km s'. [Om] B45669 5688 [Nu] B4... 5847 -1 1 * Notcorrectedforheliocentric orgalactocentricredshift. Line Hnregion(czinkms) inkms“)* Spectra ofHiiRegionsB3andB4inNGC7318 b) SpectraoftheHigh-RedshiftRegions B3 55945559 B3 57005520 B3 5637 B3 ...5742 HALTON ARP TABLE 1 Plate Q2271Q2272 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem

Fig. 2.—Photograph (PH5693A) with image tube through interference filter which registers hydrogen-«, with redshift between - 1500 and +2100 km s-1. Photograph printed lightly to show emission knots in body of NGC 7320. 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem

Fig. 3.—-Combination print of three image-tube photographs through interference filter which registers hydrogen-a, with redshift between 1900 and 6500 km s_1. Note absence of emission knots in NGC 7320, their presence in NGC 7318 A and B, and long, slightly curved filament of emission which runs southwest of NGC 7319. 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem

Fig. 4.—Direct photograph (PH5679A) with Eastman Kodak 098-04 plate behind RG2 filter. Hydrogen-« emission regions from both redshift systems register and are designated by series A, B, and C (see text). 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1973ApJ. . .183 . .411A 1 presence ofstrongHaemissionandyieldredshiftsthatarealmostidenticaltothe mm~ itisseenthatiftheseeingimagewereappreciablylessthan2",andinaddition accepted ascompletelycertain.Thewideslitusedtoencompasstheknotsonblind between twoemissionknotsinNGC7318. broken upbyaperturbingevent.Therefore,thisisbelievablevelocitydifference differently illuminatedinthetwocasesandcauseanappreciablespeciouseffect the twoknotswerenotexactlyeast-westfromeachother,thenslitcouldbe offset projectsontotheplateatawidthof0.09mm.Atscaleabout200Â photographs infigure2or4.Second,thereisatendencyforthehigh-redshift emission regions. redshift listedforNGC7318B. measured redshifts. Another wayofstatingthisisthattheoutlineluminousmatteringalaxies frequent onthesidetowardNGC7320.Theneteffectofthisdistributionistoyield filled circlesfallonappropriatearmsandfeaturesofNGC7318,butseemtobemore northwest halfofNGC7320.Thissameeffectcanbeseenvisuallybystudyingthe represent thelow-redshiftHnregions,andfilledcircleshigh-redshift systems ofhigh-andlow-redshiftregionswhichappeardisplacedtowardeachother. H iiregionstobemorenumerousinthesoutheastsideofNGC7318(fig.3).These high-redshift regions.Thescale andorientationarethesameasinfig.1. It seemssignificant,however,thatthetwoindividualknotsobservedshow On theotherhand,redshiftdifferencebetweentwoknotsshouldnotbe © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The moststrikingresultisthatthenumberofopencirclesdefinitelyricherin Figure 6showstheidentifiedHnregionsplottedonsky.Theopencircles Fig. 6.—Plotonskyofidentified Hnregions.Opencircles,low-redshiftregions;filled circles, c) DistributionoftheHiiregionswithinNGC7320and7318 STEPHAN’S QUINTET423 N 1973ApJ. . .183 . .411A the actualphysicalextentofaresolvedHnregion. diameters oftheHnregionsrangeuptonearly5",soitisclearthatweareseeing from figures3and4,theseeingdisksofsmallestimagesareaboutT'.But in anX,Ymeasuringmachine,andtheresultsaregiventable2.Ascanbeseen redshift galaxyshouldbeabout8timesmoredistantthanthelow-redshiftgalaxy, redshift ofNGC7320.Ifoneappliestheredshift-distancerelation,thenhigher- regions haveidenticaldistributionsofapparentdiameters! result oftheinvestigationsofar.Bothhigh-andlow-redshiftsystemsHn and 4490weremuchstrongerontheirinteractingsides. Additionally, VeronandSauvayre(1965)showedthattheHnregionsinNGC4485 is moreseparatedthanthetwosystemsofHnregions.Theplotgivesimpression, may bemeasured.Thebestimage-tubeplatesineachredshiftsystemweremeasured regions inM51fellonthesideofgalaxytowardinteractingcompanion. since ChuvaevandPronik(1970)pointedoutthateightofthe10brightestHn for NGC7320and7318,thatonthesidesnearestneighboringgalaxymore 424 B7. B4. A20 A14 and thereforeitsHnregionsshouldbe8timessmallerinapparentlineardimension. B16. B15. B10. A12 A9. A8. A6. A5. A4. A3. A2. 7320 andNGC7318.Theempiricalprecedentforthiskindofinteractionisstrong H iiregionsareproduced. A21 Al. B3. Table 2andtheplotofthosemeasuresinfigurelademonstratemoststartling The reasonthisresultissounexpectedthatNGC7318hasabout8timesthe Once theHnregionshavebeenidentifiedasinfigure4,theirapparentdiameters This resultwouldargueforsomekindofinteractionbetweenthegalaxiesNGC © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem No. High Region Diameter (arc sec) 4.25 2.57 2.19 2.33 2.70 3.66 3.58 2.39 2.13 3.47 2.13 2.35 1.77 3.75 1.91 3.54 1.62 1.88 d) TheApparentDiametersoftheHnRegions Surface BrightnessofHiiRegionsin NGC 7318and7320 A7. A24 A13 B2., A19 B9. B8., Bll HALTON ARP No. Medium Region TABLE 2 Diameter (arc sec) 2.29 2.10 2.15 3.40 3.39 3.05 3.23 1.62 A22 A18 A17 A10 A23 A16 A15 All B18. B17. B14. B13. B12. B6.. B5.. Bl.. No. Low Region Diameter (arc sec) 4.95 4.79 2.66 2.64 2.13 2.00 2.88 2.33 2.37 1.99 1.98 1.42 1.78 1.94 1.69 1.84 1973ApJ. . .183 . .411A regions; dashedlines,high-redshiftHnregions. Fig. lb.—FrequencydistributionofsizesHnregions inNGC7331. Fig. la.—FrequencydistributionofsizesHiiregions intable2.Solidlines,low-redshiftHn © American Astronomical Society 10 10 10 0 0 0 5 5 5 1 T : LOW SURFACEBRIGHTNESST0TAL33 J. HIGH SURFACEBRIGHTNESSTOTAL-'30 MEDIUM SURFACEBRIGHTNESSTOTAL:34 DIAMETER (arcsec) il 4 68 wmm APPARENT DIAMETER 0 mm Provided bytheNASA Astrophysics DataSystem & Fig. lb Fig. la HIE REGIONSINNGC7331 10 _0_ _ 12 1973ApJ. . .183 . .411A -1 _1- mean, galactocentricredshiftof6200kms+277s“=6477[see 426 legend offig.1].)Atthisdistance122MpcanHnregion5"indiameterwouldhave we computeadistancetoNGC7318of122Mpc.(Forthiscomputationuse latest evaluationoftheHubbleconstantat53kmsMpc(Sandage1972),then intrinsic diameterwecomputeadistanceofjustabout10MpcforHnregionwith and Tammannclaiminanygalaxyareabout600pc.Evenifwholesegmentsofspiral an absolutesizeof2960pc!ThelargestintrinsicHnregiondistancesthatSandage dimensions ofthefewlargestHnregionsinNGC7320and7318are redshift systemsofHnregions.Wecanshowthisfirstbyassumingthattheabsolute closer distanceofNGC7331and7320isrequiredforboththehigh-low- recall thatmanydwarfgalaxiesareoftheorderorlessthan5kpcindiameter. arms areconsideredasHnregions(ArpandBrueckel1972),themaximumdiameter 7331 (Holmberg1964). an apparentdiameterof5".Thisisroughlythedistancequoted(11Mpc)forNGC (Sersic 1960;Sandage1962)andinSbgalaxies(ArpBrueckel1972).Withthis order of200pc.ThisseemstobeaboutthesizebiggestHnregionsinScgalaxies believe thatNGC7318containedemissionknotsinitsarmswereessentiallythe If NGC7318wereatadistancecorrespondingtoitsredshift,wecouldbeasked farther tothered.(ThanksareextendedFrankBrueckelwhomadeactualX regions inNGC7320and7318.Image-tubeplateswiththe200-inchtelescope NGC 7318aresosimilarastoarguestronglyforbeingatroughlythesame number, size,andgeneralappearanceoftheHnregionsinbothNGC7320 size ofsmallergalaxies. one canattaininagiantSblikeM31is,again,justabout600pc. of theHiiregionsindicatetobetruedistance?”Theanswerisclearlythat distance asNGC7320. figure lb.ThesimilaritybetweenthisdistributionandtheoneinM31(Arp and YmeasuresonthemeasuringmachineofNGC7320,7318, were madebehindthe6585Âinterferencefilterandcomparedwithimagetubeplates us differsbyjustaboutafactorof10. Brueckel 1972)isstriking.InM31atotalof93Hnregionswerefoundlargerthan 7331 plates.) 7331 havebeenmeasuredwiththesametechniquesaswereusedtomeasureHn then wecomputeadistanceofabout7.8Mpcto NGC7320andabout7.0Mpcto in M31areabout60"diameter;thelargestof theremainingHnregionsinNGC dominant Sbspiralsintheirgroups,andthelisted distanceofthetwosystemsfrom NGC 7318.Thesedistancesareslightlysmallerthan thelisteddistancetoNGC7331 size ofabout200pcforthatkindHnregion(57" diameteratthedistanceofM31), derive adistanceof9.7MpctoNGC7331.The largest oftheremainingHnregions size of10''7forthoseinNGC7331,andadistance toM31ofD=0.61Mpc,thenwe adopt anapparentangularsizeof170"forthetwo largestinM31,anapparentangular 7331 areabout7".Thisyieldsadistanceof 5.3Mpc.Ifweadoptanintrinsic of 11Mpc.Buttheyare muchclosertothelisteddistanceofNGC7331 thanthey Mpc. are totheredshiftdistance ofNGC7318which,asmentionedpreviously, is122 10". InNGC7331atotalof97werefoundgreaterthan1".Thetwosystemsareboth In ordertorealizehowunlikelyanHnregiondiameterof3kpcis,oneneedbut On theotherhand,ifwetakeHubblelawtoapplyNGC7318,anduse Actually, justavisualinspectionoffigures2,3,4,and6showsthatthebrightness, One wouldthenbelednaturallytoask:“Whichdistancedotheobserveddiameters The resultingfrequencydistributionofHnregionsizesinNGC7331isshown In ordertomakeanadditionalcheckonthisconclusion,theHnregionsinNGC © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Just asinM31,therearetwooutstandinglylarge HnregionsinNGC7331.Ifwe HALTON ARP Vol. 183 1973ApJ. . .183 . .411A No. 2,1973 which emergesfromthesoutheastendofnucleusNGC7319.Theposition combination ofplatesobtainedatAsiagoandPalomar.Thejournalapparent near maximumlight.Theobservations,astheyareknowntothisauthor,consistof plates plusWratten2Cfilteriswavelengthsensitivefromabout3900to5400Â,with photographs mostlyinvolve103a-Jemulsions.UnpublishedworkatCaltechbyMark necessarily uncertainatfaintermagnitudesthanaroundmaximum.ThePalomar letters infigure4.SincetheNGC7331photoelectricsequencereachedonlyasfaint had beenmeasuredbyArp(1961).Rosino’ssequencestarsareidentifiedsmall magnitudes isgivenintable3. about onemonthafteritsoutburst,andconsequentlyfewobservationsareavailable the supernovaismarkedinfigure4.Itwasnotnoticedondiscoveryplateuntil Aaronson andJ.Kormendyshowsthatthephotometricsystemdefinedby103-J as i?=16.86mag,themagnitudescalesetuparoundsupernovainNGC7319is Rosino, whobasedituponanearlierphotoelectricsequencearoundNGC7331which then, theBsystemisabout0.1magfainterthanrrijsystem.Thecolorof plate plusWr2Cfilteris:(R—ra)=0.22(RF).Forstarsofintermediatecolor with A~5000Â.)ThecolorequationfromtheBphotographicmagnitudestoJ an effectivewavelengthabout4600Â.(WithWr4filtertherangeisto5400Â supernova wasnotmeasured,soitisimpossibletotransformallthemagnitude y e The magnitudesequenceusedforcomparisontothesupernovawasonesetupby During thesummerof1971asupernovawasdiscoveredbyRosinoneararm © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 4668 Sept.2118.9 4634 Sept.17-18.8 4618 Sept.11-18.6 4751 Oct.17>18.8 4702 Oct.1018.8 PH 5960Sept.21...18.8 4620 Sept.1218.6 PS 7088Aug.30...18.0 4606 Aug.1916.85 4903 Nov.7>19 4808 Oct.21-18.9 PS 7237.Oct.19...19.0 PS Nov.21-20 4932 Nov.16>19 Comparison Starsa16.40 PS 7238Oct.19...19.1 nations byKormendywithPalomarSchmidt(103a-J + Wr the AsiagoSchmidt(113a-0+GG13).FirstthreePSdesig- 5081 Dec.14>19.5 and lastPSdesignationby Brucato withPalomarSchmidt estimated byArpfromlisted sequence. (103a-O +GG13).Magnitudes fromPalomarplateseye- 2C), PHdesignationbyArpwith200-inchtelescope(103a-J), * PlateswithnumbersonlyarefromRosinotaken Plate* Date(1971)Bm. III. SUPERNOVAINNGC7319 STEPHAN’S QUINTET Supernova inNGC7319 TABLE 3 d 18.6: c 17.20 b 16.65 427 1973ApJ. . .183 . .411A magnitudes werevisuallyinterpretedfromthissequence. sequence isextrapolatedfainterthanaboutB=16.8mag andthatanumberofthesupernova Ciatti (unpublished)fitsallbutfainterpoints.Seetextfor discussionofspectrumphase. Fig. 8a.—LightcurveofsupernovadiscoveredinNGC 7319byRosino.Notethatmagnitude Fig. 8b.—AlternatelightcurvesuggestedbyRosino.Standard curvefromRosino,Barbon,and © American Astronomical Society Provided bytheNASA Astrophysics DataSystem Fig. 8a Fig. 8b 1973ApJ. . .183 . .411A 1 constant brightness.Theuncertaintiesofestimatingsupernovabrightnessrelativeto photographs takenbothatAsiagoandPalomarwithdifferenttelescopesplate for thenext30days.Infigure8Stillstandappearstoberealbecauseitcomesfrom in thatspan.However,theStillstandorshouldertolightcurveataboutB—18.9 is about2maginthefirst30days.Typically,aTypeIsupernovawilldecline that thesupernovaappearedtostandataboutsameapparentmagnitudefor the magnitudesequencecouldintroducescatterfromlightcurve.Theuncertainties filter combinationswhichindependentlygivethesame30-daydurationofroughly mag isnotanormalfeatureofsupernova(seelightcurvesinZwicky1965).TypeI curve whichareavailable. time ofmaximum,estimatingbrightnessesonanapproximatemagnitudesystemis curve reasonablyaccurately.Consideringthattherearenoobservationsofthesuper- observations toasinglesystem.Itisclearfromfigure8,however,thattreatingallthe municated thefollowingconclusions:“ThislookstobeanormalTypeIsupernova in themagnitudescaleofsequencestarscouldchangelevelatwhichStill- supernovae, aftertheinitial30-daydecline,typicallyfallatconstantrateof^mag a TypeIsupernova.Ifthefirstobservationisnearmaximum,theninitialdecline sufficiently accuratetogivetheessentialfeaturesofthosepartssupernovalight nova ontheriseor,asfarisknown,forseveralmonthspriortosuspected magnitude estimatesassomethingclosetoBmagnitudesgivestheshapeoflight given byMinkowski(1939),Searlewasabletosaythatthespectrumagoodmatch the spectrumwithchronologyofchangesonaTypeIsupernovaas taken around20daysaftermaximum.Theredshiftisaboutz=0.02.”Bycomparing which isnotcharacteristicofsupernovae. as unusual. stand occurred.Butthereseemstobenoscaleerrorthatwouldaltertheobservedfact was clearlydifferentfromphasesasearly18daysorlate44aftermaxi- be nearÆ=21mag.Therefore,evenifthefinalpointisassignedanuncertaintyof be seenclearlyonthatplate,butitisnottoofarabovetheplatelimit,whichlimitmust one 48-inchplatetakenbyBrucatoon1971November21.Thesupernovaimagecan 30 daysinSeptemberandOctober.Itisthislatterfeaturewhichmarksthelightcurve £max =16.3mag. reached byusingtwodifferenttechniques.First we canreadtheapparentmagnitude mum. Wehaveseenfromfigure8thattheinitialphaseoflightcurvedoesresemble to thestandardspectrumatabout2024daysaftermaximumbutthat exhibited behaviordifferentfromthetypicalType Isupernova. then isnotinanysenseconclusive,butitraises thepossibilitythatspectrum least 33daysandprobablymorelike38to40days. Theevidencefromthespectrum mum magnitude2^magbrighterthanthebeginning oftheStillstand.Thatyields I supernovaandidentifythebeginningofStillstand astheendoffirstphase a TypeIsupernova.However,thephase,fromhighcurve,isindicatedtobeat maximum magnitudesobserved forTypeIsupernovaeinothergalaxies (Kowal and 33 dayspriortotheStillstand;thisgivesi?= 16.9 mag.Or,wecantakethemaxi- of thestandardlightcurve,thenwecanestimate themaximumapparentmagnitude 1971 September21.LeonardSearlekindlyagreedtoevaluatethespectrumandcom- Sargent 1971).Fromthose dataweseethatforagalaxyofredshift 6935 kms“ max ± \mag,thelightcurvestillshelvesofftoosteeply,leavingaconspicuousStillstand The brightestportionofthelightcurveasshowninfigure8«resemblesmoderately © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The finaldeclineinNovemberseemsmuchtoosteep.estimatedfalloffrestson Figure 9showsaspectrumofthesupernovaasobtainedbyArponnight If weassumeforthesakeofcomputation,however, thatwearedealingwithaType We cancomparethiscalculatedapparentmagnitude atmaximumlightwith STEPHAN’S QUINTET 429 1973ApJ. . .183 . .411A -1 -1 is somewhatpeculiarforagalaxyatredshiftdistancecorrespondingto6700kms, taken. Butthespectrumismuchearlierinphasethanthis,andwewouldtherefore yield a47-dayspanbetweenmaximumlightandthedatethatArpspectrumwas then thesupernovashouldhavereachedamaximumapparentmagnitudebetween it ismuchmorepeculiarforasupernovaatthecloserdistanceofNGC7331and (3) Themaximummagnitudereachedseemstobeunusuallyfaint.Butifthesupernova by about^mag.AdoptingthemaximumtobesoearlyinAugust,however,would bright by0.25magontheaverage,andpointinNovemberisestimatedtoofaint is anacceptablelight-curvefitifthefourmeasuredmagnitudesinOctoberaretoo NGC 7320.If7319wereatadistancecorrespondingtoredshiftof800kms, spectrum appearstobethatofaTypeIsupernovabutpossiblypeculiarforitsphase. the measuredpointsintable3.Thelatterfitgivesamaximumapparentmagnitudeof to beobscured. the expectedabsolutemagnitudeofaTypeIsupernovaobservedinNGC7319andat tional interpretationoftheirredshift.Thiswouldnotstronglyviolate runs countertotheevidencethatNGC7319isatcloserdistanceof7331and B =\0and11mag.Clearlythefaintapparentmagnitudeinferredforthissupernova have astrongpeculiarityinthatregard. about 15.8magatanepochofmaximum1971August5-6.Asfigure%bshows,that novae lightcurves.Usingthisstandardcurve,Rosinosuggestsadifferentfitto in absolutemagnitude.Thisturnwouldforceone oftwoconclusions:{a)thatType NGC 7319withtherestofQuintet,however, asachanceevent. the sametimewouldbesupportedbyfailuretoresolveHnregionsinNGC7319. NGC 7319andpossibly7317areatthelargedistanceindicatedbyaconven- action withthelowredshiftNGC7320isaccidental,andhigh-redshiftmembers NGC 7320. supernovae observedreachbrightermaximafortheredshiftoftheirsystemsthan in NGC7319isseentobeabout1.5magfainterthanitshouldforagalaxyofthat one inNGC7319,andthesupernovaeothersystemspresumablyalsohavechances tude. Thislastexplanation wouldneedtobereconciledwiththeresultthat whatever its intrinsicredshift,thisalsocausesthesupernova tobereducedinabsolutemagni- therefore wehaveenteredacircularloopofreasoning); (b)theNGC7319objectis not atypicalsupernova,andwhateveritisabout thesystemofNGC7319thatcauses regions inNGC7318.Itwouldleavethesupernova inNGC7319asanomalouslylow NGC 7320.Thisexplainstheproximity,interaction, andapparentsizesoftheHn It wouldstillattributetheclosespatialprojection andtheapparentinteractionof the sameareaofskyasNGC7320. of theQuintetareallreallydistantsupergiantgalaxiesjustaccidentallyprojectedinto redshift. ^max -15.2magwithanextremerangeofabout±1mag.Therefore,thesupernova 430 been calibratedhavehadtheirdistancesestimated primarilyfromredshifts,and I supernovaehavebeengenerallymiscalibrated(perhaps allsystemsinwhichtheyhave obscured bydustinthegalaxytosomeextent.However,about97percentof (galactocentric) themaximummagnitudereachedbyaTypeIsupernovaisabout magnitude ofitsHnregions. it isthatgivesNGC7318 itsintrinsicredshiftdoesnotreducetheabsolute sizeor There arethreepossibleresolutionsofthiscontradiction: 2. NGC7320and7318areatthesameclosedistanceas7331,but In summarythen:(1)Thelightcurveofthesupernovaseemstobepeculiar.(2) 3. AllmembersoftheQuintetareatsame close distanceofNGC7331and © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Recently Rosino,Barbon,andCiattihavecompletedananalysisofTypeIsuper- Of course,itcanbearguedthatthesupernovainNGC7319mighthavebeen 1. TheHiiregionsinNGC7318areintrinsicallyverylarge,theevidenceforinter- HALTON ARP 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1973ApJ. . .183 . .411A -1 _1 -1 trail ofmaterial.Therealitythisluminouscanbeconfirmedonthedeepcom- magnitude andsurfacebrightnessfeaturesintheneighborhoodof27-29magper plates shows,asreproducedinfigure10,starstheneighborhoodof24thapparent significant. Adirect,three-hourexposurewiththe200-inchtelescopeonbakedIIIa-J square arcsecond.EmergingfromthesoutheastendofNGC7320isafaintluminous from thesoutheastendofNGC7320,itsweepsawaytoeastandbecomesnarrower from establishingtheappreciablesizeandlengthofthisfeature,tracingestablishes the JetPropulsionLaboratoryforallowinguseoftheirJoyce-Loeblisodensitytracer.) mendy (1972).Onthe48-inchtelescopematerialreferredto,luminoustailcurves bination printoftheregionobtainedfrom48-inchSchmidtplatesbyArpandKor- around tothenorthandcomescloseconnectingasmall,well-definedbarred centered approximatelyalongtheapparentmajoraxisofNGC7320.Afteritemerges another veryimportantpoint;namely,thetrailisaboutasbroadNGC7320and analyze thisplatewithanisodensityrecordingphotometer.(Thanksareextendedto light ofanytheothermembersStephan’sQuintet.Consequently,tailisshown comparison showsthatthisisthesamekindofresolutiontakesplaceinedges The pointisthatthedeep200-inchphotographreproducedinfigure10shows and attenuated.ButthewayfeatureisrelatedtobodyofNGC7320argues to consistofapparentlythesamekindmaterialasbodyNGC7320itself. of thegalaxyNGC7320itself.Nocomparableresolutiontakesplaceincontinuum material inthetailtoberesolvedintoluminousconcentrationsandpatches.Direct strongly foritsconnectionwiththatgalaxy. Figure 11showsthenthefaintestoutercontoursoftailfromNGC7320.Aside spiral. Thatbarredspiralhasaredshiftofz=6000kms(Lynds1972). been exactlyprojected—wemustinsteadacceptthelong,luminoustailasafeature members oftheQuintetonwhichNGC7320,bysomeimprobablecoincidencehad actually belongingtoNGC7320itself. long luminoustailonNGC7320isthemosttangiblekindofevidenceforinter- in Stephan’sQuintetarguesthatNGC7318Aand BareatthesamedistanceasNGC action ofNGC7320withthehigh-redshiftmembersStephan’sQuintet,and interpretation ofitsredshiftz=6700kms . TheabsenceofHnregions,except strongest argumentthatallmembersoftheQuintetareatsamedistance. NGC 7319arguesforthatgalaxybeingatadistance correspondingtoacosmological Assuming theconventionalluminositycalibration, thepresenceofasupernovain resolved contradictionhere.Intheopinionof the author,however,overriding for asingleoneofverysmallapparentdiameter, wouldalsoplaceNGC7319ata 7318 AandBthenwouldbeindicatedasnot due tocosmologicalDopplershifts. 7320 (about10Mpc).Thez=5700and6700km sredshiftsmeasuredforNGC distance. in theQuintet,whichargue stronglythatatleastthefourgalaxiesmentioned, NGC considerations aretheopticalformsandfaint outerluminousfeaturesobserved much greaterdistancethanNGC7320or 7318.Undeniablythereisanun- 7320, NGC7319,and 7318AandBaremutuallyinteracting atthesame The lastpieceofevidencewhichthispaperpresentsis,inmyopinion,themost Returning tothe200-inchplatereproducedinfigure10,however,itispossible There isonefinalpoint,however,whichmustbeemphasizedasbeingquitecrucial. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem We cannowarguethatthetailcouldnotbeafeaturebelongingtohigh-redshift This enablesustoconsiderthefinalargument,whichisthatpresenceofthis The newobservationalevidenceonthedistribution anddiametersofHnregions IV. PHOTOGRAPHYTOLIMITINGMAGNITUDESINSTEPHAN’SQUINTET V. SUMMARYANDDISCUSSION STEPHAN’S QUINTET 433 1973ApJ. . .183 . .411A o 434 possibility ofashockwavebeingexcitedbythemovementtogethertwogalaxies. paper hasalreadypointedtothisinterpretation. more interestingisthesuggestionthatthisinterfaceregionofradioemissionextends NGC 7319and7318fromwhichradioemissionemanates.Allensuggeststhe down towardNGC7320.Inmyopinion,fromtheradioevidence,itlooksasif In anycasethereisclearevidenceforinteractionofthetwosystems.Perhapseven synthesis radiotelescope(Allen1972)addsmoreevidencefortheinteractionofNGC it isseenthattherealong,almoststraightlineofhydrogen-aemissionbetweenNGC apparently connectedwiththenewradioobservations.Infigure3ofpresentpaper 7319 withNGC7318AandB.Thereclearlyseemstobeaninterfaceregionbetween 7320 isalsoinvolvedintheinteraction.Ofcourse,opticalevidencepresent toward thesoutheast.Butcloserinspectionshowsthathydrogen-aregisteredin registration ofthelongarmNGC7319thatsweepsawayfromgalaxyandacross real becauseitcanbeseenonalltheseparateplates,butnaturallyreproducesbetter 7319 andthenearestQuintetmembers,NGC73207318.Thefilamentis figure 3isalmostperfectlystraight,whereasthecontinuumarm(asseeninfig.9, for example)curvesmoresharplyandhascusps.Thefilamentinfigure3seemsalmost on thecombinedprint.Atfirstluminousfilamentseemstobejustaslightlydifferent to representastraightleadingedgeagainstwhichthecontinuumopticalarmistangent. in thesameregionascrescentofradioemissionbetweenNGC7319and7318. remainder ofthefilamentturnsoffeastward,whereasradiocrescentrunsmore This wouldlendsupporttoAllen’ssuggestionofaninteractiondisturbance.Butthe true, however,thatthelong,straighthydrogen-aemissionfilamentisaverypeculiar because itisnotclearwhyshock-excitedsynchrotronradiationand formation; andwhetheritisashockphenomenonornot,justitsgeneralvisual hydrogen-a shouldbecoincidentinsomeregionsandseparateotherregions.Itis NGC 7319andboththelow-redshift7320high-redshift7318. southerly towardNGC7320andisconsiderablyshorter.Thisverypuzzlingbehavior projected radiusofabout0.2Mpc.Suchadistanceisrelativelysmallcomparedtothe appearance stronglysuggeststhatitrepresentssomekindofinteractionfrontbetween distance, existforaradiusofapproximatelyIaroundNGC7331.Thismeans Kormendy (unpublished)hasshownthatdwarfgalaxies,obviouslyatthesame that theStephan’sQuintetsituationisfarfromunique. Infact,allthemultipleinter- at theprojecteddistance(about^degree)ofStephan’sQuintet. size ofourownLocalGroupgalaxies(0.6MpcbetweenGalaxyand M31). Therefore,itisquiteexpectedthatcompanionstoNGC7331wouldbefound pass thisarrayofobservationaldataandimplied connectionsisthattheQuintethas to verymuchhigherthanthatoftheparentgalaxy. contain peculiarandinteractingcompanionsof redshift rangingfromslightlyhigher acting systemslikeStephan’sQuintet(atotalof sixwereadoptedasthebest-known shown thattheneighborhoodsoflargespiralgalaxies likeNGC7331characteristically objects comprisingthisclass)wereshowntofall close tolargegalaxies.Itwasfurther been ejectedfromNGC 7331, andthatthehigherredshiftofsomeits membersis galaxies. due toanintrinsicredshift connectedwiththerecentoriginofmatter withinthese sources existsinthissameregion.Thehypothesis whichhasbeensuggestedtoencom- 7331 andStephan’sQuintet.Arp{\912d)hasshown thataconcentrationofradio © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Recent high-resolutionradiomappingofStephan’sQuintetwiththeWesterbork But thereisavailableinthepresentpaperadditionalopticalevidencewhich The northwestendofthisopticalhydrogen-afilamentliesclosely,andmoststrongly, As forthelargerpictureofStephan’sQuintetanditspossiblerelationtoNGC7331, Within groupsofgalaxiesthisordersizeit has beenindicatedbyArp(1972c) Arp andKormendy(1972)havediscussedthefaint nebulousmaterialbetweenNGC HALTON ARP 1973ApJ. . .183 . .411A © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem

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