19 68ApJ. . .151. .105D o 2 2_11 in arcsecandmin. The AstrophysicalJournal,Vol.151,January1968 Reference Catalogue(G.andA.deVaucouleurs1964)arecentre-evaluationofdata posed night-skyluminosity.Untilthepresentstudyonlyavailableinformationonto- plate. grain focusofthe36-inchreflector(scale1mm= 16'H9) oftheMcDonaldObservatory. October 5and6,1961,December27,1962,with theUBVphotometeratCasse- photographs. Wereporthereonourattemptstoderiveintegratedmagnitudesandcolors in thesesystems. on theLocalGroup(deVaucouleurs1967). reflector ofMcDonaldObservatory. in theUBVsystemfromdirectphotoelectricscansacrosswith36-inch known becauseoftheextremefaintnessevenbrightertheseobjects;typically The trueabundanceofthisgalaxytypeinspacemaybemuchlargerthanonemight detected outtoanaveragediameterinexcessofI. The effectiveequivalentradiusis(r*)=9Í4wherefi(B)25.2magsec“andthesystemmaybe of theNavalObservatoryisshowninFigure1(Plate 5). other nearbygroups(vandenBergh1959)andclusters(Reaves1956,1966).Thevalu- po(B) =24.6magsec“inanenvelopeofgradientUCa)d(logI)/da——0.077minJ—1.15kpc“. scans withthe36-inchreflectorareB—9.04{M—13.0ifmAT=220),{BV)+063, able studiesbyHodge(1961a,6),includingstarcountsinFornax,haverenewedinterest tal magnitudewastheearlyestimatebyShapley(1939)ofwg=9.0,fromsmall-scale the surfacebrightnessincentralregionsisonlyone-tenth(orless)ofsuperim- assume fromtherelativelysmallsampleofadozenorsoatpresentknowninLocal the Sculptorsystem,typeexampleoflow-luminositydwarfspheroidalgalaxies. Group (deVaucouleurs1967).Similarsystemsareapparentlypresentinabundance —34°43', —34°44',—34°46'(1950)nearthecenterof thesystemmadeduringthreenights, e T P (U —B)=+008.. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The observationsconsistofphotoelectricscans(96, 9v,6u)atdeclinations—34°41', % Exceptwhereunitsoftimeare explicitlystated,allangulardimensionsinthispaperare expressed A photographofthe40'X47'centralregion systemwiththe40-inchreflector The luminositydistribution,integratedmagnitudes,andcolorsarestillverypoorly * ContributionsfromtheMcDonaldObservatoryofUniversity ofTexas,No.414. The Fornaxsystem(Shapley1938,1939;BaadeandHubble1939)is,togetherwith The photometryiscomparedwithstarcountsbyHodgeandnewona40-inchreflector The luminositydistributionshowsanearlyuniformcentralcoreof(r)6'andmaximumbrightness General informationontheFornaxsystemisgiveninTable1,mainlyfrom Integrated magnitudesandcolorsoftheFornaxdwarfspheroidalgalaxyderivedfromphotoelectric f NowatU.S.NavalObservatory,FlagstaffStation. INTEGRATED MAGNITUDESANDCOLORINDICES II. INSTRUMENTALANBOBSERVATIONALBATA OF THEFORNAXDWARFGALAXY* G. BEVaUCOTJLEURSANBH.D.ABLEsf Received October24,1966;revisedMay31,1967 McDonald Observatory,UniversityofTexas I. INTROBUCTION ABSTRACT 105 PLATE 5
Fig. 1.—Fornax dwarf galaxy. 40-inch reflector, U.S. Naval Observatory, December 1, 1964; 2 h.; 103a-D + GG 11; north at top, east at left. Dark lines show declination limits of photoelectric scans. Area of figure is approximately 40' X 47'. de Vaucotjleurs and Ables (see page 105)
© American Astronomical Society Provided by the NASA Astrophysics Data System 19 68ApJ. . .151. .105D Different fieldapertureswereusedforthe1961and1962observations.For The imageofthegalaxywasallowedtodriftacrossfieldapertureatdiurnalrate by Mr.F.Lopez.TheextinctioncoefficientsinmagnitudesarelistedTable2B. magnitudes persquaresecondofarc. parent magnitudemeasuredthroughtheholetocorrespondingsurfacebrightnessin in Table2A.Thelastcolumngivestheadditiveconstant2.5logAforconvertingap- range fromabout2to3mm. while theBrownrecorderwasdrivenatrateof0.5inch/mintime.Thiscombina- night werereducedseparatelytothel/BVsystembyacomputerprogramdeveloped ured throughairmassesgreaterthanthatoftheFornaxdwarf.Themeasuresforeach diameters oftheholeswereadopted(Nos.6and10).Theapertureconstantsarelisted observations theeffectivediametersofscanningholesweredeterminedbyrecording tion gaveascaleofabout62mm/degreeonwhichthediametersscanningholes 106 G.DEVAUCOULEURSANDH.D.ABLESVol.151 and timingstartransitsathighdeclinations;forthe1962observationsgeometric © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The photometerusedfortheobservationshasbeendescribedbyJohnson(1962). Four ofthesixtoeightJohnson-Morganstandardsobservedeachnightweremeas- I II11 I, P.. Type ... I, b. S (1950). L, B. Vo (km/sec) a (1950).. V (km/sec) ♦AfterHodge (19616)andShapley(1939). t MountWilsonvalueforglobularclusterNGC1049. Í Inglobularclusters,afterHodge(1961a). Oct. 6,1961. Oct. 5,1961. Dec. 27,1962 10.. 6.. 8 . 7.. Hole No h Date dE3 237™7 266?0, —30?2 237?3, —65?7 203?1, —64?5 _34°44' -141t/ - 7in Elements oftheFornaxSystem Effective Diameter Extinction Coefficients Instrumental Constants 161.0 115?0±0?5 114 5 148 4+05 TABLE 2B TABLE 2A TABLE 1 ¿1(6-*) 0 062 0 086 .084 Absorption correction Magnitude (pg)of Mean axisratio Mean diameter,envelope Mean diameter,core Distance (kpc) Distance modulus brightest star kz(u—b) 0.255 0 259 10 772 10 034 10.595 +0010 10 041+0010 272 2 5logA 0.168 0 135 h(v) .129 [19 6-19.7f Í19.1-19 3* 22 0: 65' -95'* 15' -20' 0.23 0 2 0 65* 19 68ApJ. . .151. .105D No. 1,1968FORNAXDWARFGALAXY107 each scanasaprovisionaloriginoftheabscissascale.The“average”centerwasdeter- recorded onthetracing,andacontinuoustracewasdrawnthroughundisturbed zenith distancez*,amplifiergainG*,andaperture A*bytherelation mined byfoldingthebestscans,assumingasymmetricaleast-westluminositydistribu- each colorwasdeterminedbyaveragingthereduced scans,andfinallyageneralmean rejected becauseoflargefluctuations. passing throughthezeropoint.Thelevelschosen by thismethodagreedverywellwith with thevaluechosenfromvisualinspectionofmeancurves. scan. Asomewhatmoreobjectivedeterminationoftheskylevelispossiblefromaplot of thethree(ô*/)profilesforagivennight,meanskylevel,<5'),waschoseneach the nightskycomplicateddeterminationoflevel.Fromavisualinspection limits shownonthephotographinFigure1(Plate5). from thecenterofgalaxy. to representthetrueluminosityprofileofgalaxy.Aprominentstarwasselectedon regions andinterpolatedthroughtheaffectedbyfieldstars.Thiscurvewastaken profile wasderivedforall threenights. were plottedversus<($&>.Theproperchoiceofthe skylevelresultedinastraightline value ofthemeanskylevelwaschosenandresultingnetdeflections<5> profiles andskylevelsforthescanswithlargefluctuations. level, andthelowerlimitisreadbyextrapolationat1/r=0.Ingeneral,moreprob- where <ô'>isthemeandeflectionaboveanarbitraryreferencelevelatdistancer stars. Finally,eachscanwasfoldedaboutitscentertogiveameanprofile(ô')=/(r), center andQistheextinctioncoefficientforagiven color.Ameannightlyprofilein where (8)isthemeannetdeflectionofgalaxy aboveskyatanydistancerfromthe by assumingthe(6—u)and{ub)colorstobeconstantinmorereliablymeasured for eachofthevanduscanswasthendeterminedfrommeanbprofilenight for theapplicationofthismethod,upperlimitskylevelisincloseragreement outer regionsofthegalaxy,thisplotwilldefineastraightlinewhichgivesupperand of log<ô&'>versus\¡rforeachscan.Foranexponentialluminositydistributioninthe respect tothesizeofgalaxyand(2)scansshowhavea“flat”cen- the initialvisualestimatesfrommeanluminosity profiles.Twoofthevscanswere central andintermediateregions.Foreachofthe<£/><5')profilesanapproximate lower limitsfortheskylevel.Thelastmeasuredpointsetsupperlimit the skylevelhasbeendeterminedforeach.Allofscanswerewithindeclination duction sinceallthescansofaparticularcolorongivennightcanbeaveragedafter through thecenterofgalaxybecause(1)theirseparationindeclinationissmallwith tion. Theabscissaofeachscanwasadjustedtofitthiscenterbymeansthereference able valuewouldbesomewherebetweentheselimits;but,forthescansuniformenough each ofthesemethods.Themoreuniformscanswereusedtointerpolatetheouter each abscissarfornightbyaveragingthethreebluescans.Themeanskylevel tral regionofalmostuniformbrightnesswitharadiusabout6'.Thissimplifiesthere- S VW M © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem All ofthenetprofilesforagivennightwereinitially reducedtostandardvaluesof Each scanwasaccuratelylocatedonaprintofthegalaxybymeansfieldstars Within theaccuracyofmeasurementsallscanscanbeconsideredtopass To converttheaverage deflections tomagnitudes,thesurfacebrightness, Bi(in A meanbluenetdeflection(i.e.,abovesky)<5&>=<5&'>—<5/>wasdeterminedat In mostofthescans,particularlyvandufluctuationsinbrightness The skylevelforeachscanwasadoptedafteracarefulcomparisonoftheresultsfrom log
VII. COMPARISON WITH STAR COUNTS In Figure 5 we compare the luminosity distribution with the average star density according to Hodge (1961#, Table III). The agreement is not good: (1) In the central region the star counts indicate a rather sharp peak in the stellar density while the photometry shows a nearly constant luminosity plateau for r < 6'; (2) in the inter- mediate and outer regions the gradient of the luminosity curve is much steeper than indicated by the star counts. By integration of the star counts we derive an effective radius, r¿ = 12Í3, significantly larger than the photometric value of re* = 9Í4. Both studies agree, however, that the system is detectable to a major diameter in excess of Io = 4kpc. The discrepancy between star counts and photometry can be explained in part by the
TABLE 4 Photometric Parameters of the Fornax Dwarf Galaxy Distance modulus (pg) m — M = 22.0: Galactic absorption A. A = 0.2 Geometric distance (kpc) A = 230 Integrated luminosity! hr = 8832 Surface brightness for unit defl } £i = 27.79 Total apparent magnitude . mt(B) = 9.04 Absolute magnitude...... Mt{B) = -13.0 Integrated color indices B — V = +0.63, U - B = +0.08 Major axis position angle§|| 6 = 54° Threshold surface brightness J .. Pm — 29.5 Major axis at threshold! . 2am — 68' = 4.5 kpc Minor axis at threshold! . 2bm = 44' = 2.9 kpc Major axis at /* = 25.0 mag sec“2 . 2u(25) = 20:i = 1.3 kpc Luminosity within p = 25.0 mag sec“2 ¿(25) = 0.423 Gradient of exponential component G{a) — —0.077 min“1 = —1.15 kpc“1 Equivalent gradient of exp. comp. G(r*) = —0.098 min“1 = —1.46 kpc“1 Observed central brightness} . mo — 24.6 Parameters at & = 1/4: Semimajor axis . ui = 7 .'15 Axis ratio .. Ç! = 0.74 Equivalent radius rS = 6.'2 Surface brightness} . ju, - 24.7 Parameters at Æ = 1/2 (effective): Semimajor axis ae = ll'AS Axis ratio. . qe = 0.69 Equivalent radius Ye = 9.'4 Surface brightness} at re*. pe = 25.2 Mean surface brightness# inside pJ = 15.9 Parameters at Æ = 3/4: Semimajor axis . . us = 17.T Axis ratio . qz = 0.65 Equivalent radius rz* = 13.'9 Surface brightness} .. ... pz = 26.2 Concentration indices . C21 = Te/ri = 1.52 C32 = rz/re* = 1.48 t Units of surface brightness per square minute. t Mag (B) per square second. § Based on Hodge’s (1961) values for the orientation and ellipticity. II 40-inch plate gives 0 = 46°. # Mag (B) per square minute.
© American Astronomical Society • Provided by the NASA Astrophysics Data System 19 68ApJ. . .151. .105D -1 difference. A partialrepetitionofbothcountsandphotometrywasperformedtohelpresolvethe measurements nearthesouthernhorizon:“Theaveragesurfacebrightnesswithina us toquotepreviouslyunpublishedresultsofobservationsmadebyhimwiththe100- inherent difficultyoftheobservationsandinpartbydifferentdefinitionscenter. inch reflectorofMountWilsonObservatorywhenlowcloudsoverLosAngelesallowed No. 1,1968FORNAXDWARFGALAXY113 of thebrightestforegroundstarCD—35°919andmarkedHinFigure6. is 0.69±O.^.”Thepeakdensity,accordingtoHodge,located32"south,270"east circle of0Í93radius,centeredonthecentralpeak,is=23.78±0.14andcolor regions, butthesurfacebrightnessismorethandouble thatindicatedbyourvalueof Abscissae aremeasuredfrom(C)fortheluminosityprofileandFlagstaffcounts{H) per unitsquarevariesbetween 0and9,indicatingthatcrowdingerrorsare notserious. Naval Observatory61-inchreflectoronJanuary11, 1967.Anareanearthecenterof Hodge’s counts. /¿o =24.6.Thediscrepancyof0.8magismuch greater thantheestimatedcombined system definedbythescanswasmeasuredwith an aperture1.'9indiametercentered errors (0.3mag).Additionalindependentmeasurements weremade,therefore,withthe Figs. 2-5)arebasedonaverages of25squares(5X5countingsquares)with aGaussian and i?—F=+0.61,incloseagreementwiththe McDonalddata. 180" eastand10"southofthereferencestarCD—35°919. Theresultis£=24.71 (scale =29'7mm)overanarea37'X38'60 X62squares.Thenumberofstars © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Concerning thephotometry,Dr.P.Hodgehaskindlycommunicatedandpermitted The smoothedisopleths in Figure6(whichmaybecomparedwithHodge’s [19616] To checkthestarcounts,oneofus(H.D.A.)has countedstarsonthe40-inchplate The coloragreeswellenoughwithourmoreprecise value0.66±0.02forthecentral Fig. 5.—ComparisonofmeanluminosityIandstellardensitydistributionsTV*alongmajoraxis. B 19 68ApJ. . .151. .105D -2 -2 dental peakPor#istakenastheorigin.Similarfluctuationsareobservedinlow- west asymmetrydiscussedbyHodgevanisheswhenthecenterCratherthanacci- does notappearinHodge’scounts,noristhereanytraceofHFigure6.Theeast- density globularclusterssuchaswCentauri(BurrandGascoigne1956). various centroidswithrespecttotheCDstararelistedinTable5. weight functionofstandarddeviationcr~lunit^Oí6thatcorrespondstoabout in Figure6bycontour4.Becauseofthefluctuationssmallnumbers,center system isbetterdefinedbyconsideringthecenterofsymmetryCroughlyelliptical east, 1Í3northofthereferencestar,butseveralothersonlyslightlylowerareindicated outer isoplethssuchascontour25starsmin).Therectangularcoordinatesofthe 1Í3 resolution.ThereisamainpeakcountPinsidecontour5(N>12starsmin),75 114 G.DEVAUCOULEURSANDH.D.ABLESVol.151 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem It isdoubtfulthatthedensitypeaksHandPhavemuchsignificance;wenote 19 68ApJ. . .151. .105D -2 -2 No. 1,1968FORNAXDWARFGALAXY115 sumed foregrounddensityof0.9starminderivedbycomparisonwithHodge^data. of thetotalluminositysystemsincemt—9.0andm*=m2.SlogNt19.7 they gowiththephotoelectricluminosityprofile.Thecountswerecorrectedforanas- along themajoraxisreflectedaboutC.Thenewcountsagreereasonablywellasfar diaphragm ontheADHSchmidttelescope.Webelievethatmagnitudelimitismuch correct. NowaccordingtoHodge(1961a)thestellarpopulationinmagnituderange of 0.52starminat=—65°correspondsthroughSeares’stables(Scares,vanRhijn, fainter thanestimatedbyHodgeandintherange20.5-20.7.Thebackgrounddensity 2500 tothelimitingmagnitudeofplateADH5094exposedfor90minwitha24-inch observations. preliminary draftofthispaperandDr.P.Hodge foradditionalinformationonhis and Joyner1925)to20.3intheoldphotographicscalethatrequiresapositive 19.1 Copyright 1968. The University of Chicago. Printed in U.S.A. © American Astronomical Society • Provided by the NASA Astrophysics Data System