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1991ApJ. . .379. .177L 45 810 79 The AstrophysicalJournal,379:177-215,1991September20 © 1991.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. detected inopticalemissionlines (Phillipsetal.1986;Bettoni & Buson1987)withionized gas massesofonly10-10M. Many ellipticals havebeendetectedinX-ray emissionfrom formation inthesegalaxiesandonthesurprisinglyuniform given usmuchinformationontheglobalprocessesof contain asubstantialcoldinterstellarmedium(e.g.,Faber & values fromonegalaxytothenextofsuchparametersas rela- 10-10 Mq of hotgas(Nulsen,Stewart, &Fabian1984; (Knapp 1987,1990).Close to 60%ofellipticalsarealso in Hiemission,withtypicalatomic gasmassesof10-10M many ellipticalsdohavesome coldgas,eitheratomicormolec- Gallager 1976;Johnson&Gottsman1979).However,in the tive gasmass,ratioofatomictomolecularand ular. About15%oftheellipticals and25%ofSO’saredetected past decademoresensitivemeasurementshaveshown that observations quicklyindicatedthatellipticalgalaxiesdo not of IRorHaemissiontogasmass.Ontheotherhand,initial 0 0 Studies oftheinterstellarmediumspiralgalaxieshave line resultedin10detectionsincludingeightoutofthe20ellipticals.Eightthesegalaxieshavenotpre- detections, withobservedmoleculargas,aswell27upperlimits.Thus,ourstudyalmostdoublesthe viously beendetected.Asearchoftheliteraturerevealsatotal17ellipticalgalaxies,includingoureight (albeit withalargerdispersion),similarratiosofHaluminosityandFIRtomoleculargasmass, molecular andatomicgashavethesamemeanratiooftomassaslatertypespirals number ofellipticalgalaxiesdetectedinCOemission. intermediate betweentheellipticalandspiralsystems.The27early-typegalaxieswhicharedetectedinboth properties similartothoseinlate-typegalaxies,butonamuchsmallerscale. type ),extendingtomuchlowervaluesthanforthespirals.ThelenticulargalaxieshaveCOproperties and anapproximatelypower-lawdistributionofMJL(incontrasttotheGaussianforlater probably containamolecularinterstellarmedium.Forthoseellipticalsinwhichgasisdetected,the clusion. Thedetectedellipticalsalsotendtobemuchbrighterinthefar-,implyingpresenceofcold galaxies becauseoftheirlargerlinewidths.Furtherstudythebrightellipticalsisneededtoverifythiscon- ticals, whichhavemorethantwicethedetectionrateofbrightergalaxies.Bothdetectedandunde- galaxies indicatesthatthemoleculargasmayoccurpreferentiallyinbluer,lowerluminositydwarfellip- and identicalFIRcolors.Thus,these(mostlyFIR-bright)ellipticalsseemtohaveglobalinterstellarmedium tected ellipticalsareatthesamemeandistance;however,itmaybemoredifficulttodetectgasinbright lack ofcorrelationbetweenthemoleculargasmassandblueluminosityarguesforanexternalorigin dust and/orstarformationassociatedwiththemoleculargas. Subject headings:galaxies:interstellarmatter—interstellar:abundancesmolecules this gas. nB 78 12 The detectedellipticalshavetypicalmoleculargasmassesofabout10-10M,similartotheirHimasses, Observations of24early-typegalaxies,mostlyfar-infrared(FIR)-brightellipticals,intheCO(2-l)emission A comparisonofthepropertiesthoseellipticalswhichhavebeendetectedinCOwithundetected Our highdetectionrate,andthepower-lawdistributionofM/L,impliesthatmostFIR-brightellipticals © American Astronomical Society • Provided by the NASA Astrophysics Data System 0 H2B 1. INTRODUCTION Downs LaboratoryofPhysics,320-47,CaliforniaInstituteTechnology,Pasadena,CA91125 Harvard-Smithsonian CenterforAstrophysics,60GardenStreet,Cambridge,MA02138 MOLECULAR GASINELLIPTICALGALAXIES Peyton Hall,PrincetonUniversityObservatory,Princeton,NJ08544 Received 1990October19;accepted1991April3 Joanna F.LeesandG.R.Knapp Michael P.Rupen T. G.Phillips ABSTRACT AND 177 12 Clearly, itisimportanttoknowwhetherglobalpropertiesrela- formation andtheroleofISMingalaxyevolution,as well the verydifferentenvironmentsofspiralandellipticalgalaxies. Forman, Jones,&Tucker1985;TrinchieriFabbiano1985). (1989) listIRASFIRfluxdensities forapproximately1150 from acompilationofco-addedIRASfluxesearly-type gal- Knowledge ofthesepropertiescouldgivevitalcluestogalaxy as thermalemissionfromdustgrainsinthegas,wedecided to related totheglobalFIRemission,whichisusuallyinterpreted as tothemoredetailedphysicsofinterstellarmedium. ting theinterstellarmediumandstarformationaresimilar in early-type galaxies,findingthat about45%oftheellipticalsare observe anumberofFIR-brightellipticalgalaxies,selected emission thoseellipticalgalaxies withS>1.0Jy,about detected atboth60and100/un. WechoosetoobserveinCO axies (Knappetal.1989),in the CO(2-l)line.Knappetal. have , and fivemorehavenearby spiral companions a totalof76elliptical galaxies.Fourofthese galaxiesdonot 14% oftheellipticalsincluded inthesurveyofKnappetal.,for i0fim Since inspiralgalaxiesthecoldgasmassisveryclosely 1991ApJ. . .379. .177L 1 -1 -1 made usingthe10.4mtelescopeofCaltechSubmillimeter 400 and500K.Thespectrallinebackendisanacousto-optic with zenithopacitiesbetween0.05and0.08throughout the beam brightnesstemperatureT.Theweatherwassuperb, losses; dividingbythemain-beamefficiencygivesmain- perature (T*¡)iscorrectedforatmosphericopacityandantenna The calibrationwasperformedusingobservationsof an double-sideband mode,withreceivertemperaturesof~110 K. receiver (Ellison1991)usedliquidhelium-cooledSISmixers in the Novemberrun,somewhatworseforMayrun.The and givermspointinguncertaintiesofapproximately±4"for half-power beamwidthis32"andthemain-beamefficiency 21-28 and1990May6-11.Atthisfrequency,thetelescope Observatory onMaunaKea,Hawaii,1989November ing oneFIR-faintelliptical,IC2006.Theobservationsare early-type galaxiesNGC404 andNGC7465,whichhadbeen smoothing of0.64kms“and atotalbandwidthof660km spectrograph (Masson1982)with1024channels,each0.495 observations. Thesystemtemperaturewastypicallybetween ambient temperaturechopper.Theresultingantenna tem- et al.1987);thedistancetoVirgoClusterwastakenbe was assumed(forexample,fromTurner&Gott1976orDavies published resultsofCOobservationsE,SO,andSOagalaxies etc. Wethencombineourobservationswithpreviously presented in§2.In3wediscusstheresultsonthese24indi- Thronson etal.1989;Wiklind&Henkel1989c;Gordon emission.) AnumberofothersurveysFIR-brightearly-type detected previouslybyWiklind &Henkel(1989b)andThron- observed aschecksthatthe system wasworking,aswerethe s. 72%. NightlyobservationsofJupiterconfirmthisefficiency for oursamplethesecorrectionswouldbelessthan10%or change thedistancestogalaxiesprojectedcloseVirgo,but 20% andsocanbesafelyneglected. tions fornonuniformexpansion,exceptthosegalaxiesin velocity assumingH=100hkmsMpc,withnocorrec- to comparethedistributionofmoleculargasmassinthese those oftheatomicandionizedgas,stellarkinematics,dust, vidual systemsandcomparethemoleculargaspropertiesto these ellipticals,aswellfiveotherearly-typegalaxies,includ- galaxies havealsobeenundertaken(Sage&Wrobel1989; been detectedinCOemission[byusorbyotherobservers], MHz wide,correspondingtoavelocityresolutionbefore of 1.5Mpc.IncludingcorrectionsforVirgocentricinfallmight obvious groupswherethebestestimateforgroupdistance detected ellipticalswiththeundetectedin§4.2. nosities, colors,IRproperties,andHimassesoftheCO- systems, aswellthedistributionsofM/M,/L observed. Twenty-onegalaxiesofthe67havedetectedHi and 11havesensitiveupperlimits,leaving41galaxiesyettobe 67 FIR-brightellipticals.(Currently,15oftheseellipticalshave that maybecontributingsomeoftheIRflux.Weareleftwith Finally, conclusionsaregivenin§5. 178 13.5 Mpc(forh=1).NGC404isassumedtobeatadistance 1990b). MB 0 H2HIFIR 12 12 Observations oftheCO(2-l)lineat230.538GHzwere The starburstgalaxiesNGC 253andNGC1068were The distanceforeachgalaxywasestimatedfromtheradial In thispaper,wereportonCO(2-l)observationsof19 and infraredcolors,in§4.1.Wealsocomparelumi- © American Astronomical Society • Provided by the NASA Astrophysics Data System 2. OBSERVATIONS LEES, KNAPP,RUPEN,&PHILLIPS 1 -1 -1 integrated COlineintensity,isgiveninTable1.Thetypical four otherearly-typegalaxieswhicharemostlikelynot in Kxkms~andthemolecular hydrogencolumndensityof was estimatedusingaconversion betweentheCOlinefluxI width betweenthe50%powerpointsofglobalHiprofile, rms noiselevelsareabout5-15mK(0.2-0.6Jy).Thevelocity May run. logs (thereferencesarecitedbyPalumbo,Tanzella-Nitti,& (1982, hereafterESO).Velocitiesweretakenfromthesecata- leurs, deVaucouleurs,&Corwin(1976,hereafterRC2), parts ofthesegalaxies.PositionsweretakenfromDressel& its opticalcenterinposition-switchedmodewithathrowof ellipticals) wereobserved.Eightdefinitelydetected,and centered onthevelocityofgalaxy.Itisclearfrom the increased by50kms(togivethefullwidthofline), and was obtainedfromHidataandtakentobethevelocity range usedfortheintegrationthosegalaxiesnotdetected carefully, wealsosawsomeevidenceforthesameproblem,but (although whenwecheckedthe1989Novemberdatamore Vettolani 1983),orfrommorerecentstudiesintheliterature. Condon (1976)whereavailable;otherwise,fromdeVaucou- tributed inaringlikeorclumpyconfigurationtheouter arcminutes insize,wewouldhavemissedmoleculargasdis- approximately 40%onbothruns.Eachgalaxywasobservedat several morewereprobablydetected.Thedetectionratewas impressive fortheobservationsofsouthernobjectslike IC profiles andthermsnoisevaluesinTable1thatwewere able which arejusttheformalvalues.Wewereabletobypassthis in mindwheninterpretingourquotedupperlimitsanderrors, noise doesseemtobeuncorrelated.Thiseffectshouldkept what lessthanwewouldexpect,althoughoverlargescalesthe atic channelnoise,duetoproblemswiththeAOSspectrometer to integratedownthetheoreticalnoiselimitinthese long meant asubstantiallossofobservingefficiencyforthe1990 difficulty tosomeextentbyshiftingthecentralobservingfre- smooth overfivechannels,thermsnoisedecreasesbysome- at alowerlevelthaninthe1990Mayrun).Asresult,whenwe observations thanthe1989Novemberdatabecauseofsystem- Sandage &Tammann(1987,hereafterRSA),orLauberts observations. Thesensitivityandstabilityareparticularly were alsoexcised.Inthecourseofobservations,wefound again beforesummingthespectra.Unfortunately,thisalso quency bysmallamountseveryfewscans,thenshiftingback for eachgalaxy,weightingscanbyitsrmsnoise,anda waited for30saftereachcalibrationscan.Inall,about15%of bration scan,sofortheremainderofobservationswe noise; afewverynoisyscans(rmsnoise2-16timestheaverage) showed baselinecurvaturegreaterthantheirpeak-to-peak son etal.(1989),respectively.Inall,20ellipticals(aswellas emission. TheobservedlineprofilesoftheellipticalsandSO’s, linear baselinewasfittedthroughchannelsjudgedtobefreeof the datawererejected.Theremainingscansthensummed that theseoftenseemedtooccurimmediatelyafteracali- profiles showtheentirevelocityrangeobserved.Thechannel- binned in3.2kmschannels,areshownFigure1;the to-channel rmsnoiseofthebinneddata,togetherwith the 1459 andNGC2328. co ± 10'inazimuth.Sincethesegalaxiesaretypicallyacoupleof The massofmolecularhydrogen forthedetectedgalaxies The individual10minuteobservationswererejectedifthey The noiselevelsweresomewhatworseforthe1990May 20 ATh =4x10a/cm“, (1) 2co 1991ApJ. . .379. .177L brightness temperature,^MB’inmK; the abscissaisheliocentricvelocityinkmsThefullrangeof spectraisshown. - Fig. 1.—TheobservedCO(2-l)line profiles smoothedtoaresolutionof3.2kms^forthe24early-typegalaxies listedinTable1.Theordinateisthemainbeam © American Astronomical Society • Provided by the NASA Astrophysics Data System v [km/sec] hel v [km/sec] hel 1991ApJ. . .379. .177L © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 1—Continued v [km/sec] v [km/sec] hel hel 180 1991ApJ. . .379. .177L © American Astronomical Society • Provided by the NASA Astrophysics Data System v [km/sec] hel 181 1991ApJ. . .379. .177L © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 1—Continued vi [km/sec] he vi [km/sec] he 182 1991ApJ. . .379. .177L 2 -1 1 12 where aisconstantoforderunity[Scoville&Sanders1987; The beamareaisn0/4In2,where6theGaussianhalf-power independent, thenthe3aupper limittotheintegrationis without. Assumingthechannel tochannelfluctuationsare beamwidth, takentobe32".Then true forspiralgalaxies(Knappetal.1980;Maloney1990)]. those galaxieswithHidetections, and300kmsforthose velocity width,chosentobe the Hiwidthplus50kms~for Upper limitsweretakenas3aintegratedoveranexpected Sanders, Solomon,&Scoville1984;thisassumesthat the CO(2-l) intensityissimilartothatatO(1-0),which is a a a a a a a a a a a a a a from thoseinallotherpartsofthispaper—cf§2. detection [singleparentheses]).Theuncertaintiesintheformerareobviouslylarge. listed inparentheses(e.g.,NGC4742);nondetections,double216). NGC216 NGC 855 NGC404 NGC 1400 NGC 1052 NGC 855(30"E) NGC 2814 NGC 2768 NGC 2328 IC 2006ring IC 2006 NGC 3265 NGC 2814(5'N) IC3370 NGC 4278 NGC 3273 NGC 5018 NGC 4742 NGC 4581 NGC 4476 NGC 5666(15"SE). NGC 5666 NGC 5363 NGC 5666(30"NW) NGC 7468 NGC 7465 NGC 7464 IC 1459 NGC 6654 NGC 5666(30"SE). NGC 5666(15"NW) a _ 1 8_2 Thesegalaxieswereobservedinthesecondrun1990May.Allotherfirst1989November. Col. 3.—Declination. Col. 1.—Galaxyname. Col. 7.—TheintegratedCOintensity.ThisistheintegralofunbinnedlineprofileinaregionwidthÁVcenteredonV.Possibledetections are Col. 6.—Rootmeansquarenoiseofthebinneddata(i.e.,per3.2kmsbin). Col. 5.—Morphologicaltype. Col. 4.—ThecentralheliocentricvelocityoftheCOline(ifdetected),orourobservationsnotdetected,ifonlyatentative Col. 2.—Rightascension. Col. 9.—ThemassofmoleculargasderivedfromIusingeq.(2)inunits10a/iM.Notethatupperlimitsthistablearecalculateddifferently Col. 8.—Thevelocityintervaloverwhichthespectrumwasintegratedtoobtain/. Q coo co © American Astronomical Society lo52a/ M =1.7x(^)coAÍ0•(2) H2 (1) 1 'cc^^p^Kkms-, (3) hm 003859?6 03 3715.4 02 3837.0 02 1111.0 01 0639.3 03 5226.3 03 5235.9 09 1709.2 09 0745.2 07 0101.0 23 0030.2 22 5931.8 22 5924.7 25 5423.0 10 2814 10 2819.1 12 2459 12 1736.5 12 3531.5 12 2726.7 13 5336.3 13 1020 12 49 14 3043.3 18 2514.4 (1950) R.A. (2) MOLECULAR GASINELLIPTICALGALAXIES 12 New ObservationsofCO(2-l)inEllipticalGalaxies -18 5056 -08 2806 + 273836 -36 0647 -36 0647 + 352710 + 642750 + 601440 -41 5942 — 21°19T2" -39 0342 -35 2124 + 290313 + 123727 + 293326 -19 1512 -10 1100 + 014509 +10 4347 + 052958 + 162008 + 154217 -36 4348 + 730911 + 154150 (1950) Decl. (3) Ye (km s Provided bythe NASA Astrophysics Data System 2000 2970 2419 2897 1558 1400 1400 1478 2245 2245 2245 1450 1690 1363 1170 2089 2245 2155 1700 1138 1289 1818 -69 1900 1700 1820 1935 TABLE 1 (4) 602 610 565 630 _ 1 -1 where

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© American Astronomical Society • Provided by the NASA Astrophysics Data System 1991ApJ. . .379. .177L for molecularcloudsinthesolar neighborhood.Thisvalueis is thecloudradiationtemperature, sothata»1isapplicable not affectedappreciablyby changes inmetallicityofthegas where nisthehydrogenvolume densityinthecloudsandT Galaxy, orevenfornearbyspiralgalaxies.According to Maloney (1987,1990), 196 Bottom :TheratioofIRAS60¿unto100pmfluxdensity. H R N185 .. N2328 . N1400 . N1275 . N855 .. N404 .. N205 .. N3265 . U2836 . N7176 . N5666 . N5128 . N4472 . N3928 . N4649 . N4476 . 13370 .. 9-2 8 -2 82 -2-1 925 2 Col. 2.—Blueluminosityinunitsof10/iL. Col. 1.—Galaxyname. Col. 9.—Top:Theratioofmoleculartoatomichydrogengasmasses.Bottom :The1oerror. Col. 8.—Top:Themassofmolecularhydrogenfromeq.(2)inunits10 a/i M.Bottom:The1oerrorinthemolecularhydrogenmass. Col. 7.—Top:Themassofatomichydrogeninunits\0h~M.Bottom: The1oerrorintheatomichydrogenmass. Col. 6.—ThelogarithmoftheX-rayluminosityinunitssolarluminosities. Col. 5.—ThelogarithmoftheHaluminosityinunitssolarluminosities. Col. 4.—Thelogarithmoftheradioluminosityat20cminunits/iergsHzs Col. 3.—Top:FIRluminosity(40-120pm)inunitsof10h~L,derivedfromtheIRASfluxusingequation=3.65x(2.58S+ S)D. Col. 12.—Top:Theratiooftotalgasmass(excludinganypossibleX-ray-emitting gas)toblueluminosity.Bottom:The1crerror. Col. 11.—Top:Theratioofmoleculargasmasstoblueluminosity.Bottom :The1oerror. Col. 10.—Top:Theratioofatomicgasmasstoblueluminosity.Bottom 1aerror. 0 0 Q 0FIR6o 100/tin Name (1) © American Astronomical Society • Provided by the NASA Astrophysics Data System 109. a 47.9 40.9 59.3 32.9 12.6 0.110 0.291 0.277 0.749 0.322 0.860 3.35 Lb 8.28 3.05 (2) 1.00 1.79 3 .200 cm-/UW <»> V'Y^kA H ^60 ßtn/S100ßtn 20.6 22.8 0.0005 0.77 0.16 0.22 0.817 0.549 0.23 0.282 0.37 0.0948 0.50 0.0085 0.0009 0.28 0.53 0.61 0.63 0.41 0.207 0.41 0.225 0.481 0.41 0.58 0.45 0.72 8.01 3.45 1.28 1.59 (3) log (L)J <24.77 <24.91 <23.78 <27.84 rhx <26.60 All EllipticalsDetectedinCO(DerivedQuantities) 26.97 26.29 27.00 31.7 27.77 29.56 28.16 26.52 30.20 27.73 (4) (5)(6) LEES, KNAPP,RUPEN,&PHILLIPS 6.60 8.80 3.85 6.26 5.98 5.65 (4) TABLE 3 <4.48 <5.76 <5.72 10.88 8.95 7.57 7.26 unless themetallicityissubstantiallylessthansolar(Maloney molecular cloudpropertiesin suchanenvironmenttobelike that neartheSun,andhence itisunrealistictoexpectthe Cluster), theinterstellarradiationfieldwillbestronger than our beamsizeof32"correspondstoatthedistanceVirgo quite differentfromsolar.However, onamoreoptimisticnote, those intheGalaxy,especially sincethemetallicitiesmaybe observed inboththe60and 100 //mbands,andtheratiosof the dusttemperaturesof the ellipticalgalaxiesthatare 1990). Intheinner2kpcofellipticalgalaxies(whichiswhat <16.9 <6.28 <0.343 <0.618 <0.334 M, h 0.0015 0.416 0.225 0.0037 0.30 0.882 4.27 (7) 1.67 1.91 a 76.3 57.5 27.6 0.0033 0.0001 0.0026 0.054 0.835 0.0086 0.0023 0.0243 0.0010 0.0036 0.536 0.164 0.56 0.909 0.060 0.026 4.02 0.10 0.013 0.150 0.41 0.089 0.707 7.39 5.73 1.9 3.1 1.2 1.36 M, (8) (7 h 2 AVM hi >0.13 >3.40 >1.1 >4.0 >0.44 0.008 0.01 0.26 0.96 0.06 0.021 0.11 0.07 0.61 0.10 0.54 0.10 0.94 3.0 1.70 (9) (7 <0.19 <0.016 <0.0006 <0.0015 <0.019 0.0014 0.13 0.077 0.0013 0.12 0.04 0.19 0.14 0.0040 (10) l/Lß 0.002 0.0010 0.0008 0.0003 0.0013 0.0001 0.0023 MJLb 0.61 0.002 0.025 0.053 0.0027 0.0084 0.008 0.072 0.003 0.016 0.01 0.11 0.13 0.012 0.006 0.076 0.00002 0.00025 0.002 0.022 0.33 0.01 0.0002 0.0017 0.04 h (11) a MJL 0.001 0.0003 0.0001 0.0037 0.062 0.025 0.006 0.053 0.001 0.132 0.086 0.0026 0.19 0.003 0.016 0.01 0.61 0.00019 0.00025 0.002 0.022 0.04 0.30 0.01 0.33 0.27 0.016 0.0017 0.007 0.076 0.04 0.01 0.0006 %b (12) a r" r" ft TABLE 4 All Ellipticals Observed But Not Detected in CO (Observed Quantities) «iS" «H ^ (Sb gg- ^8 ^ .f-e; ^ S' lb O ^ s 13 ®<~n Q S ^ cq oq b « Ci ffi u p o Oh i I£ ; H 6 : © American Astronomical Society • Provided by the NASA Astrophysics Data System B / £ 2 CO 00 E CN Tt -rHro(N d ON '^■ ON 3; r-H o>oooopqouco izccxiucaca CN rn I O0 NO u<:u<:0>0'0u- :Z>u» tn os »O ^ O to fO tN _ .E< E r' Si d R S ^ •V m ? 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Fig. 2.—Thedistributionofblueluminosity forthecompletesampleofearly-typegalaxiesobservedin COemission(see§4.1),binnedinunitsoflog Name (1) © American Astronomical Society • Provided by the NASA Astrophysics Data System 101 10.3 12.8 16.3 Lb (2) 6.12 4.44 2.27 7.12 4.29 3.23 5.40 1.53 ^60 /un/^100/im 23.5 19.0 0.79 0.840 0.38 0.25 0.41 0.52 0.39 0.642 0.33 0.39 2.09 0.61 0.47 0.27 0.552 1.15 1.97 3.80 1.62 1.97 (3) 2 2 log (L) rHax log (Lh-/L) log (Lh-/L) 29.17 27.44 28.76 28.56 31.91 27.14 27.49 27.88 27.79 27.72 All LenticularsDetectedinCO(DerivedQuantities) b0 bG (4) (5)(6) 7.18 6.77 7.21 5.23 6.07 7.40 TABLE 7 6.71 5.97 5.46 201 <21.2 <6.63 <1.60 <0.515 <0.271 <0.773 <0.606 <4.88 m, hH2 13.4 16.0 (7) (8) 0.570 c a 3.47 29.7 50.9 0.36 2.0 0.47 0.18 0.54 0.84 2.01 2.9 5.23 0.27 0.71 5.67 0.53 0.32 0.30 0.155 0.715 7.88 3.36 7.88 5.19 1.55 M/M H2hi >12.4 >10.2 >3.3 >4.5 >0.27 >3.0 >1.2 >1.6 (9) G 0.8 0.2 0.05 0.16 3.5 3.8 0.02 0.32 <0.0016 <0.11 <0.17 <0.0072 <0.0017 <0.014 <0.32 <0.019 0.013 0.13 0.15 (10) 0.37 a mjl ub 0.49 0.0052 0.03 0.49 0.010 0.045 0.03 0.0004 0.007 0.044 0.008 0.024 0.010 0.022 0.002 0.021 0.15 0.02 0.52 0.01 0.005 0.022 0.01 0.12 (11) a M/L gasB 0.05 0.49 0.058 0.62 0.0006 0.0052 0.010 0.03 0.18 0.056 0.044 0.022 0.01 0.021 0.010 0.01 0.15 0.002 0.11 0.01 0.008 0.022 0.52 0.49 (12) G r"

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© American Astronomical Society • Provided by the NASA Astrophysics Data System 1991ApJ. . .379. .177L N216 .. N274 .. N632 .. N244 .. N2902 . N2685 . U3426 . N1023 . N3245 . N3011 . N4503 . N4435 . N4429 . N4203 . N4150 . N3870 . N7332 . N4694 . N4596 . Note.—Explanation ofcolumnsisgiveninnotestoTable3. Name (1) complete sample.Thenumbersinparentheses arethevaluesfor our observationsalone. type thatareobservedand/ordetected inCOemissionfromour Sa 13(0) 12 (0)92 SOa 19(1) 10(0) 53 SO 31(2) 12 (1)39(50) E 44(20)17(8)39%(40%) a Listedarethenumbersofgalaxies ofeachmorphological Total 107(23) 51 (9)48(39) Type ObservedDetectedRate © American Astronomical Society Summary ofAllEarly-TypeGalaxies 54.7 14.9 0.559 9.09 6.19 5.22 0.741 2.09 4.10 4.93 0.0896 0.472 5.33 6.69 3.55 5.58 7.30 7.81 3.74 (2) (3) 2 Observed inCOEmission J 60 100pm TABLE 10 0.36 0.467 0.67 7.27 0.15 0.210 0.19 0.0791 0.53 0.119 0.160 0.631 0.0492 0.41 0.414 0.43 0.653 0.30 0.610 0.27 0.46 0.0122 0.128 0.51 8.69 0.51 0.47 1.22 All LenticularsObservedButNotDetectedinCO(DerivedQuantities) MOLECULAR GASINELLIPTICALGALAXIES log (L,)(LJ <28.48 h <25.94 <28.01 29.12 30.34 (4) (5) 27.09 27.29 27.06 26.50 26.98 26.95 Detection <5.58 <3.10 7.01 6.01 6.40 5.61 log (L) x 8.15 5.40 TABLE 9 7.48 (6) Provided bythe NASA Astrophysics Data System 12 12 12 12 (1-0) intensitiesofabout0.8 (Maloney1990).Ontheother (Knapp etal.1980;Casoli 1990; Maloney1990).Optically (1-0) intensitiesofapproximately 0.5(Wiklind1990),similarto NGC 3928,5128,and 7465)withratiosof(2-1)to in bothCO(1-0)and(2-1)emission(NGC404,NGC1275, ellipticals, 23havebeenobservedinCO(1-0)and21 in widely differentfromthevaluestypicallyfoundforspiral gal- hand, optically thingaswilltendtohave ratios muchlarger thick coldgaswithTä10 K shouldhavearatioof(2-1)to the ratioofclosetounityinspiral galaxiesandtheMilkyWay same COtransitionsobservedineachgalaxy.Ofthesample of axies. the CO(2-l)to(1-0)intensities(seebelow),seemnot be CO(2-l). Fiveoftheearly-typegalaxieshavebeendetected ex <0.258 <0.258 <0.344 <0.0061 <0.310 <0.386 m The secondpointisthatwearenotevencomparingthe 24.5 30.8 12.2 11.4 h\ MJM 0.17 0.63 9.08 6.1 7.91 0.73 0.31 0.808 5.64 0.090 0.284 0.191 7.67 1.48 hhi 1.3 1.05 1.43 (7) (8)(9) g a <256. <375 <153 <104 <22.9 <63.9 <52.3 <56.7 <17.4 <17.4 <75.9 <18.3 <2.50 <8.01 <1.59 <0.619 <2.92 <0.456 <0.603 <366 <11. <16. <10. <12. <14. <23. <0.20 <8.1 <5.8 <7.7 <9.9 <0.42 <0.0023 <0.0068 <0.0046 <0.0049 <0.0063 <0.0052 0.47 0.03 0.26 0.11 0.22 0.58 0.007 0.087 0.15 0.32 0.27 0.10 0.14 0.04 0.01 0.14 0.17 (10) 0.0039 0.038 0.04 0.0012 a <0.84 <4.9 <4.1 <0.046 <7.0 <0.70 <4.3 <2.7 <1.1 MJL <0.22 <0.41 <0.0042 <1.4 <3.3 <3.9 <0.0068 <1.4 <0.016 <0.032 nB (H) G MJL <0.0048 <3.3 <0.0082 <0.22 <0.42 <0.033 b 0.47 0.26 0.12 0.22 0.58 0.234 0.087 0.28 0.15 0.32 0.90 0.27 0.45 0.14 0.17 0.37 0.14 2.35 0.48 0.0039 0.005 0.038 (12) 1.64 1.36 1.44 1.29 G 203 1 991ApJ.. . .379. . 177L 1 1/2 ferent observationalstudiesistheuncertaintyintroducedby ences betweenthevariousstudies. possible, however,thatthiscouldintroducesystematicdiffer- than unity(Knappetal.1980).Sincetheobservedratioseems 204 more diligentinlistingtherelevantinformationthanwere lar gasmassesviaequation(2)(whichisvalidforour have lookedatslightlydifferentpositionsonthegalaxy(many the opticalnucleus).Furthermore,differentobserversmay tions reportmolecularmasseswithinasinglebeamcenteredon about 15"and60"(thisisrelevantsincemostoftheobserva- the useofdifferenttelescopes,withbeamsizesvaryingbetween to derivemoleculargasmassesfromeithertransition.Itis to becloseone,weuseequation(2)withthesamevalueofa paper justhowmuchthedatahadbeencorrected(seeVerter authors ofthepapersreportingCOobservationswere when theauthorshavenotdoneso.Needlesstosay,some able tocorrectforatmosphericopacityandantennalosses over, toconverttheCOfluxesgivenbyauthorsmolecu- do notgivetheprecisepositionatwhichtheypointed).More- for thevarioustelescopes,orvaluesgivenbyauthors of others, andinsomecasesitwasverydifficulttoinferfrom the and main-beamefficiencyofthetelescopeused,wemustbe observations only),weneedaccuratevaluesforthebeamsize used theconversionfromKkms“toJygivenby the channel tofluctuations areindependent(seeeq.[3]). increases withthenumberof channels,AT,asaocVifthe narrow linesfortworeasons. First,thermsnoise,

log

Fig. 4.—As in Fig. 2, for the distribution of the observational sensitivities- 1of the CO studies, in units of M(H2)/LB, for both the upper limits and detections. The observational sensitivity, a, is the rms noise in a velocity width of 300 km s , assuming Gaussian noise. [We have assumed that errors in the luminosity, LB, are insignificant compared to those in M(H2).]

with type (e.g., that we do not have more low CO flux detec- it is for the Sa’s and later type spirals. Instead, the ratio of tions of ellipticals because the observers have just looked at molecular gas mass to blue luminosity shows a wide range of them harder). In Figure 4 we show the distributions of the values extending to the lowest detectable bin with no clear sensitivity of the observations (in distance-independent units of falloff. In fact, the probability density function for the ellipticals molecular gas mass per unit blue luminosity), including both continues to rise toward the lowest values of MHJLB. The detections and upper limits, for each of the morphological progression from the Sa’s to E’s seems to be a gradual change types E-Sa. The sensitivity is defined as the rms noise in a from a Gaussian distribution of MnJLB for the Sa’s and later velocity bin of width 300 km s- \ assuming that the channel to types, to an approximately power-law distribution for the ellip- channel fluctuations are uncorrelated. The distributions are ticals, as Knapp et al. (1985) and Wardle & Knapp (1986) quite similar. found for the atomic gas. This progression is quite striking in The distributions of MH2/LB for each morphological type are the plots of Figure 5. The distribution of the ratio of molecular shown in Figures 5-7. Since the CO detection rate for these gas mass to blue luminosity for the elliptical galaxies is quite early-type galaxies is typically less than 50%, we use the similar to that which Wardle & Knapp (1986) found for the Kaplan-Meier estimator which Wardle & Knapp (1986) ratio of atomic gas mass to blue luminosity. The best-fitting applied to the distribution of atomic hydrogen in early-type power law for both the atomic and molecular gas is approx- 1/3 galaxies (see their Appendix C for a clear discussion of Kaplan- imately PK oc (Mgas/Lß)~ . (Knapp et al. 1985 found a sub- Meier statistics applied to this type of data). The Kaplan-Meier stantially steeper distribution for the ratio of H i gas mass to estimator (Kaplan & Meier 1958) includes the information blue luminosity than did Wardle & Knapp 1986. This is contained in the upper limits as well as the detections, the because they used the Turner estimator, PT, which is much less result being that individual detections that are lower than stable at low values of MH JLß) many of the upper limits are weighted more than detections at The cumulative probability distributions, SK, are shown in higher values. Using this estimator, the lowest detections are Figure 6. The difference between Figures 6a and 6b is in our weighted by about 1.6 for the ellipticals, and the highest by treatment of the upper limits, and a comparison between the about 0.6. two figures will give the reader some idea of the influence of The Kaplan-Meier estimator for the probability density dis- using the upper limit data on our conclusions. In Figure 6a, we tribution of MH2/Lb is shown in Figure 5. The Sa’s, and to a have used an overly conservative estimate for the upper limits lesser extent the SOa’s, show a centrally peaked, approximately (as was also used for Fig. 5, and in all subsequent discussion), Gaussian distribution of MU2/LB similar to that of later type multiplying the noise per channel by the total number of chan- spirals, but peaking at a lower value [Sb-Sc galaxies, for nels integrated (i.e., we assume the noise is correlated over a example, have a mean of log (MH2/LB)= —0.8; Young & scale of a hundred or so channels—we hope that this is not Knezek 1989]. Like the atomic gas content studied by Knapp, actually the case for any of these data!): Ico < 3

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1991ApJ. . .379. .177L The ordinateistheprobabilitydensitydistribution,P,andabscissalogarithmofratioM(H)/L.Thus,xA[M(H]expected numberof galaxies havingavaluefortheratioM(H)/Linthatrange,(a)Ellipticals;(h)SO’s;(c)SOa’s;(d)Sa’s. (b) usingthemostoptimisticupper limits.Thetruedistributionshouldlie fraction ofgalaxieshavingaratiomolecular gasmasstoblueluminosityless between thesetwoestimates. than M(H)/L.(a)Usingthemostconservative upperlimits(seetextin§4.1); tribution, S,asafunctionofM(H)/L . Thus,S\_M(H)/L]istheexpected K2B 2B 2b K2 BK2 Fig. 5.—TheKaplan-Meierestimatorfortheprobabilitydensitydistributionofmoleculargasmasstoblueluminosity(see§4.1),smoothedaresolution of0.5. Fig. 6.—TheKaplan-Meierestimator forthecumulativeprobabilitydis- © American Astronomical Society • Provided by the NASA Astrophysics Data System log (M(H)cX*/Lb)(Mq/Lq) 2 log (MÍHJoTVLb)(M/L)(Mq/Lq) o0 galaxies, asinFig.6a,showingthe1aerrorregionsshaded.Theseareonly the vations. {b)Thecumulativeprobability distributionfortheellipticals,showing statistical errorsanddonotincorporate theindividualerrorsinCOobser- the probabilitystatisticsfromFig.la, soitisclearthattheobservationalerrors the cumulativeprobabilitydistribution. Thedashedlinesaretheerrorsfrom these 20runsshowstheeffectof errors intheindividualCOdetectionson the 20MonteCarlosimulationsdescribed inthetext(§4.1).Acomparisonof are onlyimportantfortheverylowest valuesofM(H)/L. 2B Fig. 7.—(a)Thecumulativeprobabilitydistributions,S,fortheEand SO K _1 log (M(H)a/LB)(Mo/Lo) 2 1991ApJ. . .379. .177L (since theearliertypeshavemoreupperlimitsperdetection), ive changes.Weexpectthe“true”cumulativedistributionlies evident alsoinFigure6abutdoesnotintroduceanyqualitat- this exaggeratesthedifferencebetweenmorphologicaltypes molecular gascontentprogressivelydecreasesgoingfromthe somewhere betweenthesetwoplots.Itcanbeseenthatthe version oftheKolmogorov-Smirnovstatisticapplicableto between thecumulativeprobabilitydistributionsusinga the resultofWardle&Knapp(1986)foratomicgascontent Kaplan-Meier estimator(Koziol1980;Wardle&Knapp of thesegalaxies. SOa/Sa galaxiestotheSO’sellipticals.Thisissimilar pared inFigurela.Itcanbeseenthatthereisverylittle limits forthedata(asinFig.6a).The1aerrorsoncumula- better thanthe99%level,usingmostconservativeupper between theSOandSOa/Sa,galaxiestobehighlysignificantat type, smoothedto a resolutionof0.5.(a)Ellipticals;(b)SO’s; (c)SOa’s;(d)Sa’s. cally theformulaeforerrorsinP(seeAppendixCof tive probabilityfunctions,S,fortheE’sandSO’sarecom- Wardle &Knapp1986)areaccurateonlyinthelimitofalarge amount ofuncertaintyinvolvedinthis,however,sincetechni- overlap, confirmingtheaboveconclusion.Thereisafair which thevaluesfordetectionswerepickedrandomlyfrom vidual observations,weran20MonteCarlosimulationsin larger thanthosedisplayedinFigurela.Totestfortheimpor- detection arenotconsidered.Thus,thetrueerrorswillbe ranking statistic,sothattheuncertaintiesineachindividual based purelyonthestatisticalerrorsfromanincompletesam- tance oftheerrorsintroducedbyuncertaintiesinindi- C ofWardle&Knapp(1986),theKaplan-Meierestimatorisa pling oftheprobabilitydistribution.AsisshowninAppendix sample. Gaussian distributionscenteredontheobservedvalueand 1986). WefindthedifferencesbetweenEandSO, K K We analyzedthestatisticalsignificanceofdifference Fig. 8.—TheKaplan-Meier estimatorfortheprobability distributionoftheratiomolecular toatomicgasmass,M(H)/M(H i), asafunctionofmorphological Furthermore, theconclusionsinaboveparagraphare 2 © American Astronomical Society • Provided by the NASA Astrophysics Data System MOLECULAR GASINELLIPTICALGALAXIES log (MCH^orVMtHI)) log (M(H)a-VM(HI)) 2 with avarianceequaltothe1errorforeachobservation.We fications oftheindividualgalaxies.Totestmagnitude from thenumberstatistics,andnotuncertaintiesin then calculatedSwitheachsetofnumbersandcompared Clearly, forthissmallasamplethelargestsourceoferroris a Kaplan-MeiererrorsfromFigurela,fortheellipticals. them. TheresultisshowninFigurelb,andcomparedtothe1 the differencebetweenSforellipticalsandlenticulars previously placedintheEclass,toSOsample.Thisreduced the uncertainty,wemoved10E/SOgalaxies,whichhad the individualobservations,exceptforverysmallestvalues tioned above,hadasimilareffectsincetheRSAtendstoclas- Tammann 1987),ratherthantheclassificationschememen- slightly. UsingRSAclassificationsconsistently(Sandage& of MJL,wheretheyarecomparable. sify galaxiesassomewhatlatertypesthantheRC2or introduced byerrorsinclassification,buttheycouldeasilybe vious paragraph. comparable totheobservationalerrorsdiscussedinpre- UGC/ESO catalogs(Nilson1973;deVaucouleursetal.1976; Lauberts 1982).Itisveryhardtoquantifytheuncertainties as afunctionoftype,whichisformallyquitesignificant. gas mass-to-blueluminosityratiooftheseearly-typegalaxies However, theconsiderationoferrorsduetoindividual and alargersampleofgalaxiesisneededtoverifyit. observations, classification,etc.makesthisresultuncertain, K function ofmorphologicaltypefordiskgalaxies.However, variation oftheratiomoleculartoatomicgasmassasa K compare theKaplan-Meierprobabilitydistributionsof they includeveryfewearly-typegalaxies,soinFigure8we hb Another sourceofuncertaintyisinthemorphologicalclassi- In conclusion,weseemtoseeadifferenceinthemolecular 4.1.4. MoleculartoAtomicGasMassRatioasaFunctionofType Young &Knezek(1989)andVerter(1987)havestudiedthe 207 1991ApJ. . .379. .177L identical. WithintheerrorsmeanvaluesofM/M,areall rather smallnumberofearly-typegalaxiesthataredetectedin finds forhersampleofdiskgalaxies1.9a,andthatYoung both COandHiemission,thedistributionsarestatistically Mh/Mh ifortheE,SO,SOa,andSagalaxies.Considering early-type galaxieshavesimilarratiosofmoleculartoatomic & Knezek(1989)of1.4aforalargergalaxysample.Thus,these approximately 1.4a,similartothemeanthatVerter(1987) masses areanorderofmagnitudeormorelower. gas massestolate-typesystems,eventhoughthetypical 208 early- tolate-typegalaxies.Asaresult,theyfindfactorof20 molecular gascontent,MJL,actuallydecreasesgoingfrom from typesSatoSdm(asRoberts1969found20yearsago),the logical type.Theyfindthatalthoughtheratioofatomicgas mass toblueluminosity,M/L,increasesbyafactorof7 sample of142diskgalaxiesfromtheFCRAOExtragalactic molecular toatomicgasmassratiowithincreasingmorpho- molecular toatomicgasmass. variation intheratioM/MgoingfromSO/SatoSd/Sm CO surveytostudythevariationofM/Mjwithmorpho- (1.0 ±0.9)a(E’s),or(1.4l.l)aforthewholesampleof27 (1.5 ±l.l)a(Sa’s),(1.41.4)a(SOa’s),(1.91.7)a(SO’s),and logical type,eventhoughoursampleisstillquitesmall.Young systems, withtheearly-typegalaxieshavinghighestratioof early-type galaxiesdetectedinbothHiandCO.Unfor- tunately, YoungandKnezekdonotlistwhich11SO-Sagal- Although itispossiblethatweunderestimatethemassesof are alatertypesamplethanours,sincetheyfindmeanvalue axies theyobserved,soitisdifficulttocomparetheirsample S0-Sa galaxies.Wefindmeanvaluesforoursampleof H2H of M/L0.32a(comparethistothevaluesinFig.5). ours tounderstandthisdifference.Itseemsthattheirgalaxies & Knezekfindavalueof=(5.7±2.7)afor11 2 HB HIB H2HI H2H H2b H2HI 12 Young &Knezek(1989)usedCO(1-0)observationsofa Our resultsseemtodisagreewiththistrendofdecreasing Fig. 9.—Asin8,forthedistribution oftheratiomoleculargasmasstoFIRluminosity(40-120/mi),M(H )/L, smoothedtoaresolutionof0.5. 2FIR © American Astronomical Society • Provided by the NASA Astrophysics Data System log (Mh/cxLfîr)(M/Lo) log (M/cxLfir)(Mq/L©) 20 Ha LEES, KNAPP,RUPEN,&PHILLIPS Therefore, wefindnosignificantdifferenceintherelative molecular gasdistribution.Thus,themassesare galaxies alongthemajoraxis),mostofearly-type of thedata(whereasYoungandKnezekhavemappedtheir unlikely tobeunderestimatedbymorethanafactorof2. that havebeenmappedshowaverycentrallyconcentrated molecular gasbyusingonlyasingleon-nucleusbeamformost possible thattheratiopeaksattypeSaanddecreasestoward Young &Knezek(1989)ofveryhighratiosmolecularto type galaxies,andwedonotconfirmthetrendsuggestedby amounts ofmolecularandatomicgasbetweenearly-late- (Young etal.1989),andthisisalsotrueofthe(generallyFIR- between theirFIRluminositiesandmoleculargasmasses to confirmthisresult. both earlierandlatertypes.Furtherobservationsarerequired atomic gasmassesinearly-typegalaxies.Itis,ofcourse,still There isnosignificantvariationwithmorphologicaltypes bright) early-typegalaxiesstudiedinthispaper.InFigure9we which rangedoverfourordersofmagnitudefortheelliptical show theKaplan-MeierprobabilitydistributionsofM/L. strongly withthedistributionsofM/LshowninFigure5, sions oflessthananordermagnitude.Thisresultcontrasts E-Sa, andthedistributionsareallsharplypeakedwithdisper- cold interstellardust. ation fromtheseellipticalscomesthermalemission galaxies. ThisFIR-COcomparisonshowsthattheFIRradi- earliest typestohavelargervaluesofL/M.Inlatertype within theerrors,theredoesseemtobeaslighttrendfor shown inFigure10.Althoughthedistributionsaresame H2FIR HlB HaH2 Late-type spiralgalaxiesshowafairlytightcorrelation The Kaplan-MeierprobabilitydistributionsofLJMare HH2 4.1.6. HaandCOLuminositiesasaFunctionofType 4.1.5. FIRversusCOasaFunctionofType Vol. 379 1991ApJ. . .379. .177L content orIRemission. molecular gascontentandother properties,suchasatomicgas we wouldliketoknowifthere isanyclearcorrelationbetween it morelikelythatacontinuum ofpropertiesexist?Inaddition, ellipticals. Inotherwords, do theseellipticalsrepresenta way peculiarorabnormal,compared toasampleof“normal” separate classof“gas-rich”or “star-forming”ellipticals,oris galaxies inwhichCOemissionhasbeendetectedare any (This conclusionisalsonotinfluencedbythefactthatrecent and sensitivityoftheCOobservations[see§4.2.3].) axies, sincethereisnocorrelationbetween100pirnfluxdensity between theinfraredcolorsanddetectionofCOemission. phological type,andthatthereseemstobenocorrelation Thronson &Bally,thatthereisnocleardependenceonmor- observations havepreferentiallyobserved100/mi-bright gal- (with S¿JS>0.5)havecolorsthatmaybeindicative galaxies issimilartothesampleofearly-type of & Bally1987b).Wenotethattheoveralldistributionofthese upper left-handcorner,andgalaxiestowardthelowerright In theseplots,emissionfromcolddustwouldshowupinthe of acontributionfromdustheatedbyyoungstars(Thronson probably morelikelythattheHaemissioninellipticalsiscon- late-type spiralstoellipticals,indicatingthattheearly-type source. taminated byacontributionfromgasionizedthecentral galaxies probablyhavestrongeractivegalacticnuclei,itis strengths ofnuclearradiosourcestendtoincreasegoingfrom an indicatorofstarformationefficiency.However,sincethe spiral galaxies,itiscommonpracticetointerpretthisratioas 60f1im No. 1,1991MOLECULARGASINELLIPTICALGALAXIES209 An obviousquestiontoaddressiswhetherthe17elliptical The infraredcolorsofthegalaxiesareplottedinFigure11. 4.2. PropertiesofEllipticalsContainingMolecularGas Fig. 10.—Asin8,forthedistributionofratioHaluminositytomoleculargasmass,L(Ha)/M(H),smoothedaresolution0.75 2 © American Astronomical Society • Provided by the NASA Astrophysics Data System 4.1.7. InfraredColors PU Hh « 0.002 0.004 0.006 0.008 0.002 0.004 0.006 0 0 log (L(Ha)a/M(H)(L/M(L(Ha)«/M(H 20o data. whether theremightbetwodistinctclassesofellipticals(gas- of properties,weshouldbeable todeterminethisfromthese rich andgas-poor),orwhether thereseemstobeacontinuum “undetected” sampledoesnotnecessarilyimplythatit con- mation. tains nomoleculargas.However,sinceweareinterested in sensitivity. Thus,thefactthatagalaxyisincludedin the ticals, sincetheCOobservationshaveawidedispersion in between our“detected”and“undetected”samplesofellip- (for instance,theycalculateupperlimitstoMandjin the ment betweenthenumbersofRobertsetal.(1990)and ours up tomid-1989andsodoesnotcontainmostoftheCOinfor- ellipticals. Fortheoverlappinggalaxies,thereisgoodagree- fact thatweincludeaslightlyhigherfractionoflow-luminosity Tammann 1987),including167ellipticals.TheRSAisneithera same waywedo),althoughtheircompilationisonlycomplete the overallluminositydistributionsaresimilarexceptfor About 60%ofourgalaxiesarelistedintheRSAcatalog,and from theobviousbiasesthatoursampleof44ellipticalsshows. magnitude- noravolume-limitedcatalog,butitdoesnotsuffer early-type (E-Sa)galaxiesintheRSAcatalog(Sandage& cataloged thepropertiesofinterstellarmediumall et al.(1990)havedoneacarefulsearchoftheliteratureand included thanareinthegeneralellipticalpopulation.Roberts in mindduringthiscomparison,however,thatisnot galaxies witha detectablemolecularinterstellar medium,with of ellipticals.Forexample,manymoreFIR-brightgalaxiesare any wayacompleteornecessarilyevenrepresentativesample detected withthosegalaxiesthatweredetected.Wemustbear properties oftheellipticalsthatwereobservedinCOandnot H2H In Figures12-17wecompare thepropertiesofelliptical We mustalsoemphasizethatthereisasubstantialoverlap In responsetothefirstquestion,wecancompareglobal 4.2.1. DistributionsoverDistances andLuminosities 1991ApJ. . .379. .177L 92 92 102 92 102 9102 dust emissionistypicallyintheupperleft-handportionofthesediagrams,andstarformation(ornuclearemission)likelytobeasignificantheat sourceforthose is anX-raysourcewithapowerfulSeyfert2nucleuswhichpresumablyheatingthedustandcausinghigh60to100/anratio,aswell ratioofFIRto ratio of12to25/anfluxdensitybecauseitisaluminousgalaxywithrelativelylowFIRflux(L/L=0.005),andthestarsdominatelightat 12/an.UGC3426 galaxies withS/S>0.5(Thronson&Bally1987b).TheCO-detectedaremarkedasfilledcircles,andtheCO-undetected ascrosses,(a) the sample).However,distributionsoverdistanceare blue luminosity(L/L=1.6). Ellipticals; {b)SO’s;(c)SOa’s;(d)Sa’s.NGC4649andUGC3426(Mrk3)aremarkedsincetheyhaveratheranomalousIRcolors.probably hasahigh 210 ellipticals (excludingIC1182,NGC6166,andCygnusAwhich the undetectedellipticals.Thedistributionsoverdistanceare contain moleculargashaveawiderluminositydistribution The plotseemstoindicateadifferencebetweenthedetected galaxies detectedatD<5Mpc.Ourconclusionsarenotsub- are allundetectedandatdistancesmuchlargerthantherestof shown inFigure12.Themeandistancesareidentical,being17 ellipticals showanarrower,brighterluminositydistribution which peakstowardthefaintend,whereasundetected axies. TheSO’s,SOa’s,andSa’salsoshowsimilardistributions somewhat different,sincethereisamuchhigherfractionof NGC 205,and404,thedistributionfordetected identical tothatforthesampleofRSAellipticalsRoberts et and undetectedellipticals,inthesensethatgalaxies that stantially affectedifwedonotconsidertheseverynearbygal- Mpc forthedetectedgalaxies,and16undetected (9 x10h~L,excludingIC1182,NGC6166,andCygnus ellipticals detectedinCOemissionisabout3x10h~ L. galaxies becomesmoresimilartothatoftheundetectedellip- al. (1990).ExcludingthethreeLocalGroupdwarfs,NGC 185, over distanceforthedetectedandundetectedgalaxies. The undetectedgalaxieshaveamedianofabout10h~ L ticals, butstilldiffersquiteabit.Themedianluminosityof the (60% whenweexcludethenearby LocalGroupdwarfs,NGC tively lowluminosityellipticals (<10h~L)is75%±31% A), over3timeshigher.Thus, thedetectionrateofrela- brighter than10h~L thedetectionrateisonly FIRB 60/im100/im FIRB 185, NGC205,and404). Forellipticalsintherange 10-10 h~L,thisfallsto 35%±14%,andforgalaxies q Q e 0 0 0 Fig. 11.—TheIRASinfraredcolorsofthegalaxiesinsample:ratio12to25//mfluxdensitiesversus60100/¿m densities.Cold The distributionoverblueluminosityisshowninFigure13. © American Astronomical Society • Provided by the NASA Astrophysics Data System m 0.5 m LEES, KNAPP,RUPEN,&PHILLIPS Seo/x/Sioo^ each bin.(a)EllipticalsdetectedinCO emission;(b)undetectedellipticals. , binnedinunitsof5Mpc,and theordinateisnumberofgalaxiesin sample ofCO-observedellipticals.The abscissaisthedistance,D,inmega- Fig. 12.—Thedistributionofthedistances ofthegalaxiesincomplete 100/i Vol. 379 r"

^ No. 1, 1991 MOLECULAR GAS IN ELLIPTICAL GALAXIES 211

channel, the error in the integrated CO intensity, Ico = Í ^MB ÔV, will increase as dK0,5 if the channel-to-channel fluc- tuations are independent, where dV is the full velocity width of the line. Furthermore, the Faber-Jackson relation gives dV oc I?’25 (Binney & Tremaine 1987). Thus, the smallest detectable CO intensity, /min, will increase only quite slowly 0125 with luminosity: Jmin ocL . The elliptical galaxies in our sample range over a factor of 103 in luminosity, but this will affect /min by only a factor of 2, or 0.4 dex. Since the detected CO fluxes range from 5 to 5000 Jy km s_1, it is unlikely that such a small change in /min could affect the percent detected by the factor of more than 2 that is observed. However, it is quite possible that the observations are very restricted by the rela- tively short velocity baselines of many receivers, since many observed H i line widths are above 500 km s_1. To check this possibility, a survey of the higher luminosity galaxies with a long velocity baseline (preferably more than 1000 km s-1) is needed. If this difference between the ellipticals that are detected in CO and those that are not detected is real, and if it is not an observational bias as discussed above, then it could be due to differing gas distributions in the high- and low-luminosity gal- axies. The high radiation field in the central regions of the brighter ellipticals, and the energy input from an (which are stronger in the giant ellipticals than in the dwarfs) could prevent cold gas from accumulating in the 2 log (Lb h /Lo) central parts, and in the high-luminosity ellipticals the gas could be distributed in a ring-like configuration in the outer Fig. 13.—As in Fig. 12, for the distribution of blue luminosities, LB, binned in units oflog(L ) = 0.3333. parts of the galaxy. Such gas would have remained undetected, B since most CO observations, ours included, are on-nucleus with beam sizes typically less than half the optical diameter of the galaxy. 26% ± 12% (the quoted errors just being from the number On the other hand, if the high-luminosity ellipticals really do statistics). [However, the mean values of MH2/LB for the gal- not contain much molecular gas compared to their low- axies which are detected in each of these three groups are luminosity counterparts, this effect leads to a number of inter- 9 2 comparable: (0.03 ± 0.04)a for LB < 10 h~ L0, esting possibilities. Sandage et al. (1985), in their survey of the (0.12 ± 0.12)a for galaxies with luminosities of 109-1010 h~2 Cluster, find very different luminosity distributions for 10 2 Lq, and (0.12 ± 0.22)a for LB > 10 h~ L0.] This difference the morphologically distinct dwarf ellipticals and “ true ” ellip- is unlikely to be only a distance effect since the large difference ticals. The latter galaxies have a fairly broad, flat luminosity 8 11 2 in median luminosity would have to be produced by a factor of distribution between about 4 x 10 and 10 h~ L0. The two difference in median distance. In fact, the median distances dwarf ellipticals, on the other hand, increase exponentially are identical. This effect is not evident in the luminosity dis- toward lower luminosities, dominating the luminosity distribu- 9 -2 tributions of the S0-Sa’s, where the median luminosities of the tion fainter than about 2 x 10 /i L0. It is very interesting detected and undetected galaxies are virtually identical. One that the luminosity distribution of the detected ellipticals (Fig. possible explanation for the preferential detection of low- 13) is quite similar to that of the dwarfs, and that of the unde- luminosity ellipticals is that more luminous ellipticals will on tected galaxies more like the “true” ellipticals. However, as average have larger line widths, since their rotational velocities mentioned above, a better survey of the higher luminosity gal- will be higher. Because of both the increased noise associated axies is needed before this possibility can be verified. with integrating over a larger velocity range, and the relatively small velocity baseline available with most current CO recei- 4.2.2. Colors vers (660 km s"1 at the CSO, for example), these lines would be The colors of the ellipticals are plotted in Figure 14 as a harder to detect in a brighter galaxy, even if the corresponding function of their luminosity. The results agree fairly well with molecular mass were identical. (However, we would also expect the mean color-magnitude relation for early-type galaxies, this to affect the other morphological types.) For example, one which is also shown, along with its 2 a dispersion (de Vaucou- 12 of the bright galaxies that we did detect in CO(2-l) emission, leurs & de Vaucouleurs 1972; Visvanathan & Sandage 1977; NGC 1400, probably has an almost face-on gas disk (see dis- Sandage & Visvanathan 1978a, b). There may be a larger cussion in § 3). If the disk had been closer to edge-on, the true scatter in our plot, but this is most likely due to the fact that rotational velocity of the gas of about 470 km s-1 which we the galaxy colors are from a number of different sources, with calculated from the stellar velocity dispersion would give a line varying corrections. The existence of some galaxies scattered -1 width of over 900 km s , and we would clearly not have below the line, “blue stragglers,” was also noted in the field detected it. galaxy sample of Sandage & Visvanathan (1978a); however, Let us check how important these sources of bias might be. these galaxies do not seem to be preferentially detected in CO. For a given rms noise in brightness temperature, TMB, per In fact, the two bluest galaxies, NGC 7468 and Haro 20, are

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1991ApJ. . .379. .177L 212 is clearthatthegalaxiesdetectedinCOaregeneralbluer not detected,butthereisevidencethattheyareprobablyboth {B-V);(b)(U-V). dispersion theyfoundforfieldgalaxiesalsomarkedasdashedlines.Thetwo Visvanathan &Sandage(1977)forabout100EandSOgalaxies,withthe2a crosses theundetectedones.Thesolidlineismeanrelationshipfoundby quite lowmetallicity.Excludingthesetwopeculiargalaxies,it of CO-observedellipticals.Thesoliddotsarethedetectedgalaxies,and NGC 7468andHaro20).Itistemptingtointerpretthisdiffer- but theremayalsobeaslighttendencyforthedetectedgal- to theluminosityeffectdescribedinprevioussubsection, than thosenotdetected.However,thisdifferenceismostlydue very peculiarUV-excessgalaxiesNGC7468andHaro20aremarked,(a) ence asstarformationoccurringinthemolecular gas; axies tobesomewhatbluerthantheundetectedgalaxiesof issues willbeaddressedmorethoroughlyinalaterpaper. metallicity variation(Faber1983;Bursteinetal.1988).These however, thecolor-magnitudeeffectisoftenexplained as a same luminosity(excludingthepeculiarUV-excessgalaxies flux density.Asexpected,wefindamuchhigherCOdetection cates thedustcontentofgalaxy.InFigure15weshow the gas contentistheinfraredfluxdensity,whichsupposedlyindi- rate forellipticalswithhigherIRflux.Ofthe with Kaplan-Meier probabilitydistributionsoverIRAS100 ¿¿m ellipticals (Knappetal.1989) andsincemanyofthesewere been doneafterthepublication ofco-addedIRASdata Sioo/xm >2Jy,86%±25%are detectedinCOemission,and are detected.Sincethemost sensitiveCOobservationshave only 20%±12%ofthosewith lowIRfluxdensities,<2Jy, looking atFIR-brightgalaxies, asweare,theabovestatistics Fig. 14.—Thecolorvs.blueluminositydiagramsforthecompletesample Another interestingquantitytocomparewiththemolecular © American Astronomical Society • Provided by the NASA Astrophysics Data System 4.2.3. InfraredFluxes 3 log (Lh/L) b0 LEES, KNAPP,RUPEN,&PHILLIPS -1 1- could bebiasedtowardthedetectionofIR-brightgalaxies. equal withintheerrors.TheKaplan-Meiercumulativeprob- s interval)oftheellipticalswithS>2Jyis37km However, themeansensitivityinCOintensity(overa300km s", whereasforthelow-S^o/imgalaxiesitis26Jykms ability distributionsareshownwiththe1aerrorsinFigure16. Ellipticals detectedinCOemission;(h)undetectedellipticals. shaded regions. CO emissionvs.thosethatarenotdetected. The1aerrorsarealsomarkedas tribution of100fiminfraredfluxdensity, S,fortheellipticalsdetectedin 100 fiminfraredfluxdensity,Ssmoothedtoaresolutionof1.0.(a) 10fim l00fim i00fimi Fig. 15.—TheKaplan-Meierestimatorfortheprobabilitydistributionof Fig. 16.—TheKaplan-Meierestimator ofthecumulativeprobabilitydis- Vol. 379 1991ApJ. . .379. .177L 782 No. 1,1991MOLECULARGASINELLIPTICALGALAXIES213 emission containontheaveragemoreatomicgasthan in Table3andFigure8impliesthattheatomicmolecular undetected ellipticals.ThelargevariationinM/M,evident hence thepropertiesofinterstellarmediumellipticals gas contentsmaynotbecloselyrelatedinthesegalaxies, and with thehighincidenceofopticaldustlanesseeninellipticals we hadassumedwhenstartingoursurvey.Thisalsoagrees emission. Thereisnosignificantdifferencebetweenthe dis- in Figure17fortheellipticalsdetectedandundetected CO quite heterogeneous. detected byIRAS(Walsh&Knapp1990). bright ellipticalsareindeedmuchmorelikelytobedetected,as and withoutCOemissionishighlysignificant.SotheFIR- Clearly, thedifferencebetweenFIRfluxesofellipticalswith tributions, consideringthesmallnumberofgalaxies.Ofthose- detected inHi,and59%±16%oftheglaxiesnot in galaxies detectedinCOandobservedHi,62%±22%were a resolutionof0.3. CO andobservedinHiweredetectedi.Thus,whether or not anellipticalhasbeendetectedinHinoeffecton the about 40%)are detected.Thetypicalmolecular massesare the 19FIR-brightellipticalgalaxies (S>1Jy),eight(or type galaxies(Table1),wedetect emissionfrom10systems.Of probability thatthegalaxywill bedetectedinCO. H2H 10-10