Dark and luminous matter in the NGC 3992 group of

arXiv:astro-ph/0204335v119Apr20021Abstract.DetailedneutralhydrogenobservationshavebeenobtainedofthelargebarredspiralgalaxyNGC3992anditsthreesmallcompaniongalaxies,UGC6923,UGC6940,andUGC6969.Forthemaingalaxy,theHidistri-butionisregularwithalowlevelradialextensionoutsidethestellardisc.However,atexactlytheregionofthebar,thereisapronouncedcentralHiholeinthegasdistribu-tion.Likelygashasbeentransportedinwardsbythebarandbecauseoftheemptynessoftheholenolargeaccre-tioneventscanhavehappenedinrecentgalactictimes.Thegaskinematicsisveryregularanditisdemonstratedthattheinfluenceofthebarpotentialonthevelocityfieldisnegligible.Apreciseandextendedrotationcurvehasbeenderivedshowingsomedistinctfeatureswhichcanbeexplainedbythenon-exponentialradiallightdistributionofNGC3992.Thedecompositionoftherotationcurvegivesaslightpreferenceforasubmaximaldisc,thougharangeofdisccontributions,uptoamaximumdiscsit-uationfitsnearlyequallywell.Forsuchamaximumdisccontribution,whichmightbeexpectedinordertogener-ateandmaintainthebar,therequiredmass-to-lightratioislargebutnotexceptional.Keywords:galaxies:individual:NGC3992–ISM:kine-maticsanddynamics–galaxies:kinematicsanddynamics–galaxies:spiralA&Amanuscriptno.(willbeinsertedbyhandlater)Yourthesauruscodesare:missing;youhavenotinsertedthemASTRONOMYANDASTROPHYSICSDarkandluminousmatterintheNGC3992groupofgalaxiesI.ThelargebarredspiralNGC3992RoelofBottema1andMarcA.W.Verheijen21KapteynAstronomicalInstitute,P.O.Box800,NL-9700AVGroningen,TheNetherlands,e-mail:[e-mail]2UniversityofWisconsin,DepartmentofAstronomy,475NorthCharterStreet,Madison,WI53706,U.S.A.,e-mail:[e-mail]1;accepteddate21.IntroductionRotationcurvesderivedfromneutralhydrogenobserva-tionsattheouterregionsofspiralgalaxiesunambiguouslyshowthatsubstantialamountsofdarkmatterarerequired(Bosma1978;Begeman1987,1989).Anyphysicallyrea-sonabledistributionofthisdarkmatternecessitatesthepresenceofatleastsomeofthatintheinneropticaldiscregion,contributinginsomedegreetothetotalrotationinthatregion.Unfortunately,fromtheobservedrotationcurveandlightdistributiononecannotapriorideterminetheratioofdarktoluminousmatter(vanAlbadaetal.1985).Therearearguments,mainlytheoretical,thatthecontributionofthedischastobemaximized,leadingtothesocalledmaximumdischypothesis(vanAlbada&Sancisi1986;Saluccietal.1991;Sellwood&Moore1999).Ontheotherhand,observationsofdiscstellarvelocitydis-persions(Bottema1993,1997)leadtotheconclusionthatthedisccontributes,onaverage,63%tothetotalrotationatthepositionwherethedischasitsmaximumrotation.Thisfindingissupportedbyastatisticalanalysisofrota-tioncurveshapesinrelationtothecompactnessofdiscs(Courteau&Rix1999).Everydetailedrotationcurveofanygalaxymaygivecluesastotheratioofdarktolu-minousmatterinagalaxy.Ithasbeenarguedthattheobservedcorrelationoffeaturesintherotationcurvewithfeaturesinthephotometryexcludesasubmaximumdisccase(vanAlbada&Sancisi1986).SinceitwasalreadyknownthatNGC3992exhibitssuchspecificrotationcurvefeatures,thiswarrantedmoredetailedobservationsandaproperdecompositionofthecurveintothecontributionsofthegalacticconstituents.Atleastasubstantialfractionofspiralgalaxieshasnobar.Yetanisolatedcoldstellardisccanneverbesta-ble(Ostriker&Peebles1973).Variouscriteriaforgalaxieshavebeenputforwardthatshouldbeobeyedinordertoavoidabarinstability.Toomre’s(1964)Qcriterionforlocalstabilitycanbeappliedforaglobalsituationas-Sendoffprintrequeststo:R.BottemasumingaminimumQvalueforastellardisc(Sellwood&Carlberg1984).Alreadyin1973Ostriker&Peeblesshowedbynumericalexperimentsthatasubstantialspher-icaldarkhalocanstabilizeadisc.TheircriterionstatesthathEkini/Epot0.14,ortheratiooftheaverageki-neticenergyofadisctoitspotentialenergyshouldbelessthan0.14inordertobestable.Asanalternativetoadarkhalo,adisccanbestabilizedbyasubstantialbulge(Sellwood&Evans2001).Inadditiontoaddingpotentialenergythebulgealsomakestherotationcurveflatintheinnerregions,creatinganILRwhichmayinhibitthebarformationmechanism(Toomre1981).CalculationsforaspecificsetofgalaxiesbyEfstathiouetal.(1982)showedthatforε=vmaxpGMdisc/h∼1.1,(1)thediscwasunstabletobarformation.ThiscriterionhasrecentlybeenreviewedandreanalysedbySyeretal.(1999)fora3Dsituation.Fordiscswitharatioofscale-length(h)tosech-squaredthicknessz0offiveandQ=1.2atR=2.4htheyfoundthatthenumericalvalueof1.1inEq.(1)hastobedecreasedto0.7.Thoughthiscri-terionmayhaveitsmerits,itisdifficulttoimaginethatdiscstabilitydoesnotdependonitsstellarvelocitydisper-sion.Henceitmaybedangeroustoapplyittoarandomlyobservedgalaxy.Asonecansee,arobust,wellestablishedstabilitycri-teriondoesnot(yet)exist.Nevertheless,fromthecriteriajustmentionedsomegeneralrelationscanbeextracted.Agalaxybecomesmoreunstablewhenitsdiscmasstodarkmatterratio,oritsdisctobulgemassratioislarger.Itisalsomoreunstablewhenitsstellarvelocitydispersionislower.Intheextremeonethenhasacold,thin,maximumdiscsituation,whichiscertainlyunstable.Evenadiscwithh/z0offive,arepresentativevalueforspiralgalax-ies(vanderKruit&Searle1982),atthemaximumdisclimitformsalargebar(Bottema&Gerritsen19