Contrary to traditional models of galaxy formation, recent observations suggest that some bulges form within preexisting disk galaxies. Such late-epoch bulge formation within disks seems to be linked to disk gas inflow and central star formation, caused by

ACCEPTED

BY

AJ2003DECEMBER16

APreprinttypesetusingLTEXstyleemulateapjv.11/12/01

FORMINGYOUNGBULGESWITHINEXISTINGDISKS:

STATISTICALEVIDENCEFOREXTERNALDRIVERS

SHEILAJ.KANNAPPAN1,ROLFA.JANSEN2,ANDELIZABETHJ.BARTON3

AcceptedbyAJ2003December16

arXiv:astro-ph/0312431v1 16 Dec 2003

ABSTRACT

Contrarytotraditionalmodelsofgalaxyformation,recentobservationssuggestthatsomebulgesformwithinpreexistingdiskgalaxies.Suchlate-epochbulgeformationwithindisksseemstobelinkedtodiskgasin owandcentralstarformation,causedbyeitherinternalsecularprocessesorgalaxymergersandinteractions.Weidentifyapopulationofgalaxieslikelytobeexperiencingactivebulgegrowthwithindisks,usingthecriterionthatthecolorwithinthehalf-lightradiusisbluerthantheouterdiskcolor.Suchblue-centeredgalaxiesmakeup>10%ofstar-formingdiskgalaxieswithintheNearbyFieldGalaxySurvey,abroadsurveydesignedtorepresentthenaturaldiversityofthelow-zgalaxypopulationoverawiderangeofluminositiesandenvironments.Blue-centeredgalaxiescorrelateat99%con dencewithmorphologicalpeculiaritiessuggestiveofminormergersandinteractions.Fromthisandotherevidence,wearguethatexternaldriversratherthaninternalsecularprocessesprobablyaccountforthemajorityofblue-centeredgalaxies.Wegoontodiscussquantitativeplausibilityargu-mentsindicatingthatblue-centeredevolutionaryphasesmayrepresentanimportantmodeofbulgegrowthformostdiskgalaxies,leadingtosigni cantchangesinbulge-to-diskratiowithoutdestroyingdisks.Ifthisviewiscorrect,bulgegrowthwithindisksmaybeanaturalconsequenceoftherepeatedgalaxymergersandinteractionsinherentinhierarchicalgalaxyformation.

Subjectheadings:galaxies:bulges—galaxies:evolution—galaxies:interactions—galaxies:starburst

1.INTRODUCTION

Traditionalwisdomholdsthatgalaxybulgesareold,red,andspheroidal.Inthisview,bulgesareessentiallytinyellipticalgalaxiesthatformedearlyon,viaeitherprimordialcollapseorhierarchicalmergers(e.g.Eggenetal.1962;Kauffmann1996).Thefactthatwecallthesesmallspheroids“bulges,”accordingtothispicture,simplyre ectsthefactthattheylateraccreteddisks.

However,bulgescanalsobeyoung,blue,anddisky.SomebulgesdisplayoffsetsfromtheFaber-Jacksonrelation(Faber&Jackson1976)thatareindicativeofyouthfulmass-to-lightratiosordiskykinematics(Kormendy&Illingworth1983;Ko-rmendy1993).Studiesofbulgecolorssuggestyoungeragesand/orlowermetallicitiesforlate-typegalaxybulgescomparedtoearly-typegalaxybulges(Peletieretal.1999;Carolloetal.2001),andforbulgescomparedtoellipticalgalaxies(Ellisetal.2001).Bulgecolorsandinnerdiskcolorscorrelateclosely,hint-ingatconnectedformationhistories(Peletier&Balcells1996).Late-typegalaxybulgestendtohaveexponentialratherthanr1/4-lawpro les(e.g.,Andredakisetal.1995),withbulgescalelengthspossiblylinkedtoouterdiskscalelengths(Courteauetal.1996;MacArthuretal.2002,andreferencestherein).Prugnieletal.(2001) ndthattheMg2indexforSa–Scbulgescorrelatesbetterwiththeouterdiskrotationvelocitythanwiththebulgevelocitydispersion.

Thisbodyofevidenceimpliesthatmanybulgesprobablyformedorgrewsubstantiallyatlateepochs,evolvingbothwithinandtogetherwithpreexistingdisks(“insitu”bulgefor-mation).Twopossiblescenariosforinsitubulgeformationexist.Onepossibilityisinternalsecularevolutiondrivenbybarinstabilitiesorothernon-axisymmetricdistortions,whichcancausediskgasin ow,centralstarformationthatenhancesthebulge-to-diskratio,andverticalresonancesthatdiffusedisk

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starsintothebulge(e.g.,Pfenniger&Norman1990;Friedli&Benz1993).Alternatively,thesamephenomena—non-axisymmetricdistortions,diskgasin ow,andcentralstarfor-mation—canbetriggeredbyneighborinteractionsorminormergers(e.g.,Mihos&Hernquist1994;Steinmetz&Navarro2002).Evena10×lower-masscompanioncantriggerdiskgasin owfardisproportionatetotheperturber’smass(Hernquist&Mihos1995).Accretionofsmallcompanionscanalsocon-tributetoinsitubulgegrowthdirectly(e.g.,Walkeretal.1996;Aguerrietal.2001).Instudiesofindividualgalaxiesandnon-statisticalsamples,internallyandexternallydrivenscenariosforinsitubulgegrowtharevirtuallycompletelydegenerate.Onecanrarelyruleoutfaintcompanionsoverawide eld,andev-idenceforminormergersisevenmoredif culttodetectandinterpret.

Inthispaper,wefocusonaclassofgalaxieslikelytobecur-rentlybuildingbulgesviainsitumechanisms,andweattempttoresolvetheinternal/externaltriggerdegeneracyforthesegalax-iesbystatisticalmeans.WeidentifylikelyexamplesofinsitubulgegrowthbyapurelyoperationalcriterionadaptedfromJansenetal.(2000b):welookforgalaxiesinwhichthecolorwithinthehalf-lightradiusisbluerthantheouterdiskcolor.Insuch“blue-centered”galaxies,newlyformedstarsdominatethecentralcolorsandmayplausiblybechangingtheconcen-trationindexandbulge-to-diskratio,throughbothtemporarydecreasesincentralstellarmass-to-lightratioandpermanentbuild-upofstellarmassatthecenter(Schweizer1990;Wyseetal.1997;BartonGillespieetal.2003).Theblue-centercri-terionallowsustoexaminestronglyevolvingsystemsthatmaycontainbars,mergerdebris,orotherunrelaxedstructuresthatprecludereliablebulge-to-diskdecomposition.Weshowthatthespatialscalesofstarformationintheseobjectsareconsis-tentwithbulgegrowth.

HarlanSmithFellow,McDonaldObservatory,TheUniversityofTexasatAustin,1UniversityStationC1402,Austin,TX78712-0259;sheila@astro.as.utexas.eduDepartmentofPhysicsandAstronomy,ArizonaStateUniversity,Box871504,Tempe,AZ85287-1504;Rolf.Jansen@asu.eduHubbleFellow,StewardObservatory,UniversityofArizona,933N.CherryAve.,Tucson,AZ85721;ebarton@as.arizona.edu

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