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

2Kannappan,Jansen,&Barton

Todeterminethefrequencyandphysicaldriversofblue-centeredgalaxiesinthegeneralgalaxypopulation,weexam-inetheirdistributionwithintheNearbyFieldGalaxySurvey(NFGS,Jansenetal.2000a,b;Kannappan2001),astatisti-callyrepresentativesampleoflow-redshiftgalaxiesspanningallHubbletypesoverawiderangeofluminositiesandenviron-ments.Blue-centeredgalaxiesmakeup>10%ofstar-formingdiskgalaxiesbrighterthanMB= 17.5intheNFGS,andtheyoccurwithincreasingfrequencyatlowerluminosities(Tullyetal.1996;Jansenetal.2000b).Basedonindependent,uniformmorphologicalpeculiarityclassi cationsforthesurvey,wewillarguethatexternaltriggersprobablyaccountforthemajorityofblue-centeredgalaxiesintheNFGS.Uniformbutshallowcompaniondatasupportthesameresult,withsomeinterpretiveambiguity.Thisanalysisextendspreviousworklinkinggalaxyencounterstostarburstsandcentralstarformation(e.g.,Lar-son&Tinsley1978;Kennicutt&Keel1984;Keeletal.1985;Schweizer1990;Bartonetal.2000;BartonGillespieetal.2003)inthatweassesshowoftencentralstarformationcanbetracedtogalaxyencountersasopposedtootherprocessesatworkinthegeneralgalaxypopulation.Ofcourse,wecon-sideronlyaspecialclassofcentralstarformation,intenseandwidespreadenoughtocauseabluecenter.Theblue-centercri-terionisnotsensitivetosmall-scalenuclearstarformation,starformationontopofveryredunderlyingpopulations,orcentralstarformationsurroundedbycomparableamountsofdiskstarformation.Nonetheless,boththeabundanceofblue-centeredgalaxiesandtheintensityandextentoftheircentralstarforma-tionsuggestthatunderstandingthesegalaxiesmaybeimportanttounderstandingthephenomenonofinsitubulgegrowth.

Theremainderofthispaperisorganizedasfollows.In§2webrie ydescribetheNearbyFieldGalaxySurveyaswellasasecondsample,theClosePairsSurvey(Bartonetal.2000),whichweuseforfurtheranalysisofblue-centeredgalaxieswithcompanions.In§3wede netheouter-minus-innercolordiffer-ence (B R),discuss (B R)trendswithluminosityandmor-phology,andsummarizethepropertiesofblue-centeredgalax-ieswithintheNFGS.In§4wepresentstatisticalcorrelationsbetweenblue-centeredgalaxies(aswellas (B R))andfac-torsthatmayindicateexternaldisturbance,i.e.,morphologicalpeculiaritiesandclosecompanions.In§5wepresentase-riesofquantitativeplausibilityargumentstosupportthesug-gestionthatblue-centeredphasesrepresentanimportantmodeofinsitubulgegrowth.Finally,in§6wediscussopenques-tionsraisedbytheseresultsregardingtherelationshipofblue-centeredgalaxiestothephenomenonofdiskybulgesandtotheparadigmofhierarchicalgalaxyformation.Wesummarizeourresultsin§7.

2.SURVEYDATA

TheNearbyFieldGalaxySurveyisidealforthisanalysisbe-causeitwasdesignedtoprovideastatisticallyrepresentativesampleofthegeneralgalaxypopulation,withoutexplicitse-lectionbiasincolor,morphology,oranyothergalaxyproperty(Jansenetal.2000b,hereafterJ00).Imaging,spectrophotom-etry,andkinematicdatafortheNFGScomefromJ00,Jansenetal.(2000a),andKannappan(2001),respectively.The196galaxiesintheNFGSweredrawnfromtheCfA1redshiftsur-vey(Huchraetal.1983)innumbersroughlyproportionaltothelocalgalaxyluminosityfunctionandspanthefullrangeofHubbletypesandenvironments(withtheexclusionoftheVirgo

4

Wehavereclassi edoneNFGSgalaxy,A11332+3536,fromS0toSB0/a.

Clustertoavoidoverrepresentationofclustergalaxies).How-ever,akeypointfortheanalysisinthispaperisthefactthatthepropertiesoftheNFGSdonotre ectthoseofa ux-limitedsample.Moreluminousgalaxieswereselectedatprogressivelylargerdistances(asdescribedindetailinJ00),suchthatallap-parentmagnitudesmZfallinaverynarrowrange14–14.5,setbytheCfA1magnitudelimit(wheremsimilartoaB-bandmagnitude).Z,theZwickymagni-tude,isInturn,theCfA1surveyisasubsetoftheUpdatedZwickyCatalog(UZC,Falcoetal.1999),whichisaneditedandupdatedversionoftheCfA2redshiftsurvey,withmagnitudelimitmustoperformsystematicsearchesZ=15.5.ThereforetheUZCallowsforcompanionstoNFGSgalaxiesdownto～1magfainterthantheprimary.Thistypeofsearchavoidsdistance-dependentsamplingbiases:ratherthansearchingforcompanionsdowntoaspeci edab-solutemagnitude,wesearchforcompanionsdownto～1magfainterthantheprimary,regardlessofprimaryluminosityorredshift.Physically,suchasearchissensible,asthedetectionthresholdre ectsanapproximatelyconstantinteractionmassratio,regardlessofabsolutemassscale.

Unfortunately,aswewillseein§4.1,manyofthecompan-ionsrelevanttoblue-centeredgalaxiesmaybetoofaint,ortoocloselyblended,tobeseparatelycataloguedintheUZC.Tode-tectdisturbancesfromsmallcompanionsormergersalreadyinprogress,weusetheuniformsetofmorphologicalpeculiarity agsde nedfortheNFGSbyKannappanetal.(2002).These agsdonotimplyatype“Pec”classi cation.Toavoidcon-fusiononthispoint,wegroupbothtype“Im”andtype“Pec”galaxiesundertype“Irr”inthispaper.Thisrede nitionisinci-dental,becauseexceptforabrieflookatthefullNFGSin§3,wewillfocusmostofourattentiononasamplethatexcludesnearlyalltypeIrrgalaxiesviaamagnitudecutatMonasecondsurvey,the191-galaxyB= 17.5.WealsodrawClosePairsSurveyofBartonetal.(2000),tosupplementouranalysisofblue-centeredgalaxiesknowntohaveclosecompanions.TheClosePairsSurveywasselectedfromtheCfA2redshiftsur-vey(Geller&Huchra1989)asastatisticalsampleofgalaxypairswithline-of-sightvelocityseparation V<1000kms 1andprojectedspatialseparation X 100kpc(Bartonetal.2000,the Xlimitdiffersfromtheoriginalreferencebecausewehaverecomputedalldistancesasdiscussedbelow).UnliketheNFGS,theClosePairsSurveyre ectstheinherentlumi-nositydistributionofitsmagnitude-limitedparentsurvey.Mor-phologicaltypesfortheClosePairsSurveyarebasedoninde-pendentestimatesby4

eachofus,usingtheNFGSasacommoncalibrationsample.AllmagnitudesanddistancesinthispaperarecomputedusingH0=75kms 1Mpc 1,withacorrectionforVirgocen-tricinfallbasedonthemodelofKraan-Kortewegetal.(1984,seeJ00).WeuseGalacticextinctioncorrectionsderivedfromSchlegeletal.(1998)forbothsurveys,andwequotemagni-tudeswithoutinternalextinctioncorrectionsexceptwhereoth-erwiseindicated.Wehavecheckedthataddinginternalextinc-tioncorrectionsdoesnotin uenceourresults.

3.CHARACTERIZINGBLUE-CENTEREDGALAXIES

Fig.1showstheimagesandsurfacebrightnesspro lesforallblue-centeredgalaxiesbrighterthanMthesegalaxiesbythecriterionB= 17.5intheNFGS.Weidentify (B R)>0,wherethecolordifference (B R)equalstheouterdiskcolor(fromthehalf-lightradiusretothe75%-lightradius)minusthecentral