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Multiwavelength

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MultiwavelengthMultiw

    Multiwavelength

    ResearchinAstron.Astrophys.2010Vo1.10No.12,12231241

    http.'//www,raa-journa1.orghttp."fwww.iop.org/journals/raa Researchin

    Asgonomyand

    Astrophysics

    Multiwavelengthstudyofnearlyface-onlowsurfacebrightness diskgalaxies

    DongGaol,2,

    YanChunLiang1,

    Shun.FangLiu1,2,

    GuoHuZhong1,2,

    XiaoYanChenI,2,

    YanBinYangl,2,

    FrancoisHammer4,

    GuoChaoYang1,2,v,

    LiCaiDengl,2andJingYaoHu1,2

    1

    2

    NationalAstronomicalObservatories,ChineseAcademyofSciences,Beijing100012,Chin

    a;

    ycliang@nao.cas.cn;dgao@nao.cas.cn

    KeyLaboratoryofOpticalAstronomy,NationalAstronomicalObservatories,ChineseAcad

    emy

    ofSciences.Beiiing1000l2,China

    GraduateUniversityofChineseAcademyofSciences,Beijing100049,C:hina GEPI.ObservatcIiredeParisMeudon,92195Meudon,France

    DepartmentofPhysics.HebeiNormalUniversity,Shijiazhuang050016,China Received2010March29;accepted2010July20

    AbstractW_estudytheagesofalargesample(1802)ofnearlyfaceondisklowsur

    facebrightnessgalaxiesfLSBGs)usingtheevolutionarypopulationsynthesis(EPS) modelPEGASEwithanexponentiallydecreasingstarformationratetofittheirmul

    tiwavelengthspectralenergydistributions(SEDs)fromfarultraviolet(FUV)tonear

    infi'ared(NIR).ThederivedagesofLSBGsare1-5Gyrformostofthesampleno matterifconstantorvaryingdustextinctionisadopted.whicharesimilartomostof thepreviousstudiesonsmallersamples.ThismeansthattheseLSBGsformedthema

    jorityoftheirstarsquiterecently.However,asmallpartofthesample(2%3%)has

    largeragesof5-8Gyr,meaningtheirmajorstarformingprocessmayhaveoccurred earlier.Atthesametime,alargesample(5886)ofhighsurfacebrightnessgalaxies (HSBGs,areselectedandstudiedusingthesamemethodforcomparisons.Thede

    rivedagesare1-5Gyrformostofthesample(97%)aswel1.Theseresultsprobably meanthattheseLSBGshavenotmuchdifferentstarformationhistoriesfromtheir HSBGscounterparts.However,weshouldnoticethateHSBGsaregenerallyabout

    0.2Gyryounger,whichcouldmeanthattheHSBGsmayhaveundergonemorerecent starformingactivitiesthantheLSBGs.

    Keywords:galaxies:evolution?——galaxies:formation--——galaxies:photometry..

    ——

    galaxies:spiral---——galaxies:statistics'_.——ultraviolet:galaxies

    1INTRoDUCTIoN

    LowSurfaceBrightnessGalaxies(LSBGs)wereoftenhardtofindowingtotheirfaintnesscompared

    withthenightsky,thustheirpropertieswereseldomstudiedandtheircontributionstothegalaxy

    SupportedbytheNationalNaturalScienceFoundationofChina.

    l224D.Gaoetal

    populationwereunderestimatedforalongtime.However,ithasbeensuggestedthatLSBGsmay

compriseupto1/2ofthelocalgalaxypopulation(McGaugheta1.1995).

    AninitialquantitativestudyaboutLSBGswasdonebyFreeman(1970),whonoticedthatthe centralsurfacebrightnessof28outoftheir36diskgalaxiesfe11withinarathernarrowrange, /znfB1=21.650.3magarcsec

    .Thiscouldbecausedbyselectioneffects(Disney1976;Zwicky

    1957).Sincethen.manyeffortshavebeenmadetosearchformoreLSBGsfromsurveys(seethere

    viewsbyBothuneta1.1997:Impey&Bothan1997:andtheintroductionpartinZhongeta1.2008).

    TheseincludegiantLSBGs(Sprayberryeta1.1995).redLSBGs(0'Neileta1.1997)andthemost

    commoncasesof"blueLSBGs"aslate

    type,disk.dominatedspiralswith/z0(B)>22magarcsec

    (Zhongeta1.2008andthereferencestherein).TheLSBGsweregenerallythoughttobeunevolved

    systemswithlowmetallicity(McGaugh1994),lowstarformationrate(vanderHulsteta1.1993),

    arelativelyhighgasfraction(deBloketa1.1996)andlargeamountsofdarkmatter(deBlok&

    McGaugh1997).TheageofLSBGsisalsoanimportanttopictostudy.

    TheagesoftheLSBGshavelongbeenamatterofcontroversy.Broadbandphotometilestudies,

    complementedbyHemissionlinedataandsyntheticstellarpopulationmodeling,predictquitea

    widerangefortheagesofblueLSBGs:from1-2Gyr(Zackrissoneta1.20051to7

    9Gyr(Padoan

    eta1.1997;Jimenezeta1.1998).Ronnback&Bergvallfl994)carriedoutthemulticolorstudies

    ofasampleofextremelybluegalaxieswithonlysmallradialcolorgradients.Intheirwork,BVI photometrywasinterpretedusingspectralevolutionarymodels,andagestypicallyhighertha

n3Gyr

    werefound.Usingoptical/near-infrared(NIR1broadbandphotometrytogetherwithHolemission

    linedataforasubsetofthissample,Zackrissoneta1.(2005)foundthatthecurrentobservations cannotruleoutthepossibilitythattheseblueLSBGsformedasrecentlyas1-2Gyrago.Schombert

    eta1.(2001arguedonthebasisoftheirVIcolorsandrelativeHIcontentthatthemostgas

    rich

    LSBGsshouldtypicallyhavemeanstellaragesbelow5Gyr.Padoaneta1.f1997)andJimenezeta1.

    f1998)usedUBVRIphotometrytoconcludethatmostofthegalaxiesintheirsampleappearedto

    beolderthai1about7GvLv0robyoveta1.(2009)usednumericalhydrodynamicmodelingtostudy

    the1ong

    termf13Gyr)dynamicalandchemica1evolutionofblueLSBGs,adoptingasporadic scenarioforstarformation.Theirmodelingstronglysuggestedtheexistenceofaminimumagefor

    blueLSBGs:1.5-3.0Gyr(frommodelBVcolorsandmeanoxygenabundances),or5-6Gyr

    (frommodelHoLequivalentwidths).ThelatervaluemaydecreaseslightlyifLSBGshavetruncated

    initialmassfunctions(IMFs)withasmalleruppermasslimit.Haberzettleta1.f2005)studiedthestar

    formationhistoryofasampleofsevenLSBGsintheHDF

    S.Thecomparisonofmeasuredspectral

    energydistributions(SEDs,withthesyntheticspectraextractedfromthesynthesisevolutionmodel

    PEGASE(Fioc&Rocca

    Volmerange1997,assuminganexponentiallydecliningstarformationrate) suggestedtheagesofthedominantstellarpopulationsarebetween2to5Gyr.Thisimpliesthatt

he

    maiorstarformationeventofLSBGgalaxiestookplaceatmuchlaterstages(at0.2t0O.41.

    ThesestudiesshowthattheagesofLSBGsareatopicfordebate,andcouldcoverawide range,althoughtheworkingsampleissmallintheseinvestigations.Surelymuchmoreeffortsanda

    muchlargersampleareneededfordetailedstudiesontheagesofLSBGs.Moreover.mostofthese

    investigationsarebasedonopticalphotometrywhichhasobviousshortcomingsinestimatingages

    ofgalaxies.Weknowthattheyoungeststarsmaydominatetheultraviolet(UV)andopticalpartof

    thespectrum.andanoldstellarcomponentwilltherebybeextremelydifficulttodetect.Withthe

    inclusionofNIRbroadbandphotometry.onewouldexpecttobeabletoconstraintheagesmore

    strictly(Belleta1.2000;Zackrissoneta1.2005).TheUVdataarealsoneededtobuildSEDsof galaxiesoverawiderrange.whichwillhelptoderiveagesofgalaxiesmorereliably.Atpresent, themoderndigitalskysurveywithlargeskycoverageprovidesuswithamuchlargersampleof galaxiesfordoingdetailedandstatisticalstudiesinmanyfields.Moreover,themultiwavelength

    dataofalargesetofastronomicalobiectsfromfar-ultraviolet(FUV)toNIRhavebeencarriedout

    byhighlyemcientsurveysandhavebeenreleasedforpublicuse.suchastheGalaxyEvolution Explorer(GALEX)inFUVandnear-ultraviolet(NUV),theSloanDigitalSkySurvey(SDSS)in

    Mutiwave1engthStudyofNearlyFaceonLSBDiskGalaxies1225

    opticalandtheTwoMicronAllSkySurvey(2MASS1inNIR.Thesemultiwavelengthdatawillhelp

    ustostudythepropertiesofalargesampleofLSBGs.WlefoCHSontheagesofLSBGsinthiswork.

WewillstudytheagesofalargesampleofnearlyfaceondiskLSBGsselectedfromtheSDSS

    DataRelease7(DR7,Abazajianeta1.2009)maingalaxysample(MGS,Strausseta1.2002),and

    thenmatchedthemwiththe2MASS,NIRandGALEX/1ljVdata.sothattheirSEDscouldcoverthe

    rangefromFUVtoKsf1350Ato2.17tzm)bands.ThesizeoftheselectedsampleofLSBGsis 1802andtheirofB)>22magarcsec2.Indeed,thisisafollow

    upworkofZhongeta1.(2008)

    andLiangeta1.(2010).InZhongeta1.(2008),weselectedalargesampleofLSBGsfromthe SDSSDataRelease4(DR4,Adelman

    McCarthyeta1.2006)maingalaxysample(Straussetat.

    2002).andpresenteditsbasicphotometricproperties,includingcorrelationsofdiskscalelength

    versusB

    bandabsolutemagnitudeanddistance,andstellarpopulationsfromcolors.InLianget a1.(2010),westudiedthespectroscopicpropertiesofthislargesampleofLSBGs,includingdust

    extinction,strongemissionlineratios,metallicitiesandstellarmass

    metallicityrelations.Inthis

    work,wefirstlyextendthesamplefromDR4toDR7,thentrytoobtaintheirmultiwavelengthdata

    fromFUVtoKsbasedonthepublicsurveydatafromGALEXGR4/GR5andthe2MASSExtended

    SourceCatalog(Xsc).Then.weusetheevolutionarypopulationsynthesis(EPS)mode1PEGASE

    to6ttheirmultiwavelengthSEDsfromFUVtoNIR.andderivetheagesofthesegalaxies.Atthe

    sametime.alargesampleofnearlyface.ondiskhighsurfacebrightnessgalaxies(HSBGs,58861

    with0()<22magarcsec-2isselectedandthesameanalysesareappliedforcomparison. ThisPaperisorganizedasfollows.InSection2,wedescribethemultiwavelengthobservations

    andtheselectedsample.InSection3,wedescribetheEPSmodelPEGASE,whichiswhatweused

    tofittheSEDsofthegalaxiesandtheadoptedparameters.InSection4.wepresentthefitting methodandthederivedagesofthesamplegalaxies.InSection5.theincompletenesseffectsare

    discussed.ThediscussionsaregiveninSection6.WesummarizeandconcludetheworkinSection7.

    ThroughoutthisPaper,acosmologicalmodelwimH0=70kmsMpc

    l,QM=0.3andQA=0.7

    hasbeenadopted.A11themagnitudesareintheABsystem.

    2THESAMPLE

    Oursamplegalaxiesareselectedbymatchingthreedatabasesfrommultiwavelengthsurveys,i.e.,

    theSDSSforugrizoptical.2MASSforJHKsNIR,andGALEXforFUVandNUVbands. 2.1TheSDSSSample

    TheSDSSisthemostambitiousastronomicalsurveyeverundertakeninimagingandspectroscopy

    (Yorketa1.2000;Stoughtoneta1.2002).Thephotometricandspectroscopicobservationswere

    conductedusingthe2.5

    mSDSStelescopeattheApachePointObservatoryinNcwMexico.USA.

    Theimagingdataweredoneindrift

    scanmodeinthefivebandsofuqriz.witheffectivecentral

    wavelengthsof3551,4686,6l66,7480and8932A,respectivelyfGunneta1.1998),andthe95%

    completenesslimitsforpointsourcesare22.0,22.2,22.2,21.3and20.5mag,respectively.The

spectraareflux.andwavelengthcalibratedwith4096pixelsfrom3800to9200AatR

    1800

    (Stoughtoneta1.2002).

    Weselect21664nearlyfaceondiskLSBGsfromtheSDSS

    DR7MGSbvfollowingthecri

    teriausedinZhongeta1.(2008).ThesearefracDev<0.25(indicatingthefractionofluminosity

    contributedbythedeVaucouleursprofilerelativetoallexponentialprofileinther

    bandisvery

    smal1).b/a>0.75(fornearlyface.onones,aandbarethesemimaiorandsemi

    minoraxesof

    thefittedexponentialdisk,respectively),MB<

    18fexcludingafewdwarfgalaxieswiththisB

    bandabsolutemagnitudecutoff)and0(B)>22magarcsec.Theseselectioncriteriahave

    also

    beendescribedinZhongeta1.(20l0).Atthesametime,30896nearlyface

    ondiskHSBGswith

    1http:#www.sdss.org

    1226D.GaoetaI

    l-t0(B)<22magarcsec_.areselectedforcomparisons.Thecentralsurfacebrightnessesofour

    samplegalaxiesarecalculatedusingthecorrespondingparametersprovidedintheSDSScatalog

    followingthemethodgiveninSection2.2andequationr6)ofZhongeta1.(2008).WeusePetrosian

    magnitudes(andtheirerrors)torepresenttheopticalbrightnessoftheSDSSgalaxies.ThePetrosian

    magnitudescouldrecoveressentiallyallofthefluxofanexponentialgalaxyprofile(Stoughtonet

    a1.2002),whichareappropriateforoursample.A11themagnitudesareconvertedtotheABsy

stem

    followingHewetteta1.(2006).

    2.2TheSDSS-2MASSMatchedSample

    Wleusethe2MASSExtendedSourceCatalogfXSC)tomatchtheSDSSselectedsample.Usingthe

    Mt.Hopkinsnorthern1.3mtelescopeandtheCTIOsouthem1.3

    mtelescopeinChile.the2MASS

    coveredalmosttheentireskyinthe(1.25m),H(1.65m)andKs(2.17m)bands,witha spatialresolutionof3.Thepoint

    sourcesensitivitylimits(10cr)are15.8(0.8mJy),15.1(1.0mJy),

    and14.3(1.4mJy)maginthe,HandKs,respectively.Theextendedsourcesensitivity(10)is

    1magbrighterthanthepoint

    sourcelimits,or14.7(2.1mJy),13.9(3.0mJy),and13.1(4.1mJy)

    magattheJ,H,andKs,respectively,withtheprecisethresholddependingonthebrightnessprofile

    oftheobjects(Jarretteta1.2000).ThemagnitudelimitoftypicalextendedsourcesintheK8band

    isabout15_3ABmag(Leeeta1.2010).

    BymatchingourSDSSsamplegalaxieswiththe2MASSXSCcatalogwitl1in3following Blantoneta1.(2005)andLeeeta1.(2010),weobtain3523LSBGsand9483HSBGs.FortheJ,H,

    andKs,magnitudes(andtheirerrors)ofthegalaxies,weadoptthosefromthefitextrapolation({,h,k

    m

    ext)providedbythe2MASSdatabasej,whichareassumedtobettertracethegalacticluminosity

    (Jarretteta1.2000;Blantoneta1.2005).A11themagnitudesareconvertedtotheABsystemfollowing

    Blantoneta1.f2005).

2.3TheSDSS-?2MASS?-GALEXMatchedSample

    WleusetheGALEX(Martineta1.2005)surveyGR4/GR5datatomatchourselectedsamplefrom

    SDSSand2MASS.TheGALEXwaslaunchedin2003April.andwillcoverthewholeskyinthe

    FUVf13501750A)andNUV(17502800A)bands,withspatialresolutionsof6.0inthe

    FUVand4.5intheNUVThecentralwavelengthsofFUVandNUVare1528Aand2271A. respectively.Inthispaper.onlytheGALEXobjectsdetectedbothintheNUVandFUVbandswere

    used.TheGALEXdataproductsaremadeavailabletothegeneralpublicviatheMultiMission ArchiveattheSpaceTelescopeScienceInstitute(MAST).TheGALEXsurveydesignwasdonefor

    fivemodes.inwhichtwoofthemaremorerelatedtoourwork,i.e.,theAll

    skyImagingSurvey

    fAIS)withthegoaltosurveytheentireskysubjecttoasensitivityof

    AB20.5.andtheMedium

    ImagingSurveyfM1s).whichcovers1000degwithextensiveoverlapoftheSDSS(Martineta1.

    2005:Morrisseyeta1.2007).

    WeusetheCASJobstomatchourSDSS

    2MASSsamplewiththeGALEXGR4/GR5database

    byimportingthecoordinatelistandthenobtaintheirmagnitudesinFUVandNUV(seethehelp sectiononthewebsite6).Firstly.weimportapreparedcoordinatelistintothedatabase,whichis

    fortheRAandDECofourSDSS

    2MASSmatchedsample.Thenanewtableofthisiscreatedin

    thedatabase.Nextwesearchforneighborsoftheobjectswithinaradiusof5.andthenobtaina newtableofGALEX

    matchedobjectswhichprovidestheuniqueGALEXobiectidentifierfobjid) http..11www.ipac.caltech.edu/2mass/

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