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1、SeeingGalaxiesThroughThickandThin:I.OpticalOpacityMeasuresinOverlappingGalaxiesRAYMONDE.WHITEIII1,2,3ANDWILLIAMC.KEEL1,2,3DepartmentofPhysics&Astronomy,UniversityofAlabama,Tuscaloosa,AL35487-0324ANDCHRISTOPHERJ.CONSELICE2,4,5DepartmentofAstronomy&Astrophysics,Univers
2、ityofChicagoarXiv:astro-ph/9608113v119Aug19961Visitingastronomer,KittPeakNationalObservatory,NationalOpticalAstronomyObservatories,operatedbyAURA,Inc.,undercooperativeagreementwiththeNationalScienceFoundation.2Visitingastronomer,CerroTololoInter-AmericanObservatory,N
3、ationalOpticalAstronomyObservatories,likewiseoperatedbyAURA,Inc.,undercooperativeagreementwiththeNationalScienceFoundation.3Visitingastronomer,LowellObservatory,Flagstaff,Arizona4NSFREUsummerstudentatUniversityofAlabama5presentaddress:DepartmentofAstronomy,Universityo
4、fWisconsin–2–ABSTRACTWedescribetheuseofpartiallyoverlappinggalaxiestoprovidedirectmeasurementsoftheeffectiveabsorptioningalaxydisks,independentofassumptionsaboutinternaldiskstructure.Thenon-overlappingpartsofthegalaxiesandsymmetryconsiderationsareusedtoreconstruct,via
5、differentialphotometry,howmuchbackgroundgalaxylightislostinpassingthroughtheforegrounddisks.Extensivecatalogsearchesyield∼15−25nearbygalaxypairssuitableforvaryingdegreesofouranalysis;tenofthebestsuchexamplesarepresentedhere.Fromthesepairs,wefindthatinterarmextinctionis
6、modest,decliningfromA∼1magnitudeat0.3RBtoessentiallyzerobyRB;theB2525interarmdusthasascalelengthconsistentwiththatofthediskstarlight.Incontrast,dustinspiralarmsandresonanceringsmaybeopticallythick(AB>2)atvirtuallyanyradius.SomediskshaveflatterextinctioncurvesthantheGa
7、laxy,withAB/AI≈1.6;thisisprobablythesignatureofclumpydustdistributions.Eventhoughtypicalspiralsarenotopticallythickthroughouttheirdisks,wheretheyareopticallythickiscorrelatedwithwheretheyaremostluminous:inspiralarmsandinnerdisks.Thiscorrelationbetweenabsorptionandemi
8、ssionregionsmayaccountfortheirapparentsurfacebrightnessbeingonlymildlydependentoninclination,erroneouslyindicatingthatspiralsaregen
