Seeing Galaxies through Thick and Thin. I. Optical Opacity Measures in Overlapping Galaxies

We describe the use of partially overlapping galaxies to provide directmeasurements of the effective absorption in galaxy disks, independent ofassumptions about internal disk structure. The non-overlapping parts of thegalaxies and symmetry considerations are used to reconstruct, via differentialphotometry, how much background galaxy light is lost in passing through theforeground disks. Extensive catalog searches yield ~15-25 nearby galaxy pairssuitable for varying degrees of our analysis; ten of the best such examples arepresented here. From these pairs, we find that interarm extinction is modest,declining from A_B ~1 magnitude at 0.3 R_25^B to essentially zero by R_25^B;the interarm dust has a scale length consistent with that of the diskstarlight. In contrast, dust in spiral arms and resonance rings may beoptically thick (A_B > 2) at virtually any radius. Some disks have flatterextinction curves than the Galaxy, with A_B/A_I = 1.6; this is probably thesignature of clumpy dust distributions. Even though typical spirals are notoptically thick throughout their disks, where they {\it are} optically thick iscorrelated with where they are most luminous: in spiral arms and inner disks.This correlation between absorption and emission regions may account for theirapparent surface brightness being only mildly dependent on inclination,erroneously indicating that spirals are generally optically thick. Taken as anensemble, the opacities of spiral galaxies may be just great enough tosignificantly affect QSO counts, though not enough to cause their high redshiftcutoff.