Seeing Galaxies Through Thick and Thin: II. Direct Measures of   Extinction in Spiral Disks Through Spectroscopy of Overlapping Galaxies
  We use slit spectroscopy of overlapping pairs of galaxies to directly determine the extinction in disks of foreground spiral galaxies. The Doppler shifts of pair members are determined via cross-correlation and their relative correlation amplitudes are used to separate their contributions to the combined spectra in regions of overlap. This spectroscopic approach is less subject to stringent symmetry constraints than our previous purely photometric analyses. Extinctions of foreground members were obtained for 6 of the candidates in our sample of 18 mostly spiral/spiral pairs, when the signal to noise and velocity difference were suitable. In agreement with our previous imaging results, we find that the extinction in interarm regions is very modest, typically A_B=0.1 mag (corrected to face on), while spiral arms exhibit higher extinctions of 0.3 mag. 
Donovan L. Domingue, William C. Keel, Raymond E. White III