Photometry of Partially Overlapping Galaxies

use non-overlapping parts & symmetry to estimate

how much light from background galaxy is lost

in passing through foreground spiral

--> DIRECT measure of opacity in foreground spiral

Benefits

• direct (non-statistical)
• no selection against high opacity regions
• purely differential photometry (no calibrations)
• large area coverage
• need no corrections for:
  • internal extinction of background galaxy
  • extinction of Milky Way

Disadvantages

• few reasonable objects
• dependent on galaxy symmetry