1998AJ....116.2191P

We probe the dust extinction through the foreground disk of the overlapping galaxy pair UGC 5041 by analyzing B-, I-, and H-band images. The inclined foreground disk of this infrared-selected pair is almost opaque in B at a projected distance of ∼8 kpc. From the images, we directly estimate the area-weighted distribution of differential near-IR extinction: it is nearly Gaussian, with <τ_I - τ_H> = 0.6 and σ = 0.27. For a homogenous dust distribution and a Milky Way extinction curve, this corresponds to a face-on distribution p(τ) with a mean of <τ_V> = 0.34 and σ_V = 0.15. For a clumpy dust model, the optical depth estimate increases to <τ_V> = 0.41 and σ_V_ = 0.19. Even though the galaxy pair is subject to different selection biases and our analysis is subject to different systematics, the result is consistent with existing case studies, indicating that <τ_V> ∼ 0.3 is generic for late-type spirals near their half-light radii. We outline how to estimate from p(τ) by how much background quasars are underrepresented, where projected within ∼10 kpc of nearby spirals, such as damped Lyα absorbers or gravitational lenses; from our data we derive a factor of 2 deficit for flux-limited, optical surveys.