The Extinction Distribution in the Galaxy UGC 5041

Abstract

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 ~8kpc. From the images, we estimate directly the area-weighted distribution of differential near-IR extinction: it is nearly Gaussian with <tau_I-tau_H>=0.6 and sigma=0.27. For a homogenous dust distribution and a Milky Way extinction curve, this corresponds to a face-on distribution p(tau) with a mean of <tau_V>=0.34 and sigma_V=0.15. For a clumpy dust model the optical depth estimate increases to <tau_V>=0.41 and sigma_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 <tau_V>~0.3 is generic for late-type spirals near their half-light radii. We outline how to estimate from p(tau) by how much background quasars are underreresented, where projected within ~10kpc of nearby spirals, such as damped Ly-alpha absorbers or gravitational lenses; from our data we derive a factor of two deficit for flux-limited, optical surveys.