Seeing Galaxies though Thick and Thin

The superimposed pair of spiral galaxies comprising NGC 3314 ooers a unique opportunity to trace the dust properties in a spiral galaxy. We analyze multicolor HST imaging, supported by ground-based near-IR imaging and ber-array spectroscopy to measure dust extinction in the foreground Sc galaxy NGC 3314A, which is backlit by the Sb system NGC 3314B. The superposition allows us to measure extinctions over a wide range of galactocentric radii in the foreground galaxy, from 0.4-4.5 kpc. In the outer half of the disk, the extinction is strongly localized in discrete dust lanes, including some patches whose galactic setting is clear only because of associated H emission at the foreground velocity. These dust features show an extinction curve with a slope close to the Galactic mean (R = 3:5 0:3) over a range in galactocentric radius from 1.6 to 3.8 kpc, with no radial trend. Using the I ? K color of the background nucleus, we derive an extinction A I = 3:3 through the disk at a projected distance 400 pc from the nucleus of NGC 3314A. The extinction in even the inner disk of NGC 3314A is quite patchy, since background H emission is detected from all parts of the system. Local anticorrelations between foreground and background line emission demonstrate that the dust is concentrated to star-forming regions, as has been found for the blue light in several systems. Colors of dust lanes in NGC 3314A which are projected only partially against the background disk indicate that the dust scale height in the foreground disk is substantially smaller than that of the stars. The color-intensity behavior of the net light in these regions tracks the predictions of a thin-layer model closely.