AN EXTENDED DUST DISK IN A SPIRAL GALAXY: AN OCCULTING GALAXY PAIR IN THE ACS NEARBY GALAXY SURVEY TREASURY

We present an analysis of an occulting galaxy pair, serendipitously discovered in the ACS Nearby Galaxy Survey Treasury observations of NGC 253 taken with the Hubble Space Telescope's (HST) Advanced Camera for Surveys in F475W, F606W, and F814W (SDSS-g, broad V, and I). The foreground disk system (at z <= 0.06) shows a dusty disk much more extended than the starlight, with spiral lanes seen in extinction out to 1.5 R(25), approximately 6 half-light radii. This pair is the first where extinction can be mapped reliably out to this distance from the center. The spiral arms of the extended dust disk show typical extinction values of A(F475W) similar to 0.25, A(F606W) similar to 0.25, and A(F814W) similar to 0.15. The extinction law inferred from these measures is similar to that of the local Milky Way, and we show that the smoothing effects of sampling at limited spatial resolution (< 57 pc, in these data) flattens the observed function through mixing of regions with different extinction. This galaxy illustrates the diversity of dust distributions in spirals, and the limitations of adopting a single dust model for optically similar galaxies. The ideal geometry of this pair of overlapping galaxies and the high sampling of HST data make this data set ideal to analyze this pair with three separate approaches to overlapping galaxies: (1) a combined fit, rotating copies of both galaxies, (2) a simple flip of the background image, and (3) an estimate of the original fluxes for the individual galaxies based on reconstructions of their proper isophotes. We conclude that in the case of high-quality data such as these, isophotal models are to be preferred.