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We map the distribution of dust in M31 at 25pc resolution, using stellarphotometry from the Panchromatic Hubble Andromeda Treasury. We develop a newmapping technique that models the NIR color-magnitude diagram (CMD) of redgiant branch (RGB) stars. The model CMDs combine an unreddened foreground ofRGB stars with a reddened background population viewed through a log-normalcolumn density distribution of dust. Fits to the model constrain the medianextinction, the width of the extinction distribution, and the fraction ofreddened stars. The resulting extinction map has >4 times better resolutionthan maps of dust emission, while providing a more direct measurement of thedust column. There is superb morphological agreement between the new map andmaps of the extinction inferred from dust emission by Draine et al. 2014.However, the widely-used Draine & Li (2007) dust models overpredict theobserved extinction by a factor of ~2.5, suggesting that M31's true dust massis lower and that dust grains are significantly more emissive than assumed inDraine et al. (2014). The discrepancy we identify is consistent with similarfindings in the Milky Way by the Planck Collaboration (2015), but has a morecomplex dependence on parameters from the Draine & Li (2007) dust models. Wealso show that the discrepancy with the Draine et al. (2014) map is lowestwhere the interstellar radiation field has a harder spectrum than average. Wediscuss possible improvements to the CMD dust mapping technique, and explorefurther applications.

The Panchromatic Hubble Andromeda Treasury VIII: A Wide-Area, High-Resolution Map of Dust Extinction in M31