The Panchromatic Hubble Andromeda Treasury VIII: A Wide-Area,
  High-Resolution Map of Dust Extinction in M31
  We map the distribution of dust in M31 at 25pc resolution, using stellar
photometry from the Panchromatic Hubble Andromeda Treasury. We develop a new
mapping technique that models the NIR color-magnitude diagram (CMD) of red
giant branch (RGB) stars. The model CMDs combine an unreddened foreground of
RGB stars with a reddened background population viewed through a log-normal
column density distribution of dust. Fits to the model constrain the median
extinction, the width of the extinction distribution, and the fraction of
reddened stars. The resulting extinction map has >4 times better resolution
than maps of dust emission, while providing a more direct measurement of the
dust column. There is superb morphological agreement between the new map and
maps of the extinction inferred from dust emission by Draine et al. 2014.
However, the widely-used Draine & Li (2007) dust models overpredict the
observed extinction by a factor of ~2.5, suggesting that M31's true dust mass
is lower and that dust grains are significantly more emissive than assumed in
Draine et al. (2014). The discrepancy we identify is consistent with similar
findings in the Milky Way by the Planck Collaboration (2015), but has a more
complex dependence on parameters from the Draine & Li (2007) dust models. We
also show that the discrepancy with the Draine et al. (2014) map is lowest
where the interstellar radiation field has a harder spectrum than average. We
discuss possible improvements to the CMD dust mapping technique, and explore
further applications.
