High-Resolution Mapping of Dust via Extinction in the M31 Bulge
  We map the dust distribution in the central 180" (~680 pc) region of the M31
bulge, based on HST/WFC3 and ACS observations in ten bands from
near-ultraviolet (2700 A) to near-infrared (1.5 micron). This large wavelength
coverage gives us great leverage to detect not only dense dusty clumps, but
also diffuse dusty molecular gas. We fit a pixel-by-pixel spectral energy
distributions to construct a high-dynamic-range extinction map with
unparalleled angular resolution (~0.5" , i.e., ~2 pc) and sensitivity (the
extinction uncertainty, \delta A_V~0.05). In particular, the data allow to
directly fit the fractions of starlight obscured by individual dusty clumps,
and hence their radial distances in the bulge. Most of these clumps seem to be
located in a thin plane, which is tilted with respect to the M31 disk and
appears face-on. We convert the extinction map into a dust mass surface density
map and compare it with that derived from the dust emission as observed by
Herschel . The dust masses in these two maps are consistent with each other,
except in the low-extinction regions, where the mass inferred from the
extinction tends to be underestimated. Further, we use simulations to show that
our method can be used to measure the masses of dusty clumps in Virgo cluster
early-type galaxies to an accuracy within a factor of ~2.
