Extinction Maps towards the Milky Way Bulge: 2D and 3D Tests with APOGEE
  Galactic interstellar extinction maps are powerful and necessary tools for
Milky Way structure and stellar population analyses, particularly toward the
heavily-reddened bulge and in the midplane. However, due to the difficulty of
obtaining reliable extinction measures and distances for a large number of
stars that are independent of these maps, tests of their accuracy and
systematics have been limited.
  Our goal is to assess a variety of photometric stellar extinction estimates,
including both 2D and 3D extinction maps, using independent extinction measures
based on a large spectroscopic sample of stars towards the Milky Way bulge. We
employ stellar atmospheric parameters derived from high-resolution $H$-band
APOGEE spectra, combined with theoretical stellar isochrones, to calculate
line-of-sight extinction and distances for a sample of more than 2400 giants
towards the Milky Way bulge. We compare these extinction values to those
predicted by individual near-IR and near+mid-IR stellar colors, 2D bulge
extinction maps and 3D extinction maps. The long baseline, near+mid-IR stellar
colors are, on average, the most accurate predictors of the APOGEE extinction
estimates, and the 2D and 3D extinction maps derived from different stellar
populations along different sightlines show varying degrees of reliability. We
present the results of all of the comparisons and discuss reasons for the
observed discrepancies. We also demonstrate how the particular stellar
atmospheric models adopted can have a strong impact on this type of analysis,
and discuss related caveats.
