Mapping the stellar age of the Milky Way bulge with the VVV. III. High
  resolution reddening map
  The detailed study of the Galactic bulge stellar population necessarily
requires an accurate representation of the interstellar extinction particularly
toward the Galactic plane and center, where the severe and differential
reddening is expected to vary on sub-arcmin scales. Although recent infrared
surveys have addressed this problem by providing extinction maps across the
whole Galactic bulge area, dereddened color-magnitude diagrams near the plane
and center appear systematically undercorrected, suggesting the need for higher
resolutions. These undercorrections affect any stellar study sensitive to color
(e.g. star formation history analysis via color-magnitude diagram fitting),
either making them inaccurate or limiting them to small low/stable extinction
windows where this value is better constrained. We aim at providing a
high-resolution (2 arcmin to $\sim$ 10 arcsec) color excess map for the VVV
bulge area, in $\mathrm{J}-\mathrm{K}_s$ color. We use the MW-BULGE-PSFPHOT
catalogs sampling $\sim$ 300 deg$^2$ across the Galactic bulge ($|l| <
10^\circ$ and $-10^\circ < b < 5^\circ$) to isolate a sample of red clump and
red giant branch stars, for which we calculate average
$\mathrm{J}-\mathrm{K}_s$ color in a fine spatial grid in $(l, b)$ space. We
obtain a E$(\mathrm{J}-\mathrm{K}_s)$ map spanning the VVV bulge area of
roughly 300 deg$^2$, with the equivalent to a resolution between $\sim$ 1
arcmin for bulge outskirts ($l < -6^\circ$) to below 20 arcsec within the
central $|l| < 1^\circ$, and below 10 arcsec for the innermost area ($|l| <
1^\circ$ and $|b| < 3^\circ$). The result is publicly available at
http://basti-iac.oa-teramo.inaf.it/vvvexmap/
