A Correction to the Standard Galactic Reddening Map: Passive Galaxies as
  Standard Crayons
  We present corrections to the Schlegel, Finkbeiner, Davis (SFD98) reddening
maps over the Sloan Digital Sky Survey northern Galactic cap area. To find
these corrections, we employ what we dub the "standard crayon" method, in which
we use passively evolving galaxies as color standards by which to measure
deviations from the reddening map. We select these passively evolving galaxies
spectroscopically, using limits on the H alpha and O II equivalent widths to
remove all star-forming galaxies from the SDSS main galaxy catalog. We find
that by correcting for known reddening, redshift, color-magnitude relation, and
variation of color with environmental density, we can reduce the scatter in
color to below 3% in the bulk of the 151,637 galaxies we select. Using these
galaxies we construct maps of the deviation from the SFD98 reddening map at 4.5
degree resolution, with 1-sigma error of ~ 1.5 millimagnitudes E(B-V). We find
that the SFD98 maps are largely accurate with most of the map having deviations
below 3 millimagnitudes E(B-V), though some regions do deviate from SFD98 by as
much as 50%. The maximum deviation found is 45 millimagnitudes in E(B-V), and
spatial structure of the deviation is strongly correlated with the observed
dust temperature, such that SFD98 underpredicts reddening in regions of low
dust temperature. Our maps of these deviations, as well as their errors, are
made available to the scientific community as supplemental correction to SFD98
at http://www.peekandgraves2010.com.
