A Correction to the Standard Galactic Reddening Map: Passive Galaxies as Standard Crayons

We present corrections to the Schlegel et al. (SFD98) reddening maps over the Sloan Digital Sky Survey (SDSS) northern Galactic cap area. To find these corrections, we employ what we call the "standard crayon" method, in which we use passively evolving galaxies as color standards to measure deviations from the reddening map. We select these passively evolving galaxies spectroscopically, using limits on the Hα 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 that we select. Using these galaxies, we construct maps of the deviation from the SFD98 reddening map at 4fdg5 resolution, with 1σ error of ~1.5 mmag E(B - V). We find that the SFD98 maps are largely accurate with most of the map having deviations below 3 mmag E(B - V), though some regions do deviate from SFD98 by as much as 50%. The maximum deviation found is 45 mmag in E(B - V), and spatial structure of the deviation is strongly correlated with the observed dust temperature, such that SFD98 underpredict reddening in regions of low dust temperature. Our maps of these deviations, as well as their errors, are made available to the scientific community on the Web as a supplemental correction to SFD98.