Measuring Galactic Extinction: A Test

We test the recently published all-sky reddening map of Schlegel, Finkbeiner& Davis (1998 [SFD]) using the extinction study of a region in the Taurus darkcloud complex by Arce & Goodman (1999 [AG]). In their study, AG use fourdifferent techniques to measure the amount and structure of the extinctiontoward Taurus, and all four techniques agree very well. Thus we believe thatthe AG results are a truthful representation of the extinction in the regionand can be used to test the reliability of the SFD reddening map. The resultsof our test show that the SFD all-sky reddening map, which is based on datafrom COBE/DIRBE and IRAS/ISSA, overestimates the reddening by a factor of 1.3to 1.5 in regions of smooth extinction with A_V > 0.5 mag. In some regions ofsteep extinction gradients the SFD map underestimates the reddening value,probably due to its low spatial resolution. We expect that the astronomicalcommunity will be using the SFD reddening map extensively. We offer this Letteras a cautionary note about using the SFD map in regions of high extinction (A_V> 0.5 mag), as it might not be giving accurate reddening values there.Comment: 14 pages (which include 2 pages of figures