The effect of FIR emission from SDSS galaxies on the SFD galactic extinction map

We compare the most successful and widely used map of Galactic dust extinction, provided by Schlegel, Finkbeiner, and Davis (1998, hereafter SFD), to the galaxy number counts in the Sloan Digital Sky Survey (SDSS) photometric/spectroscopic DR4 sample. We divide the SDSS survey area into 69 disjoint subrogions according to the dust extinction provided by SFD and compare the surface number density of galaxies in each subregion. As expected, the galaxy surface number density decreases with increasing extinction, but only for SFD extinction values above about 0.1 to 0.2 magnitudes (depending on the band). At lower values of the SFD extinction, we find that the sky surface density of galaxies increases with increasing extinction, precisely the opposite of the effect expected from Galactic dust. We also find that the average color of the SDSS photometric galaxy sample is slightly bluer at higher SFD extinctions in this regime, again the opposite of the effect expected from Galactic dust. Even though these anomalies occur only for sight-lines with low SFD extinction values, they affect approximately 68% of the high galactic latitude sky in which galaxies and their clustering properties are normally studied. Although it would be possible to explain these effects with a mysterious component of Galactic dust, which is anti-correlated with the 100 μm flux on which the SFD extinction map is based, this model is not physically plausible. Moreover, we find that the surface number density of SDSS photometric quasars does not show any similar effect, as would be expected if the explanation were an unknown Galactic dust component. Considering these results, we suggest that the far infrared (FIR) brightness of the sky in regions of true low dust extinction is significantly "contaminated" by the FIR emission from background galaxies. We show that such an explanation is both qualitatively and quantitatively consistent with the available data. Based on this interpretation we conclude that systematic errors in the SFD extinction map due to extragalactic FIR emission are quite small, on the order hundredths of a magnitude, but nevertheless statistically detectable. Unfortunately, however, these errors are also entangled in a complex way with a signal of great interest to many "precision cosmology" applications, namely the large-scale clustering of galaxies.