Milky Way Tomography IV: Dissecting Dust
  We use SDSS photometry of 73 million stars to simultaneously obtain best-fit
main-sequence stellar energy distribution (SED) and amount of dust extinction
along the line of sight towards each star. Using a subsample of 23 million
stars with 2MASS photometry, whose addition enables more robust results, we
show that SDSS photometry alone is sufficient to break degeneracies between
intrinsic stellar color and dust amount when the shape of extinction curve is
fixed. When using both SDSS and 2MASS photometry, the ratio of the total to
selective absorption, $R_V$, can be determined with an uncertainty of about 0.1
for most stars in high-extinction regions. These fits enable detailed studies
of the dust properties and its spatial distribution, and of the stellar spatial
distribution at low Galactic latitudes. Our results are in good agreement with
the extinction normalization given by the Schlegel et al. (1998, SFD) dust maps
at high northern Galactic latitudes, but indicate that the SFD extinction map
appears to be consistently overestimated by about 20% in the southern sky, in
agreement with Schlafly et al. (2010). The constraints on the shape of the dust
extinction curve across the SDSS and 2MASS bandpasses support the models by
Fitzpatrick (1999) and Cardelli et al. (1989). For the latter, we find an
$R_V=3.0\pm0.1$(random) $\pm0.1$(systematic) over most of the high-latitude
sky. At low Galactic latitudes (|b|<5), we demonstrate that the SFD map cannot
be reliably used to correct for extinction as most stars are embedded in dust,
rather than behind it. We introduce a method for efficient selection of
candidate red giant stars in the disk, dubbed "dusty parallax relation", which
utilizes a correlation between distance and the extinction along the line of
sight. We make these best-fit parameters, as well as all the input SDSS and
2MASS data, publicly available in a user-friendly format.
