Using M Dwarf Spectra to Map Extinction in the Local Galaxy
  We use spectra of more than 56,000 M dwarfs from the Sloan Digital Sky Survey
(SDSS) to create a high-latitude extinction map of the local Galaxy. Our
technique compares spectra from the stars in the SDSS Data Release 7 M dwarf
sample in low-extinction lines of sight, as determined by Schlegel, Finkbeiner,
& Davis, to other SDSS M dwarf spectra in order to derive improved distance
estimates and accurate line-of-sight extinctions. Unlike most previous studies,
which have used a two-color method to determine extinction, we fit extinction
curves to fluxes across the entire spectral range from 5700 to 9200 {\AA} for
every star in our sample. Our result is an extinction map that extends from a
few tens of pc to approximately 2 kpc away from the sun. We also use a similar
technique to create a map of R_V values within approximately 1 kpc of the sun,
and find they are consistent with the widely accepted diffuse interstellar
medium value of 3.1. Using our extinction data, we derive a dust scale height
for the local galaxy of 119\pm15 parsecs and find evidence for a local dust
cavity.
