Detection of the Milky Way spiral arms in dust from 3D mapping
  Large stellar surveys are sensitive to interstellar dust through the effects
of reddening. Using extinctions measured from photometry and spectroscopy,
together with three-dimensional (3D) positions of individual stars, it is
possible to construct a three-dimensional dust map. We present the first
continuous map of the dust distribution in the Galactic disk out to 7 kpc
within 100 pc of the Galactic midplane, using red clump and giant stars from
SDSS APOGEE DR14. We use a non-parametric method based on Gaussian Processes to
map the dust density, which is the local property of the ISM rather than an
integrated quantity. This method models the dust correlation between points in
3D space and can capture arbitrary variations, unconstrained by a pre-specified
functional form. This produces a continuous map without line-of-sight
artefacts. Our resulting map traces some features of the local Galactic spiral
arms, even though the model contains no prior suggestion of spiral arms, nor
any underlying model for the Galactic structure. This is the first time that
such evident arm structures have been captured by a dust density map in the
Milky Way. Our resulting map also traces some of the known giant molecular
clouds in the Galaxy and puts some constraints on their distances, some of
which were hitherto relatively uncertain.
