Tracing the Spiral Structure of the Outer Milky Way with Dense Atomic
  Hydrogen Gas
  We present a new face-on map of dense neutral atomic hydrogen (HI) gas in the
outer Galaxy. Our map has been produced from the Leiden/Argentine/Bonn (LAB) HI
21-cm line all-sky survey by finding intensity maxima along every line of sight
and then by projecting them on the Galactic plane. The resulting face-on map
strikingly reveals the complex spiral structure beyond the solar circle, which
is characterized by a mixture of distinct long arcs of HI concentrations and
numerous 'interarm' features. The comparison with more conventional spiral
tracers confirms the nature of those long arc structures as spiral arms. Our
map shows that the HI spiral structure in the outer Galaxy is well described by
a four-arm spiral model (pitch angle of 12 deg) with some deviations, and gives
a new insight into identifying HI features associated with individual arms.
