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.