A New Multi-Tracer Approach to Defining the Spiral arm width in the
  Milky Way
  We analyze recent observations of the spiral arm width in the Milky Way, as a
function of the galactic radius, and we compare this relation with the
prediction from the density wave theory. We use the following method: in each
spiral arm, we concentrate on the separation (or offset) between the
starforming region (radio masers) near the shock front of a density wave, and
the aged star region (diffuse CO gas) near the potential minimum of a density
wave; we take this separation between these two tracers as the arm width.
  New results: we find a typical separation (maser to diffuse CO gas) near 250
+/- 50 pc, and an increase of this separation with galactic radius of about 25
+/- 5 pc per kpc. We note that, as expected, this separation is somewhat
smaller than that found earlier between the dust lane and the aged star region.
Overall, these results supports the basics of a density wave.
