The spiral structure of the Galaxy revealed by CS sources and evidence
  for the 4:1 resonance
  We present a map of the spiral structure of the Galaxy, as traced by
molecular CS emission associated with IRAS sources which are believed to be
compact HII regions. The CS line velocities are used to determine the kinematic
distances of the sources, in order to investigate their distribution in the
galactic plane. This allows us to use 870 objects to trace the arms, a number
larger than that of previous studies based on classical HII regions. The
distance ambiguity of the kinematic distances, when it exists, is solved by
different procedures, including the latitude distribution and an analysis of
the longitude-velocity diagram. The well defined spiral arms are seen to be
confined inside the co-rotation radius, as is often the case in spiral
galaxies. We identify a square-shaped sub-structure in the CS map with that
predicted by stellar orbits at the 4:1 resonance (4 epicycle oscillations in
one turn around the galactic center). The sub-structure is found at the
expected radius, based on the known pattern rotation speed and epicycle
frequency curve. An inner arm presents an end with strong inward curvature and
intense star formation that we tentatively associate with the region where this
arm surrounds the extremity of the bar, as seen in many barred galaxies.
Finally, a new arm with concave curvature is found in the Sagitta to Cepheus
region of the sky.
