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We present a map of the spiral structure of the Galaxy, as traced by molecular carbon monosulphide (CS) emission associated with  IRAS  sources which are believed to be compact H  ii  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 H  ii  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 study of the spiral structure is complemented with other tracers: open clusters, Cepheids, methanol masers and H  ii  regions. The well‐defined spiral arms are seen to be confined inside the corotation 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 (four epicycle oscillations in one turn around the galactic centre). 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 inwards 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. The observed arms are interpreted in terms of perturbations similar to grooves in the gravitational potential of the disc, produced by crowding of stellar orbits.

The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance