We investigate the structure of the Milky Way by determining how features in
a spatial map correspond to CO features in a velocity map. We examine
structures including logarithmic spiral arms, a ring and a bar. We explore the
available parameter space, including the pitch angle of the spiral arms, radius
of a ring, and rotation curve. We show that surprisingly, a spiral arm provides
a better fit to the observed molecular ring than a true ring feature. This is
because both a spiral arm, and the observed feature known as the molecular
ring, are curved in velocity longitude space. We find that much of the CO
emission in the velocity longitude map can be fitted by a nearly symmetric 2
armed spiral pattern. One of the arms corresponds to the molecular ring, whilst
the opposite arm naturally reproduces the Perseus arm. Multiple arms also
contribute to further emission in the vicinity of the molecular ring and match
other observed spiral arms. Whether the Galactic structure consists primarily
of two, or several spiral arms, the presence of 2 symmetric logarithmic
spirals, which begin in the vicinity of the ends of the bar, suggest a spiral
density wave associated with the bar.Comment: 7 pages, 2 figures, accepted by MNRA
