  We explore the development of structures in molecular gas in the Milky Way by
applying the analysis of the brightness distribution function (BDF) and the
brightness distribution index (BDI) in the archival data from the Boston
University-Five College Radio Astronomy Observatory 13CO J=1-0 Galactic Ring
Survey. The BDI measures the fractional contribution of spatially confined
bright molecular emission over faint emission extended over large areas. This
relative quantity is largely independent of the amount of molecular gas and of
any conventional, pre-conceived structures, such as cores, clumps, or giant
molecular clouds. The structured molecular gas traced by higher BDI is located
continuously along the spiral arms in the Milky Way in the longitude-velocity
diagram. This clearly indicates that molecular gas changes its structure as it
flows through the spiral arms. Although the high-BDI gas generally coincides
with H II regions, there is also some high-BDI gas with no/little signature of
ongoing star formation. These results support a possible evolutionary sequence
in which unstructured, diffuse gas transforms itself into a structured state on
encountering the spiral arms, followed by star formation and an eventual return
to the unstructured state after the spiral arm passage.
