Structured Molecular Gas Reveals Galactic Spiral Arms

Abstract

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.

6 pages, 4 figures, accepted for publication in ApJL

References in corpus (7)

• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• Resolution of the Distance Ambiguity for Galactic HII Regions
• Dynamically Driven Evolution of the Interstellar Medium in M51
• The formation of molecular clouds in spiral galaxies
• The Distance to SS433/W50 and its Interaction with the ISM
• Instabilities of Spiral Shocks -- II. A quasi-steady State in the multi-phase inhomogeneous ISM
• Structural Variation of Molecular Gas in the Sagittarius Arm and Inter-Arm Regions

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