Structured Molecular Gas Reveals Galactic Spiral Arms

TL;DR

This study analyzes molecular gas structures in the Milky Way using brightness distribution metrics, revealing that gas becomes more structured as it passes through spiral arms, which may influence star formation processes.

Contribution

It introduces the use of brightness distribution function and index to quantify molecular gas structure, showing how gas evolves structurally across spiral arms in the galaxy.

Findings

Structured gas correlates with spiral arms in the Milky Way.

High BDI regions sometimes lack star formation signatures.

Gas transitions from unstructured to structured as it encounters 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.