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We have carried out survey observations toward the Galactic plane at l~38 degin the 12CO and 13CO J=1-0 lines using the Nobeyama Radio Observatory 45-mtelescope. A wide area (0.8 x 0.8 deg) was mapped with high spatial resolution(17"). The line of sight samples the gas in both the Sagittarius arm and theinter-arm regions. The present observations reveal how the structure andphysical conditions vary across a spiral arm. We classify the molecular gas inthe line of sight into two distinct components based on its appearance: thebright and compact B component and the fainter and diffuse (i.e., moreextended) D component. The B component is predominantly seen at the spiral armvelocities, while the D component dominates at the inter-arm velocities and isalso found at the spiral arm velocities. We introduce the brightnessdistribution function and the brightness distribution index (BDI, whichindicates the dominance of the B component) in order to quantify the map'sappearance. The radial velocities of BDI peaks coincide with those of high 12COJ=3-2/12CO J=1-0 intensity ratio (i.e., warm gas) and H II regions, and tend tobe offset from the line brightness peaks at lower velocities (i.e., presumablydownstream side of the arm). Our observations reveal that the gas structure atsmall scales changes across a spiral arm: bright and spatially confinedstructures develop in a spiral arm, leading to star formation at downstreamside, while extended emission dominates in the inter-arm region.

Structural Variation of Molecular Gas in the Sagittarius Arm and Inter-Arm Regions