The Distant Outer Gas Arm Between l=35d and l=45d

TL;DR

This study maps the outer Galactic gas arm using CO emission, revealing its structure, displacement from the IAU plane, and properties of associated molecular clouds, providing insights into the Galaxy's large-scale structure and the Sun's position.

Contribution

First unbiased CO survey of the outer Galactic arm segment, identifying molecular clouds and analyzing their spatial distribution and physical parameters.

Findings

The Outer Arm is traced by CO emission beyond the solar circle.

The physical mid-plane of the Outer Arm is slightly displaced from the IAU plane.

An upper limit of ~17.1 pc is estimated for the Sun's offset from the Galactic mid-plane.

Abstract

The Galactic plane has been mapped from l=34.75d to 45.25d and b=-5.25d to 5.25d in the CO (J=1-0) emission with the 13.7 m telescope of the Purple Mountain Observatory. The unbiased survey covers a large area of 100 square degrees sampled every 30" with a velocity resolution of ~0.2km/s. In this paper, we present the result of an unbiased CO survey of this longitude and latitude range in the velocity range from -60km/s to -10km/s. Over 500 molecular clouds (MCs) are picked out from the 12CO emission, and 131 of these MCs are associated with 13CO emission.The distant MCs, which lie beyond the solar circle and are mostly concentrated in the Galactic plane, trace the large-scale molecular gas structure over 10 degrees of Galactic azimuth. We suggest that the CO emission of the segment is from the Outer Arm. The physical mid-plane traced by the Outer Arm seems to be slightly displaced from the IAU-defined plane on a large scale, which could be explained by the warped plane at large Galactocentric distances of >~10 kpc and the apparent tilted mid-plane to the projected IAU-defined plane caused by the Sun's z-height above the disk for distances near and within the Solar circle. If the inner plane of our Galaxy is flat, we can derive an upper limit of the Sun's offset of ~17.1 pc above the physical mid-plane of the Milky Way. We also discuss the correlations between the physical parameters of the distant MCs, which is quite consistent with the result of other studies of this parameter.