THE DISTANT OUTER GAS ARM BETWEEN l = 35° AND l = 45°

Abstract

ABSTRACT The Galactic plane has been mapped from l = 34.°75 to 45.°25 and b = −5.°25 to 5.°25 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 110 square degrees sampled every 30″ with a velocity resolution of . In this paper, we present the result of an unbiased CO survey of this longitude and latitude range in the velocity range from −60 to . Over 500 molecular clouds (MCs) are picked out from the 12CO (J = 1–0) 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 find that the distribution of the distant MCs can be well fitted by a Gaussian function with a full width at half maximum (FWHM) of 0.°7 with the Galactic latitude. 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. After removing the effect of the warp and tilted structure, the scale height of the MCs in the Outer Arm is about 0.°6 or 160 pc at a heliocentric distance of 15 kpc. 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.