An Outer Arm in the Second Galactic Quadrant: Structure

TL;DR

This study uses high-sensitivity CO survey data to map the Outer arm of the Milky Way in the second galactic quadrant, revealing its structure, molecular cloud content, and physical properties for the first time on such a large scale.

Contribution

It provides the first large-scale molecular gas map of the Outer arm in the second galactic quadrant, identifying numerous molecular clouds and analyzing the arm's structure and physical characteristics.

Findings

Detected 481 molecular clouds, 69% of which are new.

Estimated the total mass of Outer arm clouds as ~3.1 million solar masses.

Fitted the spiral arm's pitch angle as approximately 13.1 degrees.

Abstract

The lack of arm tracers, especially the remote tracers, is one of the most difficult problems preventing us from studying the structure of the Milky Way. Fortunately, with its high-sensitivity CO survey, the Milky Way Imaging Scroll Painting (MWISP) project offers such an opportunity. Since completing about one-third of its mission, an area of l=[100,150] deg, b=[-3,5] deg has nearly been covered. The Outer arm of the Milky Way first clearly revealed its shape in the second galactic quadrant in the form of molecular gas --- this is the first time that the Outer arm has been reported in such a large-scale mapping of molecular gas. Using the 115 GHz 12CO(1-0) data of MWISP at the LSR velocity ~= [-100,-60] km s^-1 and in the area mentioned above, we have detected 481 molecular clouds in total, and among them 332 (about 69\%) are newly detected and 457 probably belong to the Outer arm. The total mass of the detected Outer arm clouds is ~ 3.1*10^6 M_sun. Assuming that the spiral arm is a logarithmic spiral, the pitch angle is fitted as ~ 13.1 deg. Besides combining both the CO data from MWISP and the 21 cm HI data from the Canadian Galactic Plane Survey (CGPS), the gas distribution, warp, and thickness of the Outer arm are also studied.