A new view of the Spiral Structure of the Northern Outer Milky Way in Carbon Monoxide

TL;DR

This study presents new face-on molecular gas maps of the northern outer Milky Way, revealing extended spiral arms and connecting previously identified structures, thereby enhancing understanding of the galaxy's spiral structure.

Contribution

It provides the first detailed CO-based face-on maps of the northern outer Galaxy, extending the known spiral arms beyond previous optical and radio observations.

Findings

Identified three spiral segments extending 16-43 kpc.

Connected two previously separate spiral arm segments.

Extended the known CO spiral arms to about 22 kpc radius.

Abstract

Based on 32162 molecular clouds from the Milky Way Imaging Scroll Painting project, we obtain new face-on molecular gas maps of the northern outer Galaxy. The total molecular gas surface density map reveals three segments of spirals, extending 16-43 kiloparsecs in length. The Perseus and Outer arms stand out prominently, appearing as quasi-continuous structures along most of their length. At the Galactic outskirts, about 1306 clouds connect the two segments of the new spiral arm discovered by Dame & Thaddeus (2011) in the first quadrant and Sun et al. (2015) in the second quadrant, possibly extending the arm into the outer third quadrant. Logarithmic spirals can be fitted to the CO arm segments with pitch angles ranging from 4 to 12 degree. These CO arms extend beyond previous CO studies and the optical radius, reaching a galactic radius of about 22 kiloparsecs, comparable to the HI radial range.