A Possible Extension of the Scutum-Centaurus Arm into the Outer Second Quadrant

TL;DR

This study combines HI and CO data to identify a new segment of the spiral arm extending into the outer second Galactic quadrant, revealing its structure, molecular cloud content, and pitch angle.

Contribution

It presents the first identification of a new spiral arm extension beyond the Outer Arm in the second Galactic quadrant using combined HI and CO data.

Findings

Discovered a new spiral arm segment between 15-19 kpc from the Galactic center.

Identified 72 molecular clouds associated with the new arm.

Estimated the arm's pitch angle as approximately 9.3 degrees.

Abstract

Combining HI data from the Canadian Galactic Plane Survey~(CGPS) and CO data from the Milky Way Imaging Scroll Painting (MWISP) project, we have identified a new segment of a spiral arm between Galactocentric radii of 15 and 19~kpc that apparently lies beyond the Outer Arm in the second Galactic quadrant. Over most of its length, the arm is 400-600 pc thick in $z$. The new arm appears to be the extension of the distant arm recently discovered by Dame & Thaddeus (2011) as well as the Scutum-Centaurus Arm into the outer second quadrant. Our current survey identified a total of 72 molecular clouds with masses on the order of 10$^2$-10$^4$M$_{\sun}$ that probably lie in the new arm. When all of the available data from the CO molecular clouds are fit, the best$-$fitting spiral model gives a pitch angle of 9.3$^{\circ}$ $\pm$0.7$^{\circ}$.