Investigating the Edge of High Mass Star Formation in the Milky Way Galaxy

The Outer Scutum-Centaurus arm (OSC) is the most distant molecular spiral arm known in the Milky Way. The OSC was first revealed in 2011 by Dame and Thaddeus using HI and CO data as a possible extension of the Scutum-Centaurus arm. Here we use the Arizona Radio Observatory (ARO) 12m telescope to observe the 12CO (J = 1-0) and 13CO (J = 1-0) transitions towards 78 HII region and HII region candidates in the First Galactic quadrant, and towards 63 in the Second and Third Galactic quadrants, all selected from the WISE Catalog of Galactic HII Regions. Within the First Galactic quadrant, we detect 17 CO clouds associated with HII regions in the OSC's locus as defined by HI emission. Using the same techniques in the Second and Third Galactic quadrants, we did not detect any CO clouds definitively associated with HII Regions past the Outer arm. CO clouds have also been observed in the First and Second Galactic quadrant coincident with a potential extension of the OSC as defined by HI emission by the Milky Way Imaging Scroll Painting (MWISP) project. Examining these data with the same techniques we used to reduce our observational data produces similar findings, 4 CO clouds associated with HII regions in the first Galactic quadrant and none in the Second or Third. The relative lack of CO molecular clouds associated with HII regions in the Second and Third Galactic quadrants compared with the First Galactic quadrant indicates that star formation in the OSC spiral arm does not follow a log-spiral fit, consistent with the HI emission. Based on our data, we estimate the edge of high high mass star formation in the Milky Way occurs on the OSC, near the border of the First and Second Galactic quadrant, at a Galactocentric distance of approximately 14.5kpc.