The Outer Scutum-Centaurus arm (OSC) is the most distant molecular spiral arm known in the Milky Way. The OSC may be the very distant end of the well-known Scutum-Centaurus arm, which stretches from the end of the Galactic bar to the outer Galaxy. At this distance the OSC is seen in the first Galactic quadrant. The population of star formation tracers in the OSC remains largely uncharacterized. Extragalactic studies show a strong correlation between molecular gas and star formation, and carbon monoxide (CO) emission was recently discovered in the OSC. Here we use the Arizona Radio Observatory (ARO) 12-m telescope to observe the $^{12}$CO J = 1-0 and $^{13}$CO J = 1-0 transitions toward 78 HII region candidates chosen from the WISE Catalog of Galactic HII Regions. These targets are spatially coincident with the Galactic longitude-latitude ($ell, b$) OSC locus as defined by HI emission. We detect CO emission in $sim 80$% of our targets. In total, we detect 117 $^{12}$CO and 40 $^{13}$CO emission lines. About 2/3 of our targets have at least one emission line originating beyond the Solar orbit. Most of the detections beyond the Solar orbit are associated with the Outer Arm, but there are 17 $^{12}$CO emission lines and 8 $^{13}$CO emission lines with LSR velocities that are consistent with the velocities of the OSC. There is no apparent difference between the physical properties (e.g., molecular column density) of these OSC molecular clouds and non--OSC molecular clouds within our sample.
تحميل البحث