\label{CO} CO observations trace gas with mean density around 100 cm\(^{-3}\). CO emission associated with the Scutum-Centaurus Arm of the Milky Way \citep{Dame2011}\citep{Dame2001} is shown in Figure \ref{fig:COarm}, which presents a plane-of-the-sky map integrated over \(-50 <v_{LSR}< -30\) km s\(^{-1}\). The velocity range is centered on -40 km s\(^{-1}\), the average velocity of the Scutum-Centaurus Arm in Nessie’s longitude range (see Figures \ref{fig:topview} and \ref{fig:coloredlines}). The white chalk line superimposed on Figure \ref{fig:COarm} is the same tracing of “Nessie Optimistic" shown in Figure \ref{fig:FindingChart}. The black feature labeled “Nessie" refers to “Nessie Classic."
Judging by-eye vertical (latitude) centroid of the CO emission in Figure \ref{fig:COarm} appears to follow Nessie remarkably well, even out to the full \(8^\circ\) (430 pc) extent of Nessie Optimistic. We have also calculated a curve representing the locus of latitude centroids for CO in this velocity range, and even at this coarse resolution, a curve following Nessie’s shape is clearly a better fit than a straight line passing through the CO centroids.
Table 1 estimates that the Nessie IRDC has a typical \({\rm H_2}\) column density of \(\sim 10^{23}\) cm\(^{-2}\) and a typical volume density of \(\sim 10^5\) cm\(^{-3}\). Thus, the plane-of-the-sky coincidence of the line-of-sight-velocity-selected “Scutum-Centaurus" CO emission and the mid-IR extinction suggests that the Nessie IRDC may be a kind of dense “spine" or “bone" of this section of the Scutum-Centaurus Arm, as traced by much-less-dense (\(\sim 100\) cm\(^{-3}\)) CO-traced gas. But, the spatial resolution of the CO map is too low (\(8'\)), and the 20 km s\(^{-1}\) velocity range associated with the Arm in CO is too broad to decide based on this evidence alone whether Nessie is a well-centered “spine" or just a long skinny feature associated with, but potentially significantly inclined to, the Scutum-Centaurus Arm.