\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 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."
The vertical (latitude) centroid of the CO emission attributed to the Scutum-Centaurus Arm \citep{Dame2011} shown in Figure \ref{fig:COarm} appears to follow Nessie remarkably well, even out to the full \(8^\circ\) (430 pc) extent of Nessie Optimistic. Table 1 estimates Nessie’s typical \({\rm H_2}\) column density at \(\simgreat 10^{23}\) cm\(^{-2}\) and its typical volume density at \(\simgreat 10^5\) 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 “spine" or “bone" of this section of the Scutum-Centaurus Arm. 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.