deletions | additions       diff --git a/CO_velocities.tex b/CO_velocities.tex  index 642a565..bfaf326 100644  --- a/CO_velocities.tex  +++ b/CO_velocities.tex 
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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. This coincidence of features can be seen by-eye in Figure \ref{fig:COarm}, but we have also compared Nessie's shape with a trace of the CO distribution's latitude centroid, and we have confirmed, at least at the coarse resolution of the CO map, that a curve following Nessie's shape is a better fit than a straight line passing through the CO centroids.   Table 1 estimates Nessie's typical ${\rm H_2}$ column density at $\gtrsim 10^{23}$ cm$^{-2}$ and its typical volume density at $\gtrsim 10^5$ 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, as traced by \sim $\sim  100$ 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.