deletions | additions       diff --git a/CO_velocities.tex b/CO_velocities.tex  index add7632..214e5c9 100644  --- a/CO_velocities.tex  +++ b/CO_velocities.tex 
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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 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 dense ``spine" or ``bone" of this section of the Scutum-Centaurus Arm, as traced by $\sim much-less-dense ($\sim  100$ cm$^{-3}$ 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.