deletions | additions       diff --git a/NH3_velocities.tex b/NH3_velocities.tex  index a4036ad..4d33972 100644  --- a/NH3_velocities.tex  +++ b/NH3_velocities.tex 
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The H$_2$O Southern Galactic Plane (HOPS) Survey \citep{Purcell2012b} has surveyed hundreds of sites of massive star formation visible from the Southern Hemisphere for ${\rm NH}_3$ emission, which traces gas at densities $n\gtrsim 10^4$ cm$^{-3}$. The HOPS targets were selected based on H$_2$O maser emission, thermal molecular emission, and radio recombination lines, so as to include nearly all known regions of massive star formation within the surveyed area. These ``massive-star-forming region" selection criteria mean that the HOPS database includes ${\rm NH}_3$ spectra for dozens of positions within the longitude range covered by Nessie.   Figure \ref{fig:HOPSoverlay} shows an overlay of HOPS sources' ${\rm NH}_3$-determined LSR velocities on the Spitzer image of Nessie used in Figure \ref{fig:coloredlines}. The (color-coded) velocities of the HOPS sources, for both Nessie Classic, and Nessie Extended (see Figure \ref{fig:FindingChart}), agree remarkably well with what is predicted for the Scutum-Centaurus Arm (color-coded lines). Note that agreement of the ${\rm NH}_3$ and predicted velocity to within 5 km s$^{-1}$ is indicated bya  light-colored circle circles  around the HOPS symbol. symbol (see caption for details).  White circles correspond to the Nessie Extended sources also shown in Figure \ref{fig:pvdiagram}, below, and grey circles mark other points, in Nessie Optimistic, also likely (based on their velocity) to be associated with the Scutum-Centaurus arm. The velocities of sources at latitudes much different from Nessie's within this longitude range largely do {\it not} agree, and those sources are unlikely to associated with the near-side of the Scutum-Centaurus Arm. For Nessie Classic, Jackson et al. (2010) had already noted a very narrow velocity range for dense gas associated with the IRDC, based on HNC observations. What is new here is the three-dimensional (latitude, longitude, and \textit{and}  velocity) association of a {\it longer} Nessie's dense gas with predictions for where the centroid of the Milky Way's Scutum-Centaurus Arm's ``middle" would lie. Figure \ref{fig:pvdiagram}, which offers a position-velocity diagram of CO (color)  and ${\rm NH}_3$ emission (black dots)  together, shows the association of the Nessie-HOPS sources with the Scutum Centaurus Arm most clearly. What is most remarkable about Figure \ref{fig:pvdiagram} is that the black line sloping through the figure is {\it not} a fit to the black dots representing the HOPS sources. Instead, that line indicates the position-velocity trace of the Scutum-Centaurus Arm based on \citep{Dame2011} data for the full Galaxy, not just this small longitude range. Figure \ref{fig:pvdiagram} implies that Nessie goes right down the ``spine" of the Scutum-Centaurus Arm, as best we can measure its position in CO position-velocity space.