deletions | additions       diff --git a/NH3_velocities.tex b/NH3_velocities.tex  index 7ea0686..6808b59 100644  --- a/NH3_velocities.tex  +++ b/NH3_velocities.tex 
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\subsubsection{NH$_3$ Velocities} \label{ammonia} To estimate the 3D orientation of Nessie more precisely, we need to employ a gas tracer whose emission is sparser than CO's in position-position-velocity space. Many recent studies have shown that IRDCs typically host over-dense blobs of gas (often called ``clumps" or ``cores") that provide the gaseous reservoirs for the formation for massive stars. Thus, several studies have been undertaken to survey IRDCs and their ilk for emission in molecular lines that trace high-density ($\gg 10^3$ cm$^{-3}$), potentially star-forming, gas. 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 throughout this paper (see Figures \ref{fig:FindingChart} and \ref{fig:coloredlines}). It is clear that the velocities of the HOPS sources within Nessie Extended (xxshould we show more length of Nessie?xx) agree remarkably well with what is predicted for the Scutum-Centaurus Arm (color-coding of dashed line). The velocities of sources at other latitudes within this longitude range do {\it not} agree, as they are not part of the Nessie feature, or, in most cases, not even associated with the (near-side of the) Scutum-Centaurus Arm (should we color-code the white lines, so we can show which of the higher $b$ sources could be at the far part of the Arm? likely no, because the sensitivity of HOPS should not really see many of those?xx). For the innermost part of Nessie (Nessie Classic), Jackson et al. (2010) had already noted a very narrow velocity range for dense gas associated with the IRDC, based on HCN HNC  observations. What is new here is the three-dimensional (latitude, longitude, and velocity) association of (an even 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 and ${\rm NH}_3$ emission 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 indicated 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.