\label{spine} All the evidence presented in this paper, taken together, strongly suggests that Nessie forms a spine-like feature that runs down the center of the Scutum-Centaurus Arm of the Milky Way. How did it get there? Is it the crest of a classic spiral density wave \citep{Lin1964}, or does it have some other cause? Any feature this long and skinny that is not controlled by Galactic-scale forces will be subject to a variety of instabilities, and cannot last long. It would be great if we could look to numerical simulations for answers, but today’s simulations can, alas, only give hints. Nessie is so skinny, and so much denser than its surroundings that no extant numerical simulation has the combination of spatial resolution and dynamic range in density needed to produce a feature like it.
Figure \ref{fig:simulation} offers a snapshot of a numerical simulation \cite{2013MNRAS.432..653D} that represents the state of the art at present (available as a movie at http://empslocal.ex.ac.uk/people/staff/cld214/movies.html). One can see density features that are highly elongated, both within the spiral arms, and also between the arms. Many of the features between the arms in Figure \ref{fig:simulation} are similar to the ‘spurs’ and ‘feathers’ that have been simulated and observed by E. Ostriker and colleagues \citep{Shetty2006,Vigne2008,Corder2008}. Figure \ref{fig:IC342} (discussed below) shows a recent WISE image of the galaxy IC342 \citep{Jarrett2013}, and it is clear from that image that some ‘spiral’ galaxies also exhibit inter-arm filaments that are even more pronounced than the simulated spurs and feathers.
In the case of Nessie, the velocity information analyzed in §\ref{CO} and \ref{ammonia} seems to very strongly favor Nessie’s being oriented exactly along (within, as the backbone of) an arm (Scutum-Centaurus) over the idea that Nessie is a spur or interam filament.
Estimates for the mass of Nessie under various assumptions are given in Table 1. Jackson et al. 2010 model Nessie as a(n unmagnetized) self-gravitating fluid cylinder supported against collapse by a “turbulent" analog of thermal pressure, undergoing the sausage instability discussed in \citet{1953ApJ...118..116C}. For the observed line width of HNC, the theoretical critical mass per unit length, \(m_l\), is 525 \({\rm M}_\odot/{\rm pc}\) for Nessie. But, if, as Jackson et al. explain, one estimates \(m_l\) using HNC emission itself and (uncertain) abundance values for HNC, then \(110 < m_l < 5 \times 10^4 {\rm M}_\odot {\rm pc}^{-1}\). Given that the low end of this range (\(110 {\rm M}_\odot {\rm pc}^{-1}\), favored by Jackson et al.) gives a very low value for extinction toward Nessie (\(A_V \sim 4\) mag), we favor higher values \(m_l\), needed to be consistent with the observed IR extinction. Recent LABOCA observations of dust continuum emission from pieces of Nessie (Kauffmann, private communication), suggest that \(m_l \simgreat 10^3\) in the mid-IR-opaque portions of Nessie. So, at present, it would appear that there is at least an order-of-magnitude uncertainty in \(m_l\). Some of this uncertainty is caused by the definition of Nessie’s shape, which makes it unclear which “mass" to measure in calculating \(m_l\), but more is due to the vagaries of converting molecular line emission and/or dust continuum to true masses.
It is not the goal of this paper to produce a more definitive estimate of Nessie’s \(m_l\) or total mass, or to model Nessie’s internal density structure. Instead, here, we only seek to estimate the total mass of Nessie in order to consider its mass as a fraction of that in the Galaxy or in a spiral arm. So, Table 1 offers rough estimates of the mass of cylinders, whose (constant) average density is set so that the typical extinctions associated with Nessie’s IR-dark (\(A_v\sim 100\)) and HCN bright (\(A_V \simgreat\) a few mag) radii are sensible. Assuming a mean density for the mid-IR opaque material of \(10^5\) cm\(^{-3}\), then Nessie Classic is \(1 \times 10^5\) M\(_\odot\), Nessie Extended is \(2 \times 10^5\) M\(_\odot\) and Nessie Optimistic is \(5 \times 10^5\) M\(_\odot\). If one assumes that the envelope traced by the HNC observations of Jackson et al. (2010) for Nessie Classic continues along Nessie’s length, then the mass of a \(n\sim 500\) cm\(^{-3}\) cylindrical tube (see Table 1) associated with Nessie would be \(5 \times 10^4\) M\(_\odot\) for Classic and \(3 \times 10^5\) M\(_\odot\) for Optimistic. For the Optimistic case, this mass amounts to 2 millionths of the total baryonic mass (assuming \(\sim 10^{11}\) M\(_\odot\) total) of the Milky Way. To use this fraction in order to estimate the total number of “Nessies" discoverable in the Milky Way’s ISM, we need to remember that the HCN mass is likely a lower limit (meaning the mass fraction is an upper limit), and that most of the gas mass in the ISM is at low density, following a log-normal-like density distribution. Given those caveats, we estimate that of order thousands of additional Nessie-like features should be discoverable, if they are characteristic of spiral arms.