Catherine Zucker edited sectionMethdology_To.tex  almost 9 years ago

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Of the ten filaments with velocities consistent with galactic rotation, \textbf{six} of these meet all six bone criteria: \textbf{filament 1 ("BC\_026.94-0.30"), filament 2 ("BC\_025.24-0.45"), filament 5 ("BC\_018.88-0.09"), filament 7 ("BC\_04.14-0.02"), filament 9 ("BC\_335.31-0.29"), and filament 10 ("BC\_332.21-0.04")}, to varying degrees of excellence. We note that filament 10 has likely been disrupted by stellar feedback, making its aspect ratio and velocity structure more difficult to define. Since we predict that all galactic bones will likely be destroyed by stellar feedback and/or galactic shear, we include it here as part of a larger attempt to build a catalog of bones at all stages of their evolution. We also include filament 9, even though it has a $p-v$ orientation perpendicular to predicted fits of the Scutum-Centaurus arm (see Figure \ref{fig:skeleton}). As spurs and inter-arm structures are likely to lie close to the physical Galactic mid-plane, but with velocity gradients angled with respect to predicted arm fits, we do {\it not} require that a bone be parallel to arm $p-v$ traces, so as not to exclude potential spurs, feathers, or other inter-arm features.  Of the four remaining filaments that do not meet all six criteria---filament 8 ("BC\_357.62-0.33"), filament 6 ("BC\_11.13-0.12"), filament 3 ("BC\_24.96-0.17"), and filament 4 ("BC\_21.25-0.15")---all of them fail criterion 6 (aspect ratio $\ge 50:1$). As our criterion 6 does not allow for projection effects in imposing an aspect ratio limit, we emphasize that those filaments lying more tangential to our line-of-sight will appear foreshortened, and could very well meet the 50:1 minimum limit if projection effects were removed. We plan to examine the the aspect ratios of all our candidates in a follow-up study, accounting for expected projection effects if they lie along the nearest spiral arm. The first of these candidates, filament 8, shows particular promise, lying within 2-3 pc of the physical Galactic midplane and tracing a prominent peak of CO emission in both \textit{p-p} and \textit{p-v} space (see appendix section on filament 8). The second filament, filament 6 ("the snake"), has already been well-studied from a star formation perspective, hosting over a dozen protostellar cores likely to produce regions of high-mass star formation \citep{Wang_2014,Henning_2010}. From a Galactic bone perspective, the snake strongly satisfies all criteria except number 6---it lies within 15 pc of the physical galactic midplane and 5 km/s from the \citet{Dame_2011} Scutum-Centaurus global-log fit to CO, also tracing a prominent peak of CO emission in \textit{p-v} space (see appendix section on filament 6). The remaining two filaments, filaments 3 and 4, are both awarded a quality rating of "C." Filament 3 lies 10 km/s from the \citet{Shane_1972} fit to HI for the Scutum-Centaurus arm (at the upper limit of criterion 4) while filament 4 appears to be a potential interarm filament, lying between the Scutum-Centaurus and Norma-4kpc arms in \textit{p-v}. \textit{p-v} space.  We also note a small break in the extinction feature of filament 4, though the filament has been confirmed to be contiguous in velocity space as traced by $^{13}\rm{CO}$ from the GRS Survey. In summary, it is important to emphasize that some of the above criteria will likely be modified in the long run, as we learn more about the Skeleton of the Milky Way. Given our limited a priori knowledge of the Galaxy's structure, it is presently easier to confirm Bones that are spine-like, lying along arms with velocities predicted by extant modeling (criteria 1, 5), and harder to find spurs off those arms or inter-arm features (e.g. filament 9, the velocities of which are hard to predict well. Similarly, since criterion 6 does not allow for projection effects in imposing an aspect ratio limit, bones which otherwise meet all criteria could fail if they lie close to the tangents of spiral arms. As we learn more about spiral structure from simulations and modeling, these criteria will also be adjusted to allow for bone-like features that represent spurs, inter-arm structures, and/or foreshortened structures lying close to our line of sight.