Catherine Zucker edited sectionAnalysis_of_N.tex  about 9 years ago

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Since \citet{Ragan_2014} find little or no association with their filaments and Galactic structure, they speculate that perhaps we are not as sensitive to spiral arm filaments in the first quadrant, or that the frequency and orientation of spiral arm filaments in the first quadrant is different than the fourth. Since three of our strongest bone candidates (BC\_18.88-0.09, BC\_26.94-0.30, BC\_25.24-0.45) all lie in the first quadrant, we argue against the above speculations. Once again, we attribute the discrepancy not to a difference in frequency or sensitivity, but to a difference in methodology. Unlike \citet{Ragan_2014}, we never used minimum angular length as a criterion because no bone is likely to be as long as Nessie, given its extremely favorable position on the sky. As a result, we argue that bones are not necessarily the longest features on the sky, but the longest features that have incredibly high (>50:1) aspect ratios when compared to the typical Giant Molecular Cloud. Since parts of GMFs from \citet{Ragan_2014} are too diffuse to allow for the measurement of their widths, no complete GMF from that study could reasonably meet our bone definition. Thus, a clear and consistent description of a bone is critical, and, in future studies, we plan to continue to apply the criteria above to achieve consistency.   Like \citet{Ragan_2014}, Wang et al. (2015) search for large-scale filaments and establish their relationship to Galactic structure after the fact. Rather than searching for filaments elongated along the Galactic plane, Wang et al. (2015) search for the longest, coldest, and densest filaments (aspect ratio >>10) in Hi-GAL images, within the longitude range of $15^\circ < l < 56^\circ$. Wang et al. (2015) highlight nine filaments as their most prominent, with one of the nine being Nessie. Only one of their filaments (BC\_11.13-0.12, the "snake") overlaps with our sample. Five other Wang et al. (2015) filaments fail one or more of our bone criteria. G24, G26, and G47 lie between 39-62 pc above the physical Galactic midplane (significantly outside our $\pm 20$ pc criteria), while G28 and G65 have aspect ratios of only around 19:1 and 38:1 (less than our 50:1 minimum aspect ratio criteria). G29 and G49 could be potential bone candidates, but without seeing p-v \textit{p-v}  diagrams of the selected filaments (overlaid on log-spiral p-v \textit{p-v}  fits to the Milky Way's arms) it is difficult to determine whether they would satisfy our criterion 4 (within 10 km/s of the global-log spiral fit to any Milky Way arm) and criterion 5 (no abrupt shifts in velocity, of more than 3 km/s per 10 pc, within extinction feature). Though Wang et al. (2015) do say that that 5/9 of the Herschel filaments are associated with the \citet{Vallee_2008} arm models in p-v space, without a precise definition of "association" and an explanation of how filaments are plotted in p-v space, it is difficult to ascertain to what extent these filaments satisfy our criterion 5. Additionally, though Wang et al. (2014) do find velocity coherence along their filaments, they cite that, in certain clumps, the velocity gradients are much larger, creating the possibility that they might exceed our criterion 5. In future studies, we plan to follow-up on G29 and G49 to determine a) the precise number of bone criteria they satisfy and b) their association with a spiral arm in p-v space, taking into account updated spiral p-v fits from \citet{Dame_2011}, \citet{Sanna_2014}, and Reid and Dame (2015), in prep. In judging whether a filamentary cloud lies within an arm, or is highly inclined to it, the velocity of the associated gas offers the most relevant evidence. Since Wang et al. (2015) only show the correlation between filaments and spiral structure in X-Y space using the \citet{Reid_2014} model, we will reanalyze the most promising filaments (G29 and G49) in longitude-velocity space to more accurately determine their correlation with a spiral arm. An in-depth analysis of the other nine bone candidates can be found in the appendix.