Catherine Zucker edited sectionAnalysis_of_N.tex  almost 9 years ago

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\citet{Ragan_2014} find little or no association with their GMFs and Galactic structure, suggesting that we are perhaps 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. Our three bone candidates with an "A" quality rating (filaments 1,2, and 5) all lie in the first quadrant, so we speculate that Galactic bones are not subject to the same fourth quadrant bias that GMFs are potentially prone to. We also emphasize that the GMFs from \citet{Ragan_2014} and our Galactic bones should be classified as fundamentally different objects. The lengths of the GMFs range from 60-230 pc, while our longest bone candidates is only 52 pc. By definition, GMFs are meant to be larger structures composed of several smaller, high-contrast elements, so no bone in itself will realistically be classified as a GMF. As is the case with filament 5, we expect that there will be significant overlap between the GMFs and Bone catalogs in the future, as our Galactic bones should be a subset of any spiral tracing GMFs yet to be discovered.   Like \citet{Ragan_2014}, Wang et al. (2015) \citet{Wang_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 $15^\circ < l < 56^\circ$. Filaments were initially identified using Hi-GAL 350 and 500 $\mu\rm{m}$ emission. Temperature and column density maps were created for each candidate, and those which exhibited systematically lower temperatures with respect to the background were selected. As in our study, \citet{Wang_2014} confirmed velocity contiguity by extracting a \textit{p-v} slice along the curvature of each filament. Wang et al. (2015) highlight nine filaments as their most prominent, with one of the nine being Nessie. Only one of their filaments (Filament 6, or BC\_011.13-0.12, the "snake") overlaps with our sample, and is not classified as a bone due to its short aspect ratio ($\approx 25:1$). Seven 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 (outside our $\pm 20$ pc criterion), while G28 and G65 have aspect ratios of around 19:1 and 38:1 (less than our 50:1 minimum aspect ratio criterion). Additionally, G29 and G49 are not largely continuous mid-infrared extinction features, violating our criterion 1. This is not surprising, as the \citet{Wang_2014} study was designed to identify filaments emitting at longer Hi-GAL wavelengths, which are not necessarily seen continuously in absorption at mid-infrared wavelengths. In future studies, \citet{Wang_2014} plan to extend their search to the entire Galactic plane. Despite differences in methodology, there should be some degree of overlap between the Wang et al. (2015) catalog and our bones catalog, and these, along with the \citet{Ragan_2014} catalog, are expected to be highly complementary of each other.