Catherine Zucker edited sectionAnalysis_of_N.tex  almost 9 years ago

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Our study is not the first follow up to the Nessie work in \citet{Goodman_2014} to look for more long filaments associated with spiral structure. \citet{Ragan_2014} and Wang et al. (2015) have undertaken similar studies. However, ours is the first study to specifically look for bones in regions we are most likely to find them, that is, elongated along the Galactic plane. Moreover, ours is the only study to create a quantitative set of criteria capable of defining this new class of objects (i.e. galactic "bones").   \citet{Ragan_2014} undertook a blind search ({\it not} restricted to latitudes where the mid-plane should lie) for long thin filaments (> $1^\circ$) in the first quadrant of the Milky Way, using near and mid-infrared images. In addition to confirming that Nessie lies along the Scutum-Centaurus arm, \citet{Ragan_2014} find seven Giant Molecular Filaments (GMFs) of which only one, GMF 20.0-17.9, is said to be associated with Galactic structure (declared a spur of the Scutum-Centaurus arm). Our strongest bone candidate, filament 5, is a subsection of GMF 20.0-17.9, but, unlike \citet{Ragan_2014}, we argue that filament 5 runs right down the spine of the Scutum-Centaurus arm in \texit{p-v} space. We believe the discrepancy arises due to a difference in methodology. \citet{Ragan_2014} group neighboring IRDCs into a single filament, despite breaks in the extinction feature and kinks in velocity structure. Since grouping several IRDCs to make a longer structure violates our criteria 1 and 6, we only consider the continuous and kinematically coherent part of the filament, which is remarkably parallel to the Scutum-Centaurus arm in \textit{p-v} space. Likewise, in figure 4 from \citet{Ragan_2014} (analogous to our figure \ref{fig:skeleton}), they represent filaments as straight lines connecting velocities measured at the tips of the filaments while we represent filaments as sets of points whose velocities are determined by the BGPS, HOPS, MALT90, and GRS surveys. We compare our p-p and \textit{p-v} analysis of BC\_18.88-0.08 filament 5  with the analysis from \citet{Ragan_2014} in figure \ref{fig:ragan_comp}. \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.