Catherine Zucker edited sectionMethdology_To.tex  about 9 years ago

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Next, we ensure that the candidates are contiguous in velocity space as traced by high-density emission from the HOPS, MALT90, and BGPS surveys. In cases where HOPS, MALT90, and BGPS catalog data are not available along the extinction feature, we also extract spectra from GRS (high resolution $^{13}$CO (1-0) data) and MALT90 \textit{p-p-v} cubes using the spectrum extracter tool in Glue. We once again link spectral \textit{p-p-v} cubes from the GRS survey with GLIMPSE-Spitzer mid-infrared images and use the spectrum-extractor tool to obtain velocities along different regions of the extinction feature; a demonstration of the procedure used to extract velocities in Glue is shown in the appendix. Since CO traces lower density gas ($\approx10^2 \textrm{ cm}^{-3}$) and $\mathrm{N_2H+}$, $\textrm{HCO}^+$, and $\textrm{NH}_3$ trace high density gas ($\approx10^4 \textrm{ cm}^{-3}$), the dense gas sources provide much stronger evidence for the velocity of cold, dense, filamentary IRDCs. However, where dense gas sources are not available, the complete and unbiased high resolution GRS survey, although less desirable, allows us to roughly gauge the velocity along entire lengths of filaments.   By overlaying the HOPS, MALT90, BGPS, and GRS determined velocities on a p-v diagram of CO emission, we establish whether these filaments are associated with an existing spiral arm trace. For this study, we use the whole-galaxy \citet{Dame_2001} CO survey to locate each of the arms in \textit{p-p-v} space and determine whether these filaments are consistent with global-log fits to CO for various spiral arms. Of the approximately fifteen candidates identified visually, ten of these candidates are within 10 km/s of the Scutum-Centaurus and Norma-Cygnus arms. We plot these ten candidates in \textit{p-p-v} space, as shown in figure \ref{fig:skeleton}. In addition to showing our Bone candidates, we show several different predictions of the positions of two spiral arms toward the inner Galaxy in longitude-velocity space, from \citet{Dame_2011}, \citet{Sanna_2014}, \citet{Shane_1972}, and \citet{Vallee_2008}. We also include Scutum-Centaurus and Norma fits from Reid and Dame 2015 (in prep), derived from trigonometric parallax measurements of high-mass star forming regions taken as part of the BeSSeL survey \citep{Reid_2014}. Reid and Dame (2015), in prep, produce fits with (l,b,v) loci that closely follow GMCs that trace the arms, producing a rough log-spiral approximation determined by trigonometric parallax rather than an assumed galactic rotation curve. After narrowing down our list to ten filaments with kinematic structure consistent with existing spiral arm models, we develop a set of quantitative criteria for objects to be called "bones:" \begin{enumerate}  \item{Largely continuous mid-infrared extinction feature}