Cara Battersby a trace of the mid-IR extinction feature of filament 5  about 9 years ago

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\section{Analysis of New Bones}  Of the six newly-identified bones, filament 5 is the most Nessie-like, in that it is highly elongated (.8 degrees or 52 pc) and \textit{exactly} along a previously-claimed spiral arm trace in p-p-v space, although its orientation makes it less elongated than Nessie on the sky. We show a close-up shot of the p-v diagram for filament 5 in figure \ref{fig:Candid5_pos_vel}. Like Nessie, filament 5 also forms a spine of the Scutum arm as traced by lower density CO gas (figure \ref{fig:Candid5_pos_pos}). By overlaying amid-IR  trace of the mid-IR extinction feature of  filament 5 on a plane of the sky map (integrated in Scutum's velocity range in the region around filament 5) we see that filament 5 lies in the center of the most intense CO emission. Finally, figure \ref{fig:Candid5_with_tilt} shows that filament 5 lies within $\approx$ 10 pc of the true physical mid-plane. All these figures taken together indicate that filament 5 is Nessie's counterpart in the first quadrant, suggesting that Nessie is not a curiosity, but one of several bones that trace significant spiral features. We note that filament 5 is the same IRDC identified as GMF 20.0-17.9 in a \citet{Ragan_2014} study of Giant Molecular Filaments (GMFs). \citet{Ragan_2014} undertook a blind search (not restricted to latitudes where the mid-plane should lie) for long thin filaments in the first quadrant of the Milky Way, using near and mid-infrared images. In addition to confirming that Nessie lies along the Scutum arm, \citet{Ragan_2014} find seven GMFs, of which one (GMF 20.0-17.9, our Filament 5) is said to be a spur of the Scutum-Centaurus arm. Unlike \citet{Ragan_2014} we argue that filament 5 runs right down the spine of the Scutum-Centaurus arm in p-v space. We believe the discrepancy arises due to a difference in methodology. \citet{Ragan_2014} group neighboring IRDCs into a single filament, while we determine that there is a significant kink in velocity structure associated with a dramatic plane-of-the-sky bend at a longitude of $\approx 18.5^{\circ}$. Since grouping both IRDCs to make a longer structure violates our criterion 5, we only consider the kinematically coherent part of the filament (yellow boxed region in figure \ref{fig:Candid5_with_tilt}), which is remarkably parallel to the Scutum arm in p-v space. Likewise, in figure 6 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. Thus, a clear and consistent description of a bone is critical, and, in future studies, we plan to continue to apply the criteria like the ones listed above to achieve consistency.