Catherine Zucker edited sectionMethdology_To.tex  about 9 years ago

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\section{Methodology}  To search for more bones, we visually inspect regions (|$l$|\textless 30$^{\circ}$, |$b$|\textless 1$^{\circ}$) where arms are predicted to lie according to our current understanding of the Milky Way's structure; the expected ($l,b,v$) paths of the Galactic arms are calculated using a log-spiral approximation as described in recent literature \cite{Dame_2011,Vallee_2008}. The predicted positions of the Galactic arms (Scutum-Centaurus, Carina-Sagittarius, Norma-Cygnus, and Perseus) are overlaid on three-color Spitzer GLIMPSE \citep{Benjamin_2003,Churchwell_2009} images in World Wide Telescope (WWT)---a tool that facilitates easy visualization of several layers of data at scales from the full sky down to the highest-resolution details. As part of our initial criteria, we search for long, largely continuous, filamentary mid-infrared extinction features that are near and roughly parallel to the Galactic mid-plane. This initial inspection yielded about fifteen Bone candidates (a video showing how this search worked in WWT is available \href{http://tinyurl.com/morenessies}{on YouTube}, and the original WWT Tour, of which the video shows a capture, is available at the \href{http://dx.doi.org/10.7910/DVN/29934}{Bones of the Milky Way Dataverse}).  For features that appear associated with spiral arms on the 2-D plane of the sky, radial velocity data is needed to establish whether 3-D association with a spiral feature is likely. The filament must have similar line-of-sight velocities along its length. Moreover, the measured radial velocities should be very close to those predicted by the Milky Way's rotation curve for arms at a known distance. To probe the velocity structure of these fifteen filaments, we employ radial velocity data from five separate radio surveys: HOPS \citep{Purcell_2012,Walsh_2011}, MALT90 \citep{Foster_2011,Jackson_2013}, BGPSspectral line follow-up  \cite{Schlingman_2011,Shirley_2013,Ellsworth_Bowers_2013}, GRS \citep{Jackson_2006} and ThrUMMS \cite{Barnes_2011}. The HOPS, MALT90, and BGPS surveys are all geared towards probing dense regions hosting the early stages of high mass star formation. We utilize $\textrm{NH}_3$ emission from HOPS, $\mathrm{N_2H^{+}}$ from MALT90, and $\textrm{HCO}^{+}$ from BGPS. All three of these thermal emission lines trace dense molecular gas ($\approx10^{4}\textrm{ cm}^{-3}$) and are often found in dense, cool clouds with temperatures less than 100 K \citep{Purcell_2012,Shirley_2013}. As infrared dark clouds tend to harbor cool, high density clumps of gas which fuel the formation of massive stars, all three of these data sets contain spectra for hundreds of regions within the longitude range of the potential Galactic bones. We investigate the velocity structure of our filaments in two ways: first, whenever possible, we establish the velocity coherence of our candidates by performing a slice extraction along each filamentary extinction feature in \href{http://www.glueviz.org/en/stable/index.html}{Glue}, a visualization tool that facilitates the linking of data sets. We link spectral \textit{p-p-v} cubes from the GRS and ThrUMMS survey with GLIMPSE-Spitzer mid-infrared images and obtain velocity as a function of position along a path that traces the entire extinction feature. The results of the slice extraction along the path of one of our strongest bone candidates is shown in figure \ref{fig:filament5_slice}. We are able to establish velocity coherence for all candidates lying within the coverage range of the GRS survey ($ 18^\circ < l < 56^\circ$) and the ThrUMMS survey ($ 350^\circ < l < 358^\circ$).