Catherine Zucker edited sectionDiscussion_Th.tex  about 9 years ago

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In future, we plan to test and apply algorithms that "connect the dots" between markers of high density IRDC peaks, in a search for more skeletal features. Lenfestey, Fuller and Peretto (2014) (in prep) have recently undertaken such a study, utilizing an IRDC catalog of $\approx$ 11,000 high density peaks from \citet{Peretto_2009}. Lenfestey et al. have grouped these catalog objects into long filamentary structures, using a Minimum Spanning Tree (MST) algorithm, identifying 100 structures in the region $|l| < 65\degr, |b|<1\degr$. Of these structures, 22 are linear features similar to the Nessie nebula. We plan to investigate the Lenfestey filaments, as well as apply MST and related structure-finding procedures to additional surveys \citep[e.g. ATLASGAL or Hi-GAL,][]{Csengeri_2014,Molinari_2010}, applying our initial bone criteria to all candidates found, thereby producing a larger population of bones capable of pinning down galactic structure.   Along with increasing our bone population, we plan to improve simulations in hopes of answering key questions about bones' origin and evolution. For instance, what fraction of highly-elongated dense clouds appear to be a) aligned with arms b) spur-like c) inter-arm and d) random long thin clouds unaligned with Galactic structure? What are the likely origins of these types of objects, and do they have different properties (e.g. velocity and density profiles, mass per unit length)? We know that not all long skinny filaments are expected to be associated with galactic structure. Studies prior to \citet{Ragan_2014} offer at least two examples of long molecular clouds that are not obviously Bone-like. The ***INVALID BYTE SEQUENCE HERE***Massive Molecular Filament***INVALID BYTE SEQUENCE HERE*** G32.02+0.06, studied by \citet{Battersby_2014}, does not appear to be tracing an arm structure. Likewise, the 500-pc long molecular ***INVALID BYTE SEQUENCE HERE***wisp***INVALID BYTE SEQUENCE HERE*** discussed by \citet{li13} \citet{Li_2013}  also does not presently appear directly related to Galactic structure. Neither of these two clouds currently lies in any special position in p-p-v space. It is possible that these are Bone remnants, disrupted by feedback or Galactic shear, but, without better Galaxy modeling, it is very hard to speculate on what fractions of long thin clouds were formerly bones, are currently bones, or were never bones. While the \citet{Smith_2014} galaxy models are the first that provide high enough resolution to simulate our incredibly long and thin bones, they do not include stellar feedback nor magnetic fields---either of which could cause disruptions in the appearance of the simulated bone-like features. In the future, we hope to utilize more comprehensive, targeted high-resolution synthetic observations (e.g. of dust absorption and emission and of CO spectra), based on high-resolution simulations like the ones in \citet{smith14}. Finally, we hope to use simulations to estimate the biases inherent in our selection criteria (how many spurious "Bones" should we expect to find randomly, by the chance alignment of discontinuous IRDC peaks?).