There are potentially thousands of bone-like filaments observable in the Milky Way. If we find enough of them, we can piece them together to delineate the major structural features of our galaxy. Astronomers have been trying to accurately model the location of the spiral arms in position-velocity space for decades. A bones-based approach should be able to resolve some of the discrepancies amongst the many arm models shown in figure \ref{fig:skeleton}. The level of disagreement on arm locations is large enough that finding even just a handful of Nessie-like structures marking sections of the spines of spiral arms will tie arm fits down with high fidelity at particular positions in p-p-v space. These “spinal” anchors will have especially large weights in statistical fits that seek to combine many measures of the Milky Way’s skeletal structure.
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 \(|l| < 65 ^{\circ}, |b|<1^{\circ}\). 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 “Massive Molecular Filament” G32.02+0.06, studied by \citet{Battersby_2014}, does not appear to be tracing an arm structure. Likewise, the 500-pc long molecular “wisp” discussed by \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{Smith_2014}. 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?).
Though challenging, we plan to combine future high resolution synthetic observations with a wealth of existing data sets to build a skeletal model of the Milky Way. When used in conjunction with BeSSeL maser-based rotation curves \citep{Reid_2014}, CO \citep{Dame_2001} and HI \citep{Shane_1972} p-v fitting, 3D extinction mapping \citep{Schlafly_2014b}, HII region arm mapping \citep{Anderson_2012}, and GAIA results, bones have the potential to not only redefine Galactic structure at unprecedented resolution, but also to resolve fundamental questions that have been plaguing Galactic astronomers for decades.