Abstract
Recently, Goodman et al. (2014) argued that the very long, very thin infrared dark cloud "Nessie" lies directly in the Galactic mid-plane and runs along the Scutum-Centaurus arm in position-position-velocity ($p-p-v$) space as traced by lower density $\rm {CO}$ and higher density ${\rm NH}_3$ gas. Nessie was presented as the first "bone" of the Milky Way, an extraordinarily long, thin, high-contrast filament that can be used to map our Galaxy's "skeleton." Here, we present evidence for additional bones in the Milky Way Galaxy, arguing that Nessie is not a curiosity but one of several filaments that could potentially trace Galactic structure. Our ten bone candidates are all long, filamentary, mid-infrared extinction features which lie parallel to, and no more than 20 pc from, the physical Galactic mid-plane. We use $\rm {CO}$, ${\rm N}_2{\rm H}^+$, $\rm {HCO}^+$, and ${\rm NH}_3$ radial velocity data to establish the three-dimensional location of the candidates in ${\it p-p-v}$ space. Of the ten candidates, six also: have a projected aspect ratio of $\geqq$50:1; run along, or extremely close to, the Scutum-Centaurus arm in ${\it p-p-v}$ space; and exhibit no abrupt shifts in velocity. The evidence presented here suggests that these candidates are marking the locations of significant spiral features, with the bone called filament 5 ("BC_18.88-0.09") being a close analog to Nessie in the Northern Sky. As molecular spectral-line and extinction maps cover more of the sky at increasing resolution and sensitivity, it should be possible to find more bones in future studies.
Highlights
Many surprisingly fundamental questions remain about the structure of the Milky Way
They do identify the Carina, Perseus and Local arms as well-defined spiral segments, suggesting that the Milky Way is a grand design spiral composed of several prominent sub-features
We show the results of extracting a slice along the red path in the upper panel, which runs through the Ragan et al (2014) infrared dark clouds (IRDCs) and our filament 5
Summary
Many surprisingly fundamental questions remain about the structure of the Milky Way. For instance, does the Milky Way have two (Jackson et al 2008; Francis & Anderson 2009; Dobbs & Burkert 2012) or four (Reid et al 2009; Bobylev & Bajkova 2014; Urquhart et al 2013) major spiral arms? What is the precise location of these arms in position-position-velocity (p-p-v ) space? What is the nature of inter-arm structure—is it made of well-defined spurs or more web-like structures? Does it even make sense to describe the Milky Way as a grand design spiral, and is it fruitful to count the number of spiral arms or prescribe terms such as “log-spiral”, “spur”, “feather” or “pitch angle” to structure whose detailed nature is not yet known? put, the spiral structure of the Milky Way is far from solved, and an understanding of its true structure continues to elude us, largely due to the fact that we are embedded in the galaxy we are attempting to delineate.To supplement existing extinction maps and observations of cold gas tracers and high-mass star forming regions, observations of star-forming complexes, young open clusters, embedded clusters, and Classical Cepheids can constrain the substructure of the Milky Way. What is the nature of inter-arm structure—is it made of well-defined spurs or more web-like structures? Does it even make sense to describe the Milky Way as a grand design spiral, and is it fruitful to count the number of spiral arms or prescribe terms such as “log-spiral”, “spur”, “feather” or “pitch angle” to structure whose detailed nature is not yet known? When taking into account error bars, they do not find any visual evidence of large-scale spiral structure. They do identify the Carina, Perseus and Local arms as well-defined spiral segments, suggesting that the Milky Way is a grand design spiral composed of several prominent sub-features
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