The Skeleton of the Milky Way

TL;DR

This paper presents evidence for multiple filamentary structures in the Milky Way that act as 'bones', tracing the Galaxy's spiral arms and structure, expanding on Nessie's role as a galactic skeleton marker.

Contribution

It identifies ten new filamentary 'bones' in the Milky Way, demonstrating their potential to map Galactic structure beyond Nessie.

Findings

Six filaments have aspect ratios ≥50:1

Six align with the Scutum-Centaurus arm in p-p-v space

Filament 5 closely resembles Nessie in the Northern Sky

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.