The Skeleton of the Milky Way

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 $\mathrm{CO}$ and higher density ${\mathrm{NH}}_3$ 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 $\mathrm{CO}$, ${\mathrm{N}}_2{\mathrm{H}}^{+}$, ${\mathrm{HCO}}^{+}$, and ${\mathrm{NH}}_3$ radial velocity data to establish the three-dimensional location of the candidates in $p-p-v$ 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 $p-p-v$ 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.

11 pages, 7 figures, plus appendix of bone candidates

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