2015ApJ...815...23Z

Recently, Goodman et al. argued that the very long, very thin infrared dark cloud ''Nessie'' lies directly in the Galactic midplane and runs along the Scutum-Centaurus Arm in position-position-velocity (p-p-v) space as traced by lower-density CO and higher-density NH₃ 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, arguing that Nessie is not a curiosity but one of several filaments that could potentially trace Galactic structure. Our 10 bone candidates are all long, filamentary, mid-infrared extinction features that lie parallel to, and no more than 20 pc from, the physical Galactic mid-plane. We use CO, N₂H⁺, HCO⁺ and NH₃ radial velocity data to establish the three-dimensional location of the candidates in p-p-v space. Of the 10 candidates, 6 also have a projected aspect ratio of {#8805}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 mark 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.