The Skeleton of the Milky Way
  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.
