The Bones of the Milky Way
  The very long and thin infrared dark cloud "Nessie" is even longer than had
been previously claimed, and an analysis of its Galactic location suggests that
it lies directly in the Milky Way's mid-plane, tracing out a highly elongated
bone-like feature within the prominent Scutum-Centaurus spiral arm. Re-analysis
of mid-infrared imagery from the Spitzer Space Telescope shows that this IRDC
is at least 2, and possibly as many as 5 times longer than had originally been
claimed by Nessie's discoverers (Jackson et al. 2010); its aspect ratio is
therefore at least 300:1, and possibly as large as 800:1. A careful accounting
for both the Sun's offset from the Galactic plane ($\sim 25$ pc) and the
Galactic center's offset from the $(l^{II},b^{II})=(0,0)$ position shows that
the latitude of the true Galactic mid-plane at the 3.1 kpc distance to the
Scutum-Centaurus Arm is not $b=0$, but instead closer to $b=-0.4$, which is the
latitude of Nessie to within a few pc. An analysis of the radial velocities of
low-density (CO) and high-density (${\rm NH}_3$) gas associated with the Nessie
dust feature suggests that Nessie runs along the Scutum-Centaurus Arm in
position-position-velocity space, which means it likely forms a dense `spine'
of the arm in real space as well. The Scutum-Centaurus arm is the closest major
spiral arm to the Sun toward the inner Galaxy, and, at the longitude of Nessie,
it is almost perpendicular to our line of sight, making Nessie the easiest
feature to see as a shadow elongated along the Galactic Plane from our
location. Future high-resolution dust mapping and molecular line observations
of the harder-to-find Galactic "bones" should allow us to exploit the Sun's
position above the plane to gain a (very foreshortened) view "from above" of
the Milky Way's structure.
