Day 715

Monday.

1408.0001
The bones of the Milky Way
Goodman, et al

The very long and think IR dark cloud "Nessie" is longer than previously claimed, and an analysis of its Galactic location suggests that it lies directly in the MW's mid-plane, tracing out a highly elongated bone-like feature within the prominent Scutum-Centaurus spiral arm.  Re-analysis of MIR imagery from Spitzer show that this IRDC is at least 2, and possibly as many as 5 times longer than had originally been claimed by Nessie's discoverers; 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 form the Galactic plane (~25 pc) and the Galactic center's offset from the (l,b)^H=(0,0) position shows that the latitude of the true Galactic mid-lane 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 (NH3) 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 the 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 MW's structure.