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 , ; 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 ( { 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 .