The Bones of the Milky Way

TL;DR

This paper reveals that the infrared dark cloud Nessie is an extremely elongated structure tracing the Milky Way's Scutum-Centaurus arm, providing insights into Galactic structure and the potential for future detailed mapping of Galactic 'bones.'

Contribution

It demonstrates Nessie's true length, confirms its alignment with the spiral arm, and refines its Galactic position, highlighting its role as a Galactic 'bone' and proposing future high-resolution studies.

Findings

Nessie's length is 2 to 5 times longer than previously thought.

Nessie aligns with the Scutum-Centaurus spiral arm in position-velocity space.

Nessie's aspect ratio is at least 300:1, possibly up to 800:1.

Abstract

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.