From AAS NOVA: “Companions for “Nessie” in the Milky Way’s Skeleton”


American Astronomical Society

7 December 2015
Susanna Kohler

The long, thin, dark filament that runs across this infrared image from the Spitzer Space Telescope is “Nessie”, a bone of the Milky Way. Nessie is more than 300 light-years long, but only 1 or 2 light-years wide. Now researchers have found more bones like Nessie, which will help us to learn about the Milky Way’s structure. [NASA/JPL/SSC]

The recent discovery of a purported “bone” of the Milky Way, a dark cloud nicknamed “Nessie”, has provided us with new clues for mapping out the spiral structure of our galaxy. It turns out that Nessie may not be alone: a follow-up study has identified more bones, potentially making up a skeleton of the Milky Way that traces out the densest parts of its spiral arms.

Inconvenient Vantage Point

How many spiral arms does the Milky Way have? Where are they located? What does the structure look like between the arms? It may seem surprising that these fundamental questions don’t yet have clear answers. But because we’re stuck in the galaxy’s disk, we’re forced to piece together our understanding of the Milky Way’s structure based primarily on measurements of position and radial velocity of structures within the galactic plane.

Milky Way map
Proposed view of the Milky Way

The discovery of Nessie presents an intriguing new tool to identify the layout of the galaxy. Nessie is a very long, thin, infrared-dark filament that runs along the modeled position of the Scutum-Centaurus arm — and is believed therefore to trace the structure of the arm. In a new study led by Catherine Zucker (University of Virginia, Harvard-Smithsonian Center for Astrophysics), the authors have searched for additional bones like Nessie, hoping to use them to map out the skeleton of the Milky Way.

New Bones Discovered

In this map of radial velocity vs. galactic longitude, the bone candidates are indicated by the numbered points. The colored lines indicate the positions of two of the galactic spiral arms, according to various models.[Zucker et al. 2015]

Zucker and collaborators began by using World Wide Telescope, a tool that facilitates visualization of multiple layers of data at a variety of scales, to search through Spitzer infrared data for additional structures like Nessie.

NASA Spitzer Telescope

Searching specifically along the predicted positions of galactic arms, they found 15 initial bone candidates.

Next, the team obtained radial-velocity data for the candidates from five separate radio surveys. Five of the candidates did not have radial velocities consistent with the galactic rotation curve at the predicted positions of the nearby arms.

Hope for a Skeleton

The authors used the remaining ten candidates to construct rough criteria for an object to be a Milky Way “bone”:

Largely continuous mid-infrared extinction feature
Parallel to the galactic plane, to within 30°
Position within 20 pc of the galactic mid-plane
Radial velocity within 10 km/s of the predicted velocity of a Milky Way arm
No abrupt shifts in velocity within the extinction feature
Projected aspect ratio of ≥50:1

Of the ten candidates, six meet all the criteria and are thought to mark the location of significant spiral features in the galaxy. The authors believe that this method may be used to identify hundreds of Milky Way bones in the future. Combining this skeleton with other tracers of galactic structure will ultimately help us to piece together a more accurate map of our galaxy.

Check out this video, produced by the authors using World Wide Telescope, that shows the locations of the newly discovered bone candidates within Spitzer images of the Milky Way galactic plane. [Credit: Zucker et al. 2015]

download mp4 video here.


Catherine Zucker et al 2015 ApJ 815 23. doi:10.1088/0004-637X/815/1/23

See the full article here .

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