The Bones of the Milky Way

Instructions for Co-Authors

The full file repository for this paper is at a shared Google Drive directory,, shared with all co-authors.

NOTE: The “aas” (press conference) slides at give a better idea of where this draft is going than the text/figures here as of now... AG will update all by c.1/1/13!

The Mendeley Library “Nessie and Friends” used to house references used in this work, at:, but since Authorea works more directly with ADS links, we’ll use the ADS Private Library at instead. The Mendeley library is the source of the nessie.bib file in the “Bibliography” folder here on Authorea, but I am not sure how to get the ADS references out as a .bib file. xxAlberto?xx

The Glue software used to intercompare data sets used in this work is online through:

We are using as an experimental platform to compile this paper. The manual steps we will need to take before submission include:

  • download LaTeX file

  • modify LaTeX file to use aas macros

  • insert needed information (e.g. about authors, running header) into was version of LaTeX manuscript

  • extract needed figures from relevant folders here & bundle them with LaTeX manuscript & macros

  • create .bib file from ADS Private Library

  • add .bib file to folder with manuscript & figures

  • fix in-line referencing so that \(\citet\) and \(\citep\) commands work


Determining the structure of the Milky Way, from our vantage point within it, is a perpetual challenge for astronomers. We know the Galaxy has spiral arms, but it remains unclear exactly how many xxTD favorite referencexx. Recent observations of maser proper motions give unprecedented accuracy in determining the three-dimensional position of the Galaxy’s center and rotation speed (Brunthaler 2011). But, to date, we still do not have a definitive picture of the Milky Way’s three dimensional structure.

The analysis offered in this paper suggests that some infrared dark clouds–in particular very long, very dark, IRDCs–appear to delineate the major features of our Galaxy as would be seen from outside of it. In particular, we study a \(>3^{\circ}\)-long cloud associated with the IRDC called “Nessie"(Jackson 2010) and we show that it appears to lie parallel to, and no more than just few pc from, the true Galactic Plane.

Our analysis uses diverse data sets, but it hinges on a modern understanding of the meaning of Galactic coordinates. When, in 1959, the IAU established the current system of Galactic \((l,b)\) coordinates (Blaauw 1959), the positions of the Sun with respect to the “true" Galactic disk, and of the Galactic Center, were not as well determined as they are now. As a result, the Galactic Plane is typically not at \(b=0\), as projected onto the sky. The exact offset from \(b=0\) depends on distance, as we explain in §xx. Taking these offsets into account, one can profitably re-examine data relevant to the Milky Way’s 3D structure.

To date, much of the study of IRDCs has focused on their star-forming properties xxAG put ref to review paper herexx. IRDCs’ very high mean densities make them analogous to Orion, the (only) nearby region forming O stars, so the IRDC regions and the bright, massive, stars they can form, are what observers of the Milky Way from beyond it would see as the predominant mode of star formation here. Thus, if one can deduce the pattern of IRDCs that an observer outside the Milky Way would see, one can determine the Milky Way’s structure, from inside.

The traditional ISM-based probes of the Milky Way’s structure have been HI and CO. Emission in these tracers gives line intensity as a function of velocity, so the position-position-velocity data resulting from HI and CO observations give three dimensional views of the Galaxy, if a rotation curve is used to translate line-of-sight velocity into a distance. Unfortunately, though, the Galaxy is filled with HI and CO, so it is very hard to disentangle features when they overlap in velocity along the line of sight, especially with low spatial-resolution data, which typifies all-sky surveys. Nonetheless, much of the basic understanding of the Milky Way’s spiral structure we have now comes from HI and CO observations, and most notably from the compilation of CO data presented in Dame xx2001xx.

Recently, several groups have targeted high-mass star-forming regions within our Galaxy for high-resolution ISM observations. In their BeSSeL Survey, xxReid et al. are using hundreds of hours of VLBA time to observe hundreds of regions for maser emission, which can give both distance and kinematic information for very high-density gas (n>xx). In the HOPS Survey, hundreds of positions associated with the dense peaks of infrared dark clouds have now been surveyed for NH3 and xx emission, which yields high-spectral resolution velocity measurements towards gas whose density typically exceeds 10^4 particles/cc. In follow up spectral-line surveys to the ATLASGAL dust-based survey of the Galactic Plane xx, xxx has measured NH3 emission in xx locations. The ThrUMMs Survey is targetting xx regions in the xx quadrant with linesxx, which also holds the potential for more high-resolution velocity measurements.

Many of these high-resolution velocity studies are based on continuum surveys, which show the locations of the highest column-density regions, either as extinction features (“dark clouds” in the optical, see xx; “IRDCs” in the infrared) or as emission features (e.g. as seen in new WISE and Herschel surveysxx).

Great power lies in the careful combination of continuum and spectral-line data when one wants to understand the structure of the ISM in three-dimensions. Thus, there have already been several efforts to combine IRDC maps with spectral-line data. The goal of those studies has typically been distance determination for entire clouds or regions, so as to better understand the conversion of measured quantities (e.g. fluxes) into physical ones (e.g. mass). xx refxx

In this study, our aim is to combine morphological information from large-scale continuum maps of the Galactic Plane with both low-resolution CO maps and high-spectral resolution maps of dense gas, so as to understand the nature of very long infrared dark clouds that appear parallel to the Galactic Plane. We focus in particular on the cloud named “Nessie. ” Jackson et al. 2008xx find that the filamentary cloud (see Figure 1) was a least 1 degree longxx, and that it exhibits the after-effects of a sausage instability which led to several massive-star-forming peaks forming along the filament regular intervals. Here, we extend the work of Jackson et al. by first (Sectionxx) literally extending the cloud, to a length of at least 3 degrees. In Section xx, we show that a careful accounting for the modern measures of the Sun’s height off of the Galactic mid-plane, and of the true (not l,b=0,0) position of the Galactic Center, means that Nessie lies not just parallel to the Galactic Plane, but in the Galactic Plane. In Section xx, we consider what velocity-resolved measures of the material associated with Nessie tells us about its three-dimensional position in the Galaxy, and we conclude that Nessie likely marks the “spine” of the Scutum-Centaurus arm of the Milky Way in which it lies. In Section xx, we consider, in the light of future computational and observational capabilities, the likelihood of finding more “Nessie-like” structures in the future, and of using them to map out the skeleton of our Milky Way.

Nessie is Longer than We Thought

\label{fig:NessieonSky}: Nessie “Classic,” “Extended,” and “Optimistic”

Nessie was named and discovered in the 2010 paper by Jackson et al., using Spitzer images that show the cloud as a very clear absorption feature at mid-infrared wavelengths (Jackson 2010). The column densities for the mid-IR absorption features known as “Infrared Dark Clouds” or “IRDCs” are typically between \(10^{23}\) to \(10^{25}\) \({\rm cm}^{-3}\). Using observations of the dense-gas tracer HCN, Jackson et al. show that the section of the cloud from \(l=337.85\) to \(339.1\) (\(\sim\)“Nessie Classic” in Figure 1) exhibits very similar line-of-sight velocities, ranging from \(v_{LSR}-40\) km s\(^{-1}\) to \(-36\) km s\(^{-1}\). The similarities of these line-of-sight velocities is taken to mean that the cloud is a coherent, long, structure, and not a chance plane-of-the-sky projection of disconnected features. Thus, “Nessie Classic” is taken to be a \(\sim 1^\circ\) long very narrow (\(<0.1^\circ\)) filament, and Jackson et al. (2010) conclude that it is undergoing a sausage instability leading to density peaks hosting active sites of massive star formation.

Our purpose in looking at Nessie again here is not to analyze the star formation taking place in the cloud. Instead, we want to reconsider just how long the full Nessie feature might be. Casual inspection of Spitzer imagery given in Figure 1 suggests that Nessie is at least \(3^\circ\) long (see “Nessie-Extended” labeling), nearly 3 times longer than originally claimed in Jackson et al. (2010). And, very careful inspection (see xxinteractive figure1?xx) of the Spitzer images suggests that Nessie could be even longer, as much as \(9^\circ\) long, if one optimistically connects what appear to be all the relevant pieces (light white “chalk” line in Figure 1).

Determining the physical, three-dimensional, nature of extensions to the Nessie cloud requires a detailed analysis of the velocity of the gas associated with the dust causing the mid-IR extinction observed. We offer such an analysis in the following Section, but here we note that if Nessie lies (as is nearly certain given its velocity range) in or near the Scutum-Centaurus Arm of the Milky Way, then its distance is roughly 3.1 kpc (cf. Jackson et al. 2010). At that distance, “Nessie Classic” is roughly 80 pc long, Nessie “Extended” is 160 pc long, and Nessie “Optimistic” is 430 pc long. For any length, the dark filament’s width is of order 0.01 degrees (or less). Thus, clouds’s axial ratio is 150 for Nessie Classic, 300 for Nessie-Extended, and nearly three times more, 800, for Nessie-Optimisitc. (These calculations are based on Table 1, a publicly-available interactive spreadsheet, at

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Looking “Down” on the Galaxy

\label{fig:galplane}: Schematic side-view of the tilt of the “True" Galactic Plane with respect to the IAU-defined plane. The drawing is not to scale. The offset of the Sun above the True Galactic Plane, and of the Galactic Center (Sgr A*) below the IAU-defined plane, conspire to tilt the IAU system with respect to the True Plane (in this simplified view) by roughly \(0.13^\circ\), meaning that the red and blue lines shown here cross at a distance roughly 12 kpc from the Sun. Any feature nearer than that to the Sun will appear at negative \(b^{II}\), even if it lies exactly in the Galactic Plane. (Collaborators can edit this figure at

Radial Position and Orientation

This is an example of reference to Figure \ref{fig:temp}.

Figure 3: Predicted Radial Velocities for the Scutum-Centaurus Arm

3. What is the Three-Dimensional Position of Nessie within the Milky Way?

Height off the Plane

Writing in 1959 on behalf of IAU sub-commission 33b, Blaauw et al. wrote: “The equatorial plane of the new co-ordinate system must of necessity pass through the sun. It is a fortunate circumstance that, within the observational uncertainty, both the sun and Sagittarius A lie in the mean plane of the Galaxy as determined from hydrogen observations. If the sun had not been so placed, points in the mean plane would not lie on the galactic equator." Astronomers today are still using the \((l^{II}, b^{II})\) Galactic coordinate system defined by Blaauw et al. (1959), but is is not still the case, within observational uncertainty, that the Sun is in the mean plane of the Galaxy, and the true position of the Galactic Center is no longer at \((l^{II}=0, b^{II}=0)\).

Figure 4: Draft CO sky view

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Figure 5: Draft Spitzer Side-View, with HOPS colored sources

What is the Significance of Nessie-like structures within a Spiral Galaxy?

simulations/tidal effects (Andi’s simulations here)

What should look for now?

More examples, or counter examples mention value of context & wide-field imaging & tools that can make that easy

dark lanes in external galaxies (Jens’s comment about that...) MMFs from Battersby & Bally, (and any other relevant new work?)


This paper was a truly a group effort, and the author list includes only some of the many people who have contributed to it. The entire project was inspired by a question: “Is Nessie parallel to the Galactic Plane?,” asked by Andi Burkert at the 2012 “Early Phases of Star Formation” (EPoS) meeting at the Ringberg Castle in Bavaria. Two EPoS attendees beyond the author list contributed significant ideas and data to this work, most notably Steven Longmore and Eli Bressert. We are grateful to Cormac Purcell for giving us advance online access to the HOPS data, and to Mark Reid for generously sharing his expertise on Galactic structure. The text here was largely written by Alyssa Goodman; the theoretical ideas come primarily from Andi Burkert; and much of the geometrical analysis was carried out by Christopher Beaumont, Bob Benjamin, and Tom Robitaille. Tom Dame and Bob Benjamin provided expertise on Galactic structure, and the Dame provided the CO maps in Figure xx. xxMore on contributions will go here!xx

Facilities (check format, namesxx): , , .


  1. A. Brunthaler, M. J. Reid, K. M. Menten, X.-W. Zheng, A. Bartkiewicz, Y. K. Choi, T. Dame, K. Hachisuka, K. Immer, G. Moellenbrock, L. Moscadelli, K. L. J. Rygl, A. Sanna, M. Sato, Y. Wu, Y. Xu, B. Zhang. The Bar and Spiral Structure Legacy (BeSSeL) survey: Mapping the Milky Way with VLBI astrometry. Astronomische Nachrichten 332, 461 (2011). Link

  2. J. M. Jackson, S. C. Finn, E. T. Chambers, J. M. Rathborne, R. Simon. The “Nessie” Nebula: Cluster Formation in a Filamentary Infrared Dark Cloud. 719, L185-L189 (2010). Link

  3. A. Blaauw, C. S. Gum, J. L. Pawsey, G. Westerhout. Note: Definition of the New I.A.U. System of Galactic Co-Ordinates. 130, 702 (1959). Link

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