The Skeleton of the Milky Way (Fall 2014 Draft)

For the live open preprint of this paper (being updated and submitted Spring 2015), this new version.


Recently, Goodman et al. (2014) argued that a very long, very thin infrared dark cloud “Nessie” lies directly in the Galactic midplane and runs along the Scutum-Centaurus arm in position-position-velocity (p-p-v) space as traced by lower density \(\textrm{CO}\) and higher density \(\mathrm{NH_3}\) gas. Nessie was presented as the first “bone” of the Milky Way, an extraordinarily long, thin, high contrast filament that can be used to map our galaxy’s ”skeleton.“ We present the first evidence of additional bones in the Milky Way Galaxy, arguing that Nessie is not a curiosity but one of several filaments that could potentially trace Galactic structure. Our ten bone candidates are all long, filamentary, mid-infrared extinction features which lie parallel to, and no more than twenty parsecs from, the physical Galactic midplane. We use \(\textrm{CO}\), \(\mathrm{N_2H+}\), \(\textrm{HCO}^{+}\) and \(\mathrm{NH_3}\) radial velocity data to establish the location of the candidates in p-p-v space. Of the ten filaments, six candidates also have a projected aspect ratio of \(\ge 50\colon1\), run along, or extremely close to, the Scutum-Centaurus arm in p-p-v space, and exhibit no abrupt shifts in velocity. Evidence suggests that these candidates are Nessie-like filaments which mark the location of significant spiral features, with ”filament 5"


Over the past several decades, astronomers have begun to define the structure and kinematic properties of the Milky Way. Yet, despite a large conglomeration of literature on the subject, many key questions remain. For instance, how many spirals arms does the Milky Way have, cf. Vallée (2008)? What is the precise location of these arms in p-p-v space? And what is the nature of the muddled interarm structure—is it spur-like or otherwise? An understanding of the Milky Way’s three-dimensional structure has eluded us, largely due to the fact that we are embedded in the galaxy we are attempting to delineate.

Much of our current understanding of the Milky Way’s three-dimensional structure stems from radial velocity measurements of high and low density gas tracers. Making use of the Milky Way’s rotation curve (McClure-Griffiths 2007), we can translate line-of-sight velocities into a distance, thereby constructing a gross three-dimensional model of our galaxy. Thanks to a wealth of spectroscopic surveys, velocity-resolved observations are readily available for much of the Galaxy’s molecular, atomic, and ionized gas. Extended tracers, like CO (Dame et al., 2001) or HI (Shane, 1972) provide the best constraints on the Galaxy’s overall anatomy. To probe finer structure, observations of high-mass star forming regions can also provide kinematic and distance information for high-density gas. For instance, measurements of trigonometric parallaxes and proper motions of masers from the BeSSeL survey produced accurate locations for several spiral arm segments, along with their associated pitch angles (Reid et al., 2014). Likewise, the Bolocam Galactic Plane Survey (BGPS, Schlingman et al., 2011), the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90, Foster et al., 2011; Jackson et al., 2013), and the \(\textrm{H}_2\textrm{O}\) Southern Galactic Plane Survey (HOPS, Purcell et al., 2012; Walsh et al., 2011) have produced hundreds of high-spectral resolution velocity measurements of the dense gas in molecular clouds. Analyses of extinction data from surveys like Pan-STARRS1 complement this emission line data and can also be used to create three-dimensional models of the Galaxy’s structure (Green et al., 2014; Schlafly et al., 2014).

While the tools available for probing the Milky Way’s internal structure are diverse, none has especially high three-dimensional resolution over wide areas. To address this problem, Goodman et al. (2014) recently discovered that extraordinarily elongated filamentary infrared dark clouds, termed “bones” could be used to determine the structure of the Milky Way’s spiral arms, their location within the Galaxy, and their appearance as viewed by an outside observer. Goodman et al. (2014) presented Nessie as the first “bone” of the Milky Way. They found that Nessie was at least three degrees (162 pc), and possibly as long as eight degrees (431 pc) in length, while being less than 0.1 deg (0.3 pc) wide. They also conclude that Nessie lies within the Galactic mid-plane of the Milky Way Galaxy, at the 3.1 kpc distance to the Scutum-Centaurus arm. An analysis of the radial velocities of \({\rm NH}_3\) emission and CO emission confirms that Nessie runs along the Scutum-Centaurus arm in p-p-v space, suggesting it forms a dense spine of that arm in physical space as well (Goodman et al., 2014)

Until very recently, no simulations had the spatial resolution to predict that super-dense filaments should trace the middle of spiral arms. However, a new numerical simulation from Goodman et al. (2014)—using the AREPO moving mesh code outlined in Smith et al. (2014)—shows dense filaments, with aspect ratios and column densities similar to Nessie, forming within and parallel to the mean plane of a simulated spiral galaxy. A detailed analysis of Nessie’s properties, along with these new simulation results, suggests that Nessie may be the first in a class of objects that could trace our Galaxy’s densest spiral features. It is gratifying to recognize that Nessie should be the easiest object of its kind to find. Nessie is located in the closest major spiral arm to the the Sun, perpendicular to our line of sight yet slightly offset from the Galactic center. This placement means that Nessie will be clearly visible against the bright background of the Galactic center, and it will appear more elongated than objects more distant or tangential to our line-of-sight. Making use of both large scale mid-infrared surveys and molecular spectral line maps, we search for harder-to-find bones, characterize their key properties, and begin to establish their relationship to the Milky Way’s spiral structure. When used in conjunction with the kinematic, parallactic, and extinction data outlined above, these new bones have the potential to pin down the Milky Way’s galactic structure, improving the level of detail from tens of parsecs to around one parsec in regions near bones.