WORKING DRAFT authorea.com/249

The Bones of the Milky Way

This is a preprint. The published article is available at the Astrophysical Journal (ApJ 797 53) (Goodman 2014). This online version, published in December 2012, is citable as an online “Authorea” preprint, and you can use the article’s URL to do that.

Abstract

ABSTRACT The very long, thin infrared dark cloud “

Introduction

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, cf. (Vallée, 2008). Recent observations of maser proper motions give unprecedented accuracy in determining the three-dimensional position of the Galaxy’s center and rotation speed (Reid et al., 2009; Brunthaler et al., 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 Clouds1–in particular very long, very dark, clouds–appear to delineate 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 “

Our analysis uses diverse data sets, but it hinges on combining those data sets with a modern understanding of the meaning of Galactic coordinates. When, in 1959, the IAU established the current system of Galactic $$(l,b)$$ coordinates (Blaauw et al., 1959), the positions of the Sun with respect to the “

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 can 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. Nonetheless, much of the basic understanding of the Milky Way’s spiral structure we have now comes from HI and CO observations of the Galaxy, much of it from the compilation of CO data presented by Dame et al. (2001).

Recently, several groups have targeted high-mass star-forming regions in the plane of the Milky Way for high-resolution observation. In their BeSSeL Survey, Reid 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 ($$n>10^8$$ cm$$^{-3}$$) gas (Reid et al., 2009; Brunthaler et al., 2011). In the HOPS Survey, hundreds of positions associated with the dense peaks of infrared dark clouds have now been surveyed for $${\rm NH}_3$$ emission (Purcell et al., 2012), yielding high-spectral resolution velocity measurements towards gas whose density typically exceeds $$10^4$$ cm$$^{-3}$$. In follow-up spectral-line surveys to the ATLASGAL (Beuther et al., 2012) dust-based survey of the Galactic Plane, Wienen et al. (2012) have measured $${\rm NH}_3$$ emission in nearly 1000 locations. The ThrUMMs Survey aims to map the entire fourth quadrant of the Milky Way in CO and higher-density tracers (Barnes et al., 2010), and it should yield additional high-resolution velocity measurements.

Targets in high-resolution (e.g. BeSSeL) studies are usually identified based on continuum surveys, which show the locations of the highest column-density regions, either as extinction features (“

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 dust maps with spectral-line data, whose goal is often the assignment of more accurate distances to particular clouds or regions e.g. (Foster et al., 2012). These improved distances allow for more reliable conversion of measured quantities (e.g. fluxes) to physical ones (e.g. mass).

In this study, our aim is to combine morphological information from large-scale mid-infrared continuum “

1. The term “

Nessie is Longer than We Thought

\label{longer}

Nessie was discovered and named using Spitzer Space Telescope images that show the cloud as a very clear absorption feature at mid-infrared wavelengths (Jackson 2010). Using observations of the dense-gas tracer HNC, Jackson et al. (2010) further show that the section of the cloud from $$l=337.85$$ to $$339.1$$ (labelled “

Our purpose in looking at Nessie again here is not to further analyze the star-forming nature of this cloud. Instead, our focus is on how long the full Nessie feature might be, and what its length and its three-dimensional position might imply about its role in the Galaxy. Casual inspection of Spitzer imagery given in Figure \ref{fig:FindingChart} suggests that Nessie is at least two or three times longer than “

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 responsible for mid-IR extinction. We offer such an analysis below (§\ref{3D}), but here we note that if Nessie (as is nearly certain given its velocity range) lies 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 of these lengths, the dark filament’s width is of order 0.01 degrees (0.5 pc), according to Jackson et al.’s (2010) analysis of the Spitzer imagery. Thus, clouds’s axial ratio is about 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 https://docs.google.com/spreadsheet/ccc?key=0AhIRxiTe1u6BdDlXOC10Zkd3WUNNZHVnRlhfeWhJYlE a snapshot of which is shown as Figure \ref{fig:table1}.)

\label{fig:FindingChart}. Nessie “

\label{fig:table1} Estimates for the density and mass of Nessie, under various assumptions about its length. The top set of values shows estimates for a cylinder with radius, length, and average density appropriate to the Spitzer “

The Three-Dimensional Position of Nessie within the Milky Way

\label{3D}

Looking “

\label{lookingdown} Astronomers would love to travel far beyond the Milky Way, so that we could observe its spiral pattern face-on, as we do for other galaxies. But our Sun is so entrenched in the Milky Way’s plane that an “

To understand why most astronomers do not yet consider the possibility or value of an overhead view, we need to consider the origin of our current Galactic coordinate system, and our current understanding of the Sun’s and the Galactic Center’s 3D positions. Writing in 1959 on behalf of the International Astronomical Union’s (IAU’s) 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.

In a further explanation of the IAU system in 1960, Blaauw et al. explain that stellar observations did, at that time, indicate the Sun to be at $$z_{\rm Sun}=22 \pm 2$$ (22 pc above the plane), but the authors then discount those observations as too dangerously affected by hard-to-correct-for extinction in and near the Galactic Plane (Blaauw et al., 1960). Instead, the 1959 IAU system relies on the 1950’s measurements of HI, which showed the Sun to be at $$z_{\rm Sun}=4\pm 12$$ pc off the Plane, consistent with the Sun being directly in the Plane ($$z_{\rm Sun}=0$$). Interestingly, since the 1950’s, the Milky Way’s HI layer has been shown to have corrugations on the scale of 10’s of pc (Malhotra, 1995), and there may be similar fluctuations in the mid-plane of the $${\rm H_2}$$ (Malhotra, 1994), so it is still tricky to use gas measurements to determine the Sun’s height off the plane.

Astronomers today are still using the $$(l^{II}, b^{II})$$ Galactic coordinate system defined by Blaauw et al. (1959), but it 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 $$(l^{II}=0, b^{II}=0)$$. Instead, a variety of lines of evidence (Chen et al., 2001; Maíz-Apellániz, 2001; Jurić et al., 2008) show that the Sun is approximately 25 pc above the stellar Galactic mid-plane, and VLBA proper motion observations of masers show that the Galactic Center is about 7 pc below where the $$(l^{II}, b^{II})$$ system would put it, at $$b=-0.046^\circ$$ (Reid et al., 2004). These offsets, as predicted by Blaauw et al., imply that “

Figure \ref{fig:galcoords} shows a schematic (not-to-scale) diagram of the effect of the Sun’s and the Galactic Center’s offsets from the mid-plane defined by the IAU in 1959 (and still in use as $$(l^{II}, b^{II})$$ today). The tilt of the the true, physical, Galactic mid-plane to the presently IAU-defined plane means that, within about 12 kpc of Sun1 any feature that is truly “

1. 12 kpc is the approximate distance where the physical and IAU planes cross, on a line toward the Galactic Center. Along other directions toward the Inner Galaxy, as shown in the lower panel of Figure \ref{fig:topview}, it will be further to the crossing point, and toward the Outer Galaxy, for a “

\label{fig:galcoords}. Schematic side-views of the of the physical mid-plane of the Galaxy with respect to the IAU-defined mid-plane. The drawings are not to scale. In both panels, $$z_{\rm Sun}$$ represents the height of the Sun above the Galactic mid-plane. In the upper panel, and in figures labeled “no tilt of plane” below, we consider only the Sun’s offset from the plane in calculating the observed coordinates of the true “physical” mid-plane. In the lower panel, and in figures labeled “with tilt” below, we also take the offset, $$z_{\rm SgrA*}$$, of the Galactic Center (Sgr A*) into account. For reference, if the distance to SgrA*, $$d_{\rm SgrA*}$$, is 8.5 kpc and $$z_{\rm Sun}$$=25 pc and $$z_{\rm SgrA*}=7$$ pc (based on $$b=-0.046^\circ$$ for SgrA*) then the angle by which the IAU mid-plane is tilted with respect to the physical plane is $$\theta_{\rm tilt}=0.12^\circ$$ and the distance from the Sun to where the two planes cross is $$d_{\rm crossing} =12$$ kpc.

Using Rotation Curves and Velocity Measurements to Place Nessie in 3D

Ever since velocity-resolved observations of stars and gas have been possible, astronomers have been modeling the rotation pattern of the Milky Way. Using a measured rotation curve for the Milky Way’s gas, (e.g. McClure‐Griffiths et al., 2007), one can translate observed LSR velocities to a unique distance in the Outer Galaxy, and to one of two possible (“

Combining a modern estimate for the Sun’s height above the plane ($$z_{\rm Sun}\sim 25$$ pc), with the IAU Galactic coordinate definitions, we can determine where the physical mid-Plane of the Galaxy should appear in the $$(l^{II}, b^{II})$$ system at any particular distance from the Sun. Figure \ref{fig:coloredlines} shows where the Scutum-Centaurus Arm would appear on the Sky (for a distance to SrgA* of 8.5 kpc, a rotation speed for the Milky Way of 220 km s$$^{-1}$$, and (U,V,W) motion for the Sun of 11.1, 12.4, and 7.2 km s$$^{-1}$$, respectively). As its caption explains in detail, Figure \ref{fig:coloredlines}’s colored lines are associated with the near part of the Scutum-Centaurus Arm. Two versions of this plane-of-the-Sky view are shown, one only accounting for the offset of the Sun, and the other also accounting for the tilt of the coordinate system caused by the Galactic Center also not lying in the IAU plane (see Figure \ref{fig:galcoords}).

The dashed colored lines in Figure \ref{fig:coloredlines}, indicating the predicted position of the Galactic Plane on the Sky at the distance to the near side of the Scutum-Centaurus Arm, pass almost directly through Nessie, regardless of whether or not one considers the “

\label{fig:topview}.For the fourth quadrant of the Milky Way, contours of constant LSR velocity of -30,-40,and-50 km s$$^{-1}$$ (using a rotation curve from McClure‐Griffiths et al., 2007) are superimposed on a cartoon model of the Milky Way. CO and dense gas observations (see below) give LSR velocities associated with Nessie (shown here as rainbow curve) near -40 km$$^{-1}$$, placing Nessie in the Scutum-Centaurus arm (highlighted in black), about 3.1 kpc from the Sun. Yellow-highlighted curves show positions in the Galaxy that would have the labeled value of Galactic Latitude ($$b$$) when viewed from the Sun. In the top panel, only the height of the Sun off the plane (taken to be 25 pc in this example) is considered in drawing the iso-b curves, and in the bottom panel, a 7 pc offset of the Galactic Center (see text), which causes an overall tilt of $$0.12^\circ$$ is also taken into account.

\label{fig:coloredlines}. Predicted Sky Position and Radial Velocities for the Scutum-Centaurus Arm of the Milky Way. In both panels, the colored lines are color-coded by velocity, given in the color bar at the top. The colored dashed line shows the predicted position for the Galactic Plane for the near side of the Scutum-Centaurus Arm (roughly 3.1 kpc from the Sun). The solid colored lines show $$\pm 20$$ pc from the mid-Plane at the same ($$\sim3.1$$ kpc) distance. These lines and colors are calculated using a model of a (flat) Milky Way described by the parameters shown below the color bar. Background imagery is from Spitzer, as in Figure \ref{fig:FindingChart}. A black dashed line emphasizes the position of $$b=0$$. As in Figure \ref{topview}: in the top panel, only a 25 pc offset of the Sun above is taken into account; and in the bottom panel, a 7 pc offset for the Galactic Center is also used in the calculation.

CO Velocities

\label{CO} CO observations trace gas with mean density around 100 cm$$^{-3}$$. CO emission associated with the Scutum-Centaurus Arm of the Milky Way (Dame et al., 2011)(Dame et al., 2001) is shown in Figure \ref{fig:COarm}, which presents a plane-of-the-sky map integrated over $$-50 <v_{LSR}< -30$$ km s$$^{-1}$$. The velocity range is centered on -40 km s$$^{-1}$$, the average velocity of the Scutum-Centaurus Arm in Nessie’s longitude range (see Figures \ref{fig:topview} and \ref{fig:coloredlines}). The white chalk line superimposed on Figure \ref{fig:COarm} is the same tracing of “

Judging by-eye vertical (latitude) centroid of the CO emission in Figure \ref{fig:COarm} appears to follow Nessie remarkably well, even out to the full $$8^\circ$$ (430 pc) extent of Nessie Optimistic. We have also calculated a curve representing the locus of latitude centroids for CO in this velocity range, and even at this coarse resolution, a curve following Nessie’s shape is clearly a better fit than a straight line passing through the CO centroids.

Table 1 estimates that the Nessie IRDC has a typical $${\rm H_2}$$ column density of $$\sim 10^{23}$$ cm$$^{-2}$$ and a typical volume density of $$\sim 10^5$$ cm$$^{-3}$$. Thus, the plane-of-the-sky coincidence of the line-of-sight-velocity-selected “

\label{fig:COarm}. CO emission, integrated between -50 and -30 km s$$^{-1}$$, as projected on the sky, based on data from (Dame et al., 2001), with trace of Nessie Optimistic (light white line) from Figure \ref{fig:FindingChart} superimposed. The dark black squiggle labeled “

NH$$_3$$ Velocities

\label{ammonia} To estimate the 3D orientation of Nessie more precisely, we need to employ a gas tracer whose emission is sparser than CO’s in position-position-velocity space. Many recent studies have shown that IRDCs typically host over-dense blobs of gas (often called “

The H$$_2$$O Southern Galactic Plane (HOPS) Survey (Purcell et al., 2012) has surveyed hundreds of sites of massive star formation visible from the Southern Hemisphere for $${\rm NH}_3$$ emission, which traces gas at densities $$n\gtrsim 10^4$$ cm$$^{-3}$$. The HOPS targets were selected based on H$$_2$$O maser emission, thermal molecular emission, and radio recombination lines, so as to include nearly all known regions of massive star formation within the surveyed area. These “

Figure \ref{fig:HOPSoverlay} shows an overlay of HOPS sources’ $${\rm NH}_3$$-determined LSR velocities on the Spitzer image of Nessie used in Figure \ref{fig:coloredlines}. The (color-coded) velocities of the HOPS sources, for both Nessie Classic, and Nessie Extended (see Figure \ref{fig:FindingChart}), agree remarkably well with what is predicted for the Scutum-Centaurus Arm (color-coded lines). Note that agreement of the $${\rm NH}_3$$ and predicted velocity to within 5 km s$$^{-1}$$ is indicated by light-colored circles around the HOPS symbol (see caption for details). White circles correspond to the Nessie Extended sources also shown in Figure \ref{fig:pvdiagram}, below, and grey circles mark other points, in Nessie Optimistic, also likely (based on their velocity) to be associated with the Scutum-Centaurus arm. The velocities of sources at latitudes much different from Nessie’s within this longitude range largely do not agree, and those sources are unlikely to associated with the near-side of the Scutum-Centaurus Arm.

For Nessie Classic, Jackson et al. (2010) had already noted a very narrow velocity range for dense gas associated with the IRDC, based on HNC observations. What is new here is the three-dimensional (latitude, longitude, and velocity) association of a longer Nessie’s dense gas with predictions for where the centroid of the Milky Way’s Scutum-Centaurus Arm’s “

Figure \ref{fig:pvdiagram}, which offers a position-velocity diagram of CO (color) and $${\rm NH}_3$$ emission (black dots) together, shows the association of the Nessie-HOPS sources with the Scutum Centaurus Arm most clearly. What is most remarkable about Figure \ref{fig:pvdiagram} is that the black line sloping through the figure is not a fit to the black dots representing the HOPS sources. Instead, that line indicates the position-velocity trace of the Scutum-Centaurus Arm based on (Dame et al., 2011) data for the full Galaxy, not just this small longitude range. Figure \ref{fig:pvdiagram} implies that Nessie goes right down the “

\label{fig:HOPSoverlay}. Superposition of HOPS Sources, color-colored by $${\rm NH}_3$$-determined LSR velocity. Colored lines have the same meaning (predicted LSR velocity) as in Figure \ref{fig:coloredlines}, so the colorful dashed lines at $$b\sim-0.5^\circ$$, in both panels, represent the physical Galactic mid-plane, and the solid colorful lines indicate 20 pc above and below the plane. Agreement of the NH$$_3$$ and predicted LSR velocity (color) to within 2.5 km s$$^{−1}$$ is indicated by a white circle around the HOPS symbol, and grey circles indicate agreement to within 5 km s$$^{−1}$$. As in Figures \ref{fig:topview} and \ref{fig:coloredlines}: in the top panel, only a 25 pc offset of the Sun above is taken into account; and in the bottom panel, a 7 pc offset for the Galactic Center is also used in the calculations.

\label{fig:pvdiagram}. Position-velocity diagram of CO and $${\rm NH}_3$$. Colored background shows CO emission integrated over $$-1.5<b<1^\circ$$. Black dots show HOPS sources coincident with Nessie Extended, also shown in Figure \ref{fig:HOPSoverlay} as white-circle-highlighted colored points. The dots are plotted at the longitudes given in Figure \ref{fig:HOPSoverlay}, and the LSR velocities given by the centroid of the $${\rm NH}_3$$ emission for each HOPS source. The black line shown is not a fit to the HOPS or the CO data shown: it is a segment of a global log-spiral fit to CO data for the entire Scutum-Centaurus Arm, extending almost $$360^{\circ}$$ around the Galaxy (taken from Fig. 4 of (Dame 2011).

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

A Bone of the Galaxy

\label{spine} All the evidence presented in this paper, taken together, strongly suggests that Nessie forms a spine-like feature that runs down the center of the Scutum-Centaurus Arm of the Milky Way. How did it get there? Is it the crest of a classic spiral density wave (Lin et al., 1964), or does it have some other cause? Any feature this long and skinny that is not controlled by Galactic-scale forces will be subject to a variety of instabilities, and cannot last long. It would be great if we could look to numerical simulations for answers, but today’s simulations can, alas, only give hints. Nessie is so skinny, and so much denser than its surroundings that no extant numerical simulation has the combination of spatial resolution and dynamic range in density needed to produce a feature like it.

Figure \ref{fig:simulation} offers a snapshot of a numerical simulation (Dobbs 2013) that represents the state of the art at present (available as a movie at http://empslocal.ex.ac.uk/people/staff/cld214/movies.html). One can see density features that are highly elongated, both within the spiral arms, and also between the arms. Many of the features between the arms in Figure \ref{fig:simulation} are similar to the ‘spurs’ and ‘feathers’ that have been simulated and observed by E. Ostriker and colleagues (Shetty et al., 2006; La Vigne et al., 2008; Corder et al., 2008). Figure \ref{fig:IC342} (discussed below) shows a recent WISE image of the galaxy IC342 (Jarrett et al., 2013), and it is clear from that image that some ‘spiral’ galaxies also exhibit inter-arm filaments that are even more pronounced than the simulated spurs and feathers.

In the case of Nessie, the velocity information analyzed in §\ref{CO} and \ref{ammonia} seems to very strongly favor Nessie’s being oriented exactly along (within, as the backbone of) an arm (Scutum-Centaurus) over the idea that Nessie is a spur or interam filament.

Estimates for the mass of Nessie under various assumptions are given in Table 1. Jackson et al. 2010 model Nessie as a(n unmagnetized) self-gravitating fluid cylinder supported against collapse by a “

It is not the goal of this paper to produce a more definitive estimate of Nessie’s $$m_l$$ or total mass, or to model Nessie’s internal density structure. Instead, here, we only seek to estimate the total mass of Nessie in order to consider its mass as a fraction of that in the Galaxy or in a spiral arm. So, Table 1 offers rough estimates of the mass of cylinders, whose (constant) average density is set so that the typical extinctions associated with Nessie’s IR-dark ($$A_v\sim 100$$) and HCN bright ($$A_V \simgreat$$ a few mag) radii are sensible. Assuming a mean density for the mid-IR opaque material of $$10^5$$ cm$$^{-3}$$, then Nessie Classic is $$1 \times 10^5$$ M$$_\odot$$, Nessie Extended is $$2 \times 10^5$$ M$$_\odot$$ and Nessie Optimistic is $$5 \times 10^5$$ M$$_\odot$$. If one assumes that the envelope traced by the HNC observations of Jackson et al. (2010) for Nessie Classic continues along Nessie’s length, then the mass of a $$n\sim 500$$ cm$$^{-3}$$ cylindrical tube (see Table 1) associated with Nessie would be $$5 \times 10^4$$ M$$_\odot$$ for Classic and $$3 \times 10^5$$ M$$_\odot$$ for Optimistic. For the Optimistic case, this mass amounts to 2 millionths of the total baryonic mass (assuming $$\sim 10^{11}$$ M$$_\odot$$ total) of the Milky Way. To use this fraction in order to estimate the total number of “

\label{fig:simulation}. Snapshot of a simulation of a 2 armed spiral galaxy, with heating and cooling, self gravity of the gas, and stellar feedback (Figure 1 of Dobbs & Pringle 2013). The gas is gathered together into GMCs in the spiral arms, and later becomes sheared into long, thin ’spurs’, or ’feathers’. The snapshot is taken at a time of 250 Myr, shows the galaxy out to a radius of 7 kpc, and is from a simulation with 8 million particles.

Can We Map the Full Skeleton of the Milky Way?

\label{future}

In an ideal Universe, we would be able to travel far enough outside of the Milky Way to observe it from “

Carry out the following thought experiment. Draw a rough plan of a spiral galaxy on a very flat piece of paper. Position a vantage point a tiny distance (a few hundredths of an inch) above that piece of paper, about two-thirds of the way out from the center of the galaxy. Now give the observer at that vantage point super-sharp eyesight and ask if the observer can separate the spiral arm features you drew, as they observe them. They can–if and only if the spiral you drew has very narrow features defining its arms. If the observer were exactly in the piece of paper (living in Flatland), separating the arms would be impossible, regardless of their width. We are, like your observer, are at a tiny, tiny, elevation off of a spiral galaxy, and our vision is good enough to separate very skinny arm-like features.

So, how might we use out vantage point above the Plane to map out more of the Milky Way’s skeleton? It turns out that Nessie is located in a place where seeing a very long IRDC projected parallel to the Galactic Plane should be easiest. Look again at Figure \ref{fig:topview}, and consider Nessie’s placement there. According to the current (data-based cartoon) view of the Milky Way shown in Figure \ref{fig:topview}, Nessie is in the closest major spiral arm (Scutum-Centaurus) to us, along a direction toward, but not exactly toward, the (confusing) Galactic Center. Nessie’s placement there means that it will have a bright background illumination as seen from further out in the Galaxy (e.g. from the Sun), and that it will have a long extent on the Sky as compared with more distant or less perpendicular-to-our-line-of-sight objects. It is always good when one finds what should be the easiest-to-see example of a new phenomenon first, so we are reassured that Nessie was the first “

To find more ‘Nessies,’ if such narrow “

As extinction and dust emission maps cover more and more of the sky at ever-improving resolution and sensitivity, we should be able to map more and more of the Milky Way’s skeleton. New wide-field extinction-based efforts based at first on Pan-Starrs, and ultimately on Gaia, will be tremendously helpful in these efforts in the coming decade.

Recent (e.g. Spitzer, Herschel) mid- and far-infrared imagery already suggests that: 1) not all galaxies once thought to be dominated by a spiral pattern really are; and 2) not all IRDCs are likely to be part of the Milky Way’s skeleton. As mentioned above, images like Figure \ref{fig:IC342} clearly show that spiral galaxies can be very web-like, with long, straight filaments interconnecting spiral arms. Thus, some of the features seen as long, skinny, IRDCs in the Milky Way could very well not be part of spiral arms, even if they are part of a Galaxy-wide structural pattern. This possibility will clearly complicate the modeling discussed above, but that just makes it more interesting! New data from ever-deeper and ever-sharper extragalactic observations will likely reveal even more complex galaxy strucutres. Combining ALMA thermal dust emission and molecular line observations of slightly-inclined galaxies will allow us to combine structural image with velocity information facilitating ever-improving model comparison.

Over the past 15 years, since their discovery, there have been many efforts to catalog and characterize IRDCs, and it is clear that not all IRDCs are, or should be, part of Nessie-like bones.

The catalog compiled by Peretto et al. (2009) lists 11,000 IRDCs, but none of the features cataloged will be Nessie-like on its own. The structure-finding algorithm used in the Peretto & Fuller work is biased toward finding core-like roundish peaks, so a cloud like Nessie forms a connect-the-dots pattern in the Peretto & Fuller catalog. In fact, Nessie is comprised of $$\sim 100$$ Peretto & Fuller sources. So, while the Peretto & Fuller catalog is tremendously useful to the study of the properties of massive star forming cores, it will only become useful for finding “

Some very large, and/or very extended, IRDCs, such as the so-called “

\label{fig:IC342}. IC342 as seen by WISE, reproduced from Jarrett et al. 2013. The colors correspond to WISE bands: 3.4 μm (blue), 4.6 μm (cyan/green), 12.0 μm (orange), 22 μm (red).

Contributions and Facilities

Contributions

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: “

The article you are reading now was the first to be prepared using a new online collaborative authoring system called Authorea. The early drafts of the paper, as well as the final version, were, and are, all open to the public. We thank Authorea’s founders and developers, Alberto Pepe, Nathan Jenkins, and Eli Bressert, for assistance as the work proceeded.

A.B. acknowledges support from the Cluster of Excellence “

Facilities

Data in this paper were taken with the following telescopes. The CO Survey of the Milky Way data (Dame 2001) are from the 1.2-Meter Millimeter-Wave Telescope in Cambridge, Massachusetts, USA. NH$$_3$$ observations of cores (Purcell 2012) in and near Nessie are from the Mopra 22-meter telescope near Coonabarabran, Australia. The mid-infrared images of the Galactic Plane used to define Nessie are from NASA’s Spitzer Space Telescope, and they were made as part of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE(Benjamin 2003),(Churchwell 2009)) and MIPSGAL (where MIPS=Multiband Infrared Photometer for Spitzer (MIPS)) Surveys of the Galactic Plane.

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