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# 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 “