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.

Abstract

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" replicating Nessie’s properties most strongly. As molecular spectral-line and extinction maps cover more of the sky at increasing resolution and sensitivity, we seek to find more bones in future studies, ultimately to create a global-fit to the Galaxy’s spiral arms by piecing together individual skeletal features.

Introduction

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.

Methdology

To search for more bones, we visually inspect regions (|\(l\)|<30\(^{\circ}\), |\(b\)|<1\(^{\circ}\)) where arms are predicted to lie according to our current understanding of the Milky Way’s structure; the expected (l,b,v) paths of the Galactic arms are calculated using a log-spiral approximation, as described in the literature. The predicted positions of the Galactic arms (Scutum-Centaurus, Carina-Sagittarius, Norma-Cygnus, and Perseus) are overlaid on three-color GLIMPSE Spitzer (Benjamin et al., 2003; Churchwell et al., 2009) images in World Wide Telescope—a tool that facilitates easy visualization of several layers of data at scales from the full sky down to the highest-resolution details. As part of our initial criteria, we search for long, largely continuous, filamentary mid-infrared extinction features that are near and roughly parallel to the Galactic mid-plane. This initial inspection yielded about 15 Bone candidates, and a video showing how this search worked in WWT is available at tinyurl.com/morenessies.

Regardless of this initial visual inspection, the true nature of these filaments, and their association with a spiral feature, can only be established by looking at radial velocity data. The filament must have similar line-of-sight velocities along its length. Moreover, the measured radial velocities should be very close to those predicted by the Milky Way’s rotation curve for arms at a known distance. To investigate the velocity structure of these fifteen filaments, we employ radial velocity data from four separate radio surveys: HOPS (Purcell et al., 2012; Walsh et al., 2011), MALT90 (Foster et al., 2011; Jackson et al., 2013), BGPS (Schlingman et al., 2011) and GRS (Jackson et al., 2006). The HOPS, MALT90, and BGPS surveys are all geared towards probing dense regions hosting the early stages of high mass star formation. From the HOPS survey, we utilize the thermal line from ammonia. With a critical density of about \(10^{4}\textrm{ cm}^{-3}\), ammonia traces dense molecular gas and is often found in dense, cool clouds with temperatures less than 100 K (Purcell et al., 2012). The \(\mathrm{N_2H^{+}}\) and \(\textrm{HCO}^{+}\) thermal line we utilize from the MALT90 and BGPS surveys are also particularly strong in cold dense regions. While the HOPS and BGPS surveys are complete over 100 and 170 square degrees, respectively, MALT90 was a follow-up survey targeted towards \(\approx2000\) dense molecular clumps first identified in the ATLASGAL 870 \(\mu\textrm{m}\) Galactic plane survey (Schuller et al., 2009). As infrared dark clouds tend to harbor cool, high density clumps of gas which fuel the formation of massive stars, all three of these databases contain spectra for hundreds of regions within the longitude range of the potential bone-like filaments.

In cases where HOPS, MALT90, and BGPS catalog data are not available along the extinction feature, we were also able to extract spectra from GRS (high resolution \(^{13}\)CO (1-0) data) and MALT90 p-p-v cubes using the spectrum extracter tool in Glue. A demonstration of the procedure used to extract velocities in Glue is shown in figure \ref{fig:glue}. As CO traces lower density gas (on average \(10^2 \textrm{ cm}^{-3}\)) and \(\mathrm{N_2H+}\), \(\textrm{HCO}^+\), and \(\textrm{NH}_3\) trace high density gas (\(>10^4 \textrm{ cm}^{-3}\)), the dense gas sources provide much stronger evidence for the velocity of cold, dense, filamentary IRDCs. When dense gas sources were not available, the complete and unbiased high resolution GRS survey, although less desirable, allows us to roughly gauge the velocity along entire lengths of filaments. In filaments composed entirely of GRS spectra, we took HOPS spectra over the entire filament using Glue and confirmed that this HOPS-determined velocity agreed with GRS-determined average velocity to within 5 km/s.

By overlaying the HOPS, MALT90, BGPS, and GRS determined velocities on a p-v diagram of CO emission, we determine whether these filaments are physical spines or simply a chance projection of mid-infrared extinction features along our line-of-sight. For this study, we use the whole-galaxy Dame et al. (2001) CO survey to locate each of the arms in p-p-v space. Of the approximately fifteen candidates identified visually, ten of these candidates are within 10 km/s of the Scutum-Centaurus and Norma-Cygnus arms. The central coordinates for these ten filaments, along with their average lengths, LSR velocities, and distances, are listed in figure \ref{fig:candidates}. We plot these ten candidates in p-p-v space, as shown in figure \ref{fig:skeleton}. In addition to showing our Bone candidates, we show several different predictions of the positions of two spiral arms toward the inner Galaxy in longitude-velocity space, from Dame et al. (2011), Sanna et al. (2014), Shane (1972), and Vallée (2008). For reference, we note that the new BeSSeL (maser) results from Sato et al. (2014) in the first quadrant favor the oldest, HI-based Shane (1972), fits for the Scutum arm.

After narrowing down our list to ten filaments with kinematic structure consistent with Galactic rotation, we develop a set of quantitative criteria for objects to be called “bones:”

  1. Largely continuous mid-infrared extinction feature

  2. Roughly parallel to the Galactic plane

  3. Within 20 pc of the physical Galactic mid-plane

  4. Within 10 km/s of the global-log spiral fit to any Milky Way arm

  5. No abrupt shifts in velocity (of more than  3 km/s per 10 pc) within extinction feature

  6. Projected aspect ratio \(\ge 50:1\)

We calculate the aspect ratios and masses per unit length for all ten filaments, along with other parameters, which are summarized in figure \ref{fig:mass_of_bones}. Of the ten filaments with velocities consistent with galactic rotation, six of these meet all six bone criteria: candidates 1, 3, 5, 7, 9, and 10. However, it is important to note that some of the above criteria will likely be modified in the long run, as we learn more about the Skeleton of the Milky Way. Given our limited a priori knowledge of the Galaxy’s structure, it is presently easier to find Bones that are spine-like, lying along arms with velocities predicted by extant modeling (criteria 1, 5), and harder to find spurs off those arms or inter-arm features, the velocities of which are hard to predict well. Similarly, criterion 6 does not allow for projection effects in imposing an aspect ratio limit. As we learn more about spiral structure from simulations and modeling, these criteria will also be adjusted to allow for Bone-like features that represent spurs, inter-arm structures, and/or foreshortened structures lying close to our line of sight.

\label{fig:glue}: A demonstration of how radial velocities were extracted using Glue. First, points were selected along the extinction feature (blues circles in upper left image) and overlaid on the CO (GRS) or \(\textrm{N}_2\textrm{H}^{+}\) (MALT90) fits cube (blue points in upper right image). A spectrum was extracted from the the small red boxed region around the points (bottom image). Then, a Gaussian was fitted around the highest peak in intensity.

\label{fig:candidates}: We indicate projected lengths, rather than physical lengths, so those candidates tangential to our line of sight will appear to be foreshortened. We calculate distances assuming that all filaments are associated with the Scutum-Centaurus arm

\label{fig:skeleton}: A position-velocity space summary of Bone candidates and spiral arm models. Black dots show measurements of BGPS, HOPS, MALT90, and GRS-determined velocities, with particular candidate filaments identified by numbers, or, in the case of Nessie, by name. Lines of varying color show predicted p-v spiral arm traces from the literature (see text for references).

\label{fig:mass_of_bones}: A comparison of the physical properties of Nessie and the Bone candidates, based on a similar table from Goodman et al. (2014). We assume the filaments are cylindrically shaped, with radius, length, and average density appropriate to the GLIMPSE mid-infrared extinction features. The assumed distance to Nessie was 3.1 kpc, while the assumed distances to the ten bone candidates are listed in figure \ref{fig:candidates}. Goodman et al. (2014) chose an average density of \(10^5 \textrm{ cm}^{-3}\) to produce a visual extinction of 100 magnitudes, typical for a long, filamentary IRDC.

Analysis of New Bones

Of the six newly-identified bones, filament 5 is the most Nessie-like, in that it is highly elongated (.8 degrees or 52 pc) and exactly along a previously-claimed spiral arm trace in p-p-v space, although its orientation makes it less elongated than Nessie on the Sky. We show a close-up shot of the p-v diagram for filament 5 in figure \ref{fig:Candid5_pos_vel}. Like Nessie, filament 5 also forms a spine of the Scutum arm as traced by lower density CO gas (figure \ref{fig:Candid5_pos_pos}). By overlaying a mid-IR trace of filament 5 on a plane of the sky map (integrated in Scutum’s velocity range in the region around filament 5) we see that filament 5 lies in the center of the most intense CO emission. Finally, figure \ref{fig:Candid5_with_tilt} shows that filament 5 lies within \(\approx\) 10 pc of the true physical mid-plane. All these figures taken together indicate that filament 5 is Nessie’s counterpart in the first quadrant, suggesting that Nessie is not a curiosity, but one of several bones that trace significant spiral features.

We note that filament 5 is the same IRDC identified as GMF 20.0-17.9 in a Ragan et al. (2014) study of Giant Molecular Filaments (GMFs). Ragan et al. (2014) undertook a blind search (not restricted to latitudes where the mid-plane should lie) for long thin filaments in the first quadrant of the Milky Way, using near and mid-infrared images. In addition to confirming that Nessie lies along the Scutum arm, Ragan et al. (2014) find seven GMFs, of which one (GMF 20.0-17.9, our Filament 5) is said to be a spur of the Scutum-Centaurus arm. Unlike Ragan et al. (2014) we argue that filament 5 runs right down the spine of the Scutum-Centaurus arm in p-v space. We believe the discrepancy arises due to a difference in methodology. Ragan et al. (2014) group neighboring IRDCs into a single filament, while we determine that there is a significant kink in velocity structure associated with a dramatic plane-of-the-sky bend at a longitude of \(\approx 18.5^{\circ}\). Since grouping both IRDCs to make a longer structure violates our criterion 5, we only consider the kinematically coherent part of the filament (yellow boxed region in figure \ref{fig:Candid5_with_tilt}), which is remarkably parallel to the Scutum arm in p-v space. Likewise, in figure 6 from Ragan et al. (2014) (analogous to our figure \ref{fig:skeleton}), they represent filaments as straight lines connecting velocities measured at the tips of the filaments, while we represent filaments as sets of points whose velocities are determined by the BGPS, HOPS, MALT90, and GRS surveys. Thus, a clear and consistent description of a bone is critical, and, in future studies, we plan to continue to apply the criteria like the ones listed above to achieve consistency.

An in-depth analysis of all six confirmed bones can be found in the appendix.

\label{fig:Candid5_pos_vel}: Position-velocity diagram of \(\textrm{CO}\), \(\textrm{NH}_3\), and \(\textrm{HCO}^+\) emission for filament 5. Blue background for the main panel shows \(^{12}\)CO (1-0) emission integrated between \(-1 < \textrm{b} < 1\) degrees (Dame et al., 2001), while the sub panel shows higher resolution GRS \(^{13}\)CO (1-0) emission integrated over the same latitude range. Black dots show HOPS (NH\(_3\) emission), BGPS (HCO\(^+\) emission) or GRS (high resolution CO emission) sources associated with filament 5. The black line is a global log fit to CO data for the entire Scutum-Centaurus arm, extending almost 360 degrees around the galaxy.

\label{fig:Candid5_pos_pos}: Plane of the sky map integrated between 35-55 km/s, the approximate velocity range of the Scutum-Centaurus arm in the region around Filament 5. A trace of Filament 5, as it would appear as a mid-IR extinction feature, is superimposed on the CO emission map. The correlation between Filament 5’s mid-IR extinction feature and Scutum’s most intense CO emission hints that Filament 5 may be a spine of Scutum as traced by lower density CO gas

\label{fig:Candid5_with_tilt}: Filament 5 lies within \(\approx\) 10 pc of the physical galactic midplane. The background is a GLIMPSE Spitzer 8 \(\mu\textrm{m}\) image. The dashed line is color-coded by Dame et al. (2011) LSR velocity and indicates the location of the physical galactic midplane. The solid colored lines indicate \(\pm\) 20 pc from the galactic midplane at the 3.7 kpc distance to filament 5. The colored dots are sources belonging to filament 5, labeled “H” ,“B”, and “G” for HOPS, BGPS, and GRS respectively. Note that we do not consider the lower velocity blue HOPS source near the center of the yellow box to be coincident with filament 5. The label “with tilt” indicates that both the Sun’s 25 pc elevation above the IAU mid-plane and the small tilt of the plane caused by the offset of the Galactic Center from the IAU (0,0) have been accounted for in this view. A closer look at filament 5 can be seen in the lower panel.

Discussion

There are potentially thousands of bone-like filaments observable in the Milky Way. If we find enough of them, we can piece them together to delineate the major structural features of our galaxy. Astronomers have been trying to accurately model the location of the spiral arms in position-velocity space for decades. A bones-based approach should be able to resolve some of the discrepancies amongst the many arm models shown in figure \ref{fig:skeleton}. The level of disagreement on arm locations is large enough that finding even just a handful of Nessie-like structures marking sections of the spines of spiral arms will tie arm fits down with high fidelity at particular positions in p-p-v space. These “spinal” anchors will have especially large weights in statistical fits that seek to combine many measures of the Milky Way’s skeletal structure.

In future, we plan to test and apply algorithms that “connect the dots” between markers of high density IRDC peaks, in a search for more skeletal features. Lenfestey, Fuller and Peretto (2014) (in prep) have recently undertaken such a study, utilizing an IRDC catalog of \(\approx\) 11,000 high density peaks from Peretto et al. (2009). Lenfestey et al. have grouped these catalog objects into long filamentary structures, using a Minimum Spanning Tree (MST) algorithm, identifying 100 structures in the region \(|l| < 65 ^{\circ}, |b|<1^{\circ}\). Of these structures, 22 are linear features similar to the Nessie nebula. We plan to investigate the Lenfestey filaments, as well as apply MST and related structure-finding procedures to additional surveys (e.g. ATLASGAL or Hi-GAL, Csengeri et al., 2014; Molinari et al., 2010), applying our initial bone criteria to all candidates found, thereby producing a larger population of bones capable of pinning down galactic structure.

Along with increasing our bone population, we plan to improve simulations in hopes of answering key questions about bones’ origin and evolution. For instance, what fraction of highly-elongated dense clouds appear to be a) aligned with arms b) spur-like c) inter-arm and d) random long thin clouds unaligned with Galactic structure? What are the likely origins of these types of objects, and do they have different properties (e.g. velocity and density profiles, mass per unit length)? We know that not all long skinny filaments are expected to be associated with galactic structure. Studies prior to Ragan et al. (2014) offer at least two examples of long molecular clouds that are not obviously Bone-like. The “Massive Molecular Filament” G32.02+0.06, studied by Battersby et al. (2014), does not appear to be tracing an arm structure. Likewise, the 500-pc long molecular “wisp” discussed by Li et al. (2013) also does not presently appear directly related to Galactic structure. Neither of these two clouds currently lies in any special position in p-p-v space. It is possible that these are Bone remnants, disrupted by feedback or Galactic shear, but, without better Galaxy modeling, it is very hard to speculate on what fractions of long thin clouds were formerly bones, are currently bones, or were never bones.

While the Smith et al. (2014) galaxy models are the first that provide high enough resolution to simulate our incredibly long and thin bones, they do not include stellar feedback nor magnetic fields—either of which could cause disruptions in the appearance of the simulated bone-like features. In the future, we hope to utilize more comprehensive, targeted high-resolution synthetic observations (e.g. of dust absorption and emission and of CO spectra), based on high-resolution simulations like the ones in Smith et al. (2014). Finally, we hope to use simulations to estimate the biases inherent in our selection criteria (how many spurious “Bones” should we expect to find randomly, by the chance alignment of discontinuous IRDC peaks?).

Though challenging, we plan to combine future high resolution synthetic observations with a wealth of existing data sets to build a skeletal model of the Milky Way. When used in conjunction with BeSSeL maser-based rotation curves (Reid et al., 2014), CO (Dame et al., 2001) and HI (Shane, 1972) p-v fitting, 3D extinction mapping (Schlafly, 2014b), HII region arm mapping (Anderson et al., 2012), and GAIA results, bones have the potential to not only redefine Galactic structure at unprecedented resolution, but also to resolve fundamental questions that have been plaguing Galactic astronomers for decades.

Acknowledments

This work is supported in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. 1262851 and by the Smithsonian Institution

This publication makes use of molecular line data from the Boston University-FCRAO Galactic Ring Survey (GRS). The GRS is a joint project of Boston University and Five College Radio Astronomy Observatory, funded by the National Science Foundation under grants AST-9800334, AST-0098562, and AST-0100793. This publication also makes use of data from the H2O Southern Galactic Plane Survey and the The Millimetre Astronomy Legacy Team 90 GHz Survey.

This work is based [in part] on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA

Filament 1

Top: Position-velocity diagram of CO, NH\(_{3}\), and HCO\(^+\) emission for filament 1. Blue background shows \(^{12}\)CO (1-0) emission integrated between \(-1 < \textrm{b} < 1\) degrees (Dame et al., 2001). Black dots show GRS sources associated with filament 1. The black lines are fits to CO data for the Scutum-Centaurus arm, from Dame et al. (2001) and Shane (1972). Bottom: Plane of the sky map integrated between 55-75 km/s, the approximate velocity range of the Scutum-Centaurus arm in the region around Filament 1. A trace of Filament 1, as it would appear as a mid-IR extinction feature, is superimposed on the CO emission map. The correlation between Filament 1’s mid-IR extinction feature and the Scutum-Centaurus’s arm most intense CO emission hints that filament 1 may be a spine of Scutum as traced by lower density CO gas

Filament 1 lies within \(\approx\) 10 pc of the physical galactic midplane. The background is a GLIMPSE Spitzer 8 \(\mu\textrm{m}\) image. The dashed line is color-coded by Dame et al. (2011) LSR velocity and indicates the location of the physical galactic midplane. The solid colored lines indicate \(\pm\) 20 pc from the galactic midplane at the 4.5 kpc distance to filament 1. The colored dots within the yellow box are sources belonging to filament 1, labeled “G” for GRS; other nearby sources unassociated with filament 1 are labeled “H” and “B” for HOPS and BGPS, respectively. The label “with tilt” indicates that both the Sun’s 25 pc elevation above the IAU mid-plane and the small tilt of the plane caused by the offset of the Galactic Center from the IAU (0,0) have been accounted for in this view. A closer look at filament 1 can be seen in the lower panel.

Filament 3

Top: Position-velocity diagram of CO, NH\(_{3}\), and HCO\(^+\) emission for filament 3. Blue background shows \(^{12}\)CO (1-0) emission integrated between \(-1 < \textrm{b} < 1\) degrees (Dame et al., 2001). Black dots show HOPS, BGPS, and HOPS sources associated with filament 3. The black lines are fits to CO data for the Scutum-Centaurus arm, from Dame et al. (2011) and Shane (1972). Bottom: Plane of the sky map integrated between 55-75 km/s, the approximate velocity range of the Scutum-Centaurus arm in the region around Filament 3. A trace of Filament 3, as it would appear as a mid-IR extinction feature, is superimposed on the CO emission map. The correlation between Filament 3’s mid-IR extinction feature and the Scutum-Centaurus’s arm most intense CO emission hints that filament 3 may be a spine of Scutum as traced by lower density CO gas

Filament 3 lies within \(\approx\) 20 pc of the physical galactic midplane. The background is a GLIMPSE Spitzer 8 \(\mu\textrm{m}\) image. The dashed line is color-coded by Dame et al. (2011) LSR velocity and indicates the location of the physical galactic midplane. The solid colored lines indicate \(\pm\) 20 pc from the galactic midplane at the 4.4 kpc distance to filament 3. The colored dots within the yellow box are sources belonging to filament 3, labeled “H”, ”B“, and ”G“ for HOPS, BGPS, and GRS respectively. The label ”with tilt" indicates that both the Sun’s 25 pc elevation above the IAU mid-plane and the small tilt of the plane caused by the offset of the Galactic Center from the IAU (0,0) have been accounted for in this view. A closer look at filament 3 can be seen in the lower panel.

Filament 7

Top: Position-velocity diagram of CO, NH\(_{3}\), and N\(_2\)H\(+\) emission for filament 7. Blue background shows \(^{12}\)CO (1-0) emission integrated between \(-1 < \textrm{b} < 1\) degrees (Dame et al., 2001). Black dots show HOPS and MALT90 sources associated with filament 7. The black lines are fits to CO data for the Scutum-Centaurus arm, from Dame et al. (2011). Bottom: Plane of the sky map integrated between 0-20 km/s, the approximate velocity range of the Scutum-Centaurus arm in the region around Filament 7. A trace of Filament 7, as it would appear as a mid-IR extinction feature, is superimposed on the CO emission map. The correlation between Filament 7’s mid-IR extinction feature and the Scutum-Centaurus’s arm more intense CO emission hints that filament 7 may be a spine of Scutum as traced by lower density CO gas

Filament 7 lies within \(\approx\) 20 pc of the physical galactic midplane. The background is a GLIMPSE Spitzer 8 \(\mu\textrm{m}\) image. The dashed line is color-coded by Dame et al. (2011) LSR velocity and indicates the location of the physical galactic midplane. The solid colored lines indicate \(\pm\) 20 pc from the galactic midplane at the 3.1 kpc distance to filament 7. The colored dots within the yellow box are sources belonging to filament 7, labeled “H”, and ”M“, for HOPS and MALT90 ,respectively. The label ”with tilt" indicates that both the Sun’s 25 pc elevation above the IAU mid-plane and the small tilt of the plane caused by the offset of the Galactic Center from the IAU (0,0) have been accounted for in this view. A closer look at filament 7 can be seen in the lower panel.

Filament 9

Top: Position-velocity diagram of CO and NH\(_{3}\) emission for filament 9. Blue background shows \(^{12}\)CO (1-0) emission integrated between \(-1 < \textrm{b} < 1\) degrees (Dame et al., 2001). Black dots show HOPS sources associated with filament 9. The black lines are fits to CO data for the Scutum-Centaurus arm, from Dame et al. (2011). Bottom: Plane of the sky map integrated between -50-30 km/s, the approximate velocity range of the Scutum-Centaurus arm in the region around Filament 9. A trace of Filament 9, as it would appear as a mid-IR extinction feature, is superimposed on the CO emission map. The correlation between Filament 9’s mid-IR extinction feature and the Scutum-Centaurus’s arm more intense CO emission hints that filament 9 may be a spine of Scutum as traced by lower density CO gas

Filament 9 lies right on the physical galactic midplane. The background is a GLIMPSE Spitzer 8 \(\mu\textrm{m}\) image. The dashed line is color-coded by Dame et al. (2011) LSR velocity and indicates the location of the physical galactic midplane. The solid colored lines indicate \(\pm\) 20 pc from the galactic midplane at the 3.2 kpc distance to filament 9. The colored dots within the yellow box are sources belonging to filament 9, labeled “H” for HOPS. The label ”with tilt" indicates that both the Sun’s 25 pc elevation above the IAU mid-plane and the small tilt of the plane caused by the offset of the Galactic Center from the IAU (0,0) have been accounted for in this view. A closer look at filament 9 can be seen in the lower panel.

Filament 10

Top: Position-velocity diagram of CO, NH\(_{3}\), and N\(_{2}\)H\(+\) emission for filament 10. Blue background shows \(^{12}\)CO (1-0) emission integrated between \(-1 < \textrm{b} < 1\) degrees (Dame et al., 2001). Black dots show HOPS and MALT90 sources associated with filament 10. The black lines are fits to CO data for the Scutum-Centaurus arm, from Dame et al. (2011). Bottom: Plane of the sky map integrated between -55-35 km/s, the approximate velocity range of the Scutum-Centaurus arm in the region around Filament 10. A trace of Filament 10, as it would appear as a mid-IR extinction feature, is superimposed on the CO emission map. The correlation between Filament 10’s mid-IR extinction feature and the Scutum-Centaurus’s arm more intense CO emission hints that filament 10 may be a spine of Scutum as traced by lower density CO gas

Filament 10 lies within \(\approx\) 15 pc of the physical galactic midplane. The background is a GLIMPSE Spitzer 8 \(\mu\textrm{m}\) image. The dashed line is color-coded by Dame et al. (2011) LSR velocity and indicates the location of the physical galactic midplane. The solid colored lines indicate \(\pm\) 20 pc from the galactic midplane at the 3.3 kpc distance to filament 10. The colored dots within the yellow box are sources belonging to filament 10, labeled “H” and ”M“ for HOPS and MALT90, respectively. The label ”with tilt" indicates that both the Sun’s 25 pc elevation above the IAU mid-plane and the small tilt of the plane caused by the offset of the Galactic Center from the IAU (0,0) have been accounted for in this view. A closer look at filament 10 can be seen in the lower panel.

References

  1. Alyssa A. Goodman, João Alves, Christopher N. Beaumont, Robert A. Benjamin, Michelle A. Borkin, Andreas Burkert, Thomas M. Dame, James Jackson, Jens Kauffmann, Thomas Robitaille, Rowan J. Smith. THE BONES OF THE MILKY WAY. ApJ 797, 53 IOP Publishing, 2014. Link

  2. Jacques P. Vallée. NEW VELOCIMETRY AND REVISED CARTOGRAPHY OF THE SPIRAL ARMS IN THE MILKY WAY‚Äö√Ñ√ÆA CONSISTENT SYMBIOSIS. The Astronomical Journal 135, 1301–1310 (2008). Link

  3. N. M. McClure-Griffiths, John M. Dickey. Milky Way Kinematics. I. Measurements at the Subcentral Point of the Fourth Quadrant. ApJ 671, 427–438 IOP Publishing, 2007. Link

  4. T. M. Dame, Dap Hartmann, P. Thaddeus. The Milky Way in Molecular Clouds: A New Complete CO Survey. ApJ 547, 792–813 IOP Publishing, 2001. Link

  5. W.W. Shane. Neutral Hydrogen in an Interior Region of the Galaxy; the Longitude Interval 22 to 42g. III. The Scutum Arm.. Astronomy and Astrophysics 16, 118 (1972). Link

  6. M. J. Reid, K. M. Menten, A. Brunthaler, X. W. Zheng, T. M. Dame, Y. Xu, Y. Wu, B. Zhang, A. Sanna, M. Sato, K. Hachisuka, Y. K. Choi, K. Immer, L. Moscadelli, K. L. J. Rygl, A. Bartkiewicz. TRIGONOMETRIC PARALLAXES OF HIGH MASS STAR FORMING REGIONS: THE STRUCTURE AND KINEMATICS OF THE MILKY WAY. ApJ 783, 130 IOP Publishing, 2014. Link

  7. Wayne M. Schlingman, Yancy L. Shirley, David E. Schenk, Erik Rosolowsky, John Bally, Cara Battersby, Miranda K. Dunham, Timothy P. Ellsworth-Bowers, Neal J. Evans, Adam Ginsburg, Guy Stringfellow. THE BOLOCAM GALACTIC PLANE SURVEY. V. HCO \(\mathplus\) AND N 2 H \(\mathplus\) SPECTROSCOPY OF 1.1 mm DUST CONTINUUM SOURCES. ApJS 195, 14 IOP Publishing, 2011. Link

  8. Jonathan B. Foster, James M. Jackson, Peter J. Barnes, Elizabeth Barris, Kate Brooks, Maria Cunningham, Susanna C. Finn, Gary A. Fuller, Steve N. Longmore, Joshua L. Mascoop, Nicolas Peretto, Jill Rathborne, Patricio Sanhueza, Frédéric Schuller, Friedrich Wyrowski. THE MILLIMETER ASTRONOMY LEGACY TEAM 90 GHz (MALT90) PILOT SURVEY. ApJS 197, 25 IOP Publishing, 2011. Link

  9. J. M. Jackson, J. M. Rathborne, J. B. Foster, J. S. Whitaker, P. Sanhueza, C. Claysmith, J. L. Mascoop, M. Wienen, S. L. Breen, F. Herpin, A. Duarte-Cabral, T. Csengeri, S. N. Longmore, Y. Contreras, B. Indermuehle, P. J. Barnes, A. J. Walsh, M. R. Cunningham, K. J. Brooks, T. R. Britton, M. A. Voronkov, J. S. Urquhart, J. Alves, C. H. Jordan, T. Hill, S. Hoq, S. C. Finn, I. Bains, S. Bontemps, L. Bronfman, J. L. Caswell, L. Deharveng, S. P. Ellingsen, G. A. Fuller, G. Garay, J. A. Green, L. Hindson, P. A. Jones, C. Lenfestey, N. Lo, V. Lowe, D. Mardones, K. M. Menten, V. Minier, L. K. Morgan, F. Motte, E. Muller, N. Peretto, C. R. Purcell, P. Schilke, Schneider-N. Bontemps, F. Schuller, A. Titmarsh, F. Wyrowski, A. Zavagno. MALT90: The Millimetre Astronomy Legacy Team 90 GHz Survey. Publ. Astron. Soc. Aust. 30 Cambridge University Press (CUP), 2013. Link

  10. C. R. Purcell, S. N. Longmore, A. J. Walsh, M. T. Whiting, S. L. Breen, T. Britton, K. J. Brooks, M. G. Burton, M. R. Cunningham, J. A. Green, L. Harvey-Smith, L. Hindson, M. G. Hoare, B. Indermuehle, P. A. Jones, N. Lo, V. Lowe, C. J. Phillips, M. A. Thompson, J. S. Urquhart, M. A. Voronkov, G. L. White. The H 2 O Southern Galactic Plane Survey: NH 3 (1,1) and (2,2) catalogues. Monthly Notices of the Royal Astronomical Society 426, 1972–1991 Oxford University Press (OUP), 2012. Link

  11. A. J. Walsh, S. L. Breen, T. Britton, K. J. Brooks, M. G. Burton, M. R. Cunningham, J. A. Green, L. Harvey-Smith, L. Hindson, M. G. Hoare, B. Indermuehle, P. A. Jones, N. Lo, S. N. Longmore, V. Lowe, C. J. Phillips, C. R. Purcell, M. A. Thompson, J. S. Urquhart, M. A. Voronkov, G. L. White, M. T. Whiting. The H2O Southern Galactic Plane Survey (HOPS) - I. Techniques and H2O maser data. Monthly Notices of the Royal Astronomical Society 416, 1764–1821 Oxford University Press (OUP), 2011. Link

  12. Gregory Maurice Green, Edward F. Schlafly, Douglas P. Finkbeiner, Mario Jurić, Hans-Walter Rix, Will Burgett, Kenneth C. Chambers, Peter W. Draper, Heather Flewelling, Rolf Peter Kudritzki, Eugene Magnier, Nicolas Martin, Nigel Metcalfe, John Tonry, Richard Wainscoat, Christopher Waters. MEASURING DISTANCES AND REDDENINGS FOR A BILLION STARS: TOWARD A 3D DUST MAP FROM PAN-STARRS 1. ApJ 783, 114 IOP Publishing, 2014. Link

  13. E. F. Schlafly, G. Green, D. P. Finkbeiner, M. Jurić, H.-W. Rix, N. F. Martin, W. S. Burgett, K. C. Chambers, P. W. Draper, K. W. Hodapp, N. Kaiser, R.-P. Kudritzki, E. A. Magnier, N. Metcalfe, J. S. Morgan, P. A. Price, C. W. Stubbs, J. L. Tonry, R. J. Wainscoat, C. Waters. A MAP OF DUST REDDENING TO 4.5 kpc FROM Pan-STARRS1. ApJ 789, 15 IOP Publishing, 2014. Link

  14. R. J. Smith, S. C. O. Glover, P. C. Clark, R. S. Klessen, V. Springel. CO-dark gas and molecular filaments in Milky Way-type galaxies. Monthly Notices of the Royal Astronomical Society 441, 1628–1645 Oxford University Press (OUP), 2014. Link

  15. Robert A. Benjamin, E. Churchwell, Brian L. Babler, T. M. Bania, Dan P. Clemens, Martin Cohen, John M. Dickey, Rémy Indebetouw, James M. Jackson, Henry A. Kobulnicky, Alex Lazarian, A. P. Marston, John S. Mathis, Marilyn R. Meade, Sara Seager, S. R. Stolovy, C. Watson, Barbara A. Whitney, Michael J. Wolff, Mark G. Wolfire. GLIMPSE. I. An SIRTF Legacy Project to Map the Inner Galaxy. Publications of the Astronomical Society of the Pacific 115, 953–964 University of Chicago Press, 2003. Link

  16. Ed Churchwell, Brian L. Babler, Marilyn R. Meade, Barbara A. Whitney, Robert Benjamin, Remy Indebetouw, Claudia Cyganowski, Thomas P. Robitaille, Matthew Povich, Christer Watson, Steve Bracker. The Spitzer /GLIMPSE Surveys: A New View of the Milky Way. Publications of the Astronomical Society of the Pacific 121, 213–230 University of Chicago Press, 2009. Link

  17. J. M. Jackson, J. M. Rathborne, R. Y. Shah, R. Simon, T. M. Bania, D. P. Clemens, E. T. Chambers, A. M. Johnson, M. Dormody, R. Lavoie, M. H. Heyer. The Boston UniversityFive College Radio Astronomy Observatory Galactic Ring Survey. ASTROPHYS J SUPPL S 163, 145–159 IOP Publishing, 2006. Link

  18. F. Schuller, K. M. Menten, Y. Contreras, F. Wyrowski, P. Schilke, L. Bronfman, T. Henning, C. M. Walmsley, H. Beuther, S. Bontemps, R. Cesaroni, L. Deharveng, G. Garay, F. Herpin, B. Lefloch, H. Linz, D. Mardones, V. Minier, S. Molinari, F. Motte, L.-Å. Nyman, V. Reveret, C. Risacher, D. Russeil, N. Schneider, L. Testi, T. Troost, T. Vasyunina, M. Wienen, A. Zavagno, A. Kovacs, E. Kreysa, G. Siringo, A. Wei ß. ATLASGAL The APEX telescope large area survey of the galaxy at 870 \(\upmu\)m. Astronomy and Astrophysics 504, 415–427 EDP Sciences, 2009. Link

  19. T. M. Dame, P. Thaddeus. A MOLECULAR SPIRAL ARM IN THE FAR OUTER GALAXY. ApJ 734, L24 IOP Publishing, 2011. Link

  20. A. Sanna, M. J. Reid, K. M. Menten, T. M. Dame, B. Zhang, M. Sato, A. Brunthaler, L. Moscadelli, K. Immer. TRIGONOMETRIC PARALLAXES TO STAR-FORMING REGIONS WITHIN 4 kpc OF THE GALACTIC CENTER. ApJ 781, 108 IOP Publishing, 2014. Link

  21. M. Sato, Y. W. Wu, K. Immer, B. Zhang, A. Sanna, M. J. Reid, T. M. Dame, A. Brunthaler, K. M. Menten. TRIGONOMETRIC PARALLAXES OF STAR FORMING REGIONS IN THE SCUTUM SPIRAL ARM. ApJ 793, 72 IOP Publishing, 2014. Link

  22. S. E. Ragan, Th. Henning, J. Tackenberg, H. Beuther, K. G. Johnston, J. Kainulainen, H. Linz. Giant molecular filaments in the Milky Way. Astronomy & Astrophysics 568, A73 EDP Sciences, 2014. Link

  23. N. Peretto, G. A. Fuller. The initial conditions of stellar protocluster formation. Astronomy and Astrophysics 505, 405–415 EDP Sciences, 2009. Link

  24. T. Csengeri, J. S. Urquhart, F. Schuller, F. Motte, S. Bontemps, F. Wyrowski, K. M. Menten, L. Bronfman, H. Beuther, Th. Henning, L. Testi, A. Zavagno, M. Walmsley. The ATLASGAL survey: a catalog of dust condensations in the Galactic plane. Astronomy & Astrophysics 565, A75 EDP Sciences, 2014. Link

  25. S. Molinari, B. Swinyard, J. Bally, M. Barlow, J.-P. Bernard, P. Martin, T. Moore, A. Noriega-Crespo, R. Plume, L. Testi, A. Zavagno, A. Abergel, B. Ali, L. Anderson, P. André, J.-P. Baluteau, C. Battersby, M. T. Beltrán, M. Benedettini, N. Billot, J. Blommaert, S. Bontemps, F. Boulanger, J. Brand, C. Brunt, M. Burton, L. Calzoletti, S. Carey, P. Caselli, R. Cesaroni, J. Cernicharo, S. Chakrabarti, A. Chrysostomou, M. Cohen, M. Compiegne, P. de Bernardis, G. de Gasperis, A. M. di Giorgio, D. Elia, F. Faustini, N. Flagey, Y. Fukui, G. A. Fuller, K. Ganga, P. Garcia-Lario, J. Glenn, P. F. Goldsmith, M. Griffin, M. Hoare, M. Huang, D. Ikhenaode, C. Joblin, G. Joncas, M. Juvela, J. M. Kirk, G. Lagache, J. Z. Li, T. L. Lim, S. D. Lord, M. Marengo, D. J. Marshall, S. Masi, F. Massi, M. Matsuura, V. Minier, M.-A. Miville-Deschênes, L. A. Montier, L. Morgan, F. Motte, J. C. Mottram, T. G. Müller, P. Natoli, J. Neves, L. Olmi, R. Paladini, D. Paradis, H. Parsons, N. Peretto, M. Pestalozzi, S. Pezzuto, F. Piacentini, L. Piazzo, D. Polychroni, M. Pomarès, C. C. Popescu, W. T. Reach, I. Ristorcelli, J.-F. Robitaille, T. Robitaille, J. A. Rodón, A. Roy, P. Royer, D. Russeil, P. Saraceno, M. Sauvage, P. Schilke, E. Schisano, N. Schneider, F. Schuller, B. Schulz, B. Sibthorpe, H. A. Smith, M. D. Smith, L. Spinoglio, D. Stamatellos, F. Strafella, G. S. Stringfellow, E. Sturm, R. Taylor, M. A. Thompson, A. Traficante, R. J. Tuffs, G. Umana, L. Valenziano, R. Vavrek, M. Veneziani, S. Viti, C. Waelkens, D. Ward-Thompson, G. White, L. A. Wilcock, F. Wyrowski, H. W. Yorke, Q. Zhang. Clouds filaments, and protostars: The Herschel Hi-GAL Milky Way. Astronomy and Astrophysics 518, L100 EDP Sciences, 2010. Link

  26. Cara Battersby, J. Bally. An 80 pc Long Massive Molecular Filament in the Galactic Mid-Plane. 417–418 In Astrophysics and Space Science Proceedings. Springer Science \(\mathplus\) Business Media, 2014. Link

  27. Guang-Xing Li, Friedrich Wyrowski, Karl Menten, Arnaud Belloche. A 500 pc filamentary gas wisp in the disk of the Milky Way. Astronomy & Astrophysics 559, A34 EDP Sciences, 2013. Link

  28. E.F. Schlafly. A LARGE CATALOG OF ACCURATE DISTANCES TO MOLECULAR CLOUDS FROM PS1 PHOTOMETRY. The Astrophysical Journal 786, 29 (2014b).

  29. L. D. Anderson, T. M. Bania, Dana S. Balser, Robert T. Rood. THE GREEN BANK TELESCOPE H II REGION DISCOVERY SURVEY. III. KINEMATIC DISTANCES. ApJ 754, 62 IOP Publishing, 2012. Link

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