Despite decades of research, the structure of our Milky Way, such as the number and orientation of spiral arms and how they would appear to an outside observer, remains a topic of much debate. Goodman et al (2014) argue that very thin, very long Infrared Dark Clouds (IRDCs) may trace out the densest portion of the spiral structure of the Milky Way, much like the dark dust lanes seen in nearby face-on spiral galaxies. Identifying and characterizing “Bones of the Milky Way" may ultimately help assemble a global fit to the Galaxy’s spiral arm by piecing together individual skeletal features. We propose here to take advantage of IRAM’s high angular resolution mapping capabilities to produce the first-ever high-resolution (0.2 pc) CO map of a candidate ‘Bone’ of the Milky Way, while simultaneously tracing the dense gas at 3mm. The fact that Bones exist at all was a surprise, and these follow-up observations will be the first step in understanding their origins, their relationship to Galactic structure, and their role in Galactic star formation.
The idea that our Galaxy may be delineated by dense, filamentary gas clouds was suggested by Goodman et al. (2014) after the discovery of the ‘Nessie’ Infrared Dark Cloud (IRDC; Jackson et al. 2010) in the Southern Galactic Plane (l \(\sim\) 338\(^{\circ}\)). ‘Nessie’ is a striking cloud, less than 0.5 pc wide but extending for over 100 pc, directly parallel to the Galactic mid-plane and following the velocity field of the nearest spiral arm. It seems that this remarkable cloud directly traces the densest portion of the Scutum-Centaurus spiral arm. The position of the Scutum-Centaurus arm at a longitude of 338\(^{\circ}\) is nearly perpendicular to our line-of-sight meaning that this is just where we might expect to find the longest IRDC visible from our vantage point. So, is this extreme cloud an anomaly or the first discovery of a class of filamentary dark clouds that trace our Galactic spiral structure, the Bones of the Milky Way?
Preliminary investigation (Zucker, Battersby, Goodman in prep.) reveals a number of candidate Bones of the Milky Way (Bone Candidates, or BCs). These structures are identified as very thin, long IRDCs (aspect ratio \(>\) 50:1, total length \(\gtrsim\) 10 pc) which are roughly parallel to and within 20 pc of the Galactic mid-plane. By associating archival molecular line data with the IRDCs (see Figure 1), we also require that the Bone Candidates (BCs) be within 10 km/s of the global log-spiral fit to a Galactic spiral arm (e.g. Vallee 2008, Dame & Thaddeus 2011). We choose the most promising Northern Hemisphere Bone Candidate, BC1, for follow-up investigation with IRAM. BC1 is a filamentary IRDC that stretches over 60 pc near 19\(^{\circ}\) longitude roughly parallel to and within 20 pc of the Galactic mid-plane and its V\(_{LSR}\) closely matches that of the Scutum-Centaurus spiral arm (see Figure 1). Herschel (Hi-GAL, Molinari et al. 2010) modified blackbody fitting of the dust emission associated with the IRDC give a dust temperature of about 15-20 K along the filament and a column density ranging from 0.5 - 7 \(\times\) 10\(^{22}\) cm\(^{-2}\) (Zucker, Battersby, Goodman in prep.), but leaves us woefully uninformed about its kinematic contiguity and structure. While the best availalble molecular line data in this region (\(^{13}\)CO J=1-0 maps from the Galactic Ring Survey, GRS, Jackson et al. 2006, 46") show that BC1 has a contiguous velocity structure, the resolution is insufficient to resolve the ridge of the filament and determine its structure and the dense gas kinematics have yet to be measured.
Our planned mapping observations of BC1 will, first and foremost, provide the first large-area, high-resolution kinematic view of such a structure. The fact that such long, skinny, high density clouds even exist was an unexpected discovery. Mapping this Bone Candidate is new territory and understanding its basic physical and kinematic properties is crucial to uncovering its role in Galactic structure and star formation.
What are the physical properties, structure, and extent of the Bone Candidate? Mapping CO at high resolution is necessary to understand the global structure of the filament. The central ridge of the filament is unresolved in the existing molecular line observations (see Figure 2), while the infrared-extinction misses the molecular envelope. In order to understand how such a structure might have formed, we need to measure its global structure. How contiguous is the molecular gas along the ridge of the filament and what is its density profile perpendicular to the filament? Is it similar to the morphology of other GMCs or is it characterized by a Plummer profile like some smaller-scale Herschel filaments (e.g. Arzoumanian et al. 2011). While Herschel data can and will be used to help determine the column density structure of the filament, in the confused inner Galaxy, molecular line data is critically important to disentangle structure along the line of sight. Mapping perpendicular cuts across the filament (see Figure 3) is essential to obtaining these density profiles and understanding the interaction of the filament with the environment (e.g. cloud-cloud collision, pressure confinement, etc.) Simultaneous observations of high-density gas tracers allow us to see the highest-density gas associated with the filament and estimate the fraction of dense gas throughout the filament. Is the entire filament likely to be forming stars? If the fraction of dense gas is high and the free-fall time of the filament short, we may expect such structures to be short-lived and relatively rare.
What is the kinematic structure? Can we detect any flow/gradients along or across the filament? Such long, filamentary dense clouds were not predicted, and it is unclear how they may have formed. The end-to-end velocity gradient and turbulent structure of the cloud informs us as to how the filament may have collapsed, whether it be along a spiral arm, embedded within a single GMC, or through cloud-cloud collision. At smaller spatial scales, we are interested in the dynamics of molecular gas in these extended filamentary structures. Is the turbulence cascade at these small scales part of the larger cascade in the Galaxy? Has the turbulence cascade in the ion and neutral species at these densities decoupled? We will address the nature of small scale turbulence in the bones by applying the turbulence diagnostics outlined in Meyer et al. (2014) and Burkhart & Lazarian (2013). Here, the inclusion of a variety of molecular species (diffuse and dense gas tracers, ions and neutrals) is crucial as it allows us to measure the properties and turbulence at different density layers in the filament. If there are velocity gradients, are they consistent with large-scale flows along the filament? Can we detect large-scale infall signatures in the dense gas tracers (as seen in Battersby et al. in prep., Schneider et al. 2011, and Peretto et al. 2013)? At 11” (0.2 pc) scales, we will be sensitive to dense, star-forming cores and can locate early sites of star formation along the filament and their relationship to the larger-scale molecular gas structure.
How does this structure compare with Galactic-scale simulations? Can its global properties (mass, axis ratio, velocity gradient) be reproduced? Galactic-scale simulations capable of resolving such extreme filaments are only now becoming available (see, e.g. Smith et al. 2014, Goodman et al. 2014). Smith et al. (2014) were able to produce long, skinny dense clouds in their Galactic-scale AREPO simulation. We plan to produce mock observations (CO and dense gas tracers) of this simulation and compare the most extreme clouds with our Bone candidate, to see where the simulation works and where it fails. The structure it fails to reproduce will be highlighted as an important next step for simulations. However, where the structure matches BC1, we will capitalize on the wealth of information available in the simulation to investigate possible formation mechanisms and timescales for Bone-like structures.