Filament 5 is our strongest bone candidate, in that it is highly elongated (\(0.7^\circ\) or 45 pc, with an aspect ratio of 140:1) and exactly along a previously-claimed spiral arm trace in p-p-v space, although its orientation makes it less somewhat elongated than Nessie on the sky. In figure \ref{fig:Candid5_pos_vel} we show a p-v diagram in the longitude range of filament 5 and overlay fits to the Scutum-Centaurus arm from \citet{Shane_1972}, \citet{Vallee_2008}, \citet{Dame_2011}, and \citet{Reid_2015}. We see that the HOPS, BGPS, and GRS-determined velocities associated with filament 5 are highly correlated with the \citet{Dame_2011} and the \citet{Reid_2015} global-log fits to CO and HI, suggesting that filament 5 is marking a “spine” of the Scutum-Centaurus arm in this longitude range. Moreover, filament 5 also lies along a CO peak in longitude-latitude space, as evident in figure \ref{fig:Candid5_pos_pos}. By overlaying a trace of the mid-IR extinction feature of filament 5 on a plane of the sky map (integrated in Scutum-Centaurus’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.
Our study is not the first follow up to the Nessie work in \citet{Goodman_2014} to look for more long filaments associated with spiral structure. \citet{Ragan_2014} and \citet{Wang_2015} have undertaken similar studies. However, ours is the first study to specifically look for bones in regions we are most likely to find them, that is, elongated along the Galactic plane. Moreover, ours is the only study to create a quantitative set of criteria capable of defining this new class of objects (i.e. galactic “bones”).
\citet{Ragan_2014} undertook a blind search (not restricted to latitudes where the mid-plane should lie) for long thin filaments (> \(1^\circ\)) in the first quadrant of the Milky Way, using near and mid-infrared images. In addition to confirming that Nessie lies along the Scutum-Centaurus arm, \citet{Ragan_2014} find seven Giant Molecular Filaments (GMFs) of which only one, GMF 20.0-17.9, is said to be associated with Galactic structure (declared a spur of the Scutum-Centaurus arm). Our strongest bone candidate, filament 5, is a subsection of GMF 20.0-17.9, but, unlike \citet{Ragan_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. \citet{Ragan_2014} group neighboring IRDCs into a single filament, despite breaks in the extinction feature and kinks in velocity structure. Since grouping several IRDCs to make a longer structure violates our criteria 1 and 6, we only consider the continuous and kinematically coherent part of the filament, which is remarkably parallel to the Scutum-Centaurus arm in p-v space. Likewise, in figure 4 from \citet{Ragan_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. We compare our p-p and p-v analysis of filament 5 with the analysis from \citet{Ragan_2014} in figure \ref{fig:ragan_comp}.
\citet{Ragan_2014} find little or no association with their GMFs and Galactic structure, suggesting that we are perhaps not as sensitive to spiral arm filaments in the first quadrant, or that the frequency and orientation of spiral arm filaments in the first quadrant is different than the fourth. Our three bone candidates with an “A” quality rating (filaments 1,2, and 5) all lie in the first quadrant, so we speculate that Galactic bones are not subject to the same fourth quadrant bias that GMFs are potentially prone to. We also emphasize that the GMFs from \citet{Ragan_2014} and our Galactic bones should be classified as fundamentally different objects. The lengths of the GMFs range from 60-230 pc, while our longest bone candidates is only 52 pc. By definition, GMFs are meant to be larger structures composed of several smaller, high-contrast elements, so no bone in itself will realistically be classified as a GMF. As is the case with filament 5, we expect that there will be significant overlap between the GMFs and Bone catalogs in the future, as our Galactic bones should be a subset of any spiral tracing GMFs yet to be discovered.
Like \citet{Ragan_2014}, \citet{Wang_2015} search for large-scale filaments and establish their relationship to Galactic structure after the fact. Rather than searching for filaments elongated along the Galactic plane, Wang et al. (2015) search for the longest, coldest, and densest filaments (aspect ratio >>10) in Hi-GAL images, within the longitude range of \(15^\circ < l < 56^\circ\). Filaments were initially identified using Hi-GAL 350 and 500 \(\mu\rm{m}\) emission. Temperature and column density maps were created for each candidate, and those which exhibited systematically lower temperatures with respect to the background were selected. As in our study, \citet{Wang_2015} confirmed velocity contiguity by extracting a p-v slice along the curvature of each filament.
\citet{Wang_2015} highlight nine filaments as their most prominent, with one of the nine being Nessie. Only one of their filaments (Filament 6, or BC_011.13-0.12, the “snake”) overlaps with our sample, and is not classified as a bone due to its short aspect ratio (\(\approx 25:1\)). Seven other \citet{Wang_2015} filaments fail one or more of our bone criteria. G24, G26, and G47 lie between 39-62 pc above the physical Galactic midplane (outside our \(\pm 20\) pc criterion), while G28 and G65 have aspect ratios of around 19:1 and 38:1 (less than our 50:1 minimum aspect ratio criterion). Additionally, G29 and G49 are not largely continuous mid-infrared extinction features, violating our criterion 1. This is not surprising, as the \citet{Wang_2015} study was designed to identify filaments emitting at longer Hi-GAL wavelengths, which are not necessarily seen continuously in absorption at mid-infrared wavelengths. In future studies, \citet{Wang_2015} plan to extend their search to the entire Galactic plane. Despite differences in methodology, there should be some degree of overlap between the \citet{Wang_2015} catalog and our bones catalog, and these, along with the \citet{Ragan_2014} catalog, are expected to be highly complementary of each other.
An in-depth analysis of the other nine bone candidates can be found in the appendix.