THE SKELETON OF THE MILKY WAY

Abstract

Recently, Goodman et al. (2014) argued that the very long, very thin infrared\ndark cloud "Nessie" lies directly in the Galactic mid-plane and runs along the\nScutum-Centaurus arm in position-position-velocity ($p-p-v$) space as traced by\nlower density $\\rm {CO}$ and higher density ${\\rm NH}_3$ gas. Nessie was\npresented as the first "bone" of the Milky Way, an extraordinarily long, thin,\nhigh-contrast filament that can be used to map our Galaxy's "skeleton." Here,\nwe present evidence for additional bones in the Milky Way Galaxy, arguing that\nNessie is not a curiosity but one of several filaments that could potentially\ntrace Galactic structure. Our ten bone candidates are all long, filamentary,\nmid-infrared extinction features which lie parallel to, and no more than 20 pc\nfrom, the physical Galactic mid-plane. We use $\\rm {CO}$, ${\\rm N}_2{\\rm H}^+$,\n$\\rm {HCO}^+$, and ${\\rm NH}_3$ radial velocity data to establish the\nthree-dimensional location of the candidates in ${\\it p-p-v}$ space. Of the ten\ncandidates, six also: have a projected aspect ratio of $\\geqq$50:1; run along,\nor extremely close to, the Scutum-Centaurus arm in ${\\it p-p-v}$ space; and\nexhibit no abrupt shifts in velocity. The evidence presented here suggests that\nthese candidates are marking the locations of significant spiral features, with\nthe bone called filament 5 ("BC_18.88-0.09") being a close analog to Nessie in\nthe Northern Sky. As molecular spectral-line and extinction maps cover more of\nthe sky at increasing resolution and sensitivity, it should be possible to find\nmore bones in future studies.\n