Dense gas in a giant molecular filament

Abstract

Context. Recent surveys of the Galactic plane in the dust continuum and CO emission lines reveal that large (≳50 pc) and massive (≳105 M⊙) filaments, know as giant molecular filaments (GMFs), may be linked to Galactic dynamics and trace the mid-plane of the gravitational potential in the Milky Way. Yet our physical understanding of GMFs is still poor. Aims. We investigate the dense gas properties of one GMF, with the ultimate goal of connecting these dense gas tracers with star formation processes in the GMF. Methods. We imaged one entire GMF located at l ~ 52–54° longitude, GMF54 (~68 pc long), in the empirical dense gas tracers using the HCN(1–0), HNC(1–0), and HCO+(1–0) lines, and their 13C isotopologue transitions, as well as the N2H+(1–0) line. We studied the dense gas distribution, the column density probability density functions (N-PDFs), and the line ratios within the GMF. Results. The dense gas molecular transitions follow the extended structure of the filament with area filling factors between 0.06 and 0.28 with respect to 13CO(1–0). We constructed the N-PDFs of H2 for each of the dense gas tracers based on their column densities and assumed uniform abundance. The N-PDFs of the dense gas tracers appear curved in log–log representation, and the HCO+ N-PDF has the flattest power-law slope index. Studying the N-PDFs for sub-regions of GMF54, we found an evolutionary trend in the N-PDFs that high-mass star-forming and photon-dominated regions have flatter power-law indices. The integrated intensity ratios of the molecular lines in GMF54 are comparable to those in nearby galaxies. In particular, the N2H+/13CO ratio, which traces the dense gas fraction, has similar values in GMF54 and all nearby galaxies except Ultraluminous Infrared Galaxies. Conclusions. As the largest coherent cold gaseous structure in our Milky Way, GMFs, are outstanding candidates for connecting studies of star formation on Galactic and extragalactic scales. By analyzing a complete map of the dense gas in a GMF we have found that: (1) the dense gas N-PDFs appear flatter in more evolved regions and steeper in younger regions, and (2) its integrated dense gas intensity ratios are similar to those of nearby galaxies.

Nobeyama 45 m Local Spur CO survey. I. Giant molecular filaments and cluster formation in the Vulpecula OB association

“…This cloud corresponds to GMF 54.0-52.0 (Ragan et al 2014). GMF 54.0-52.0 includes dense gas (Wang et al 2020) and shows signatures of massive star formation in the N115 infrared bubble reported by previous studies (Xu & Ju 2014;Zychová & Ehlerová 2016). A detailed analysis of this cloud will be presented in a separate paper.…”

“…In particular, Ragan et al (2014) reported that filamentary structures, which they named giant molecular filaments (GMFs) exist in the inter-arm regions of the Milky Way. Long filamentary clouds like GMFs often include the dense molecular gas that forms massive stars and clusters (e.g., Jackson et al 2010;Wang et al 2020). Thus, their formation and evolution have been investigated by numerical simulations (e.g., Duarte-Cabral & Dobbs 2017) and observations (e.g., Li et al 2013;Goodman et al 2014;Zucker et al 2015;Wang et al 2015;Abreu-Vicente et al 2016;Zhang et al 2019).…”

The “Maggie” filament: Physical properties of a giant atomic cloud

“…Are the physical and kinematic properties of filamentary molecular clouds inherited from an atomic counterpart in the diffuse ISM? Giant molecular filaments are to date the largest coherent entities identified in the Milky Way and have been subject of many recent studies investigating the evolution of large filaments with respect to the global dynamics of the Milky Way on the one hand and local stellar feedback on the other hand (Jackson et al 2010;Ragan et al 2014;Goodman et al 2014;Wang et al 2015Wang et al , 2016Abreu-Vicente et al 2016;Zucker et al 2015Zucker et al , 2018Wang et al 2020b).…”

The filamentary structures in the CO emission toward the Milky Way disk

“…The estimated MC properties, such as velocity line width, size, virial mass, and luminosity, are used to evaluate the dynamical state of the molecular gas and the interplay of turbulence and self-gravity in the molecular phase of the interstellar medium (ISM). More recently, special consideration has been given to elongated structures called giant molecular filaments (GMFs), which range between 10 and 300 pc in length and 1 to 40 pc in width (Goodman et al 2014;Ragan et al 2014;Wang et al 2015Wang et al , 2020aZucker et al 2015;Abreu-Vicente et al 2017).…”