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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 2020
- DOI:
- 10.1051/0004-6361/202037928
- arXiv:
- arXiv:2003.05384
- Bibcode:
- 2020A&A...641A..53W
- Keywords:
-
- ISM: clouds;
- ISM: molecules;
- stars: formation;
- radio lines: ISM;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 17 pages, 13 figures, accepted by A&