Characterizing the Multiphase Origin of [C II] Emission in M101 and NGC 6946 with Velocity-resolved Spectroscopy
Abstract
The [C II] fine-structure transition at 158 μm is frequently the brightest far-infrared line in galaxies. Due to its low ionization potential, C+ can trace the ionized, atomic, and molecular phases of the ISM. We present velocity-resolved [C II] and [N II] pointed observations from SOFIA/GREAT on ~500 pc scales in the nearby galaxies M101 and NGC 6946 and investigate the multiphase origin of [C II] emission over a range of environments. We show that ionized gas makes a negligible contribution to the [C II] emission in these positions using [N II] observations. We spectrally decompose the [C II] emission into components associated with the molecular and atomic phases using existing CO (2-1) and H I data and show that a peak signal-to-noise ratio of 10-15 is necessary for a reliable decomposition. In general, we find that in our pointings ≳50% of the [C II] emission arises from the atomic phase, with no strong dependence on star formation rate, metallicity, or galactocentric radius. We do find a difference between pointings in these two galaxies, where locations in NGC 6946 tend to have larger fractions of [C II] emission associated with the molecular phase than in M101. We also find a weak but consistent trend for fainter [C II] emission to exhibit a larger contribution from the atomic medium. We compute the thermal pressure of the cold neutral medium through the [C II] cooling function and find $\mathrm{log}({P}_{\mathrm{th}}/k)=3.8\mbox{--}4.6\,\,[{\rm{K}}\ \,{\mathrm{cm}}^{-3}]$ , a value slightly higher than similar determinations, likely because our observations are biased toward star-forming regions.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2021
- DOI:
- 10.3847/1538-4357/abfcc6
- arXiv:
- arXiv:2104.13452
- Bibcode:
- 2021ApJ...915...92T
- Keywords:
-
- Photodissociation regions;
- Interstellar medium;
- Cold neutral medium;
- Molecular gas;
- Far infrared astronomy;
- Spiral galaxies;
- 1223;
- 847;
- 266;
- 1073;
- 529;
- 1560;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 23 pages, 11 figures, 4 tables, accepted to The Astrophysical Journal