Characterizing the Multi-Phase Origin of [CII] Emission in M101 and NGC 6946 with Velocity Resolved Spectroscopy

Abstract

The [CII] fine-structure transition at 158 micron 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 [CII] and [NII] pointed observations from SOFIA/GREAT on ~500 pc scales in the nearby galaxies M101 and NGC 6946 and investigate the multi-phase origin of [CII] emission over a range of environments. We show that ionized gas makes a negligible contribution to the [CII] emission in these positions using [NII] observations. We spectrally decompose the [CII] emission into components associated with the molecular and atomic phases using existing CO(2-1) and HI 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 greater than or equal to 50% of the [CII] 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 [CII] emission associated with the molecular phase than in M101. We also find a weak but consistent trend for fainter [CII] emission to exhibit a larger contribution from the atomic medium. We compute the thermal pressure of the cold neutral medium through the [CII] cooling function and find log(P_th/k)=3.8-4.6 [K cm^-3], a value slightly higher than similar determinations, likely because our observations are biased towards star-forming regions.

23 pages, 11 figures, 4 tables, accepted to The Astrophysical Journal

References in corpus (29)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample
• Present-day cosmic abundances. A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution and interstellar dust models
• An H-alpha Imaging Survey of Galaxies in the Local 11 Mpc Volume
• GREAT: the SOFIA high-frequency heterodyne instrument
• The Herschel Dwarf Galaxy Survey: I. Properties of the low-metallicity ISM from PACS spectroscopy
• The Spitzer Survey of the Small Magellanic Cloud: FIR Emission and Cold Gas in the SMC
• [CII] 158 $\mu$m Emission as a Star Formation Tracer
• Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas
• Detection of the 205 um [NII] Line from the Carina Nebula
• A Herschel [CII] Galactic plane survey III: [CII] as a tracer of star formation
• GREAT/SOFIA atmospheric calibration
• The upGREAT 1.9 THz multi-pixel high resolution spectrometer for the SOFIA Observatory
• The Spatially Resolved [CII] Cooling Line Deficit in Galaxies
• The Origins of [CII] Emission in Local Star-forming Galaxies
• The Herschel Dwarf Galaxy Survey: II. Physical conditions, origin of [CII] emission, and porosity of the multiphase low-metallicity ISM
• The upGREAT dual frequency heterodyne arrays for SOFIA
• [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
• An independent determination of the local Hubble constant
• Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: The KINGFISH Sample
• Measuring the mixing scale of the ISM within nearby spiral galaxies
• Thermal Pressure in the Cold Neutral Medium of Nearby Galaxies
• EDGE-CALIFA survey: Self-regulation of Star formation at kpc scales
• The Abundance Properties of Nearby Late-Type Galaxies. \\ II. The Relation between Abundance Distributions and Surface Brightness Profiles
• Using [CII] 158 micron Emission From Isolated ISM Phases as a Star-Formation Rate Indicator
• Disentangling the ISM phases of the dwarf galaxy NGC 4214 using [CII] SOFIA/GREAT observations
• SOFIA/FIFI-LS Full-disk [CII] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946
• Detection of a large fraction of atomic gas not associated with star-forming material in M17 SW

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