SOFIA/FIFI-LS Full-disk [C ii] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946

Abstract

We present SOFIA/FIFI-LS observations of the [C II] 158 μm cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H I data to compare [C II] emission to dust properties, star formation rate (SFR), H 2 , and H I at 560 pc scales via stacking by environment (spiral arms, interarm, and center), radial profiles, and individual, beam-sized measurements. We attribute 73% of the [C II] luminosity to arms, and 19% and 8% to the center and interarm region, respectively. [C II]/TIR, [C II]/CO, and [C II]/PAH radial profiles are largely constant, but rise at large radii ( 8 kpc) and drop in the center ("[C II] deficit"). This increase at large radii and the observed decline with the 70 μm/100 μm dust color are likely driven by radiation field hardness. We find a near proportional [C II]-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [C II] also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of [C II] as an H 2 and SFR tracer in such regimes. Finally, based on the observed [C II] and CO radial profiles and different models, we find α CO to increase with radius, in line with the observed metallicity gradient. The low α CO (galaxy average  2 M e pc −2 (K km s −1) −1) and low [C II]/CO ratios (∼400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.

Upgrading the Field-Imaging Far-Infrared Line Spectrometer for the Stratospheric Observatory for Infrared Astronomy with kinetic inductance detectors: enabling large sample (extragalactic) surveys

“…We can compare FIFI+LS performance improvement to recent observations of FIFI-LS. Indeed, a comparable observation has already been completed with FIFI-LS for NGC6946 in [CII] by Bigiel et al 10 This map was completed in 9.5 h total, providing a signal-to-noise ratio of 4 for a line flux of 1.3 × 10 −8 W m −2 sr −1 , and an area of 56 square arc min. The same observation would have taken <2 h with FIFI+LS factoring in the sensitivity gain of at least a factor of 1.4 (to be squared when scaling observing time), the FOV increase by a factor of 1.75 and the use of more efficient observing modes that give a reduction by another factor of 1.5.…”

“…of ultra-luminous infrared galaxies and represents an important laboratory, in which to study the origin of the [CII] deficit observed in these galaxies. Bigiel et al 10 presented the results for another full-galaxy [CII] map with FIFI-LS of the nearby spiral galaxy NGC6946. The data allowed disentangling the [CII] luminosities of the arms (73%), center (19%), and interarm regions (8%) of NGC6946.…”

“…34,35,41 This is commonly known as the "FIR line deficit" problem. In the case of the [CII] line, the relative decrease of the line with respect to the FIR continuum (by up to two orders of magnitude) was first observed in (Ultra) Luminous IR Galaxies with the Infrared Space Observatory 42,43 and later on kiloparsec scale regions in normal, star-forming galaxies 10,44,45 and starbursts. 34,35 The physical reasons behind the line deficit remain an open question, and several explanations have been proposed, including: (1) reduction of the photoelectric heating efficiency due to the charging or destruction of the small dust grains, 42,46 (2) reaching gas densities in PDR and HII regions higher than the critical density of the transitions, 47 (3) HII regions with a high ionization parameter, implying a larger fraction of the non-ionizing stellar UV is absorbed by dust, thus reducing the fraction of UV photons available to heat the neutral gas, 41,47-49 (4) a change in the ionization state of the gas due to the harsh AGN powered radiation field, 50 or (5) a combination of high ionization parameter and high FIR extinction.…”

“…36,52,53 In nearby dwarf and spiral galaxies, the observed relation between [CII] and SFR surface density is almost linear, with a scatter of 0.3 dex. 8,10,54 When including the more energetic and obscured environments found in (Ultra) Luminous IR Galaxies, both the slope and the scatter in the [CII]-SFR relation increase. This is a direct consequence of the "[CII]-deficit" discussed above since for (Ultra) Luminous IR Galaxies the bulk of the star formation activity is traced by the FIR emission.…”

[C ii] and CO Emission along the Bar and Counter-arms of NGC 7479*

“…Another excellent tracer of diffuse gas that is not significantly affected by extinction in the ISM is the [C II] far-infrared (FIR) line at 157.741 μm. Although observed in many galaxies with Herschel (see, e.g., Herrera-Camus et al 2015) and the Stratospheric Observatory for Infrared Astronomy (SOFIA; see, e.g., Pineda et al 2018;Bigiel et al 2020), it has never been used for detailed studies of galaxy bars. The only published study with Herschel data is limited to the circumnuclear region of the barred spiral NGC 1097 (Beirão et al 2012).…”