We describe the design of the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS), operated as a Facility-Class instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). FIFI-LS is an imaging spectrometer for medium resolution spectroscopy. Since being commissioned in 2014, it has performed over 50 SOFIA commissioning and science flights. After operating as a principal investigator instrument in 2014 and early 2015, it was accepted as a Facility Science Instrument in 2015. In addition to the description of the design, we report on the in-flight performance and the concept of operation. We also provide an overview of the science opportunities with FIFI-LS and describe how FIFI-LS observations complement and complete observations with the PACS instrument on the Herschel observatory.

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.

The field-imaging far-infrared line spectrometer (FIFI-LS) is a science instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA). FIFI-LS allows simultaneous observations in two spectral channels. The “blue” channel is sensitive from 51[Formula: see text][Formula: see text]m to 125[Formula: see text][Formula: see text]m and the “red” channel from 115[Formula: see text][Formula: see text]m to 203[Formula: see text][Formula: see text]m. The instantaneous spectral coverage is 1000–3000[Formula: see text]km/s in the blue and 800–2500[Formula: see text]km/s in the red channel with a spectral resolution between 150[Formula: see text]km/s and 600[Formula: see text]km/s. Each spectral channel observes a field of five by five spatial pixels on the sky. The pixel size in the blue channel is 6.14 by 6.25 square arc seconds and it is 12.2 by 12.5 square arc seconds in the red channel. FIFI-LS has been operating on SOFIA since 2014. It is available to the astronomical community as a facility science instrument. We present the results of the spectral and spatial characterization of the instrument based on laboratory measurements. This includes the measured spectral resolution and examples of the line spread function in the spectral domain. In the spatial domain, a model of the instrument’s point spread function (PSF) and the description of a second pass ghost are presented. We also provide an overview of the procedures used to measure the instrument’s field of view geometry and spectral calibration. The spectral calibration yields an accuracy of 15–60[Formula: see text]km/s depending on wavelength.