Characterizing [C ii] Line Emission in Massive Star-forming Clumps

Abstract

Because the 157.74 μ m [C ii ] line is the dominant coolant of star-forming regions, it is often used to infer the global star formation rates of galaxies. By characterizing the [C ii ] and far-infrared emission from nearby Galactic star-forming molecular clumps, it is possible to determine whether extragalactic [C ii ] emission arises from a large ensemble of such clumps, and whether [C ii ] is indeed a robust indicator of global star formation. We describe [C ii ] and far-infrared observations using the FIFI-LS instrument on the Stratospheric Observatory For Infrared Astronomy (SOFIA) airborne observatory toward four dense, high-mass, Milky Way clumps. Despite similar far-infrared luminosities, the [C ii ] to far-infrared luminosity ratio, / L FIR , varies by a factor of at least 140 among these four clumps. In particular, for AGAL313.576+0.324, no [C ii ] line emission is detected despite a FIR luminosity of 24,000 . AGAL313.576+0.324 lies a factor of more than 100 below the empirical correlation curve between / L FIR and found for galaxies. AGAL313.576+0.324 may be in an early evolutionary “protostellar” phase with insufficient ultraviolet flux to ionize carbon, or in a deeply embedded “‘hypercompact” region phase where dust attenuation of UV flux limits the region of ionized carbon to undetectably small volumes. Alternatively, its apparent lack of [C ii ] emission may arise from deep absorption of the [C ii ] line against the 158 μ m continuum, or self-absorption of brighter line emission by foreground material, which might cancel or diminish any emission within the FIFI-LS instrument’s broad spectral resolution element ( km s −1 ).