AGAL313.576+0.324: A Luminous yet "[C II] Dark" Star-Forming Clump

The 157.74 micron [C II] line is the dominant coolant of star-forming regions, and is often used to characterize the global star-formation rates of galaxies. By characterizing the [C II] and far-infrared emission from nearby Galactic star-forming molecular clumps, one can determine whether extragalactic [C II] emission arises from a large ensemble of clumps like those in the Milky Way. To begin this characterization, we have observed four high-luminosity (> 10,000 L_⊙) Galactic clumps with SOFIA/FIFI-LS. Despite similar far-infrared luminosities, the [C II] to far-infrared luminosity ratio varies by a factor of >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 L_⊙. AGAL313.576+0.324 lies a factor of more than 100 below the empirical correlation curve between the [C II]/FIR luminosity ratio and dust temperature found for galaxies. AGAL313.576+0.324 may be in an early evolutionary "protostellar" phase with significant accretion luminosity but insufficient ultraviolet flux to ionize carbon. Alternatively, its apparent lack of [C II] emission may arise from deep absorption of the [C II] line against the 158 micron continuum, which might cancel any emission within the FIFI-LS instrument's broad spectral resolution element (Delta V ~ 250 km/s).