Velocity-Resolved [C ii] Emission And [C ii]/Fir Mapping Along Orion With Herschel

International audienceWe present the first ˜7.‧5 × 11.‧5 velocity-resolved (˜0.2 km s-1) map of the [C ii] 158 μm line toward the Orion molecular cloud 1 (OMC 1) taken with the Herschel/HIFI instrument. In combination with far-IR (FIR) photometric images and velocity-resolved maps of the H41α hydrogen recombination and CO J = 2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/photodissociation region (PDR)/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [C ii] luminosity (˜85%) is from the extended, FUV-illuminated face of the cloud (G0 > 500, {n}{{H}} \gt 5 × 103 cm-3) and from dense PDRs ({G}0 ≳ 104, {n}{{H}} ≳ 105 cm-3) at the interface between OMC 1 and the H ii region surrounding the Trapezium cluster. Around ˜15% of the [C ii] emission arises from a different gas component without a CO counterpart. The [C ii] excitation, PDR gas turbulence, line opacity (from [13C ii]), and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the L[C ii]/{L}{FIR} and {L}{FIR}/{M}{Gas} ratios and show that L[C ii]/{L}{FIR} decreases from the extended cloud component (˜10-2-10-3) to the more opaque star-forming cores (˜10{}-3-10-4). The lowest values are reminiscent of the “[C ii] deficit” seen in local ultraluminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing L[C ii]/{L}{FIR} ratio correlates better with the column density of dust through the molecular cloud than with {L}{FIR}/{M}{Gas}. We conclude that the [C ii]-emitting column relative to the total dust column along each line of sight is responsible for the observed L[C ii]/{L}{FIR} variations through the cloud. Uses observations obtained with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain)