VELOCITY-RESOLVED [C ii] EMISSION AND [C ii]/FIR MAPPING ALONG ORION WITH .

We present the first 7.5'×11.5' velocity-resolved (0.2 km s) map of the [C ii] 158 m line toward the Orion molecular cloud 1 (OMC 1) taken with the /HIFI instrument. In combination with far-infrared (FIR) photometric images and velocity-resolved maps of the H41 hydrogen recombination and CO =2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/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 (>500, >5×10 cm) and from dense PDRs (≳10, ≳10 cm) 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 CO counterpart. The [C ii] excitation, PDR gas turbulence, line opacity (from [C ii]) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the [C ii]/ and / ratios and show that [C ii]/ decreases from the extended cloud component (10-10) to the more opaque star-forming cores (~10-10). The lowest values are reminiscent of the "[C ii] deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing [C ii]/ ratio correlates better with the column density of dust through the molecular cloud than with /. We conclude that the [C ii] emitting column relative to the total dust column along each line of sight is responsible for the observed [C ii]/ variations through the cloud.