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

We present the first ~7.5'×11.5' velocity-resolved (~0.2 km s^-1) map of the [C ii] 158 microm line toward the Orion molecular cloud 1 (OMC 1) taken with the Herschel/HIFI instrument. In combination with far-infrared (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/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>5×10³ cm^-3) and from dense PDRs (G≳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 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 ultra-luminous 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.