2015ApJ...812...75G

We present the first ∼7.'5x11.'5 velocity-resolved (∼0.2 km/s) 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, 5x10³/cm3) and from dense PDRs ( 10⁴, 10⁵/cm3) 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]/ and/ ratios and show that L[C ii]/ decreases from the extended cloud component (∼10^–2-10^–3) to the more opaque star-forming cores (∼1010^–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]/ 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 L[C ii]/ variations through the cloud.