VELOCITY-RESOLVED [C ii] EMISSION AND [C ii]/FIR MAPPING ALONG ORION WITH<i>HERSCHEL</i>

Abstract

We present the first 7 . 5 11 . 5 velocity-resolved (0.2 km s -1 ) map of the [C II] 158 m 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 H41 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 (G 0 > 500, > n H 5 10 3 cm -3 ) and from dense PDRs ( G 0 10 4 , n H 10 5 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 [ 13 C 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.