[O i ] 63 Micron Absorption in NGC 6334

The [O I] 63 μm transition has been imaged around five far-infrared (FIR) and radio continuum sources in the southern massive star formation region NGC 6334. The [O I] 63 μm line is found in absorption toward the FIR continuum source NGC 6334V. This is only the second case in which the [O I] 63 μm line has been seen in absorption against a continuum source. From the depth of the absorption line, the minimum column density of oxygen is calculated to be N (O 0 ) ≳ 5 × 10 18 cm -2 . This amount of oxygen is consistent with [O I] 63 μm absorption due to atomic gas in the foreground molecular cloud. The [O I] 63 μm line is found in emission toward the other four sources observed: NGC 6334, sources A, C, D, and E. Single-component photodissociation region (PDR) models suggest densities of n ~ 10 4 cm -3 for these sources, based on previously observed [O I] 145 μm and [C II] 158 μm intensities. However, unphysically large far-ultraviolet (FUV) fields are implied for three of the sources, particularly for NGC 6334A. Neither one- nor two-component photodissociation region models can explain the anomalously low [O I] 63 μm intensity toward NGC 6334A nor the absorption toward NGC 6334V. We suggest that self-absorption of the [O I] 63 μm line, such as has been suggested toward DR 21, is suppressing the observed [O I] 63 μm intensity. This underestimate leads to an overestimate of the derived FUV field strengths throughout NGC 6334. The discovery of several more star-forming sites in which the [O I] 63 μm is in absorption or is self-absorbed implies that this line is not always a reliable PDR diagnostic because the PDR models do not treat the radiative transfer through the molecular cloud.