Properties of the Photodissociated Gas in NGC 6334

Abstract

To investigate the properties of photodissociated gas in strong radiation fields, the massive star formation region NGC 6334 has been mapped with the Far-Infrared Imaging Fabry-Perot Interferometer (FIFI) instrument in the far-infrared fine-structure transitions [C II] 158 μm, [O I] 146 μm, and [O I] 63 μm. Bright [C II] 158 μm emission is found throughout the cloud, whereas the [O I] 146 μm emission is associated only with the star-forming ridge. Comparison of the relative intensities of the lines with single-component photodissociation region models suggests densities of nH ~ 104 cm-3. The models imply unphysically large radiation fields for three sources, particularly for NGC 6334A, which is probably caused by self-absorption in the [O I] 63 μm line. An alternative method for estimating physical conditions, based on the correlation between [C II] 158 μm and CO line intensities, is explored. This method implies hydrogen column densities of NH 1022-23 cm-2, which agree well with those from molecular excitation models. The relative distribution of the [C II] 158 μm and [O I] 146 μm emission supports clumpy photodissociation region models that suggest the [C II] 158 μm emission arises from interclump gas and thus should be more extended than the [O I] 146 μm emission that arises from the denser clumps. The spatial coincidence of [C II] 158 μm emission peaks with minima in the molecular gas emission indicates that at least some of the molecular holes contain photodissociated gas. The relative pressures of the ionized, photodissociated, and molecular gas are compared: the photodissociated and molecular gas are in approximate pressure equilibrium, but the ionized gas is overpressurized by at least an order of magnitude at the continuum sources.