C~+ EMISSION FROM THE MAGELLANIC CLOUDS. I. THE BRIGHT H Ⅱ REGION COMPLEXES N159 AND N160

摘要

We have mapped the [C Ⅱ] 158 μm line toward the bright Large Magellanic Cloud H Ⅱ regions N160 and N159. Both H Ⅱ region/molecular cloud complexes are associated with extended clouds of C~+. Comparison with CO observations of similar resolution shows that in both complexes peak 158 μm emission occurs at the interfaces of the H Ⅱ region and the associated molecular clouds, while more diffuse extended 158 μm emission covers the entire molecular cloud complexes. Including the results on 30 Dor published elsewhere, the ratio of [C Ⅱ] to CO intensities differs from cloud to cloud over 3 orders of magnitude, presumably reflecting evolutionary differences in cloud structure. The ratio of 158 μm to far-infrared intensities also shows some variation, but over a much smaller range. It is typically around 1% and considerably higher than in Galactic clouds and in most galactic nuclei. Thus, at least on spatial scales of tens of parsecs, the intensity of 158 μm emission correlates reasonably well, but not perfectly, with the infrared continuum intensity and very poorly with the CO intensity. The observed 158 μm emission appears to be optically thin, implying minimum column densities N_H~(min) = 3 x 10~(21) cm~(-2). In contrast to Galactic objects, in three of the four clouds observed, the total mass of the photon-dominated region (PDR) is a significant fraction of the total complex mass, although not to the extreme extent deduced for 30 Dor. The relative morphologies of [C Ⅱ], CO, and far-infrared emission, as well as derived properties such as the high PDR-to-molecular mass ratios and the high photoelectric heating efficiencies characterizing the observed clouds, can be understood as the result of the lower metallicity and lower dust-to-gas ratio in the Large Magellanic Cloud relative to those in the Galaxy. This causes the cloud volume in which CO is abundant to shrink while simultaneously increasing the PDR volume; in addition, it produces a lower mean UV radiation field in the PDR zone by increasing the UV photon mean free path lengths, resulting in greater geometric dilution of the radiation field.