We present a comparative study of CO and polycyclic aromatic hydrocarbon (PAH) emission toward a region including the massive star-forming regions of NGC 6334 and NGC 6357. We use the NANTEN 12CO(J=1{0) data and the AKARI 9 m All-Sky diffuse map in order to evaluate the calibration accuracy of the AKARI data. We conrm that the overall CO distribution shows a good spatial correspondence with the PAH emission, and their intensities exhibit a good power-law correlation with a spatial resolution down to 4′ over the region of 10◦10◦. We also reveal poorer correlation for small scale structures between the two quantities toward NGC 6357, due to strong UV radiation from local sources. Larger scatter in the correlation toward NGC 6357 indicates higher ionization degree and/or PAH excitation than that of NGC 6334.

Optical imaging and spectroscopy of G353.2+0.9, the brightest part of the giant H II region NGC 6357, shows that this H II region is optically thin, contains "300 M⊙ of ionized gas and is probably expanding into the surrounding medium. Its chemical composition is similar to that found in other H II regions at similar galactocentric distances if temperature fluctuations are significant. The inner regions are probably made of thin shells and filaments, whereas extended slabs of material, maybe shells seen edge-on, are found in the periphery. The radio continuum and Hα emission maps are very similar, indicating that most of the optical nebula is not embedded in the denser regions traced by molecular gas and the presence of IR sources. About 1050 UV photons per second are required to produce the Hβ flux from the 1l.3'x10' region surrounding the Pis 24 cluster that is south of G353.2+0.9. Most of the energy powering this region is produced by the 03-7 stars in Pis 24. Most of the 2MASS sources in the field with large infrared excesses are within G353.2+0.9, indicating that the most recent star forming process occured within it. The formation of Pis 24 preceded and caused the formation of this new generation of stars and may be responsible for the present-day morphology of the entire NGC 6357 region.