2002ApJ...574..187A

We have carried out near-infrared mapping observations of photodissociation regions in M17 with the Wide Field Cryogenic Telescope and CO (J=1-0) observations in three isotope lines with the ``NANTEN'' telescope. The observations covered an area of 20'x20' with a spatial resolution of 5".6 for near-infrared wavelengths and with a half-power beamwidth of 2'.7 for millimeter wavelengths. We detected 38 sources brighter than 7 mag at 3.67 µm (L_n band), five of which show signs of young stellar objects. We have detected two emission bars (the N bar and the S bar) in all four near-infrared bands (J, K, L_n, and 3.3 µm). Their spatial distributions differ considerably from band to band, and we have compared them with the radio continuum, the mid-infrared data, and the CO molecular line emission. The different brightness and spectral energy distributions at near-infrared wavelengths can be well explained by emission from hot dust and ionized gas together with obscuration by local cold dust with a steep gradient from north to south. In the N bar, the free-free emission from ionized gas dominates at shorter wavelengths (J and K) and there is little extinction, whereas in the S bar, the free-free emission is attenuated at shorter wavelengths by the heavy local extinction. In both the N and S bars, the thermal emission from hot dust at around 1000 K dominates in the L_n band. The 3.3 µm unidentified infrared (UIR) emission delineates photodissociation regions between the H II regions and the surrounding molecular clouds. The UIR intensity decreases exponentially from the UIR peak toward the molecular clouds, with scale lengths of 88" and 100", or 0.9 and 1.0 pc, at the N and the S bars, respectively. Far-ultraviolet photons, which excite UIR emission, penetrate into the molecular clouds for ∼1 pc, in the nearly edge-on geometry. The ¹²CO contours are elongated in the direction northwest-southeast, while the C¹⁸O contours are round. Far-ultraviolet photons erode the tenuous portions (as seen in ¹²CO) of the surface of the cloud and penetrate deeply toward the denser inside, forming complex structures in the photodissociation regions bordering the molecular cloud.