2017ApJ...841...13L

We have analyzed the temperature, velocity, and density of H₂ gas in NGC 7023 with a high-resolution near-infrared spectrum of the northwestern filament of the reflection nebula. By observing NGC 7023 in the H and K bands at R ≃ 45,000 with the Immersion GRating INfrared Spectrograph, we detected 68 H₂ emission lines within the 1'' x 15'' slit. The diagnostic ratio of 2-1 S(1)/1-0 S(1) is 0.41-0.56. In addition, the estimated ortho-to-para ratio (OPR) is 1.63-1.82, indicating that the H₂ emission transitions in the observed region arise mostly from gas excited by UV fluorescence. Gradients in the temperature, velocity, and OPR within the observed area imply motion of the photodissociation region (PDR) relative to the molecular cloud. In addition, we derive the column density of H₂ from the observed emission lines and compare these results with PDR models in the literature covering a range of densities and incident UV field intensities. The notable difference between PDR model predictions and the observed data, in high rotational J levels of ν = 1, is that the predicted formation temperature for newly formed H₂ should be lower than that of the model predictions. To investigate the density distribution, we combine pixels in 1'' x 1'' areas and derive the density distribution at the 0.002 pc scale. The derived gradient of density suggests that NGC 7023 has a clumpy structure, including a high clump density of ∼10⁵ cm^–3 with a size smaller than ∼5 x 10^–3 pc embedded in lower-density regions of 10³-10⁴ cm^–3.