H₂Line Ratios to Discriminate Dense Photodissociation Regions from Shocks: Application to NGC 2023 and NGC 7023

We measured the distribution of H 2 line ratios, 2-1 S (1)/1-0 S (1) and 4-2 S (1)/3-1 S (1), in typical dense photodissociation regions (PDRs) associated with the reflection nebulae, NGC 2023 and NGC 7023. The line images were obtained with a recently developed wide-field (3.′9 × 3.′9) Fabry-Perot imager, MUSE. In both objects, the 2-1 S (1)/1-0 S (1) ratio varies spatially between 0.2 and 0.6 as a result of the variation of gas density ( n ~ 10 5 -10 6 cm -3 ) and UV intensity ( G 0 ~ 10 3 -10 4 ) in PDRs. On the other hand, the 4-2 S (1)/3-1 S (1) ratio is nearly constant (~1) in both objects. This constancy of the 4-2 S (1)/3-1 S (1) ratio agrees with model predictions. Since the observed 4-2 S (1)/3-1 S (1) ratio is significantly larger than the expected ratio for shock excitation, the 4-2 S (1)/3-1 S (1) ratio is expected to discriminate both dense and diffuse PDRs from shocks.