H2 line 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-held (3'.9 x 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 similar to 10(5)-10(6) cm(-3)) and UV intensity (G(0) similar to 10(3)-10(4)) in PDRs. On the other hand, the 4-2 S(1)/3-1 S(1) ratio is nearly constant (similar to 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.