The newly-commissioned University of New South Wales Infrared Fabry-Perot ( UNSWIRF ) has been used to image molecular hydrogen emission at 2.12 and 2.25 \mu m in the reflection nebula Parsamyan 18 .
P 18 is known to exhibit low values of the ( 1 - 0 ) / ( 2 - 1 ) ~ { } S ( 1 ) ratio suggestive of UV-pumped fluorescence rather than thermal excitation by shocks .
Our line ratio mapping reveals the full extent of this fluorescent emission from extended arc-like features , as well as a more concentrated thermal component in regions closer to the central exciting star .
We show that the emission morphology , line fluxes , and gas density are consistent with the predictions of photodissociation region ( PDR ) theory .
Those regions with the highest intrinsic 1 - 0 ~ { } S ( 1 ) intensities also tend to show the highest ( 1 - 0 ) / ( 2 - 1 ) ~ { } S ( 1 ) line ratios .
Furthermore , variations in the line ratio can be attributed to intrinsic fluctuations in the incident radiation field and/or the gas density , through the self-shielding action of H _ { 2 } .
An isolated knot of emission discovered just outside P 18 , and having both an unusually high ( 1 - 0 ) / ( 2 - 1 ) ~ { } S ( 1 ) ratio and relative velocity provides additional evidence for an outflow source associated with P 18 .