Formation pumping of molecular hydrogen in the Messier 17 photodissociation region

Abstract

We have imaged the emission from the near-infrared v = 1-0 S(1), 1-0 S(7), 2-1 S(1) and 6-4 O(3) lines of molecular hydrogen in the Northern and South Western Bars of M17, together with the hydrogen Bry and Br10 lines. This includes the first emission-line image ever to be obtained of a line from the highly excited v = 6 level of molecular hydrogen. In both Bars, the H 2 emission is generally distributed in clumps along filamentary features. The 1-0 S(1) and 2-1 S(1) images have similar morphologies. Together with their relative line ratios, this supports a fluorescent origin for their emission, within a photodissociation region. The SW-Bar contains a clumpy medium, but in the N-Bar the density is roughly constant. The 1-0 S(7) line image is also similar to the 1-0 S(1) image, but the 6-4 O(3) image is significantly different from it. Since the emission wavelengths of these two lines are similar (1.748 to 1.733 μm), this cannot be due to differential extinction between the v = 6 and the v = 1 lines. We attribute the difference to the pumping of newly formed H 2 into the v = 6, or to a nearby, level. However, this also requires a time-dependent photodissociation region (where molecule formation does not balance dissociation), rather than it to be in steady state, and/or for the formation spectrum to vary with position in the source. If this interpretation of formation pumping of molecular hydrogen is correct, it is the first clear signature from this process to be seen.