  We report the detection of a rich spectrum of more than one hundred optical
emission lines of vibrationally hot molecular hydrogen (H2) from the
photodissociation region (PDR) around the mini-starburst cluster NGC 346 in the
Small Magellanic Cloud. The lines are concentrated in the spectral range 6000
to 9300 Angstrom and have observed brightnesses ranging from 0.01% to 0.4%
times that of the H beta lambda 4861 hydrogen recombination line. Analysis of
the spatial distribution of the H2 lines shows that they originate from a range
of depths in the PDR, intermediate between the shallow layers probed by known
fluorescent lines of neutral nitrogen and oxygen, and the more shielded layers
probed by neutral carbon recombination lines. Comparison with other PDRs shows
that the relative strength of the H2 lines with respect to the [C I] lambda
8727 line increases rapidly with decreasing metallicity, being at least 40
times larger in NGC 346 than in the prototypical PDR of the Orion Bar. The
internal PDR dust extinction is also found to be anomalously low in NGC 346. A
separate result is the discovery of a high-ionization bow shock around the O2
star Walborn 3.
