One hundred optical emission lines of molecular hydrogen from a low-metallicity photodissociation region

Abstract

ABSTRACT 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 $6000 \, {{\mathring{\rm A}}}$ to $9300 \, {{\mathring{\rm A}}}$ and have observed brightnesses ranging from 0.01 per cent to 0.4 per cent times that of the H β λ4861 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 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 [$\mathrm{C\, \scriptstyle I}$] λ8727 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.