One hundred new unidentified optical emission lines from a low-metallicity photodissociation region

Author:

William J. Henney, Mabel Valerdi

Keyword:

Astrophysics, Astrophysics of Galaxies, Astrophysics of Galaxies (astro-ph.GA)

journal:

date:

2023-05-31 16:00:00

Abstract

We report the detection of a rich spectrum of more than one hundred optical emission lines from neutral/molecular gas in the photodissociation region (PDR) around the mini-starburst cluster NGC 346 in the Small Magellanic Cloud. We propose the term Deep Red Line (DRL) for these lines, which are concentrated in the spectral range 6000 Angstrom 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. The vast majority of the DRLs have never previously been detected from astronomical nebulae. Some of them may be due to neutral atoms, but most have no credible identifications in databases of atomic line transitions, and it is possible that some may correspond to transitions in molecules or molecular ions. Analysis of the spatial distribution of the DRLs shows that they originate from a range of depths in the PDR, providing a missing link 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 DRLs with respect to the [C I] lambda 8727 line increases rapidly with decreasing metallicity.

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