Near-infrared [Fe II] emission of M82 supernova remnants: Implications for tracing the supernova content of galaxies

We report near-infrared [Fe II] and Paβ spectra of two young supernova remnants and one H II region within the central 15 kpc of M82. Comparison of these spectra reveal a [Fe II]/Paβ ratio similar to that observed in Galactic supernova remnants and H II regions, and show that the [Fe II] background emission in M82 has local maxima at the position of the supernova remnants observed in the radio continuum. The new observations show that the enhanced infrared [Fe II] emission observed in Galactic supernova remnants occurs in remnants spanning a wide range of radio luminosities. These results, and a growing body of Galactic and extragalactic observations by others, suggest that the total [Fe II] luminosity of galaxies is uncontaminated by emission due to H II regions, their associated photodissociation regions, and other ionized regions which do not contain shock-processed grains or enriched supernova ejecta. The available data on infrared [Fe II] emission in galaxies suggests that (1) the iron depletion of the interstellar medium of galaxies is controlled by supernova activity independent of their nuclear classification; (2) the infrared [Fe II] luminosity of galaxies traces their relative supernova content; and (3) the [Fe II] 1.644 μm and υ = 1 → 0 S(1) H₂ emission of galaxies are correlated, suggesting that the latter is also related to supernova activity. We conclude that infrared [Fe II] lines can provide a new relative measure of supernova activity in galaxies. Comparison of infrared [Fe II] and H II luminosities may be useful for determining the relative age of starbursts, and comparison of [Fe II] and H₂ luminosities may provide new insight into understanding the production of global H₂ emission in galaxies.