Near-infrared and optical emission-line structure of the Keyhole Nebula in NGC 3372

Abstract

Narrow-band infrared and optical images of the Keyhole Nebula in NGC 3372 reveal which structures are caused by extinction, and show the underlying morphology of photoionized and shock-excited gas. Dark clouds conspire with ionized gas to create the apparent keyhole shape which is prominent at blue wavelengths and less apparent in the infrared. The Paβ/Hα line ratio shows the spatial distribution of foreground extinction. The wavelength dependence of this extinction indicates a reddening law with R ≈ 4.8, different from the normal interstellar medium. This confirms previous estimates of reddening toward the Carina Nebula determined from stellar photometry, and reveals that the anomalous extinction is patchy and within the Hn region. The morphology of the ionized gas is different from the extinction clouds; it shows an edge-on ionization front running NE to SW, with a limb-brightened indentation that forms the upper outline of the keyhole shape. A fast polar wind from η Carinae may have punctured the ionization front, since the indentation is directly along a projection of the polar axis of the star. This is supported by the morphology of shock-excited gas revealed by a high [S II]/Hα ratio. High-excitation gas emitting [O III] and He I has a smoother distribution. Molecular clumps in the region are also discussed.