Ultraviolet Mg II emission from fast neutral ejecta around Eta Carinae

We present the first images of the nebula around η Carinae obtained with the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST), including an ultraviolet (UV) image in the F280N filter that traces Mg II emission, plus contemporaneous imaging in the F336W, F658N, and F126N filters that trace near-UV continuum, [N II], and [Fe II], respectively. The F336W and F658N images are consistent with previous images in these filters, and F126N shows that for the most part, [Fe II] λ12567 traces clumpy shocked gas seen in [N II]. The F280N image, however, reveals Mg II emission from structures that have not been seen in any previous line or continuum images of η Carinae. This image shows diffuse Mg II emission immediately outside the bipolar Homunculus nebula in all directions, but with the strongest emission concentrated over the poles. The diffuse structure with prominent radial streaks, plus an anticorrelation with ionized tracers of clumpy shocked gas, leads us to suggest that this is primarily Mg II resonant scattering from unshocked, neutral atomic gas. We discuss the implied structure and geometry of the Mg II emission, and its relation to the Homunculus lobes and various other complex nebular structures. An order of magnitude estimate of the neutral gas mass traced by Mg II is 0.02 M☉, with a corresponding kinetic energy around 10⁴⁷ erg. This may provide important constraints on polar mass-loss in the early phases of the great eruption. We argue that the Mg II line may be an excellent tracer of significant reservoirs of freely expanding, unshocked, and otherwise invisible neutral atomic gas in a variety of stellar outflows.