JWST Reveals a Highly Ionized Nebula Powered by an Ultraluminous X-Ray Source with a Massive Black Hole in the Metal-poor Galaxy Holmberg II

Abstract Recent James Webb Space Telescope (JWST) observations have revealed fine-structure lines of ions with ionization potentials ≳50 eV, such as [Ne v ] and [O iv ], in nebular spectra of some low-metallicity galaxies, raising questions about the nature of the ionizing sources. Ultraluminous X-ray sources (ULXs) have been considered as potential contributors, but direct evidence linking individual ULXs to highly ionized nebulae is limited. Here, we present JWST Integral Field Unit (IFU) spectroscopy of the nebula surrounding the template ULX Ho II X–1 in the low-metallicity galaxy Holmberg II, already known to emit He ii λ 4686 and [O iv ] 25.89 μ m lines. We resolve the infrared nebula on spatial scales of ∼15−25 pc and report the first detections of [Ne v ] and [Ne vi ] in this source. We find anisotropic ionizing conditions: emission from the highest-ionization species arises from compact regions offset from the ULX toward the previously detected radio knots, whereas lower-ionization species trace the wider nebula. The preferential alignment of the highest-ionization emission with the radio axis suggests anisotropic escape of the ionizing continuum and a physical connection with jet-ejection activity. Spectral lines across the nebula are narrow and consistent with the instrumental resolution, indicating kinematically quiescent gas and disfavoring shocks as the dominant ionization mechanism. Together, these results provide direct evidence linking an individual ULX to a highly ionized nebula in a low-metallicity galaxy. These JWST observations of Ho II X–1 show that an accreting black hole in a low-metallicity galaxy is responsible for the extreme ionization of the surrounding nebula on ∼100 pc scales.