Can nebular HeII emission be explained by ultra-luminous X-ray sources?

Abstract: The shape of the ionising spectra of galaxies is a key ingredient to reveal their physical properties and to our understanding of the ionising background radiation. A long-standing unsolved problem is the presence of HeII nebular emission in many low-metallicity star-forming galaxies. This emission requires ionising photons with energy >54 eV, which are not produced in sufficient amounts by normal stellar populations. To examine if high mass X-ray binaries and ultra-luminous X-ray sources (HMXB/ULX) can explain the observed HeII nebular emission and how their presence alters other emission lines, we compute photoionisation models of galaxies including such sources. We combine spectral energy distributions (SEDs) of integrated stellar populations with constrained SEDs of ULXs to obtain composite spectra with varying amounts of X-ray luminosity, parameterised by Lx/SFR. With these we compute photoionisation models to predict the emission line fluxes of the optical recombination lines of H and He+, and the main metal lines of OIII, OII, OI, and NII. The predictions are then compared to a large sample of low-metallicity galaxies. We find that it is possible to reproduce the nebular HeII and other line observations with our spectra and with amounts of Lx/SFR compatible with the observations. Our work suggests that HMBX/ULX could be responsible for the observed nebular HeII emission. However, the strengths of the high and low ionisation lines, such as HeII and OI, depend strongly on the X-ray contribution and on the assumed SEDs of the high energy source(s); the latter are poorly known.