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