Published online by Cambridge University Press: 10 November 2010
ROSAT and optical observations of NGC 4861 and NGC 5408, two HII galaxies exhibiting both narrow Hell λ4686 emission line and luminous soft X-ray emission reveal that X-ray and line emitting regions are spatially distinct. This demonstrates that the X-ray ionization mechanism which accounts for the HeIII regions around luminous X-ray sources in the LMC is not at work on a large scale in these two HII galaxies. We Briefly review alternative explanations for the formation of the narrow Hell λ4686 line. The X-ray spectrum of NGC 4861 is soft and probably not dominated by young accreting neutron stars. Accreting black holes of stellar masses, hot gas and SNRs may account for the integrated X-ray energy distribution.
Although the strength of most emission lines observed in HII galaxies are in excellent agreement with those expected from the ionization by clusters of massive O stars, the presence of narrow, presumably nebular HeII λ4684 emission with an intensity of ≈ 1 % of that of Hβ in some HII galaxies is not yet well understood (see e.g. Campbell 1988; Conti, 1991). There are strong indications that the gas is photoionized and that shock excitation, if present at all, is not responsible for the emission of this high-ionization line. The origin of the luminous He+ Lyman continuum is currently a matter of debate since normal O stars have generally been thought to emit only negligible amounts of He+ ionizing photons.