2023MNRAS.519.5492M

We here report the detection of the nebular He II λ4686 line in 32 H II regions in the metal-poor collisional ring galaxy Cartwheel using the Multi-Unit Spectroscopic Explorer (MUSE) data set. The measured I(He II λ4686)/I(H β) ratio varies from 0.004 to 0.07, with a mean value of 0.010 ± 0.003. Ten of these 32 H II regions are coincident with the location of an ultra luminous X-ray (ULX) source. We used the flux ratios of important diagnostic lines and results of photoionization by simple stellar populations (SSPs) to investigate the likely physical mechanisms responsible for the ionization of He⁺. We find that the majority of the regions (27) are consistent with photoionization by star clusters in their Wolf-Rayet (WR) phase with initial ionization parameter -3.5 < log 〈U〉 < -2.0. Blue bump (BB), the characteristic feature of the WR stars, however, is not detected in any of the spectra. We demonstrate that this non-detection is due to the relatively low equivalent width (EW) of the BB in metal-poor SSPs, in spite of containing sufficient number of WR stars to reproduce the observed I(He II λ4686)/I(H β) ratio of ≤1.5 per cent at the Cartwheel metallicity of Z = 0.004. The H II regions in the WR phase that are coincident with a ULX source do not show line ratios characteristic of ionization by X-ray sources. However, the ULX sources may have a role to play in the ionization of He⁺ in two (#99, 144) of the five regions that are not in the WR phase. Ionization by radiative shocks along with the presence of channels for the selective leakage of ionizing photons are the likely scenarios in #17 and #148, the two regions with the highest observed I(He II λ4686)/I(H β) ratio.