Gas excitation in galaxies and active galactic nuclei with He IIλ4686 and X-ray emission

Abstract

The origin of He II emission in galaxies remains a debated topic, requiring ionizing photons with energies exceeding 54 eV. While massive stars, such as Wolf-Rayet stars, have been considered potential sources, their UV flux often fails to fully explain the observed He II emission. Recent studies suggest that X-ray binaries (XRBs) might contribute significantly to this ionization. We explore the relationship between X-ray and $\mathrm{He}\mathrm{II}\lambda 4686$ emission in a statistically significant sample of galaxies, investigating whether X-ray sources, including active galactic nuclei (AGNs) and XRBs, serve as the primary mechanism for He II ionization across different galactic environments. We cross-matched a sample of known well-detected He II galaxies with the Chandra Source Catalog, yielding 165 galaxies with X-ray and $\mathrm{He}\mathrm{II}\lambda 4686$ detections. The sources were classified into star-forming galaxies (SFGs) and AGNs based on the BPT diagram and a classification scheme defined for He II galaxies. We find a strong, linear correlation between X-ray and He II luminosity across AGNs and SFGs spanning over seven orders of magnitude. AGNs generally exhibit higher He II/H$\beta$ flux ratios, stronger extinction, and harder X-ray spectra. The O32 ratio of SFGs is tightly correlated with the H$\beta$ equivalent width ($\mathrm{E}{\mathrm{W}}_{\mathrm{H}\beta }$) but not with the He II/H$\beta$ ratio, suggesting a different excitation mechanism. We derive an O32--$\mathrm{E}{\mathrm{W}}_{\mathrm{H}\beta }$ line above which only AGNs of our sample reside. The tight correlation between X-ray and He II luminosity supports X-rays as the primary driver of He II excitation. While AGNs have one common ionization source, the central black hole, in SFGs low-energy species are mainly excited by UV emission related to star-forming activity, however, high-energy species like He II require the presence of XRBs.

14 + 2 appendix pages, 7 figures, 1 table. Accepted for publication in A&A on March 3, 2025. Updated to match the published version

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