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

Astronomical Institute, Academy of Sciences, Boční II 1401, CZ-14131 Prague, Czech Republic

^⋆ Corresponding author; konstantinos.kouroumpatzakis@asu.cas.cz

Received: 27 November 2024
Accepted: 3 March 2025

Abstract

Context. 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.

Aims. We explore the relationship between X-ray and He IIλ 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.

Methods. We cross-matched a sample of known well-detected He II galaxies with the Chandra Source Catalog, yielding 165 galaxies with X-ray and He IIλ 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 analyzed the correlation between X-ray and He II luminosity across different energy bands and other parameters.

Results. The comparison between X-ray and He II luminosity shows a strong, linear correlation across AGNs and SFGs spanning over seven orders of magnitude. AGNs generally exhibit higher He II/Hβ flux ratios, stronger extinction, and harder X-ray spectra. The O32 ratio of SFGs is tightly correlated with the Hβ equivalent width (EW_Hβ) but not with the He II/Hβ ratio, suggesting a different excitation mechanism. We derive an O32–EW_Hβ line above which only AGNs of our sample reside.

Conclusions. 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.