The significant contribution of supersoft X-ray Sources to the nebular HeII line emission

TL;DR

This work addresses the long-standing puzzle of nebular He II emission in star-forming galaxies, where stellar photons alone fail to supply photons with $E>54.4$ eV. By modeling SSSs as blackbody components with $kT$ in the $5$–$100$ eV range and combining them with BPASS stellar spectra in the photoionization code MAPPINGS, the authors reproduce observed He II/Hβ ratios in both local and high-redshift galaxies, identifying an optimal SSS temperature window around $20$–$40$ eV for efficient He II production. They show that a modest soft X-ray component can account for local He II enhancements, while higher-redshift galaxies require a substantially larger SSS contribution, implying larger SSS populations or higher luminosities in the early universe. The analysis yields phenomenological estimates for SSS populations (roughly $10^{3}$–$10^{5}$ sources depending on redshift and assumed luminosities) that are broadly consistent with observed counts once absorption biases are considered, and highlights the need to include diverse soft X-ray sources in population synthesis models to reliably predict a galaxy's ionizing radiation field. This work thus provides a feasible, physically motivated pathway to reconcile He II observations with galaxy evolution and motivates further census of SSS-like populations and their role across cosmic time.

Abstract

Nebular spectral lines provide insight into the properties of the interstellar medium (ISM) and the ionizing radiation within galaxies. The presence of high-energy ionization lines such as \heii indicates the existence of ionizing photons with energies exceeding the second ionization energy of helium ($54 \mathrm{eV})$. There is an enigma surrounding the origin of these lines observed in star-forming galaxies because stellar ionization cannot account for such high energy emission. This paper proposes that supersoft X-ray sources (SSSs) may produce the \heii ionization lines in star-forming galaxies. We model the spectra of SSSs using blackbody radiation and add them to the young stellar population spectra to represent the overall spectra of galaxies. Using a photoionization model, we predict the resulting \heiioptic and \hbeta line fluxes and inspect the contribution of SSSs to the elevation of the \heiioptic/\hbeta ratio in star-forming galaxies, both at low and high redshifts. We find that incorporating a blackbody with temperatures between $kT = 10-100 \mathrm{eV}$ can boost the \heiioptic/\hbeta line ratio to the levels observed in local galaxies by SDSS and in early galaxies by NIRSpec. This blackbody temperature range aligns with the observed temperatures of SSSs. The number of SSSs in spiral galaxies listed in Chandra catalogues, and our estimates of the total population, confirms that SSSs are promising candidates for the source of the \heii ionization.

Figures (5)

• Figure 1: The spectral energy distribution of binary stellar populations from BPASS with ages of 1Myr - 1Gyr. The vertical lines show the ionization potential of $\mathrm{H_{I}\ (13.59 eV),\ He_{I}\ (24.59 eV),\ O_{II}\ (35.12 eV)}$ and He ii$\mathrm{(54.42 eV)}$
• Figure 2: The spectral energy distribution of a 1 Myr stellar population from BPASS (blue, showing emission and absorption lines) and blackbody spectra of $kT=5, 10, 20, 30, 50, 80, 100$ eV (purple to yellow, as indicated in the legend). The total luminosity of each blackbody is set equal to the luminosity of the 1 Myr stellar population from BPASS, assuming a total stellar mass of $10^6$$\mathrm{M}_{\odot}$. The vertical lines show the ionization potential of $\mathrm{H_{I}\ (13.59 eV),\ He_{I}\ (24.59 eV),\ O_{II}\ (35.12 eV)}$ and He ii$\mathrm{(54.42 eV)}$
• Figure 3: He ii/$\mathrm{H{\beta}}$ as a function of blackbody temperature for the MAPPINGS model with pure blackbody spectra as an input. The bolometric luminosity is kept constant at $L = 10^{39} \mathrm{erg/s}$. Symbols with different colors marks different ionization parameters as indicated in the colorbar. Crosses marks the MAPPINGS output from the 1 Myr stellar population synthesis from BPASS with a metallicity of $0.4\ \mathrm{Z}_{\odot}\xspace$. The gas-phase metallicity is assumed to be similar to the stellar metallicity and the ionization parameter of the BPASS model is kept at a similar range with the pure blackbody model. The blue shaded region shows the 16th and 84th percentile of the He ii/$\mathrm{H_\beta}$ ratio observed in the SDSS sample and the orange shaded regions shows the 16th and 84th percentile in the NIRSpec sample.
• Figure 4: He ii/$\mathrm{H_\beta}$ as a function of blackbody temperature for the MAPPINGS model with a combination of a stellar population spectrum and a blackbody as an input. The bolometric luminosity is kept constant at $L = 10^{39} \mathrm{erg/s}$ and the ionization parameter is kept at $\log U = -3$. Symbols with different colors marks different luminosity fraction of blackbody to the BPASS spectra as indicated in the legend. Red cross marks the MAPPINGS output from the 1 Myr stellar population synthesis from BPASS with a metallicity of $12+\log(\mathrm{O/H})=8.47$ and the gas-phase metallicity is assumed to be similar to the stellar metallicity. The blue shaded region shows the range of the He ii/$\mathrm{H_\beta}$ ratio observed in the SDSS sample and the orange shaded regions shows the observed range in the NIRSpec sample.
• Figure 5: Number of observed supersoft sources as a function of temperature in eV based on Greiner96 catalog.