X-ray binaries as the origin of nebular HeII emission in low-metallicity star-forming galaxies

TL;DR

The paper addresses the origin of nebular He II emission in low-metallicity star-forming galaxies, a longstanding puzzle given that normal stellar populations struggle to supply sufficient He+-ionizing photons. It proposes that high-mass X-ray binaries (HMXBs) are the dominant source of these hard photons, tying the He II emission to the metallicity-dependent X-ray luminosity per star formation rate via a single parameter q. By combining XRB population synthesis (Fragos et al.) with BPASS stellar population models, the authors reproduce the observed $I(4686)/I(Hβ)$ ratios and their metallicity and age dependencies across diverse galaxies, including I Zw 18 as an empirical testbed. While most systems are well-explained, some very young or peculiar cases may require earlier XRB activation or additional mechanisms such as shocks, warranting further targeted multiwavelength studies to refine the relative roles of XRBs and shocks in shaping nebular He II emission.

Abstract

The origin of nebular HeII emission, which is frequently observed in low-metallicity (O/H) star-forming galaxies, remains largely an unsolved question. Using the observed anticorrelation of the integrated X-ray luminosity per unit of star formation rate ($L_X/{\rm SFR}$) of an X-ray binary population with metallicity and other empirical data from the well-studied galaxy I Zw 18, we show that the observed HeII 4686 intensity and its trend with metallicity is naturally reproduced if the bulk of He$^+$ ionizing photons are emitted by the X-ray sources. We also show that a combination of X-ray binary population models with normal single and/or binary stellar models reproduces the observed $I(4686)/I(Hβ)$ intensities and its dependency on metallicity and age. We conclude that both empirical data and theoretical models suggest that high-mass X-ray binaries are the main source of nebular HeII emission in low-metallicity star-forming galaxies.

Figures (3)

• Figure 1: Observed and predicted $I(4686)/I(\rm H H$β$)$ relative nebular line intensities as a function of metallicity. Observations of low-metallicity star-forming galaxies are shown as red plusses, and different measurements of the NW region of I Zw 18 with blue diamonds. The yellow line shows the linear regression to the data points. Assuming a default value of $q=2\times 10^{10}$ photon/erg, the empirical $L_X/{\rm SFR}$--O/H relations of Douna2015Metallicity-dep and Brorby2016Enhanced-X-ray- translate into He ii intensities shown by the blue and magenta solid lines, respectively. The black lines show the predicted He ii intensity adopting $L_X/{\rm SFR}$ predicted from the XRB synthesis models described in Sect. \ref{['s_models']} for a constant SFR over 10 Myr (dotted) and 0.1 Gyr (solid), and the same value of $q$. The blue dashed line differs from the solid line by assuming a value of $q$ that is a factor of two higher.
• Figure 2: Predicted X-ray luminosity as a function of age for simple stellar populations with different metallicities.
• Figure 3: Observed and predicted $I(4686)/I(\rm H H$β$)$ relative nebular line intensities as a function of the β H$\beta$ equivalent width. Observations are the same as in Fig. 1. Blue crosses show the predictions from the BPASS models of Xiao2018Emission-line-d, which fail to reproduce the observed He ii$\lambda$4686 intensities. Predictions for simple stellar populations of different metallicities (shown by the solid lines), derived from the combination of BPASS + XRB models, show a fair agreement with the observations.