Low metallicity natal environments and black hole masses in Ultraluminous X-ray Sources

We review the available estimates of the masses of the compact object in Ultraluminous X-ray Sources (ULXs) and critically reconsider the stellar-mass versus intermediate-mass black hole interpretations. Black holes of several hundreds to thousands of $M_\odot$ are not required for the majority of ULXs, although they might be present in the handful of known hyper-luminous ($\sim 10^{41}$ erg s$^{-1}$) objects and/or some sources showing timing features in their power density spectra. At the same time, however, stellar mass BHs may be quite a reasonable explanation for ULXs below $\sim 10^{40}$ erg s$^{-1}$, but they need super-Eddington accretion and some suitable dependence of the beaming factor on the accretion rate in order to account for ULXs above this (isotropic) luminosity. We investigate in detail a ’third way’ in which a proportion of ULXs contain $\approx 30-90 M_\odot$ black holes formed in a low metallicity environment and accreting in a slightly critical regime and find that it can consistently account for the properties of bright ULXs. Surveys of ULX locations looking for a statistically meaningful relationship between ULX position, average luminosity and local metallicity will provide a definitive test of our proposal.

💡 Research Summary

The paper provides a comprehensive reassessment of the mass estimates for compact objects in ultraluminous X‑ray sources (ULXs) and revisits the long‑standing debate between stellar‑mass black holes (BHs) and intermediate‑mass black holes (IMBHs). The authors first review the three principal methods used to infer BH masses in ULXs: (i) spectral fitting of multicolor disk models, which often yields unusually high inner‑disk temperatures that are difficult to reconcile with standard sub‑Eddington accretion onto massive BHs; (ii) timing analysis, especially the detection of quasi‑periodic oscillations (QPOs) whose frequencies have been linked to BH mass in Galactic binaries, yet whose physical origin in ULXs remains ambiguous; and (iii) luminosity‑variability correlations combined with beaming models, where super‑Eddington accretion can produce anisotropic emission that mimics higher isotropic luminosities. Each of these approaches has limitations that prevent a definitive conclusion about the presence of IMBHs in most ULXs.

The authors then introduce a “third way” that invokes black holes of 30–90 M⊙ formed in low‑metallicity environments. Stellar evolution calculations show that reduced metal‑driven winds at Z ≲ 0.2 Z⊙ allow massive progenitors to retain enough mass to collapse directly into BHs of this size. Such BHs, accreting at modestly super‑Eddington rates (≈1–3 L_Edd), can naturally produce X‑ray luminosities up to ∼10^40 erg s⁻¹ without invoking extreme beaming. This scenario simultaneously accounts for several observational trends: (a) ULXs preferentially reside in star‑forming, metal‑poor regions of galaxies; (b) the ULX luminosity function shows a steep drop above 10^40 erg s⁻¹, with only a handful of hyper‑luminous sources (∼10^41 erg s⁻¹) that may indeed require IMBHs or very strong beaming; and (c) timing features that have been interpreted as evidence for IMBHs can also arise from modestly super‑critical accretion flows around 30–90 M⊙ BHs.

Nevertheless, the authors acknowledge that a small subset of ULXs—particularly the hyper‑luminous objects and those exhibiting low‑frequency QPOs—might still demand IMBHs or highly collimated outflows. They argue that the ULX population is heterogeneous, with mass, metallicity, accretion rate, and beaming all contributing to the observed diversity.

To test the low‑metallicity, high‑mass stellar BH hypothesis, the paper proposes systematic surveys correlating ULX positions with local metallicity measurements derived from nebular spectroscopy or integral‑field unit (IFU) data. By quantifying the relationship between average ULX luminosity and the metallicity of their host environments across a statistically significant sample (e.g., nearby spirals, dwarf irregulars, and starburst galaxies), one can directly assess whether the proposed mass–metallicity link holds. Confirmation would solidify the view that most ULXs are powered by massive stellar‑origin black holes formed in metal‑poor regions, while any residual outliers would point to genuine IMBH candidates. The authors conclude that such observational campaigns will provide a decisive test of their “third way” and help resolve the nature of ULXs once and for all.