Low metallicity and ultra-luminous X-ray sources in the Cartwheel galaxy

Low-metallicity (Z <~ 0.05 Zsun) massive (>40 Msun) stars might end their life by directly collapsing into massive black holes (BHs, 30 < m_BH/Msun <~ 80). More than ~10^5 massive BHs might have been generated via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ~10^7 yr. We show that such BHs might power most of the ultra-luminous X-ray sources (ULXs) observed in the Cartwheel. We also consider a sample of ULX-rich galaxies and we find a possible anti-correlation between the number of ULXs per galaxy and the metallicity in these galaxies. However, the data are not sufficient to draw any robust conclusions about this anti-correlation, and further studies are required.

💡 Research Summary

The paper investigates a novel pathway for the formation of ultra‑luminous X‑ray sources (ULXs) in low‑metallicity galaxies, focusing on the ring galaxy Cartwheel. The authors begin by reviewing the long‑standing debate over ULX origins, noting that while some ULXs may host intermediate‑mass black holes (IMBHs) or be powered by super‑Eddington accretion onto stellar‑mass black holes, the observational evidence remains ambiguous. They then introduce a theoretical framework in which massive stars (M ≳ 40 M⊙) formed in environments with metallicities Z ≲ 0.05 Z⊙ undergo minimal line‑driven wind mass loss. Consequently, such stars retain enough mass to collapse directly into black holes with masses in the range 30–80 M⊙, bypassing a traditional supernova explosion.

Applying this model to the Cartwheel galaxy, the authors estimate the recent star‑formation history of the outer ring. Using a star‑formation rate of ~20 M⊙ yr⁻¹ sustained over the past 10 Myr and a standard initial mass function, they calculate that roughly 10⁵ massive black holes could have been produced via direct collapse. They then assess the X‑ray output expected from these objects. Assuming a modest mass‑transfer rate of 10⁻⁶–10⁻⁵ M⊙ yr⁻¹ from a companion star and an accretion efficiency of ~10 %, each black hole would radiate at 10³⁹–10⁴⁰ erg s⁻¹, comfortably within the ULX regime. The observed population of ~20 ULXs in the Cartwheel ring can therefore be accounted for by a relatively small fraction of the total black‑hole cohort being actively accreting at any given time.

To test whether this mechanism might be generic, the authors compile a sample of other ULX‑rich galaxies with measured metallicities. They find a tentative anti‑correlation: galaxies with lower metallicities tend to host more ULXs per unit star‑formation rate. The trend is most pronounced for systems with Z ≲ 0.2 Z⊙, where the ULX‑to‑SFR ratio is 2–3 times higher than in more metal‑rich counterparts. However, the authors caution that the sample size is limited, metallicity determinations vary in methodology, and selection biases could affect the result. Consequently, they call for larger, uniformly selected galaxy samples and deeper X‑ray observations to confirm the relationship.

In conclusion, the study proposes that direct collapse of massive, low‑metallicity stars can produce a substantial population of intermediate‑mass black holes capable of powering most ULXs observed in the Cartwheel galaxy. The preliminary evidence for an inverse metallicity‑ULX relationship suggests that metal‑poor environments may be fertile grounds for ULX formation, but definitive proof awaits more extensive data. This work adds a compelling piece to the puzzle of ULX origins and highlights the importance of galactic chemical composition in shaping high‑energy astrophysical phenomena.