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In the last few years multiwavelength observations have boosted ourunderstanding of Ultraluminous X-ray Sources (ULXs). Yet, the most fundamentalquestions on ULXs still remain to be definitively answered: do they containstellar or intermediate mass black holes? How do they form? We investigate thepossibility that the black holes hosted in ULXs originate from massive (40-120$M_\odot$) stars in low metallicity natal environments. Such black holes have atypical mass in the range $\sim 30-90 M_\odot$ and may account for theproperties of bright (above $\sim 10^{40}$ erg s$^{-1}$) ULXs. More than $\sim10^5$ massive black holes might have been generated in this way in the metalpoor Cartwheel galaxy during the last $10^7$ years and might power most of theULXs observed in it. Support to our interpretation comes from NGC 1313 X-2, thefirst ULX with a tentative identification of the orbital period in the opticalband, for which binary evolution calculations show that the system is mostlikely made by a massive donor dumping matter on a $50-100 M_\odot$ black hole.

Ultraluminous X-ray Sources forming in low metallicity natal environments