We have studied an ultraluminous X-ray source ( ULX ) in NGC 4559 with XMM-Newton , and its peculiar star-forming environment with HST/WFPC2 .
The X-ray source is one of the brightest in its class ( L _ { x } \approx 2 \times 10 ^ { 40 } erg s ^ { -1 } ) .
Luminosity and timing arguments suggest a mass \ga 50 M _ { \odot } 
for the accreting black hole .
The ULX is located near the rim of a young ( age < 30 Myr ) , large ( diameter \approx 700 pc ) ring-like star forming complex possibly triggered by the impact of a dwarf satellite galaxy through the gas-rich outer disk of NGC 4559 .
We speculate that galaxy interactions ( including the infall of high-velocity clouds and satellites on a galactic disk ) and low-metallicity environments offer favourable conditions for the formation of compact remnants more massive than “ standard ” X-ray binaries , and accreting from a massive Roche-lobe filling companion .