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 ~ 2 x 10^{40} erg/s). Luminosity and timing arguments suggest a mass >~ 50 M_sun for the accreting black hole. The ULX is located near the rim of a young (age < 30 Myr), large (diameter ~ 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.
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