The Sc galaxy M 99 in the Virgo cluster has been strongly affected by tidal interactions and recent close encounters , responsible for an asymmetric spiral pattern and a high star formation rate .
Our XMM-Newton study shows that the inner disk is dominated by hot plasma at kT \approx 0.30 keV , with a total X-ray luminosity \approx 10 ^ { 41 } erg s ^ { -1 } in the 0.3 – 12 keV band .
At the outskirts of the galaxy , away from the main star-forming regions , there is an ultraluminous X-ray source ( ULX ) with an X-ray luminosity \approx 2 \times 10 ^ { 40 } erg s ^ { -1 } and a hard spectrum well fitted by a power law of photon index \Gamma \approx 1.7 .
This source is close to the location where a massive H I cloud appears to be falling onto the M 99 disk at a relative speed > 100 km s ^ { -1 } .
We suggest that there may be a direct physical link between fast cloud collisions and the formation of bright ULXs , which may be powered by accreting black holes with masses \sim 100 M _ { \odot } .
External collisions may trigger large-scale dynamical collapses of protoclusters , leading to the formation of very massive ( \ga 200 M _ { \odot } ) stellar progenitors ; we argue that such stars may later collapse into massive black holes if their metal abundance is sufficiently low .