A ULX associated with a cloud collision in M99

The Sc galaxy M99 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 ~ 0.30 keV, with a total X-ray luminosity ~ 10^{41} erg/s 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 ~ 2 x 10^{40} erg/s and a hard spectrum well fitted by a power law of photon index Gamma ~ 1.7. This source is close to the location where a massive HI cloud appears to be falling onto the M99 disk at a relative speed > 100 km/s. 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 ~ 100 Msun. External collisions may trigger large-scale dynamical collapses of protoclusters, leading to the formation of very massive (>~ 200 Msun) stellar progenitors; we argue that such stars may later collapse into massive black holes if their metal abundance is sufficiently low.