A ultraluminous X-ray source 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 disc is dominated by hot plasma at kT ~ 0.30 keV, with a total X-ray luminosity of ~10⁴¹ergs^-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 of ~2 × 10⁴⁰ergs^-1 and a hard spectrum well fitted by a power law of photon index Γ ~ 1.7. This source is close to the location where a massive HI cloud appears to be falling on to the M99 disc at a relative speed of >100kms^-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 ~100M_solar. External collisions may trigger large-scale dynamical collapses of protoclusters, leading to the formation of very massive (>~200M_solar) stellar progenitors; we argue that such stars may later collapse into massive black holes if their metal abundance is sufficiently low.