A ULX associated with a cloud collision in M 99

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 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 × 10 erg s 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 H I 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∼ 100M⊙. External collisions may trigger large-scale dynamical collapses of protoclusters, leading to the formation of very massive (& 200M⊙) stellar progenitors; we argue that such stars may later collapse into massive black holes if their metal abundance is sufficiently low.