A ULX associated with a cloud collision in M99

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 ≈ 0 . 30 keV, with a total X-ray luminosity ≈ 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 ≈ 2 × 10 40 erg s − 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 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 ∼ 100 M ⊙ . External collisions may trigger large-scale dynamical collapses of protoclusters, leading to the formation of very massive ( & 200 M ⊙ ) stellar progenitors; we argue that such stars may later collapse into massive black holes if their metal abundance is sufficiently low.