A ULX associated with a cloud collision in M99

Abstract

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.