摘要

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 disc is dominated by hot plasma at kT approximate to 0.30 keV, with a total X-ray luminosity of approximate to 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 of approximate to 2 x 10(40) erg s(-1) and a hard spectrum well fitted by a power law of photon index Gamma approximate to 1.7. This source is close to the location where a massive H I cloud appears to be falling on to the M 99 disc at a relative speed of > 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 similar to 100 M(circle dot). External collisions may trigger large-scale dynamical collapses of protoclusters, leading to the formation of very massive (greater than or similar to 200 M(circle dot)) stellar progenitors; we argue that such stars may later collapse into massive black holes if their metal abundance is sufficiently low.

  • 出版日期2006-11-11