The star-forming environment of an ultraluminous X-ray source in NGC 4559: an optical study

We have studied the candidate optical counterparts and the stellar population in the star-forming complex around the bright ultraluminous X-ray source (ULX) in the western part of the spiral galaxy NGC 4559, using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/Optical Monitor and ground-based data. We find that the ULX is located near a small group of OB stars, but is not associated with any massive young clusters nor with any extraordinary massive stars. The brightest point source in the Chandra error circle is consistent with a single blue supergiant (BSG) of mass approximate to20 M-circle dot and age approximate to10 Myr. A few other stars are resolved inside the error circle: mostly BSGs and red supergiants (RSGs) with inferred masses approximate to10-15 M-circle dot and ages approximate to20 Myr. This is consistent with the interpretation of this ULX as a black hole (BH) accreting from a high-mass donor star in its supergiant phase, with mass transfer occurring via Roche-lobe overflow. The observed optical colours and the blue-to-red supergiant ratio suggest a low metal abundance for the stellar population: 0.2 less than or similar toZ/Z(circle dot)less than or similar to 0.4 (using the Padua tracks), or 0.05 less than or similar toZ/Z(circle dot)less than or similar to 0.2 (using the Geneva tracks). The age of the star-forming complex is less than or similar to30 Myr. Halpha images show that this star-forming region has a ring-like appearance. We propose that it is an expanding wave of star formation, triggered by an initial density perturbation, in a region where the gas was only marginally stable to gravitational collapse. We also suggest that the most likely trigger was a collision with a satellite dwarf galaxy going through the gas-rich outer disc of NGC 4559 less than 30 Myr ago. The culprit could be the dwarf galaxy visible a few arcsec north-west of the complex. If this is the case, this system is a scaled-down version of the Cartwheel galaxy. The X-ray data favour a BH more massive (M > 50 M-circle dot) than typical Milky Way BH candidates. The optical data favour a young BH originating in the recent episode of massive star formation; however, they also rule out an association with young massive star clusters (none are present in the X7 field). We speculate that other mechanisms may lead to the formation of relatively massive BHs (perhaps Msimilar to 50-100 M-circle dot) from stellar evolution processes in low-metallicity environments, or when star formation is triggered by galactic collisions.