| I will discuss far-ultraviolet (FUV) and optical observations performed with the Hubble Space Telescope and the Very Large Telescope of the Brightest Cluster Galaxies (BCG) in the Abell 2597 and Abell 2204 galaxy clusters.
FUV and U band continuum emission is found to be enhanced in clumpy, filamentary structures centered on the BCG nuclei. These structures can be traced out to 20 kpc and their location agrees with previously known gas filaments.
Excess FUV and U band light is determined by removing emission due to the underlying old stellar population. We find the excess FUV and U emission to be spatially coincident. A naive stellar interpretation of this emission requires the existence of a large amount of very hot 30000-50000 K stars.
Correcting for nebular continuum emission and dust implies that normal stars can not explain the observations, unless very special circumstances apply. Other, likely non-stellar, processes are required, but the most obvious ones fail. What produces the extreme FUV to U colors in these BCGs?
|Oonk et al. 2011 (arXiv:1103.1765, MNRAS in publication)