ON THE NATURE OF THE BLUE-LIGHT IN CENTRAL CLUSTER GALAXIES

An excess blue component of light, extended over the innermost few kpc, is observed in several central cluster galaxies. Using a combination of blue optical spectra and IUE data, we show that the spectrum of the excess blue light is consistent with both star formation dominated by the light of B5 stars, and power-law spectra with slopes between lambda-1.5 and lambda-2.5, even allowing for intrinsic reddening. If the blue continuum is emitted by stars, we find that the data are more consistent with a fading starburst model than with continuous star formation. We compare possible origins for the blue continuum, such as the scattering of a hidden active nucleus, or a starburst induced by either the radio jets or a subcluster merger. Scattering of nuclear light requires a significant beaming factor if due to hot electrons, whilst dust-scattering is more efficient. Observations necessary to discriminate between the competing theories are discussed. We note the similarities to the extended blue continuum in distant radio galaxies, and suggest that the origin of the blue light is common to both types of system.