We use the Hubble Space Telescope Advanced Camera for Surveys (ACS)/Solar Blind Channel (SBC) and Very Large Telescope (VLT) Focal Reducer/low dispersion Spectrograph (FORS) cameras to observe the brightest cluster galaxies (BCGs) in Abell 2597 and Abell 2204 in the far-ultraviolet (FUV) F150LP and optical U, B, V, R, I Bessel filters.
The FUV and U-band emission is enhanced in bright, filamentary structures surrounding the BCG nuclei. These filaments can be traced out to 20 kpc from the nuclei in the FUV. Excess FUV and U-band light is determined by removing emission due to the underlying old stellar population and mapped with 1-arcsec spatial resolution over the central 20-kpc regions of both galaxies.
We find the FUV and U excess emission to be spatially coincident, and a stellar interpretation requires the existence of a significant amount of 10 000–50 000 K stars. Correcting for nebular continuum emission and dust intrinsic to the BCG further increases the FUV to U-band emission ratio and implies that stars alone may not suffice to explain the observations. However, lack of detailed information on the gas and dust distribution and extinction law in these systems prevents us from ruling out a purely stellar origin.
Non-stellar processes, such as the central active galactic nucleus, scattering, synchrotron and Bremsstrahlung emission are investigated and found to not be able to explain the FUV and U-band measurements in A2597. Contributions from non-thermal processes not treated here should be investigated.
Comparing the FUV emission to the optical Hα line emitting nebula shows good agreement on kpc-scales in both A2597 and A2204. In concordance with an earlier investigation by O’Dea et al. (2004), we find that O stars can account for the ionizing photons necessary to explain the observed Hα line emission.