The UV-optical colours of brightest cluster galaxies in optically and
  X-ray selected clusters
  Many brightest cluster galaxies (BCGs) at the centers of X-ray selected
clusters exhibit clear evidence for recent star formation. However, studies of
BCGs in optically-selected clusters show that star formation is not enhanced
when compared to control samples of non-BCGs of similar stellar mass. Here we
analyze a sample of 113 BCGs in low redshift (z<0.1), optically-selected
clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in
X-ray selected clusters. We convolve the SDSS images of the BCGs to match the
resolution of the GALEX data and we measure UV-optical colours in their inner
and outer regions. We find that optically-selected BCGs exhibit smaller scatter
in optical colours and redder inner NUV-r colours than the control galaxies,
indicating that they are a homogenous population with very little ongoing star
formation. The BCGs in the X-ray selected cluster sample span a similar range
in optical colours, but have bluer NUV-r colours. Among X-ray selected BCGs,
those located in clusters with central cooling times of less than 1 Gyr are
significantly bluer than those located in clusters where the central gas
cooling times are long. Our main conclusion is that the location of a galaxy at
the centre of its halo is not sufficient to determine whether or not it is
currently forming stars. One must also have information about the thermodynamic
state of the gas in the core of the halo.
