2002MNRAS.337..470B

In a programme of deep imaging of quasar fields, we have found an unusual compact group of galaxies associated with the radio-loud quasar MRC B0941-200 at z = 0.715. The group of galaxies comprises at least five obvious members projected within 10 arcsec (∼75 kpc) of the quasar, all with very similar optical and infrared colours. Spectroscopy has confirmed that the brightest galaxies are at z = 0.715, and infers a minimum age of 3 Gyr for their dominant stellar populations. In addition, the galaxy group is embedded in diffuse continuum emission, visible from B through to K band and with observed B - I colours 2 mag or more bluer than the galaxies. Extended narrow-line emission is also detected across the whole region of the diffuse continuum emission. At greater distances (∼500 kpc) from the quasar, we have also detected an excess of faint, red galaxies with colours identical to those of the group galaxies. This suggests that the group, including MRC B0941-200, lies at the edge of a larger structure of galaxies, akin to a loose group or cluster. We show how the characteristics of the field of MRC B0941-200 can be understood using our knowledge of extended emission-line regions in high-redshift radio sources, and by extrapolating our understanding of nearby compact groups to high redshift, allowing for increased star formation rates and galactic gas content in high-redshift galaxies.