Redshift histogram (top panel) and colour–magnitude diagram (bottom) of 2XMMp J092120.2+371735. Objects with spectra are marked green, while the members of the high-redshift group are shown in red. Blue refers to the foreground population of galaxies, which is unlikely to be responsible for the X-ray emission. Error bars plotted in panel b indicate the photometric uncertainties and are visible when they exceed the symbol size.