  We use the semi-analytical model of galaxy formation GALFORM to characterise
an indirect signature of AGN feedback in the environment of radio galaxies at
high redshifts. The predicted environment of radio galaxies is denser than that
of radio-quiet galaxies with the same stellar mass. This is consistent with
observational results from the CARLA survey. Our model shows that the
differences in environment are due to radio galaxies being hosted by dark
matter haloes that are ~1.5 dex more massive than those hosting radio-quiet
galaxies with the same stellar mass. By running a control-simulation in which
AGN feedback is switched-off, we identify AGN feedback as the primary mechanism
affecting the build-up of the stellar component of radio galaxies, thus
explaining the different environment in radio galaxies and their radio-quiet
counterparts. The difference in host halo mass between radio loud and radio
quiet galaxies translates into different galaxies populating each environment.
We predict a higher fraction of passive galaxies around radio loud galaxies
compared to their radio-quiet counterparts. Furthermore, such a high fraction
of passive galaxies shapes the predicted infrared luminosity function in the
environment of radio galaxies in a way that is consistent with observational
findings. Our results suggest that the impact of AGN feedback at high redshifts
and environmental mechanisms affecting galaxies in high halo masses can be
revealed by studying the environment of radio galaxies, thus providing new
constraints on galaxy formation physics at high redshifts.
