| Radio outflows of active galactic nuclei (AGN) are invoked in cosmological models as a key feedback mechanism in the latest phases of massive galaxy formation. However, from an observational point of view, the impact of such a mechanism on galaxy formation and evolution is still poorly understood. I will present our results, based on radio-selected samples at low (SDSS/NVSS and 3CRR surveys; z<0.3) and high redshifts (COSMOS survey, z<3), that for the first time observationally test the importance of radio-mode feedback in massive galaxy formation (out to z~3).
In particular, in the context of the commonly adopted blue-to-red galaxy evolution scenario we find that the two major radio AGN populations -- the powerful high-excitation, and the weak low-excitation radio AGN -- represent two, earlier and later, stages of massive galaxy build-up. To expand this study to higher redshifts, we developed a new method that efficiently selects weak AGN (such as Seyfert, LINER, and absorption line AGN) based only on their NUV-NIR photometry. This method allowed us to study, for the first time, the cosmic evolution of weak radio AGN out to z~3, which can directly be linked to the radio-mode feedback prediction in cosmological models.