Why radio-loud AGN?

In recent years, active galactic nuclei (AGN) have become more popular among a wider community. The possibility of using them to produce feedback effects that would help solve some of the questions connected to the hierarchical scenario of galaxy formation and evolution has made them particularly popular among theorists. Feedback effects associated with AGN-induced outflows are now routinely incorporated in models of galaxy evolution. Indeed, gas outflows may have a wide range of effects. For example, clearing up the circumnuclear regions and halting the growth of the supermassive black holes (see e.g. Silk & Rees 1998), as well as providing the mechanism that can cause the correlations found between the mass of the central super-massive black hole and the properties of the host galaxies. Outflows can also prevent the formation of too many massive galaxies in the early universe and can inject energy and metals into the interstellar and intergalactic medium. AGN-driven outflows could be a major source of feedback in the overall galaxy formation process. However, the characteristics of such feedback are poorly constrained and the exact relevance of gaseous outflows in galaxy evolution still needs to be evaluated.

AGN-driven outflows can have different origins. Here, I will concentrate on the role that the radio-loud phase of nuclear activity (and the presence of radio plasma jets) can play in the evolution of a galaxy. Radio-loud AGN are preferentially hosted by massive early-type galaxies.