The interplay between radio jets and ISM in sub-kpc radio sources

This paper reviews the evidence for interaction between radio jets and their environment in small (sub-kpc) and intermediate scale radio sources. Observations of gas (both neutral hydrogen and ionised gas) have shown the presence of fast (> 1000 km/s) outflows likely originating from this interaction. The characteristics of these AGN-driven outflows (e.g. mass outflow rate) indicate that they may have a relevant impact on the evolution of the host galaxy. We also report on the detection of large H I disks found around the host galaxies of nearby compact radio sources. Similar structures have so far not been detected around large radio sources. The presence of these structures in relation to the evolution of young compact radio sources is discussed. 1. Interaction between the radio jets and the ISM: why is important? In a hierarchical scenario, galaxy mergers play a major role in the formation of early-type galaxies and they may also provide the way to bring gas to the central regions, fuel the super-massive black hole (SMBH) and make it active. However, the hierarchical scenario would not fully explain the properties of the observed structures unless strong feedback occurs dumping large amount of energy on the interstellar and intergalactic medium. The onset of the quasar activity (including the radio-loud phase) is now considered as one of the important source of feedback, driving gas out of the central regions of galaxy, regulating start formation and the growth of the SMBH (Silk & Rees 1998; Fabian 1999; di Matteo et al. 2005; Hopkins et al. 2005). Thus, the initial phase of AGN can be crucial in the evolution of the host galaxy. In the radio-loud phase of the AGN, radio jets and the surrounding interstellar medium (ISM) can interact and affect each others. If the ISM is rich and dense, the effect of the surrounding medium on the radio jet can, for example, momentarily disrupt the jet but it can also frustrate or destroy it. On the other hand, the radio jets may have a profound influence on the medium. It can produce gas outflows that could clear the circum-nuclear regions. Thus, the interaction between radio-jets and ISM is one of the possible mechanisms for triggering outflows (together with radiation pressure and starburst winds). The radio jets can, therefore, provide one of the source of feedback. How relevant these outflows are, is the topic of this paper.