Active galactic nuclei jet-induced feedback in galaxies - I. Suppression of star formation

Relativistic jets originating from supermassive black holes can have a considerable impact on the interstellar/intergalactic medium within which they propagate. Here, we study the interaction that a relativistic jet, and the cocoon associated with its penetration into the ISM, has on the evolution of a dense cloud, placed very near the cocoon's path, by analysing a series of high-resolution numerical simulations, and studying the dependence on jet input power, between P-jet = 10(41) and 10(47) erg s(-1). The density probability distribution function within the cocoon can be described in terms of two distinct components, which are also spatially distinct: a low-and a high-density component. The former is associated with the shocked gas within the internal region of the cocoon, while the latter is associated with the outer, shocked region of the cocoon itself. The PDF of the post-shocked region is well approximated by a modified lognormal distribution, for all values of Pjet. During the active phase, when the jet is fed by the AGN, the cloud is subject both to compression and stripping, which tend to increase its density and diminish its total mass. When the jet is switched off (i.e. during the passive phase), the shocked cloud cools further and tends to become more filamentary, under the action of a backflow which develops within the cocoon. We study the evolution of the star formation rate within the cloud, assuming this is determined by a Schmidt - Kennicutt law, and we analyse the different physical factors which have an impact on the star formation rate. We show that, although the star formation rate can occasionally increase, on time- scales of the order of 10(5) - 10(6) yr, the star formation rate will be inhibited and the cloud fragments. The cooling time of the environment within which the cloud is embedded is, however, very long: thus, star formation from the fragmented cloud remains strongly inhibited.