6.1. Toward a Working Model of AGN Jets
Relativistic jets are the conduits that connect supermassive, spinning black holes and their atten- dant disks to their host galaxies and the Universe beyond. They were once seen as exhausts that, like their automotive counterparts, remove excess heat from powerful machines. However, the ob- servations that we have reviewed suggest a rather different metaphor. Black holes are turbines that are spun up by orbiting gas to generate high-voltage electrical power, and AGN jets are lossy, glow- ing coaxial cables that ultimately heat their surroundings. This change of viewpoint is supported by observations of selected local AGNs, from which we tentatively generalize to AGNs in general.
Nonetheless, this view suggests causal and testable mechanisms through which the type of AGN that is produced is determined by the black hole mass, spin, gas supply rate, and strength of the magnetic field, which in turn may depend upon the manner by which gas is supplied to the nucleus at Rinf . In particular, it appears that only powerful jets can escape the nucleus as su- personic, relativistic flows. This model also suggests that AGN jets can accelerate protons to EeV energy and that they could be significant sources of high-energy cosmic rays. This further allows jet γ -ray emission to be mostly synchrotron instead of Compton radiation. In toto, AGNs con- tribute to and are constrained by the γ -ray background (Di Mauro et. al. 2018). Understanding the content, power, electrical current, and duty cycles of AGN jets will help quantify their role in galaxy formation and evolution.
