Theoretical Study of Radio Galaxies

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We have made theoretical investigations on the physical properties and bulk acceleration mechanism of relativistic jets in radio galaxies.1. We have solved the outflow of electron-positron pairs from hot accretion disks under the two-fluid and one-dimensional approximations. We have found that a major portion of the dissipated energy in the disk is converted to the internal and kinetic energies of pairs when the accretion rate is high, which confirms our previous result based on the one-zone approximation. This specific mechanism is the first successful one which connects gravitational energy release with jet formation.2. As for the bulk acceleration mechanism, we have investigated thermal acceleration of electron-positron pairs in a Wien equilibrium with photons at a relativistic temperature. We solved steady and spherically symmetric outflows and found that the terminal Lorentz factor easily reaches a few tens with a quite large efficiency. This mechanism is shown to naturally avoid previous difficulties of pair annihilation and radiation drag and thus proves to be very promising. The collimation problem remains to be open. This mechanism predicts strong MeV gamma-ray emission accompanying the jet formation, which can be checked by future observations.3. While a fair fraction of the energy of the relativistic jets is radiated in the internal shocks near the central engine, the remaining fraction is converted to the kinetic and internal energies of extended radio sources. We have examined the physical quantities such as magnetic field and relativistic electrons in the hot spots and lobes and found that they are compatible with our jet model consisting of mainly electron-positron pairs.