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Cosmic ray acceleration of gas in active galactic nuclei Eilek, Jean Anne

Abstract

Dynamical models of Seyfert nuclei and quasi-stellar objects are presented. The central energy source often postulated for these active objects provides a means of heating and ionizing the nuclear gas, and also exerts an outward force on the gas. Since the gas will be fully ionized, it will be nearly transparent to X-rays, while cosmic rays will interact strongly with it. Preliminary calculations of this "ionization" pressure on discrete clouds show that photons are unlikely to produce the high gas velocities relative to the nucleus which are indicated by the emission line profiles in Seyfert nuclei and the blueshifted quasar absorption lines, but that cosmic rays can accelerate the clouds up to these velocities. A more detailed calculation taking into account the dynamics of the gas is called for. A computer code was written to solve the spherically symmetric hydrodynaraic equations numerically. It uses a finite difference, implicit Eulerian scheme to solve the time dependent equations. As well as the mass conservation and momentum transfer equations, the numerical system includes an energy equation which allows for ionization and Coulomb heating, and radiative cooling. The code was used to obtain a set of nuclear evolutionary models. These models involve a static gas surrounding a quiescent energy source which turns on suddenly. A range of input physical parameters is represented: for sizes 0.1 to 1 pc, a total cosmic ray flux from 10⁴³ ergs s⁻¹ to 10⁴⁸ ergs s⁻¹, a gas density of 10⁴ to 10⁸ cm⁻³, a lowest particle energy in a power law spectrum of 0.1 to 10.0 MeV, and a central mass of 10⁸ or 10⁹ M⃙. Such soft cosmic rays have a very short absorption length in the nuclear gas. This means a narrow region in radial extent will gain the momentum of the cosmic ray beam, and an outward moving shell will form. It snowplows the cooler gas ahead of it and leaves a less dense, hot cavity behind. This thin cavity reaches temperatures of 10⁸ K, and the dense shell reaches an equilibrium temperature in the range 10⁴-10⁵ K. The shell velocities increased as the cosmic ray flux was increased, ranging from 500 to 8000 km s⁻¹. The lifetime of this phenomenon is the time for the shell to escape the nuclear region, which is only a few parsecs across. At these velocities, the timescale is only 10³ to 10⁴ years. This suggests repetitive rather than continuous activity of the central source. A quiescent phase would allow replenishment of the gas from extra-nuclear stellar sources. The interface between the hot cavity and the shell is Rayleigh-Taylor unstable with a fragmentation time approximately equal to the shell escape time. This may explain the cloud structure observed in these objects. Thermal instabilities may also arise if the central source turns off. Prediction of the sources of the permitted and forbidden emission lines is dependent on the behavior of the instabilities. The very dense shell suggests a physical distinction between the regions producing the two types of spectra, which may explain the wider permitted lines in some sources. The hot gas near the energy source will produce thermal X-rays. The luminosity and temperature predicted for the X-rays is consistent with observations.

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