Award Date

August 2017

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Physics and Astronomy

First Committee Member

Daniel Proga

Second Committee Member

Bing Zhang

Third Committee Member

Stephen Lepp

Fourth Committee Member

Darrell Pepper

Number of Pages



Active galactic nuclei (AGN) are among the most luminous objects in the universe and are known to be powered by accretion onto supermassive black holes in the centers of galaxies. AGN clouds are prominent components of successful models that attempt to unify the diversity of AGN. These clouds are often hypothesized to be the source of the broad and narrow line emission features seen in AGN spectra. Moreover, the high column densities of gas needed to account for broad absorption lines has been attributed to the same population of clouds, while the motion of AGN clouds has been invoked to explain the spectral variability observed in both broad absorption lines and warm absorbers.

Despite the importance of AGN clouds for explaining phenomena associated with AGN, we still lack a comprehensive understanding of the origin, dynamics, lifetime, and properties of these clouds. This thesis is an attempt to lay the groundwork for such a comprehensive model. After summarizing the known physics of AGN clouds and our modeling framework (i.e. the equations of hydrodynamics), we review the linear theory of the thermal instability (TI), which provides a natural mechanism to form clouds. We then extend this theory of cloud formation to account for the role of cloud acceleration, which must accompany the nonlinear regime of TI. After presenting hydrodynamical simulations that demonstrate how cloud formation and acceleration are intertwined processes, we explore how the efficiency of cloud acceleration is affected by the inclusion of flux variability. We find that the acceleration can more than double when the period of flux oscillations is longer than the thermal timescale of the gas. Next we calculate synthetic absorption line profiles to determine how clouds evolving along the line of sight would appear to a distant observer. We identify a spectral signature for cloud acceleration in the case of absorption line doublets. Finally, we show how global hydrodynamical simulations can be used to make predictions for the observables obtainable from reverberation mapping campaigns. We conclude with a summary of our findings and the next steps needed to further develop a comprehensive model of AGN clouds.


Active galaxies; Hydrodynamics; Instabilities; Numerical methods; Photoionization; Radiative transfer


Astrophysics and Astronomy | Physics