Partial Obscuration and Reddening of Active Galactic Nuclei by Outflowing Dust
Physical and Biological Sciences
Department of Physics. Independent study (PHYS 199) and senior thesis
At the center of many galaxies lies an extremely bright region, called an Active Galactic Nucleus (AGN), that is powered by a supermassive black hole consuming nearby matter. Studying the light emitted by gas close to the black hole is our main way of knowing the masses of black holes and studying how they grow by swallowing gas. Gases such as hydrogen give out light at very precise wavelengths, but motion of the gas shifts the wavelength (the so-called Doppler shift). The intensity of hydrogren emission as a function of wavelength (what is called the “line profile”) gives information about the distribution of gas at different velocities. A long-standing problem, however, is that these line profiles are unexpectedly asymmetric and difficult to interpret with most expected distributions of gas velocities around the black hole. This has been a major unsolved problem in AGN research and is a major barrier to getting masses of black holes and understanding what is going on around them. An additional problem is that a recent major Hubble Space Telescope study has shown that the gas doesn't seem to respond as expected to changes in the overall brightness of the AGN. This work presents a novel solution to these issues by showing how small, compact dust clouds close to the black hole can temporarily block our view of parts of the region of gas close to the black hole. We have shown that this changes the characteristics of the light emitted by the gas in a way that naturally reproduces the observed asymmetry and complexity. By developing a computational model of the gas and obscuring dust clouds, I successfully modeled general asymmetries observed in many AGNs as well as those observed in two particularly well-studied ones. This model is consistent with a recently emerging picture of AGNs where dust close to the inner regions of an AGN is expelled outwards from the black hole by radiation pressure. The effects of the patchy obscuration modeled in this work make several testable predictions, and in general have important implications for the study of AGNs. A paper on this has been accepted for publication in Monthly Notices of the Royal Astronomical Society.
