2018 Winner: The Location and Quantity of the Dust Causing Internal Reddening of Active Galactic Nuclei

Project Information
The Location and Quantity of the Dust Causing Internal Reddening of Active Galactic Nuclei
Physical and Biological Sciences
Department of Physics. Independent study (PHYS 199) and senior thesis.
In this two-part research project, we studied the cosmic dust that surrounds active galactic nuclei (AGNs). The accepted standard model of an AGN has a supermassive black hole in the center, which is constantly drawing in matter from the surrounding galaxy. As matter spirals into the black hole, gravitational energy released causes the gas to glow. While black holes themselves cannot emit light, we can still make spectroscopic analyses of the light emitted by the infalling matter. Dust along the line of slight affects spectroscopic measurements because it makes the light emitted look redder and fainter, as it is scattered and absorbed by the dust. This is known as "reddening." In the past the effects of cosmic dust have rarely been taken into account.

The first part of our study was an investigation to determine the location of the dust. We studied several reddening indicators and intercompared them. Within AGNs, high-density gas orbits close to the black hole (typically light weeks away from it), and low-density, outflowing gas is located much further away (typically tens to hundreds of light years from the black hole itself). We established that the probable location of the majority of the dust is between these two regions of gas.

The second part of the study was an analysis of the color variability of over 6000 AGNs. We applied a novel method to determine a) the amount of dust along the line of sight, and b) how the color of the accretion disk around a black hole changes as its brightness changes. This was the largest such study to date by more than an order of magnitude. It is also the first systematic of changes in the ultraviolet light.
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  • Clio Zane Powell Heard (Merrill)