Date on Senior Honors Thesis
11-2025
Document Type
Senior Honors Thesis
Degree Name
B.S.
Cooperating University
University of Louisville
Department
Physics and Astronomy
Degree Program
College of Arts and Sciences
Committee Chair
Dr. Benne Holwerda
Committee Member
Dr. Timothy Dowling
Committee Member
Dr. Thomas Riedel
Author's Keywords
Active Galactic Nuclei, galaxies, Large-scale structure, BPT diagrams,
Abstract
We examine how the presence of active galactic nuclei (AGN) correlates with location in large-scale cosmic structures using the Galaxy and Mass Assembly (GAMA) survey across the G09, G12, and G15 fields. Our sample contains 18,927, 9,273, and 1,148 galaxies {with} highly dense filaments, moderately dense tendrils, and highly underdense voids, respectively. AGN galaxies were identified using Baldwin-Phillips-Telverich (BPT) diagnostic diagrams based on [NII], [SII], and [OI]. We compare AGN fractions in filament, tendril, and void regions, and as a function of distance from the nearest filament centerline. Our results reveal a mild excess in filament compared to void, when using [SII]- and [NII]-based classifications, while no significant environmental dependence is found for [OI]-based classifications. Overall, we find weak environmental trends with AGN activity, which suggests that {the} local environment does not always dominate AGN activity; instead, secular processes are likely to be at play. Our findings are consistent with previous studies reporting only marginal overdense preferences for AGN.
Recommended Citation
Patel, Divya A., "Active Galactic Nuclie in Diverse Galactic Environments" (2025). College of Arts & Sciences Senior Theses. Paper 349.
Retrieved from https://ir.library.louisville.edu/honors/349
Lay Summary
Billions and billions of galaxies make up our Universe, like the trillions of cells make up us. The various types of galaxies in the Universe are defined and characterized by mechanisms and properties unique to each. For instance, our Milky Way galaxy is a star-forming spiral galaxy: a large galaxy with spiral arms populated with hot, dusty regions where stars are born. Other types, such as quiescent galaxies, are defined by their lack of star-forming regions. But every massive galaxy, like our Milky Way, contains a mysterious beast in its center: a supermassive black hole (SMBH). Oftentimes, SMBHs are found in a sleeping state with no gas accretion taking place. However, due to the high gravitational well created by SMBH, SMBH experiences an accretion disk created by infalling gas that circulates the SMBH until it is consumed. In this process, the sleeping SMBH is now awakened. Galaxies that host accreting SMBH are called Active Galactic Nuclei (AGN). AGN galaxies, our focus for this study, are defined by a powerful, active supermassive black hole at their center that blows violent jets of electromagnetic radiation, which can be observed across billions of light-years.}\\
{The central puzzle we address is what controls the awakening of SMBH. In other words, how does the gas flow from the outskirts or outside of the host galaxy to inwards? We investigate whether a galaxy's activity is determined solely by its internal gas supply or if its cosmic neighborhood plays a role. Galaxies are not randomly spread out; they are organized into a colossal network called the Cosmic Large-Scale Structure (LSS), held by a "superficial" glue called Dark Matter. LSS consists of three environments: dense filaments, smaller tendrils, and vast, isolated voids. Our research compares the fraction of AGN found in these three distinct environments to see if the awakening of SMBH depends on whether the host galaxy is part of a filament, tendril, or void.}\\
{To identify these active galaxies, we use specialized astronomical tools called BPT diagrams (Baldwin, Phillips, and Terlevich). These diagrams analyze the ratios of specific light emissions from the gas, acting like a chemical signature test. They allow us to differentiate between gas energized by young stars and gas energized by the hard, penetrating radiation of an active black hole. By examining the results from the three main BPT diagrams ($\text{NII}, \text{SII}, \text{OI}$), we not only count the number of AGN but also gain insight into the physical state of the gas around them—things like temperature, density, and ionization. This detailed analysis allows us to determine if the environment not only dictates whether a black hole is active, but also how its activity is manifested in the surrounding cosmic gas.
