X-Ray Analysis of Active Galactic Nuclei in Clusters from the Dark Energy Survey
Physical and Biological Sciences
UCSC STEM Diversity Programs Summer Research Experience (SRE) 2015 / Senior Thesis
The central supermassive black hole (SMBH) characteristic of every massive galaxy is known to have two different states: dormant and active. Active SMBHs in galaxies are known as active galactic nuclei (AGN), which are SMBHs that are accreting mass from the central region of their host galaxies. This accretion of mass occurs when there is a large influx of gas onto the central SMBH that could arise from environmental changes such as recent star formation or two galaxies merging with one another. When matter accretes onto the black hole an enveloping accretion disk is formed, and in this disk X-ray photons are released as a result of the scattering of particles. This causes AGN to be extremely luminous in X-ray. There are many fewer active galaxies than dormant ones, but every galaxy has a massive black hole at its center, so scientists theorize that AGN are just a phase of evolution that every galaxy endures. Hence, understanding the evolution of AGN is key to understanding the evolution of galaxies. My research studies AGN in galaxy clusters, so that we can study how environment affects AGN evolution. My research focuses on how galaxy cluster AGN evolve as a function of redshift and galaxy cluster mass. To obtain my sample of AGN in clusters I cross-matched the positions of documented cluster galaxies from the Dark Energy Survey (DES) with the positions of documented AGN in Chandra X-ray observations. This allowed me to observe the number of AGN detected within a given galaxy cluster and to determine how that corresponded to that cluster’s redshift or mass. Additionally, I used X-ray information from Chandra to calculate each AGN’s luminosity in order to probe their individual energy output and consequently apply appropriate luminosity cuts to my sample to ensure completeness. With this research I have reaffirmed observations in recent scientific literature that AGN activity in a galaxy cluster has a strong positive correlation with redshift, i.e. that there are more AGN in galaxy clusters at earlier times in the universe compared to the present. Secondly, I found that AGN activity in galaxy clusters has no correlation with galaxy cluster mass, which is in contrast with what has been observed in previous studies. This indicates that there may not be a clear-cut relationship between AGN activity and galaxy cluster mass as scientists previously believed. Since AGN activity is thought to be highly dependent on environment, these results reveal that future analyses of galaxy cluster AGN need to focus on the physical processes that occur in galaxy clusters and how they affect galaxy evolution.
