Dr. Brian Morsony is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at the University of Wisconsin-Madison. Dr. Morsony will simulate both cluster-center active galactic nuclei (AGN) and non-central AGN to assess their relative importance as a sources of heat and turbulence for the cluster and to determine how they change the magnetic field configuration of the cluster. Dr. Morsony will also determine how a range of different cluster environments affect the AGN, with the goal of creating a general prescription for AGN feedback that can be used in cosmological simulations where AGN jets are unresolved.
A feedback mechanism is necessary to prevent the runaway cooling and in-fall of gas in galaxy clusters. The primary candidate for feedback is AGN powered by accretion of cool gas, which create jet outflows that heat the cluster gas to form a self-regulating cycle of cooling, accretion, outflow and heating. Understanding how feedback operates is critical to our interpretation of observations of galaxy clusters and to understanding large-scale structure formation. The proposed research will carry out a series of three-dimensional numerical simulations of AGN outflows in galaxy clusters. The simulations will include magnetohydrodynamics, gas cooling and star formation, and a link between accretion rate and AGN luminosity. This will, for the first time, allow the complete feedback loop to be simulated self-consistently. Simulations will also allow the production of synthetic X-ray and radio observations for comparison with existing observations and to determine the capabilities needed for future observatories.
Dr. Morsony will also develop educational materials using black holes and associated astrophysical objects, such as AGN, gamma-ray bursts and supernova, as a tool for teaching and outreach. Materials developed will include visualization and presentation material and active-engagement questions aimed at a high-school and introductory undergraduate level. These materials will be developed through talks to the public and high school students and by teaching an undergraduate course on the Astrophysics of Black Holes. Materials will also be distributed online and by incorporation into planetarium content.
This program examined how jets interact with their environments. Jets are very fast, narrow beams of material, powered by accretion onto a black hole. Our main focus was on simulations of jets from supermassive black holes in the centers of galaxies, called Active Galactic Nuclei (AGN), and how they interact with the dense environment of groups and clusters of galaxies. Our first project was simulations of AGN jets moving through a dense background. In groups of galaxies, the galaxies orbit each other and are moving relative to the inter-galactic gas. As jets emerge from a galaxy, they encounter a head-wind which causes the jets to deflect and bend backwards. These objects are seen in nature as bent-double radio sources. How sharply the jets are bent can be used to figure out the density of the surrounding gas. This density, in turn, is used to calculate how much gas is in the galaxy group. Our simulations allowed us to refine how well the gas density can be estimated, and take into account effects such as the jets or motion of the galaxy being pointed towards or away from Earth, rather than being only in the plane of the sky (Morsony et al. 2013). Our second project involved the evolution of heavy elements in galaxy clusters and the impact of AGN jets. Most of the ordinary matter in galaxy clusters is found in hot, X-ray emitting gas in between the galaxies. All elements heavier than hydrogen and helium, collectively referred to as "metals" by astronomers, must be produced in stars and their supernova explosions. In clusters, stars are rare but the total amount of metal is still high. Overall, about 3 times as much metal must be produced per star in galaxy clusters compared to the Milky Way. Once produced, the extra metal must be moved out of the galaxies and into the hot gas between galaxies. Stars form with a distribution of masses, called the Initial Mass Function (IMF), which typically has many low-mass stars, then the number of stars at higher masses progressively decreases up to a high-mass cutoff. Because supernova only happen in higher-mass stars, but most stars are low-mass, changing the slope of the IMF and changing the high-mass cutoff can change how many supernova happen for the same number of stars created. We found (Morsony et al. 2014) that the high-mass cutoff is particularly important if a type of supernova called pair-instability supernova (PISN) are included. PISN only happen for very massive stars but they produce 100 times as much metal as a normal supernova. We found that just increasing the high-mass cutoff will increase the amount of metal produced by a population of stars by about 70%. By also making the slope of the IMF a flatter, the metal production can be increased by a factor of around 3. We also modeled how metal formed in galaxies can be mixed with the surrounding gas in galaxy clusters. We simulated AGN jets in the central galaxy in the cluster. These AGN produce large bubbles of very hot gas, seen in radio and X-ray observations of clusters. By starting with a model where the central galaxy is rich in metals, we were able to follow how the AGN lifts metals out of the cluster center and into the surrounding gas. We found that metals are dredged up behind the rising bubbles. Even after the AGN jets turn off, metal continues to be drawn into the bubbles. The AGN are very effective at moving metal out of the central galaxy, with up to 25% of the metal being removed in just 100 million years, a short time compared to the age of the cluster. The education and outreach component of the program involved using black holes as a means to generate interest in astronomy and science in general. The main component was the creation of a course on "Black Holes in the Universe" at the UW-Madison. This course was taught for the first time by Dr. Morsony in Spring 2013, with the plan of making it a regular offering. Dr. Morsony also gave presentation to physics classes and the public at Hercules High School, Wisconsin Space Place the Milwaukee Astronomical Society. Dr. Morsony helped carry out the 2013 Demographics Survey for the Committee on the Status of Women in Astronomy. This survey asked for the number of women and men as all levels between graduate student and full professor at astronomy departments and other major facilities in the United States. This is the 4th in a series of surveys dating back to 1992, providing valuable longitudinal information about how the gender balance in the field has evolved. The study will have an impact on future hiring strategies and recommendation for how to increase the participation of women astronomy.