Dr. Jones and Dr. Rudnick will conduct a coordinated program of magnetohydrodynamic (MHD) simulations and radio observations to study the Warm-Hot Intergalactic Medium (WHIM). In the early Universe, low-density gas is mixed with dark matter. As over-dense regions collapse to form the filaments of the 'cosmic web', the gas falls supersonically into these filaments, where it is shock-heated and ionized to form the WHIM. Magnetic fields will be amplified in the shocks, and electrons accelerated to relativistic speeds. This team aims to detect the filamentary WHIM by its polarized radio synchrotron radiation. They will concentrate on a redshift z ~ 0.1, just distant enough that the WHIM filaments appear narrow enough to distinguish them from the Galactic foreground, which predominates on angular scales larger than a degree across. The team will use MHD simulations to follow outflows from giant radio galaxies over megaparsec scales as they impinge on the WHIM, including a self-consistent treatment of cosmic ray acceleration and transport. The simulated objects will be compared to observed structures over the whole spectrum from radio to gamma-rays, to test models for the WHIM and the origin of intergalactic magnetic fields.
Two graduate students will be trained by participating in the research, along with undergraduate students. The team will visit schools, churches, civic organizations and state parks to share information about basic astronomy as well as their own research.
Under this award, our team carried out both theoretical and observational investigations of clusters of galaxies, the largest objects in the universe held together by gravity. These clusters are forming in today's universe from conditions that were set in place in the very early universe. Understanding those properties today helps us understand the history of the universe. But, cluster formation is very complex and sometimes very violent. So, deciphering the messages in cluster properties is challenging. Most of the atomic matter in clusters resides in the million degree, rarefied gas called the 'intracluster medium'. The structure, dynamics and history of the intracluster medium is accessible through X-ray emissions of the hot gas and also through radio emissions produced by sparse, but very high energy electrons interacting with magnetic fields. Those electrons and magnetic fields are byproducts of the violent formation of the clusters and also from supermassive black holes in cluster galaxies that drive collimated energetic, high speed jet outflows into the cluster. Those jets create huge cavities in the X-ray emitting gas that are then darker as X-ray sources, but brighter as radio sources. The research enabled by this award focused especially on understanding such processes that illuminate the clusters through radio emissions and what those emissions tell us about cluster formation. The theoretical effort in this study was based primarily on sophisticated numerical simulations of the intracluster dynamical processes outlined above, especially the interactions between jets and the gaseous media in realistic simulated clusters. Principle outcomes included discovery that even in galaxy clusters whose X-ray emissions seem to suggest they are 'quiet today' large scale gas motions may be strong enough to distort jet motions. Those distortions, in turn, are revealed through the shapes of the X-ray cavities and the radio emissions from within the cavities. The attached Figure A illustrates how such distorted X-ray cavities can be made visible by dividing a cluster image by a circularly symmetric average form. The major outcome of our observational program was the discovery that many clusters had more extensive radio emission than previously recognized. We accomplished this by developing and extending techniques, which now can be used by the broader community, to detect emission at very low surface brightness levels. We used both individual telescopes, such as the 100m Green Bank Telescope and interferometer telescopes such as the Very Large Array and the Westerbork Synthesis Radio Telescope. These results, in combination with information from various satellite programs in other wavelength bands, helped us distinguish between the various mechanisms whereby energy from the 10-100 million K plasma can get transferred to the relativistic one, and to the building up cluster magnetic fields. At present, our work supports the general idea that as clusters of galaxies grow by accreting new material from their surroundings, that turbulence develops in the hot gas, and can accelerate particles from radio galaxies and other sources until they are strong enough to produce detectable radio emission. One example of our results on the Coma Cluster is shown in the attached Figure B, made from observations with the Westerbork telescope. The colored image shows the diffuse cluster radio emission, with the contours showing all radio emission from this direction, dominated by individual compact radio galaxies whose contribution needs to be eliminated to study the diffuse emission. Our work also contributed to the professional development of the next generation of scientists, providing research experience to a number of graduate and undergraduate students. In addition, we used our experiences in research to support planetarium and a variety of other programs for the general public.
