Date of Graduation
Eberly College of Arts and Sciences
Physics and Astronomy
Daniel J Pisano
Felix J Lockman
Galaxies in our universe must acquire fresh gas to continue forming new stars. A likely source of this material may be the gas that resides between galaxies. We do not, however, have a clear understanding of the specifics, such as its distribution. The first claimed detection of this "cosmic web" of material directly in emission was published a decade ago using the Westerbork Synthesis Radio Telescope in the Netherlands while surveying neutral hydrogen in the Local Group of galaxies. Later evidence, in the form of stellar surveys and test particle simulations, showed that a tidal origin of the gas was another possibility.;More recent survey work of the Local Group, specifically between the galaxies M31 and M33, motivated us to map a section of the Westerbork emission using the Robert C . Byrd Green Bank Telescope (GBT). Our survey covers a 12 square degree area between M31 and M33, in which we reach 21 cm column density sensitivities of 1017.2 cm-2 after 400 hours of observations. These observations provide more than a factor of five better spatial resolution, and better than a factor of three in velocity resolution. Not only do we confirm the emission seen in the Westerbork data, we find that the hydrogen gas is composed of clouds a few kiloparsecs across, with properties suggesting they are a unique population to the Local Group. We conclude that the clouds are likely transient condensations from an intergalactic filament of gas, although a tidal feature cannot currently be ruled out. We also conducted GBT pointings to the northwest of M31 to search for the extended emission seen in the Westerbork data as well. What detections we find appear to be more related to the high velocity cloud population of M31. We are continuing to map other regions around M31 to search for more diffuse emission.;We also present southern sky maps of the high velocity and intermediate velocity clouds around our own Milky Way, using 21 cm survey data from the Parkes telescope in Australia. The existence of these objects have been known for over 50 years, yet there is no general consensus as to their origins. The maps we have produced are the most detailed to date, with high spatial and velocity resolution and good sensitivity. By using a model of Milky Way rotation, we more effectively filter out foreground emission from our own Galaxy to produce these maps. We also discuss the basic global properties of this gas and the features that are seen. Apart from the Magellanic system and Galactic warp, most of the emission is in the form of small clouds. Some of the emission seen in these data may be representative of the eventual fate of the M31-M33 clouds. Work with this survey is ongoing.
Wolfe, Spencer A., "Neutral Hydrogen in the Local Group and around the Milky Way" (2014). Graduate Theses, Dissertations, and Problem Reports. 6969.