On The Origin Of Neutral Hydrogen Clouds In Nearby Galaxy Groups: The Role Of Galaxy Interactions

Abstract

This thesis addresses the origin of the High Velocity Clouds (HVCs) observed around the Milky Way and other nearby galaxies. Such clouds are likely created through a combination of galactic fountains, galaxy interactions, dark matter substructure, and cold accretion. The aim is to explore the role of galaxy interactions in generating such clouds. This thesis also tests predictions for HI-embedded dark matter halos made by the Lambda Cold Dark Matter (Lambda-CDM) cosmological model.

I address these questions by using sensitive radio observations of nearby galaxy groups, to search for clouds of neutral hydrogen, and by comparing the observations to numerical simulations. The groups have varying levels of galaxy interactions by design. I find 9 clouds in the M81 galaxy group - the most highly interacting system. I also find 4 clouds that may be associated with the Milky Way HVC Complex A. I find two HI clouds in the direction of the Canes I galaxy group. However, these clouds may be HVCs associated with either the Canes I group or the more distant galaxy KUG 1218+387.

Using a statistical comparison of observed data to numerical models, I show that the M81 group HI clouds are unrelated to dark matter halos. Since the clouds are located in the most highly interacting group, it is likely that the clouds are formed via galaxy interactions. However, more simulations are needed to determine the expected properties for HI clouds generated through galaxy interactions. To this end, I have run a suite of galaxy group simulations, and have begun analysis.

Future work in determining HI cloud formation mechanisms should include high-resolution HI observations with the EVLA to determine substructure; UV and optical observations to search for a stellar component; and comparison with simulations of the M81 Group that include all the relevant gas physics and allow for cold accretion.