Collaborative Research: Star Formation Processes in Dwarf Galaxies

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AST 0204922HunterDwarf irregulars are the most common type of galaxy in the Universe. They evolve relatively slowly over time and chemically resemble the outer parts of present-day spirals and young galaxies seen at high redshift. They have no density waves and little shear. Some are very close to us, permitting the resolution of clusters and star-forming regions. As a result, dwarf irregulars are ideal laboratories for studies of star formation in a pristine environment. They are also useful for comparisons with spirals, leading to a better understanding of both.The goals of this project are to determine what regulates star formation in dwarf galaxies on a wide range of scales, and then to apply this knowledge to other galaxies. Why do star formation rates in normal dwarf galaxies span a factor of over 104, and why are young stars always limited to the inner galaxy regions, far inside the radial extent of other stars and gas? What governs how and where dense molecular clouds form? To address these questions, Dr. Deidre Hunter, at Lowell Observatory, and Dr. Bruce Elmegreen, at IBM, have observed 139 reasonably normal, non-interacting, nearby galaxies without spiral arms. The data include optical and near-infrared imagesthat allow the star formation to be determined over the lifetimes of these galaxies, atomic HI maps that show the raw cloud material and environment for cloud formation, and observations of the warm and cold molecular clouds that give birth to the stars themselves. Although a previous grant emphasized data collection and processing, these investigators have already analyzed the properties of the HII regions in the sample, including their sizes, pressures, luminosities, and locations. They studied the neutral interstellar structure in the LMC and combined all their data for a comprehensive study of NGC 2366. The results have led to a revision in the theoretical framework for star formation: global instabilities and threshold column densities for star formation are less important than local processes and conditions; the thermal equilibrium properties of the interstellar medium are as important as self-gravity; turbulence and phase changes generally structure the gas into cloud and intercloud media, yet global disturbances such as bars and interactions change this, producing anomalously massive bound clusters. The results have also enhanced our perspective on star formation processes in normal spiral galaxies by illustrating how the expansion of shells and the condensation of clouds becomes more severe at low shear. With this new award, the investigators will capitalize on their large, multi-wavelength survey to finish the project by addressing the "big-picture" questions posed above. They will find how star formation evolves over space and time in a dwarf galaxy; consider the special effects at galaxy edges; show how the star formation history relates to the gas structure and other conditions for cloud formation; determine the formation requirements for star forming regions of different sizes and densities; determine the porosity of the interstellar medium and its role in possible feedback; and determine the overall structure of the dwarfs. Many of these results will have applications to other galaxy types and to other epochs in the Universe. ***