The interstellar medium and star formation of nearby, low-mass galaxies.

Abstract

This thesis presents four different studies of the interstellar medium (ISM) and stellar content of [approximately] 40 nearby (D [approximately]< 4 Mpc), low-mass galaxies. We aim to address two fundamental questions: “How do stellar processes effect the ISM in low-mass galaxies?” and “What are the local gas conditions which lead to molecular cloud formation?” Much of the data presented here come from our survey the “Very Large Array - Advanced Camera for Surveys Nearby Galaxy Survey Treasury” (VLA-ANGST). VLA-ANGST is a targeted atomic hydrogen (H I) emission line survey directed towards 35 low-mass galaxies selected from the ANGST Hubble Space Telescope (HST) galaxy sample of the nearby universe. The VLA-ANGST project is the largest survey of its kind, demanding nearly 600 hours of VLA observing time. This unprecedented amount of observing time gives us data which has long lasting legacy value for its wealth of high resolution and high sensitivity information on the H I gas content and dynamics in a large sample of nearby, low-mass galaxies. H I data from the VLA-ANGST project will be used to explore the interactions between the gas and stellar content as well as trace the underlying dark matter distribution. Combining the H I and HST data with other tracers of recent star formation (e.g., emission processes from far ultraviolet star light, dust in the infrared, and carbon monoxide in the submillimeter) provides a comprehensive census of each galaxy, useful for understanding their evolution. We investigate the role of multiple generations of star formation in the formation of large, kiloparsec scale cavities observed in the global H I distributions of five nearby, low mass galaxies. The small gravitational potential wells of some low-mass galaxies allow the outflow of energy from stellar processes (e.g., winds, supernovae, etc.) to help shape their gas distributions. We find that stellar processes produce ample energy (at least an order of magnitude or more) to have been the dominant creation source for the observed cavities. The molecular gas responsible for the formation of stars remains elusive in many of the low-mass galaxies. We present a novel new technique to trace the immediate precursor of the molecular gas: cold H I. We apply our technique to a large sample of 31 nearby, low-mass galaxies and detect cold H I in [approximately] 85% of the final sample (23/27) after quality control cuts are applied. The cold H I discoveries presented here represent a significant step forward in our ability to study the precursory gas to star formation where standard techniques fail. We find that the cold H I occupies only a small fraction of the total H I content in each galaxy, consistent with both theory and other observational techniques in the literature. The cold Hi is typically found in higher density gas, but is markedly absent from the highest density peaks where current star formation is presumably heating the gas. Observations targeting the areas rich in cold H I gas may be the only way to study the conditions of star formation in some low-mass galaxies. Finally, we present direct observations of the molecular hydrogen content in one of the only low-mass galaxies with a molecular gas detection, NGC 4214. We use the Infrared Spectrograph onboard the Spitzer Space Telescope to measure pure rotational lines of the ground state of molecular hydrogen (H[subscript]2). These observations are some of the only direct H[subscript]2 detections in a low-mass galaxy to date. They confirm the association on the carbon monoxide (CO) molecule with the H[subscript]2 molecule assumed in the literature. We provide limits to the gas phase temperatures and column densities of the warm H[subscript]2 along the lines-of-sight of three distinct CO clouds, two of which are actively forming stars. The results presented here add to the growing understanding of how these low-mass systems form stars. This knowledge may be applicable to galaxy evolution in the early universe, which may have had similar star forming conditions.