Connecting galactic to local scales in the neutral interstellar medium across the Local Group

• Author / Creator

  Koch, Eric W

• Star formation drives secular galaxy evolution by linking stars and gas in galaxies. This link forms part of the "baryonic cycle," where stars form from interstellar gas, and a portion of that gas is returned to the interstellar medium upon the star’s death. Understanding the baryonic cycle and its role in galaxy evolution requires piecing together how the neutral interstellar medium controls where and when star formation occurs. However, our understanding remains limited because the neutral interstellar medium is affected by processes ranging from the kiloparsec scales of galaxies to the sub-parsec scales where individual stellar systems form. To advance our knowledge of the baryonic cycle and galaxy evolution, observations of the neutral interstellar medium must bridge large to small scales.

  In my thesis, I present new observations of the Local Group galaxies that connect large to small scales in the neutral interstellar medium in exquisite detail. The Local Group galaxies provide an external view to trace galaxy-scale processes but are close enough for current telescopes to resolve < 100 pc scales where key components of the baryonic cycle occur. I present the first part of an on going Local Group survey of atomic hydrogen taken with the Very Large Array. These observations provide high-spatial and -spectral resolution maps of M31 and M33 produced from my new techniques for handling massive interferometric data sets. I use these new observations to demonstrate the complex kinematics of the atomic interstellar medium and the large bias implicit inusing approximate line shape measurements. My work demonstrates that detailed spectral modeling is critically needed to guide our interpretation of the
  atomic interstellar medium. Applying detailed spectral modeling, I show that 21-cm HI emission is best modeled as a set of optically-thin Gaussians. I further show that previous results reporting opaque HI on 100 pc scales are strongly rejected by the new observations.

  I then compare tracers of the atomic and molecular interstellar medium and find strong correlations between their kinematics. The molecular interstellar medium is the direct fuel for star formation, and this correlation indicates a continued role for the atomic interstellar medium throughout the star formation process. However, these correlations are apparent when only the atomic gas spectrally associated with the molecular gas is considered. Previous studies that use all of the atomic gas along the line-of-sight are unlikely to find this association. These results further suggest the need for careful spectral modeling to study processes that link the atomic and molecular media, including how molecular gas is formed from the atomic gas.

  Finally, I demonstrate the difficulties in recovering the source of large-scale turbulence in nearby galaxies, which is a key but poorly constrained component in modern star formation theories. I show that features in the spatial power spectrum, previously interpreted as large-scale galaxy properties, are not physical and instead result from the instrument response function. Because of this, the source of turbulent driving remains ambiguous.

  By combining high-spectral resolution observations of the atomic interstellar medium with detailed modeling, my work opens new avenues for exploring the neutral interstellar medium in nearby galaxies.

• Subjects / Keywords

  • Radio Astronomy
  • Astronomy
  • Interstellar Medium
  • Local Group

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-kmsj-zr88

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.