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
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
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.
University of Minnesota Ph.D. dissertation. August 2012. Major: Astrophysics. Advisor: Dr. Evan D. Skillman. 1 computer file (PDF); xii, 222 pages.
Warren, Steven Ray.
The interstellar medium and star formation of nearby, low-mass galaxies..
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.