Thesis (Ph.D.)--Boston University / This dissertation studies the interstellar medium (ISM) in dwarf galaxies. Dwarf galaxies are important because: 1) they constitute the largest fraction of extragalactic systems, and 2) they provide templates for primordial galaxies. Indeed, local active dwarf galaxies resemble primitive systems, since they are poor in dust and heavy elements and they are profusely forming massive stars. Because dwarf galaxies are nearby, however, they can be observed in much greater detail than distant primordial systems. Therefore studies of the ISM in nearby dwarf galaxies can be used to understand the processes at work in primitive galaxies. This work focuses on the effects of low heavy element abundances (i.e., low metallicities) on the star-forming ISM. Low metallicities are known to drastically affect the ISM. With decreasing metallicity, an increasingly large fraction of the molecular ISM is photodissociated into atoms and ions. We modeled and observed the emission of a sample of low metallicity dwarf galaxies in the millimeter, submillimeter, and far-infrared wavebands. The submillimeter waveband allows us to observe the mid-J rotational transitions of carbon monoxide (CO), the usual tracer of the molecular ISM, and the fine structure transit ions of neutral carbon ([C I]), a tracer of translucent and photodissociated material. We studied regions in the Large and Small Magellanic Clouds and the Northern Hemisphere dwarf galaxy IC 10. We find that the preponderant mechanism producing neutral carbon inside molecular clouds is photodissociation. We observe a moderate increase in the ratio of [C I] to CO emission for decreasing metallicity. Our models of clumpy, unresolved photo dissociation regions explain these observations as the natural result of an augmented fraction of photo dissociated material. Finally, our observations of the submillimeter thermal dust continuum in IC 10 find an abnormally low emissivity exponent for its graybody emission. We conclude that the unusual dust continuum is caused by the selective destruction of small grains, brought about by the combined effects of low metallicities and high radiation fields.

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Where a galaxy is located has a strong effect on its properties. The dense cluster environment is home to a large population of red, quiescent elliptical galaxies, whereas blue, star-forming, spiral galaxies are common in lower-density environments. This difference is intricately linked to the ability of the galaxy to form new stars and therefore ultimately to the fuel for star formation, the atomic and molecular gas. In this thesis, I use two large JCMT surveys to explore the effects of environment on the atomic gas, molecular gas, and star formation properties of a large sample of nearby gas-rich galaxies. From the NGLS and follow-up studies, I select a sub-sample of 98 HI-flux selected spiral galaxies. I measure their total molecular gas mass using the CO J=3-2 line and combine this data with measurements of their total atomic gas mass using the 21-cm line and star formation rate using attenuation-corrected H-alpha luminosity. I find an enhancement in the mean H2 mass and a higher H2-to-HI ratio for the Virgo Cluster sample. Virgo Cluster galaxies also have longer molecular gas depletion times (H2/SFR), which suggests that they are forming stars at a lower rate relative to their molecular gas reservoirs than non-Virgo galaxies. Next, I collect VLA 21 cm line maps from the VIVA survey and follow-up VLA studies of selected galaxies in the NGLS. I measure the surface density maps of the atomic gas, molecular gas, and star formation rate in order to determine radial trends. I find that the H2 distribution is enhanced near the centre for Virgo Cluster galaxies, along with a steeper total gas (HI + H2) radial profile. I suggest that this is due to the effects of moderate ram pressure stripping, which would strip away low-density gas in the outskirts while enhancing high-density gas near the centre. There are no trends with radius for the molecular gas depletion times, but the longer depletion times for the Virgo Cluster sample is still present. Finally, I use 850 micron continuum observations for 105 star-forming galaxies and CO J=2-1 line observations for 35 galaxies in the initial data release (DR1) of the JINGLE survey. I match the JINGLE galaxies to a SDSS group catalogue and measure environmental parameters such as the host halo mass, environment density, and location in phase space. I find that the molecular gas masses estimated from the 850 μm and CO J=2-1 line observations are well-correlated. The H2-to-HI ratio and the molecular gas depletion times do not appear to vary with stellar mass. I did not find any significant variation with environment in the DR1 sample, but I will apply this framework to the full JINGLE sample once the complete dataset is available. / Thesis / Doctor of Philosophy (PhD)

Most small asteroids are defined as "rubble-piles," or bodies with zero tensile strength and large bulk porosity that are tenuously held together by cohesive forces. Improving the accuracy of predictions of asteroid strengths requires suitable laboratory measurements of relevant materials, as well as increasing the availability of materials from sample return. In this work, we use Atomic Force Microscopy (AFM) force measurements and particle characterization to characterize, evaluate, and decouple variables that affect cohesive forces that act between micron-sized grains. In our first investigation we explored interactions of JSC-1 lunar simulant grains using three sample sizes, three spherical AFM tip diameters, and varying relative humidity, observing that the results are very dependent on the RH and, by proxy, adsorbed water. We observed weaker adhesion with larger grain/tip size, which can be attributed to the changing contact area between the samples and the tips. We next performed experiments in vacuum conditions and characterized the cohesive values of a high-fidelity CI simulant (Exolith) based on the CI1 meteorite Orgueil. Our results show no significant trend in adhesion, but we do observe that some correlating characteristics of the grains, such as roughness, can dominate the work of adhesion. The chemical nature of the grains, including their affinity for water, also played a role in if they became more adhesive in vacuum conditions or less adhesive in vacuum conditions. Our studies decouple several factors that contribute to the complex physics of adhesion and even more complex idea of understanding adhesion in a space environment with irregularly shaped grains. This approach will pave the way to a better understanding of regolith surface properties, improve contact models for irregularly shaped particles, and provide suitable inputs for models of asteroid cohesion. This analysis technique can be used on future materials provided by sample return missions.

We present a range of steady-state photoionization simulations, corresponding to different assumed shell geometries and compositions, of the unseen postulated rapidly expanding outer shell to the Crab Nebula. The properties of the shell are constrained by the mass that must lie within it, and by limits to the intensities of hydrogen recombination lines. In all cases the photoionization models predict very strong emission from high ionization lines that will not be emitted by the Crab’s filaments, alleviating problems with detecting these lines in the presence of light scattered from brighter parts of the Crab. The NIR [Ne VI] λ 7.652 mm line is a particularly good case; it should be dramatically brighter than the optical lines commonly used in searches. The C IV λ1549Å doublet is predicted to be the strongest absorption line from the shell, which is in agreement with HST observations. We show that the cooling timescale for the outer shell is much longer than the age of the Crab, due to the low density. This means that the temperature of the shell will actually “remember” its initial conditions. However, the recombination time is much shorter than the age of the Crab, so the predicted level of ionization should approximate the real ionization. In any case, it is clear that IR observations present the best opportunity to detect the outer shell and so guide future models that will constrain early events in the original explosion. Infrared observations have discovered a variety of objects, including filaments in the Crab Nebula and cool-core clusters of galaxies, where the H2 1-0 S(1) line is stronger than the infrared H I lines. A variety of processes could be responsible for this emission. Although many complete shock or PDR calculations of H2 emission have been published, we know of no previous simple calculation that shows the emission spectrum and level populations of thermally excited low-density H2. We present a range of purely thermal collisional simulations, corresponding to constant gas kinetic temperature at different densities. We consider the cases where the collisions affecting H2 are predominantly with atomic or molecular hydrogen. The resulting level population (often called “excitation”) diagrams show that excitation temperatures are sometimes lower than the gas kinetic temperature when the density is too low for the level populations to go to LTE. The atomic case goes to LTE at much lower densities than the molecular case due to larger collision rates. At low densities for the v=1 and 2 vibrational manifolds level populations are quasi-thermal, which could be misinterpreted as showing the gas is in LTE at high density. At low densities for the molecular case the level population diagrams are discontinuous between v=0 and 1 vibrational manifolds and between v=2, J=0, 1 and other higher J levels within the same vibrational manifold. These jumps could be used as density diagnostics. We show how much the H2 mass would be underestimated using the H2 1-0 S(1) line strength if the density is below that required for LTE. We give diagnostic diagrams showing level populations over a range of density and temperature. The density where the level populations are given by a Boltzmann distribution relative to the total molecular abundance (required to get the correct H2 mass), is shown for various cases. We discuss the implications of these results for the interpretation of H2 observations of the Crab Nebula and filaments in cool-core clusters of galaxies.

The interstellar medium (ISM) of our Galaxy, and of others, is pervaded by ultra low-density gas and dust, as well as magnetic fields. Embedded magnetic fields have been known to play an important role in the structure and dynamics of the ISM. However, the ability to accurately quantify these fields has plagued astronomers for many decades. Unfortunately, the experimental techniques for measuring the strength and direction of magnetic fields are few, and they are observationally challenging. The only direct method of measuring the magnetic field is through the Zeeman effect. The goal of this dissertation is to expand upon the current observational studies and understanding of the effects of interstellar magnetic fields across various regions of the Galaxy. Zeeman effect observations of magnetic fields in two dynamically diverse environments in the Milky Way are presented: (1) An OH and HI absorption line study of envelopes of molecular clouds distributed throughout the Galaxy, and (2) A study of OH absorption lines toward the Galactic center region in the vicinity of the supermassive black hole Sgr A*. We have executed the first systematic observational survey designed to determine the role of magnetic fields in the inter-core regions of molecular clouds. Observations of extragalactic continuum sources that lie along the line-of-sight passing through Galactic molecular clouds were studied using the Arecibo telescope. OH Zeeman effect observations were combined with estimates of column density to allow for computation of the mass-to-flux ratio, a measurement of the gravitational to magnetic energies within a cloud. We find that molecular clouds are slightly subcritical overall. However, individual measurements yield the first evidence for magnetically subcritical molecular gas. Jansky VLA observations of 18 cm OH absorption lines were used to determine the strength of the line-of-sight magnetic field in the Galactic center region. This study yields no clear detections of the magnetic field and results that differ from a similar study by Killeen, Lo, & Crutcher (1992). Our results suggest magnetic fields no more than a few microgauss in strength.

Interstellar plasma — interstellar clouds in particular — play an important role in determining the structure and evolution of galaxies. Understanding the time evolution of such plasmas requires knowledge of the chemical processes that drive their dynamics. Two processes are studied in this dissertation: radiative electron attachment (REA) via dipole-bound states (DBSs) and dissociative recombination (DR). Of the several hundred molecules detected in the interstellar medium, only eight anions have been detected: CN-, C3N-, C5N-, C7N-, C4H-, C6H-, C8H-, and C10H-. Their production mechanism is not well known; REA was suggested as a possible formation pathway, but previous theoretical studies have found that REA rate coefficients were too low to explain the formation of CN-, C3N-, and C5N-. It was later suggested that including DBSs — an electron weakly bound at a large distance to the large dipole moment of a neutral molecule — could appreciably enhance the REA rate coefficients. The first portion of this study is dedicated to investigating the role of the large dipole moment of rotating C3N using an accurate \it ab initio approach with electronic and rotational resolution. DBS wavefunctions of C3N- are calculated and used to obtain REA cross sections that produce even smaller rate coefficients, suggesting that C3N- is efficiently formed by a different process. The second part of this study investigates DR in the difficult case of molecules with low-lying eletronic resonances, although these are not necessary for the approach. An approach to treat both direct and indirect mechanisms of DR in a diatomic ion with electronic, vibrational, and rotational resolution using R-matrix scattering calculations, frame transformation theory, and multichannel quantum defect theory is presented and applied to the CH+ and CF+ molecular ions at low collision energies. The calculated CH+ cross sections agree well with recent rotationally state-resolved experimental results and overall better than previous theoretical results. The calculated CF+ cross sections agree well with experimental results, although these do not have rotational resolution, and overall better than previous theoretical results at low energies. Additionally, the method can study rovibronic (de-)excitation — a process in competition with DR. These are calculated and compared to previous theoretical calculations for CH+, which which our results agree well with the exception of dipole-driven rotational excitation cross sections. This discrepancy is tentatively attibuted to negelcting the contribution of higher partial waves in the description of the incident electron, which will be incorporated in future studies.