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Past Dissertations

William Janesh, 2018

Title: Searching for Resolved Stellar Populations in ALFALFA Ultra-Compact High Velocity Clouds

Abstract: Large-scale surveys of the sky have recently discovered dozens of faint, low-mass dwarf galaxies in and around the Local Group. These dwarf galaxies are excellent laboratories for testing the predictions of the Lambda Cold Dark Matter theory and models of galaxy formation and evolution, star formation, and feedback. Finding and studying more dwarf galaxies will help to constrain these models, but faint, low-mass galaxies can be difficult to detect without targeted searches.

The Arecibo Legacy Fast ALFA (ALFALFA) survey was a blind neutral hydrogen (HI) survey that observed about 7000 square degrees of sky with higher sensitivity than previous blind HI surveys. Among the thousands of gas-rich objects on the nearby Universe cataloged by ALFALFA are 100 objects classified as Ultra-Compact High Velocity Clouds (UCHVCs). The UCHVCs have neutral gas properties and masses consistent with those predicted for low-mass, gas-rich dark matter halos in or near the Local Group, but have no identifiable stars associated with them in existing optical survey data. Given their properties, the UCVHCs are ideal locations for searching for previously-undetected faint dwarf galaxies. One such galaxy associated with a UCHVC has already been discovered: Leo P is a low-mass, extremely metal-poor star-forming galaxy at a distance of 1.64 Mpc, just beyond the Local Group.

I have undertaken an optical survey of a sample of UCHVCs with the WIYN 3.5-m telescope and the One Degree Imager to determine whether they contain any detectable stellar populations. The deep, wide-field imaging data are processed and then used to construct a color-magnitude diagram (CMD) from photometry of stellar sources in the images. A CMD filter is applied to select stars that have the characteristics of old, metal-poor stellar populations. The distribution of selected stars is smoothed and the regions with the highest density are identified to find the peak "overdensity". This process is repeated for distances between 250 kpc and 2.5 Mpc. I determine the significance of each peak overdensity by comparing it to a distribution of overdensities from randomly generated spatial distributions of stars.

After completing the data analysis for 23 UCHVCs, I find that five of them have highly significant stellar overdensities that overlap the HI distribution. The estimated distances of the stellar counterparts are between 350 kpc and 1.6 Mpc, and they have MV between -1.4 and -7.1, stellar masses between ~10^3 and ~10^6 M_sun and a wide range of HI-to-stellar-mass ratios, from less than one to 200. These candidate dwarf galaxies are extreme objects: their optical properties are similar to the ultra-faint dwarf galaxies discovered in large-scale optical surveys, but they are gas-rich, making them some of the most unique objects in the local Universe. If they are genuine dwarf galaxies, these objects will be valuable tools for studying star and galaxy formation and other processes (e.g., feedback, metal enrichment) at the low end of the galaxy mass function.

Maria Tiongco, 2018

Title: Kinematical Evolution of Tidally Limited Star Clusters

Abstract: Globular star clusters are traditionally pictured as dynamically simple and single stellar population systems; however, many recent results from photometric, spectroscopic, and astrometric studies are revealing that globular clusters are more complex than previously thought. In particular, kinematical features such as the presence of anisotropy in the velocity distribution and differential rotation, and the existence of multiple stellar populations characterized by variations in light element abundances among their stars, are some of the key observational findings. These new results and the forthcoming large amount of data from surveys such as the Gaia astrometric survey call for a renewed effort on the theoretical front to characterize the evolution of the internal kinematics of star clusters. My thesis work has aimed to build a theoretical framework to interpret these new observational results and to understand their link with a globular cluster's dynamical history. I have focused on the study of the evolution of the internal kinematics of star clusters as driven by the effects of two-body relaxation and the external Galactic tidal field. By means of a large suite of N-body simulations, I have explored the three-dimensional structure of the velocity space of tidally-perturbed clusters, by characterizing their degree of anisotropy and their rotational properties. These studies showed that a cluster's kinematical properties contain distinct imprints of the cluster's initial structural properties, dynamical history, and tidal environment. Finally, by relaxing some simplifying assumptions about the alignment of the rotation axis of the cluster relative to the tidal field, I have also shown how the interplay between a cluster's internal evolution and the interaction with the host galaxy can produce complex morphological and kinematical properties. Building on this fundamental understanding, I will study the dynamics of multiple stellar populations in globular clusters, with attention to the largely unexplored role of rotation. This body of results will provide essential guidance for a meaningful interpretation of the emerging dynamical complexity of globular clusters in the era of Gaia and other upcoming large spectroscopic surveys.

Karna Desai, 2017

Title: Gravitational Instabilities in a Young Protoplanetary Disk with Embedded Objects

Abstract: Gravitational Instabilities (GIs), a mechanisim for angular momentum transport, are prominent during the early phases of protoplanetary disk evolution when the disk is relatively massive. In this dissertation, I analyze GIs by inserting different objects in a disk by employing 3D hydrodynamic simulations.

GIs in a circumbinary disk are studied to determine how the presence of the companion affects the nature and strength of GIs in the disk. The circumbinary disk achieves a state of sustained marginal instability similar to an identifical disk without the companion. A realistic evolution of the binary is detected.

Planet and disk interactions play an important role in the evolution of planetary systems. To study this interaction during the early phases of planet formation, a migration study of Jovian planets in a GI-active disk is conducted. I find the migration timescales to be longer in a GI-active disk, when compared to laminar disks. The 3 M_Jupiter planet controls its own orbital evolution, while the migration of a 0.3 M_Jupiter planet is stochastic in nature. I define a 'critical mass' as the mass of an arm of the dominant two-armed spiral density wave within the planet's Hill diameter. Planets above this mass control their own destiny, and planets below this mass are scattered by the disk. This critical mass could provide a recipe for predicting the migration behavior of planets in GI-active disks.

To understand the stochastic migration of low-mass planets, I perform a simulation of 240 zero-mass planet-tracers by inserting these at a range of locations in the disk. A Diffusion Coefficient is calculated to characterize the stochastic migration of low-mass objects. The eccentricity dispersion for the sample is also studied. I find that hte diffusion of planets can be a slow process, resulting in the survival of small planetary cores.

Owen Boberg, 2017

Title: Stellar Populations and Kinematics of Peculiar Milky Way Star Clusters

Abstract: We now consider the signatures of multiple stellar populations to be a defining and universal characteristic of Milky Way globular clusters. These multiple populations are identified through light element anti-correlations and color magnitude diagram (CMD) features that cannot be explained by a simple stellar population. While these features have now been observed in nearly all Galactic globular clusters, there is still not a consensus on the mechanisms responsible for producing the observed abundance patterns and peculiar CMD morphologies. There are a number of theories that suggest these features are the result of multiple epochs of star formation after a period of self-enrichment within the cluster. Over the last two decades, large scale photometric surveys have added additional complexity to the history of globular clusters through the detection of cluster tidal tails, and the debris streams of accreted dwarf galaxies. These features highlight the complex external dynamics that are affecting the evolution of globular clusters, and in some cases, the possibility of an extragalactic origin. These external influences, as well as the internal cluster dynamics, must also be taken into consideration when attempting to explain the signatures of multiple populations in globular clusters we observe today. In this dissertation we use observational data on three unique Milk Way star clusters to explore the limitations and methods used to separate multiple stellar populations within a cluster. We used the relatively old and high mass open cluster NGC 6791 to test if molecular band strengths could be used to detect multiple populations in the cluster if they were present. Through this work we find that NGC 6791 does not host multiple population and find that molecular band strengths loose sensitivity at high metallicities like that of NGC 6791. The bulk of this thesis focuses on a pair of neighboring globular clusters NGC 5053 and NGC 5024 (M53), which provide a unique environment to test our understanding of the relationship between cluster morphology, the role of the external environment, and the degree of internal rotation within the clusters. Using chemical abundances from medium resolution spectra we find that both clusters exhibit multiple populations and do not show signs of an extra galactic origin. In M53 we found that it is dominated by the first generation of stars in the cluster, making it an outlier among other Milky Way globular clusters. Using the radial velocities we also find that M53 has a significant amount of internal rotation in its inner regions and has a changing axis of rotation. We also present the initial results of wide field photometry that will eventually be used to map out the extra tidal features of these clusters and address claims that they are connected by a tidal bridge.

Brian Brondel, 2017

Title: Populations and History in the Outer Limits of the Magellanic System

Abstract: The Magellanic Clouds (MCs) are two small galaxies that are among the nearest to the Milky Way. Because they are nearby, the Clouds are well suited to careful examination by measurement of resolved stellar populations and other techniques, yet the scientific understanding of the Clouds is only beginning to come into focus. Now, study of the Magellanic Clouds is particularly timely, in part because of the recent realization that the Clouds are only recently entering the halo of the Milky Way. Close examination of the structure and history of the Clouds has the potential to offer insights in the nature of hierarchical merging of galaxies, and study of the dynamics of the MCs and their passages through the halo of the Galaxy may yield hints about th enature of the dark matter halos generally, currently an important area of research in astronomy. The Clouds present a unique opportunity for study of stellar populations, because they are near enough that individual stars can be resolved to depths well past the main sequence turnoff. This permits analysis of stellar age and metallicity with common distance determinable by independent means.

In 2005-2011, Saha et al. (2010) conducted observations for the Outer Limits Survey (OLS) of the Magellanic Clouds, an extensive survey designed to probe the outskirts of these galaxies to fainter limits than any previous survey.

In collaboration with the OLS team I have developed methodology for obtaining high precision photometry from OLS data, and deriving star formation history and age-metallicity relations from the measurements. Detailed determination of the star formation history and age-metallicity relation in these fields requires synthesis of artificial stars and CMD fitting, and these processes will be discussed in thesis. I present the star formation history of the fields in the OLS project and confront predictions from current models of the Magellanic System.

Alec Hirschauer, 2016

Title: Metallicities of Galaxies in the Local Universe

Abstract: The degree of heavy-element enrichment for star-forming galaxies in the universe is a fundamental astrophysical characteristic which traces the amount of stellar nucleosynthesis undertaken by the constituent populations of stars. Estimating this quantity via the so-called "direct-method" is observationally challenging and requires measurement of intrinsically weak temperature-sensitive nebular emission lines, however these are typically not found for galaxies unless their emission lines are exceptionally bright. Metal abundances ("metallicities") must then therefore be estimated by empirical means utilizing ratios of strong emission lines, calibrated to sources of known abundance and/or theoretical models, which are measurable in essentially any nebular spectrum of a star-forming system. Relationships concerning metallicities in galaxies such as luminosity-metallicity and mass-metallicity are critically dependent upon reliable estimations of abundances. Therefore, having a reliable observational constraint is paramount to developing models which accurately reflect the universe. This dissertation explores metallicities for galaxies in the local universe through a variety of means. First, an attempt is made to improve calibrations of empirical relationships for estimating abundances for star-forming galaxies at high-metallicities, finding some intrinsic shortcomings but also revealing some interesting new findings regarding the computation of temperatures of the electron gas of star-forming systems, as well as detecting some anomalously under-abundant, overly-luminous galaxies. Second, a discovery is made of an extremely metal-poor star-forming galaxy, which opens the possibility to find more similar systems and to better understand star-formation in exceptionally low-abundance environments. Finally, the development of a self-consistent scale for estimating metallicities allows for the creation of luminosity-metallicity and mass-metallicity relations for a statistically representative sample of star-forming galaxies in the local universe.

Emily Richards, 2016

Title: Baryonic Distributions in Galaxy Dark Matter Halos

Abstract: In our best current understanding of the growth of structure in the Universe, visibly complex distributions of gas and stars form and evolve into a wide range of galaxies inside over densities of dark matter. Re-creating the observed diversity in the organization of baryonic mass within dark matter halos represents a key challenge for galaxy formation models. In this dissertation, I constrain the distribution of baryonic and non-baryonic matter in a statistically representative sample of 44 nearby galaxies defined from the Extended Disk Galaxy Exploration Science (EDGES) survey to address the growth of galaxy disks in dark matter halos. I trace the gravitational potentials of each galaxy using rotation curves derived from new and archival radio synthesis observations of neutral hydrogen (HI). The measured rotation curves are decomposed into baryonic and dark matter halo components using 3.6 micron images for the stellar content, the HI observations for the atomic gas component, and, when available, CO data from the literature for the molecular gas component. The HI kinematics are supplements with optical integral field spectroscopic (IFS) observations to measure the central ionized gas kinematics in 26 galaxies. Distributions of baryonic-to-toal mass ratios are determined from the rotation curve decompositions under different assumptions about the contribution of the stellar component, and are compared to global and radial properties of the dominant stellar populations extracted from optical and near-infrared photometry.Galaxies are grouped into clusters of similar baryonic-to-total mass distributions to examine whether they also exhibit similar star and gas properties. The radial distribution of baryonic-to-total mass in a galaxy does not appear to correlate with any characteristics of its star formation history. This result encapsulates the challenge facing simulations to create galaxies which evolve with different star formation histories but similar distributions of mass.

Jaime Overbeek, 2016

Title: The Evolution of Neutron-Capture Elements in the Milky Way

Abstract: Neutron-capture elements (those with Z > 30) are formed in two ways: slow neutron-capture (the s-process) and rapid neutron-capture (the r-process). The s-process is thought to mainly occur in low and intermediate-mass asymptotic giant branch (AGB) stars; the r-process site has not been conclusively identified but probably involves core collapse supernovae or neutron star mergers. A conflict has recently arisen between s-process models and observations, as [Ba/Fe] ratios appear to increase dramatically with decreasing age in open clusters to a degree not predicted by standard s-process models. Other s-process elements do not show the degree of enhancement in young clusters that Ba does. Various solutions have been proposed, including an intermediate process which may disproportionally create Ba, and an increased neutron source in low-mass (M < 1.5Msolar) AGB stars. We have assembled and analyzed a sample of 75 stars in 24 open clusters to measure abundances of three light s-process elements (Sr, Y, Zr), three heavy s-process elements (Ba, La, Ce), three r-process elements (Eu, Dy, and Gd), and three elements with significant contributions from both processes (`mixed' elements Mo, Pr, and Nd). Examining [s-process/Fe] trends with age confirms that Ba is unique in its behavior, as it has a large trend with age of -0.05 dex per Gyr but the other s-process elements have more modest trends with cluster age of -0.01 to -0.02 dex per Gyr. All of the s-process elements except Ba fit models based on an enhanced neutron source in low-mass AGB stars; we also do not see in our data the heavy s-process element trends with metallicity predicted by standard models. Problems we encountered with measuring Ba features combined with the uncertainty in predictions of i-process models require caution in interpreting the i-process as a solution to the Ba discrepancy. We do not find strong trends in s-process elements with Galactocentric radius, although there is some evidence of a break in the s-process gradient at ~11 kpc. Open cluster [Eu/Fe] abundances appear to fit with models accounting for r-process contributions from neutron star mergers and jet supernovae, but [r-/H] ratios do not appear to change with Galactocentric radius.

Michael Young, 2016

Title: Giant galaxies and their globular cluster populations: Analysis and results from a wide-field imaging survey and archive

Abstract: The globular cluster (GC) systems of giant galaxies are valuable and intriguing tools for a number of reasons, both in terms of the properties of the overall system as well as the properties of the individual GCs that make up the system. GCs are old: their ages range from a few Gyrs up to ~12 Gyrs, and they apparently form during galaxy mergers and major star formation events. The ensemble properties (including the color, metallicity, and spatial distributions) of the GC system constrain theoretical models of galaxy formation. For several years we have been carrying out a wide-field imaging survey of the GC populations of a sample of giant spiral, S0, and elliptical galaxies with distances of ~10-30 Mpc.
In this dissertation I present results and analysis of the GC systems of eight giant galaxies, representing a significant addition to the survey dataset. I also describe how the survey data and metadata was collected, homogenized, and ingested into a custom database and archive, and how a web portal was created to disseminate the survey products to the wider scientific community. I have developed and tested a probability factor to quantify the likelihood that a given GC candidate is in actuality a GC. I explored enhanced statistical methods to detect subpopulations in GC systems, and found that six of the GC systems in our survey presented with three GC subpopulations. I explored how the spatial and azimuthal distributions of these subpopulations differ in each host galaxy. I have supplemented our survey results with select GC system studies from the literature, and tested how different host galaxy properties correlate with the total number of globular clusters in a given system, finding that the combination of the dynamical mass of the galaxy and the K-band luminosity of the galaxy offered the best correlation with the number of GCs. Lastly, I applied this combination of predictors to a published catalog of GC system studies and found that the predictions were in good agreement with existing observations.

Cameron Pace, 2015

Title: Star Formation and Feedback From Radio Galaxies: Insights from Large Multiwavelength Surveys

Abstract: Active galactic nuclei (AGN) are believed to play an important role in the evolution of their host galaxies by influencing the galaxy's gas reservoirs. This may affect the growth of the galaxy's massive black hole and star formation in the host galaxy. I address two unanswered questions central to our understanding of AGN: what triggers AGN, and how and to what extent do they affect their host and neighboring galaxies? I study radio galaxies, which are a subset of AGN, because their radio jets may provide a natural feedback mechanism between the AGN and the host and neighboring galaxies. Previous studies, which were limited to small samples, produce conflicting results as to whether mergers or environmental effects lead to triggering. It is also uncertain whether radio galaxies have a net positive (via gas cloud collapse) or negative (via gas heating) effect on star formation. I use a large (~7,200) statistically significant sample of radio galaxies, for which I extract photometric information from several large-scale, multiwavelength surveys. The radio galaxies are compared to a sample of control galaxies whose properties match those of the radio galaxies, except for their lack of radio activity. This approach allows me to determine the frequency of feedback events and whether radio galaxies are responsible. I derive and compare composite spectral energy distributions (SEDs) for the radio galaxies and control sample, and find a deficit of ultraviolet and infrared emission for slow accreting radio galaxies, suggesting that they may suppress star formation in their hosts. Fast accreting radio galaxies are found to have an infrared excess, which is characteristic of their high accretion rate and not a result of AGN feedback on star formation. I compare the populations of neighbor galaxies of the two samples and find that radio galaxies have an excess of neighbors within 100 kpc, which must play a role in triggering. My results also show that radio galaxies rarely (<2%) affect star formation in neighboring galaxies. I also investigate the possibility of AGN jets triggering diffuse star formation in external gas clouds and find that such triggered star formation is uncommon.

Angela Parker Van Sistine, 2015

Title:The ALFALFA Hα Survey

Abstract:The ALFALFA Hα survey utilizes a large sample of HI-selected galaxies from the ALFALFA survey to study star formation in the local universe. ALFALFA Hα contains 1555 galaxies with distances between ~20 and ~100 Mpc. We have obtained continuum-subtracted narrowband Hα images and broadband R images for each galaxy, creating one of the largest homogeneous sets of Hα images ever assembled. Our procedures were designed to minimize the uncertainties related to the calculation of the local star formation rate density (SFRD). The galaxy sample we constructed is as close to volume-limited as possible, is a robust statistical sample, and spans a wide range of galaxy environments. In this dissertation, we discuss the properties of our galaxy sample, our procedure for deriving individual galaxy star formation rates (SFRs), and our method for calculating the local SFRD. We derive a value of log(SFRD [Msun yr-1 Mpc-3] = -1.723 +0.015 -0.017 (random) +/- 0.05 (systematic) based on the entire ALFALFA Hα survey. We also compare our Hα-based SFRs to those based on ultraviolet (UV) and infrared (IR) emission. We find that SFRs derived from a combination of UV and IR fluxes agree best with our Hα-derived SFRs.

Steven Janowiecki, 2015

Title:The Evolutionary Status of High and Extremely Low Surface Brightness Dwarf Galaxies: BCDs and Almost Dark Galaxies

Abstract: Studying dwarf galaxies can shed light on the original building blocks of galaxy formation. Most large galaxies are thought to be built up over billions of years through the collisions and mergers of smaller galaxies. The dwarf galaxies we see today are the evolved remnants of those building bloacks, and by understanding their nature and evolution, we can study the raw ingredients of galaxy formation.

Blue Compact Dwarf Galaxies (BCDs) and Almost Dark galaxies are at opposite extremes of today's population of dwarf galaxies. BCDs are exceptionally compact and host very intense starbursts, while Almost Dark galaxies are much more diffuse and have weak stellar populations.

This work studies the evolutionary context of BCDs by using deep, high-resolution images to study the detailed structure of their components, and by fitting our multi-wavelength observations with models to describe the properties of their stars, gas, and dust. BCDs appear to have exceptionally compact old stellar populations and unusually large star formation rates, when compared to typical dwarf galaxies.

By contrast, the optically faint, gas-dominated Almost Dark galaxies have extremely low star formation rates and weak stellar populations. In particular, one of the Almost Darks studied in this work has very unusual properties and is in disagreement with widely-studied scaling relations for large samples of galaxies. It appears to have too little stellar mass, a distribution of HI that is too extended to be supported by its modest rotation, and the highest well-measured gas mass-to-light ratio ever observed.

These two extreme classes may represent evolutionary stages that all galaxies pass through, and appear to be extreme ends of the broad continuum of dwarf galaxy properties. In order to use today's dwarf galaxies as windows into the building blocks of early galaxy formation, these unusual states and evolutionary pathways must be understood.

Jessica Windschitl Dowell, 2015

Title: Photometric and Kinematic Studies of Extragalactic Globular Cluster Systems

Abstract:Globular clusters (GCs) are old, luminous, compact collections of stars found in galaxy halos that formed during the early stages of galaxy formation. Because of this, GCs serve as excellent tracers of the formation, structure, and merger history of their host galaxies. My dissertation will examine both the photometric and kinematic properties of GC systems and their relationship to their host galaxies. In the first section, I will present the analysis of the GC systems of two spiral galaxies, NGC 891 and NGC 1055. I will discuss the photometric methods used to detect GCs using wide-field BVR imaging and to quantify the global properties of the system such as the total number of GCs and their radial distribution. My results for these two GCs systems were compared to other galaxies. I will also present the results of spectroscopic follow-up for two giant galaxies: the S0 galaxy NGC 4594 (M104), and the elliptical galaxy NGC 3379 (M105). I measured the radial velocities of GCs in these two galaxies, and combined them with published results to determine the mass distribution and mass-to-light (M/L) ratio profile for each galaxy out to large effective radius (7-9 Re). For both galaxies, I found that the M/L profiles increase with radius and do not flatten, which suggests that the dark matter halos in these galaxies extend to the edge of my data. I also looked for evidence of rotation in the GC systems, and found that neither system exhibits significant rotation around the host galaxy. I examined the velocity dispersion profile of each GC system and found kinematic differences between the red and blue GC subpopulations. Finally, I compared my results to mass estimates for these galaxies from other kinematic tracers and considered them in the context of galaxy formation models.

Maria Cordero, 2014

Title: Multiple Populations in More Metal-Rich Galactic Globular Clusters

Abstract: In this thesis I present chemical abundances for bright stars in the intermediate
metallicity globular cluster (GC) M5, and the relatively metal-rich GCs M71 and 47 Tuc with the goal of improving the understanding of chemical evolution in the metallicity regime sampled by these three GCs. One of the main goals of this thesis is to characterize the multiple populations hosted by these clusters using Na abundances, since Na is considered the most suitable element to separate the primordial population from younger generations. When available, oxygen abundances are used to separate the secondary generation into an intermediate and extreme populations. This chemical tagging allows the determination of population ratios in each cluster and to explore the radial distributions of the different populations, which are observational characteristics that GC formation models aim to either reproduce or predict. Moreover, we assess whether the Al scatter in these three clusters is sensitive to metallicity and compare our findings with predicted yields from intermediate-mass asymptotic giant branch stars. Furthermore, one of the predictions of mulitple pouplations GC formation models is that younger generations rotate faster than the primordial population in the early stages of the cluster.To test this idea, we selected the GC M13 since the cluster is know to host multiple populations and exhibit significant internal rotation.