Recent Dissertations

Recent dissertations

Justin Kader, 2022

Title: Multiple Populations in Globular Clusters of the Galactic Bulge

Abstract: Globular clusters (GCs) are compact assemblages of up to several million stars bound by gravity. The stellar populations of GCs are metal-poor (-2<[Fe/H]<0) and old, with ages comparable to the age of the universe. While these and other basic aspects of GCs are well established, more recent observations have revealed unexpected complexity. One example was the discovery of chemical abundance variations among the unevolved stars in several Galactic GCs, suggesting these clusters are composed of at least two stellar populations formed perhaps a few hundred million years apart. The discovery of multiple populations represented a paradigm shift, since GCs had been regarded as archetypical single-age, mono-metallicity, chemically homogeneous stellar populations.  In the roughly 20 years since this discovery, it has been demonstrated that almost all Galactic halo GCs harbor multiple stellar populations.

I focus on the stellar populations of the GCs in the Galactic bulge. These clusters have eluded close scrutiny because of extinction and reddening along their sightlines caused by interstellar dust, and because of the astrometric confusion caused by the high degree of stellar crowding in the bulge fields that these clusters inhabit. If obscurity of these clusters alone were not enough to make them tantalizing targets of study, their basic properties and proximity to the bulge warrant attention as well. The GCs of the Galactic bulge have higher metallicity than their halo counterparts for reasons that are not well understood, and there are similarities between their stars and the stars belonging to the bulge. It is possible that the evolution of these GCs is inextricably related to the assembly of the Galactic bulge itself. A detailed understanding of the stellar populations in these clusters is needed to assemble a coherent picture of the joint evolution of the bulge GCs and the bulge itself.

Chris Agostino, 2022

Title: On the Hunt for AGN: an exploration of the observational diversity of active galaxies

Abstract: Supermassive black holes are proposed to lie in the centers of most massive galaxies, and their properties have been found to correlate with those of their host galaxy, suggesting the growth of the two is intimately linked. When a central supermassive black hole grows, it is known as an active galactic nucleus (AGN), and the accretion process can drive energetic jets or radiative winds that disturb and heat the interstellar gas of their host galaxies through a process  known as ‘feedback’, which may help explain why so many galaxies have stopped forming stars. A definitive causal link between the AGN and its host galaxy’s evolution has not yet been established, and many uncertainties remain. 

One of the major challenges in the study of galaxies generally and in assessing the role of accreting supermassive black holes in the evolution of galaxies is reliably distinguishing between galaxies which do or do not have an AGN. To address this longstanding problem, I use large-scale datasets derived from observational surveys in the X-ray, Ultraviolet, optical, and Infrared to evaluate the relative merits of various AGN selection techniques in a statistical fashion, comparing the relative completenesses of the methods and investigating why there may be discrepancies between them and we particularly focus on the X-rays and the optical emission lines. We investigate the relationship between the X-ray and optical emission lines of AGNs and explore and discuss the various physical factors that drive their wide diversity. We obtained integral field spectroscopic observations of five X-ray selected AGNs that appear to lack strong AGN emission lines in SDSS spectroscopy. We look for potential hidden AGN signatures and discuss the implications for the interpretation of SDSS spectroscopy with respect to the optical emission line selection of AGNs.

Jennifer Sieben, 2022

Title: Star Formation Across Cosmic Time

Abstract: SFACT (Star Formation Across Cosmic Time) is a novel large-scale narrow-band survey designed to detect emission-line galaxies in a range of redshift windows. These data can be used to study star-formation rates at multiple epochs simultaneously. Using the WIYN 3.5m telescope, this survey detects thousands of emission-line galaxies at nine distinct redshift windows up to z=1. It does this by targeting the Hα (λ6563), [OII] (λ3727), and [OIII] (λ5007) emission lines with three custom narrow-band filters. Images are also taken with r, i, and g broad-band filters; all six filter images are combined to create deep images of each survey field. A custom pipeline has been developed to identify and measure objects of interest, resulting in a uniform selection of candidates across all narrow-band filters. Spectroscopic follow-up is also performed to confirm detections as emission-line galaxies. SFACT has detected objects ranging from HII regions in nearby galaxies to distant compact galaxies and quasars. My dissertation work includes analysis of the SFACT data in order to study star-formation rate densities of galaxies across all nine redshift windows. This includes the processing of the images, the target selection, and the photometric measurements. The data in this thesis include ~5,000 galaxies from 35 target fields.

A related parallel project is also presented in order to further refine the results achieved in the SFACT project. This project examined the luminosity functions of nearby emission-line selected galaxy samples location in the same area of the sky, but selected via different emission lines. A comparison between the samples provides a correction factor that allows us to account for the observational biases of selecting via different emission lines. Together, these two projects provide a valuable examination of the star-formation rate density of emission-line selected galaxies at a wide range of lookback times.

David Carr, 2022

Title: Active Galaxies Across Cosmic Time: AGN Demographics and Metal Abundances

Abstract: Active galactic nuclei (AGN) are the most luminous persistent sources of light in the universe. This dissertation focuses on discovering and studying AGN at different points in the universe’s history as part of the Star Formation Across Cosmic Time (SFACT) survey. SFACT is designed to discover emission-line galaxies (ELGs), primarily AGN and star-forming galaxies, at different points in the universe's history. This is accomplished using three different narrowband filters at 6590, 6750, and 7460 Å to detect emission-line galaxies at redshifts windows of up to z = 1 and beyond. These galaxies are primarily discovered via their Hα, [O III]λ5007, and [O II]λ3727 emission lines.  The imaging portion of the survey is conducted using the One Degree Imager on the WIYN 3.5m telescope. This instrument provides the survey with a wide field of view, allowing for the detection of about 200 ELGs per field. Each of these newly discovered ELGs are then reobserved using the Hydra multi-fiber positioner to obtain spectroscopic follow-up. Each spectrum tells us the detected emission line, redshift, and classification (star-forming or AGN) of each galaxy. I will use the results of the SFACT survey to study the properties of AGN, such as their luminosities and metal abundances, over a large range of redshifts. To derive the metallicity of the AGN from their spectra, I construct models of AGN and link the results of these models to the observed AGN emission line ratios, interpolating across the model grids to derive metallicity. Finally, with the sample of AGN discovered as a part of SFACT, I examine how the demographics of AGN change as the universe ages and evolves, compare the volume density of AGN to the density of star-forming galaxies, and study how the abundances of AGN change with redshift.

Nicholas John Smith, 2022

Title: A Systematic, Wide-Field Search for Stars Associated with ALFALFA Ultra-Compact High Velocity Clouds

Abstract: The Arecibo Legacy Fast ALFA (ALFALFA) survey was a blind neutral hydrogen (HI) survey that used the Arecibo radio telescope to identify 31,500 sources distributed over 7,000 square degrees of sky. Among these objects are roughly 100 Ultra Compact High-Velocity Clouds (UCHVCs), which have HI masses indicating they may reside in low-mass dark matter halos and velocities that suggest they may be located within or near the Local Group. Crucially, the UCHVCs don't have clear optical counterparts and are therefore ideal locations to search for stars associated with undiscovered dwarf galaxies.

This dissertation is a continuation of an optical survey of UCHVCs begun by our group. We observed 26 UCHVCs with the WIYN 3.5-m telescope, adding to the sample of 23 presented in Janesh et al. (2019). In addition to analyzing more UCHVCs, I corrected an error in the original star filtering method and added steps to the detection pipeline to improve the data quality and detection accuracy. I also implemented cross-matching of our source lists with Gaia and SDSS to remove possible contaminants from our star catalogs.

From the sample of UCHVCs we analyzed, we identified six dwarf galaxy candidates with absolute V-band magnitudes -2 to -8 and distances between 270 and 870 kpc. We compare the estimated optical properties of our most convincing candidate with predictions from theoretical simulations and a recent census of the Local Group and find them broadly consistent. We also investigate the possibility that one of the UCHVCs is a gas cloud associated with the Galactic globular cluster Pal 3. We identify two stellar over-densities with modest significance at similar distances to Pal 3 and near the HI source, but note that they would imply a much larger gas mass than one would expect to be associated with a globular cluster. 

Samantha Brunker, 2022

Title: Green Peas and Sam: A Multifaceted Approach to Studying Extreme, Star-forming Green Pea Galaxies

Abstract: Green Pea galaxies are compact, extreme star-forming galaxies observed at intermediate redshifts (z ~ 0.15-0.4). Here we present analyses focusing on multiple aspects of Green Peas with the goal of better understanding their nature and origins. We confirm that the galaxies in our sample are consistent with the properties of Green Pea galaxies, and we present a detailed analysis of various properties of the Green Pea galaxies in our sample. We attempt to determine if the Green Pea environments can help to explain the cause of their starburst events. We present results from a redshift survey of field galaxies within a one degree field of view around a sample of Green Pea galaxies. We measure redshifts for 1312 galaxies distributed between 13 Green Pea fields, and we combine these with redshifts from the literature in order to analyze the densities of galaxies near the Green Peas. Additionally, we present chemical compositions of the Green Pea galaxies. We have obtained deep, high quality spectra of each Green Pea in our sample. These spectra allow us to measure the temperature sensitive [O III] 4363 emission line and derive direct abundances for 9 of 13 Green Peas in our sample. Our abundance measurements are consistent with previous observations of Green Peas as low-metallicity systems. Our multifaceted approach to studying Green Pea galaxies will help us to better understand how they fit into the narrative of galaxy formation and evolution.

Madison Smith, 2022

Title: A Statistical and Multi-Wavelength Approach to Studying Star Formation Histories in Nearby Galaxies

Abstract: The way a galaxy forms and evolves depends on the laws of structure formation in the universe. The effects of these laws are imprinted on the distributions of stellar populations within a galaxy. This dissertation studies the star formation histories of a sample of 34 nearby galaxies in order to better understand how stellar disks evolve. Multi-wavelength observations are combined with modeling in order to estimate past and present star formation on global, radial, and local scales. A spatial analysis of local regions of star formation is also done in order to test for hierarchical clustering of star formation. This hierarchical clustering is detected in several galaxies. The galaxy with the strongest clustering is M63, where the clustering can be tied back to the presence of spiral density arms. I also find evidence for an age gradient in the star formation histories of the full sample. These results agree with the leading theory for structure formation in the universe, which dictates that galaxies should be forming "inside-out". As simulations continue to strive for realistic models of galaxy evolution, this research provides the observational constraints needed to better understand the distributions of young stellar populations in these nearby galaxies. 

Title: Searching for Substructure in the Stellar Populations of Dwarf and Giant Galaxies

Abstract: In the paradigm of hierarchical galaxy evolution, substructure is expected to form through tidal interactions, accretion events, or mergers of galaxies and proto-galaxies. In this thesis I investigate the spatial distribution of Red Giant Branch (RGB) stars within three Andromeda dwarf satellite galaxies (Lacerta I, Cassiopeia III, and Perseus I) and the spatial distribution of globular clusters around two massive elliptical galaxies (M86 and M84) in an effort to identify such substructure. In the process, I characterize the three Andromeda dwarf satellites by estimating their structural and positional parameters (ellipticity, position angle, half-light radius, Sersic index, Right Ascension and Declination of the galaxy centers). I find a chain of over-densities along the west side of Lacerta I and two filaments northwest and southeast of the center of Cassiopeia III that may be coherent substructures. I find no evidence of substructure within the RGB population of Perseus I, possibly due to its relatively isolated environment. Within the globular cluster systems of M86 and M84, I find three features of interest. The peak surface density of the globular cluster system of M86 is offset from the galaxy's center, which may be at least partly due to the presence of the dwarf elliptical galaxy NGC 4406B. There exists a bridge of globular clusters between the M86 and M84 with a higher surface density than would be expected. Finally a boxy iso-density contour extends along the southeast side of the globular cluster system of M86. 

Title: Stellar Physics from Lithium in Open Clusters: Fundamental Preparatory Studies and Initial Results

Abstract: Lithium can tell us about physical processes occurring inside stars, a topic that we focus on in this thesis, as we also lay the groundwork for ongoing and future work. This thesis includes study of three open clusters: NGC 188 is one of the older Open Clusters nearby that has a rich giant population and may provide clues about Li-richness; M48 has an age in between that of the Pleiades and the Hyades, and thus may help distinguish different mechanisms and clarify the timing of Li-depletion features; NGC 7142 has suggested an age and metallicity similar to M67, which might provide additional tests for the solar Li problem. We re-evaluated membership of giants in NGC 188, determined the [Fe/H] of the members, and improved the line list near the Li doublet. Although NGC 188 giants should have no Li, we detected Li in four giants that lie either slightly or substantially away from the cluster fiducial sequence, and discuss possible Li-production mechanisms for those stars. A detailed study of ~300 A, F, G, K dwarfs in M48 sets the stage for me to derive (in the future) Li abundances and to interpret the various Li-depletion features. Here, the radial velocity, multiplicity, and membership are determined for each individual star; this alone results in a minimum cluster binary fraction of 11-21%, similar to a number of other open clusters that span a variety of ages and richness. The final averaged metallicity for the M48 cluster is [Fe/H] = -0.063 ± 0.007 dex ($\sigma_{\mu}$). The stellar [Fe/H] show no temperature dependence over an unprecedently large (2500 K) range, reinforcing confidence in our methods for determining open cluster metallicities. To study the critically important question of whether the Sun's Li depletion is normal requires identification of more stars having solar age and metallicity. Toward that endeavor, my analysis of the UBVRI photometry of NGC 7142 reveals an age and metallicity indistinguishable to those of M67 and thus the Sun, suggesting that NGC 7142 may provide additional stars for comparison with the Sun.

Title:The Galactic Chemical Evolution of Chlorine and Phosphorus

Abstract: Galactic chemical evolution impacts multiple areas of research in astrophysics, from interpreting isotopic ratios of presolar grains to characterizing stellar populations in distant galaxies. Of particular interest are the light elements, from carbon to argon. While most a-elements have been well measured and have well-known nucleosynthesis, the astrophysical production of the odd-Z elements is poorly understood. This dissertation focuses on the two odd-Z elements phosphorus and chlorine in order to understand their production in the universe. To achieve this, multiple observational studies of stars within 1 kpc of the sun have been conducted using high resolution IR spectrometers Phoenix and CSHELL on the KPNO Mayall 4m telescope, the Gemini South Observatory, and the NASA Infrared Telescope Facility. I have measured 35Cl abundances in 68 M stars and 35Cl/37Cl in seven M giants using HCl ro-vibrational lines at 3.7 µm. Phosphorus abundances have been measured in 43 FGK dwarfs and giants using P I features at 1.06 µm. Abundances were determined by calculating the best fitting synthetic spectrum to observations. I conclude that Cl matches theoretical models over the metallicity range of -0.6 < [Fe/H] < 0.2. Additionally, [Cl/Ca] ratios are consistent over the metallicity range suggesting that Cl is primarily produced in core collapse supernovae with a significant contribution from Type Ia SN. The average isotope ratio measured in seven stars is 2.66 ± 0.58 with a range of 1.76 < 35Cl/37Cl < 3.42 found in the sample. The spread in Cl isotope ratios in the seven star sample is consistent with values measured in the interstellar medium. The average Cl isotope ratio most closely matches yields from massive rotating stars. For phosphorus, I confirm that a chemical evolution model with empirical yields enhanced by a factor of 2.75 best matches observations over a metallicity range of -1 < [Fe/H] < 0.2. The average abundance for the dwarf stars in the Hyades cluster was <[P/Fe]> = -0.01 ± 0.06 and <[P/Fe]> = 0.03 ± 0.03 dex for the three giants. The consistency suggests that abundances derived using the 1.06 µm P I lines are not subjected to temperature- or luminosity-dependent systematic effects at high metallicities. I find that [P/O], [P/Mg], [P/Si], and [P/Ti] ratios are consistent with the solar ratio over a range of -1.0 < [Fe/H] < 0.2 with the [P/Si] ratio increasing by ~0.1-0.2 dex at the lowest [Fe/H] ratios. Finally, the evolution of [P/Fe] with age is similar to other a-elements, providing evidence that P is produced at the same sites. I suggest phosphorus is made entirely in core collapse supernovae with no significant production in Type Ia SN. The possible corrections to chemical evolution models are constrained from the measured abundances and I suggest any adjustment must match the original metallicity dependence, with a uniform enhancement to the yields, to accurately model the Galactic chemical evolution of phosphorus.

Title: Light Element Inhomogeneities and Multiple Populations in Galactic Globular Clusters

Abstract: Originally thought of as simple, homogeneous stellar populations, Milky Way globular clusters (GCs) are now known to exhibit inhomogeneities in light elements such as C, N, O, Na, Mg, and Al. Over the last few decades spectroscopic and photometric studies have shown that these inhomogeneities reflect the existence of multiple populations possibly formed through multiple epochs of star formation causing the subsequent generations to be enriched in processed material. Recent photometric studies have gone even further and provided evidence that multiple populations are present in every GC observed in the Milky Way.

In the case of C and N abundances, interpretation of these inhomogeneities (or chemical signatures) in red giant branch (RGB) stars is complicated by evolutionary processes. Measurements of C and N in RGB stars show decreasing C and increasing N with increasing luminosity along the upper giant branch. These changes in surface abundance are a result of mixing of processed material from the stellar interior far in excess of what is predicted in canonical models of stellar evolution. The combination of these two phenomena (multiple populations in GCs and extra mixing in the bright RGB stars) complicates the C-N distributions in GCs, but also offers the possibility to use C and N abundances to explore these two effects. While past studies have explored C and N abundances in GCs, many of them lacked the sample sizes needed to model fully both multiple populations and the characteristics of deep mixing.

In this dissertation, we present spectroscopic observations of evolved red giant stars in three GCs, M10, M53, and M71 that span a range of metallicities (-2.07 < [Fe/H] < -0.78). We build sample sizes of 100-150 stars in each cluster that cover a large range in magnitudes (from the tip of the RGB to below the LFB), so that we can better constrain models of the formation of multiple populations as well as theories of extra mixing. We use measurements of the strengths of low-resolution CN and CH bands to identify CN-enhanced and CN-normal populations and determine C and N abundances in each cluster. Our large radial distributions in each cluster allow us to study the ratio of each population as a function of radius from the cluster center. We also focus on stars that have Na and O abundances measured in the literature so that we can compare our classification method with one using the Na-O anti-correlation. All of our clusters have also been studied using Hubble Space Telescope (HST) UV photometry, which allows us make comparisons between classifying stars with CN band strength and HST photometry. We find the presence of two populations in all three clusters. The populations are found to be spatially mixed in M10 and M71, while the CN-enhanced population is more centrally concentrated than the CN-normal population in M53. All three of these results are consistent with estimates of the clusters' dynamical ages. Our method of identifying multiple populations based on CN index strength agrees with both methods using Na-O and HST UV photometry for all three clusters. Finally, both populations in M10 and M53 show equal rates of C-depletion and N-enhancement as a function of magnitude, while stars in M53 experience more efficient C-depletion and N-enhancement than those in M10.

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.

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.

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.

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.

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.

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.

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

Jamie 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

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.

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.

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.

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.

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