Astronomy

Permanent URI for this collectionhttps://digital.lib.washington.edu/handle/1773/4892

Browse

Now showing 1 - 20 of 71

The Carbon Cycle of Galactic Atmospheres: Connecting Star-Formation and Cosmic Gas Flows with the Physical Conditions of the Circumgalactic Medium
(2025-10-02) Garza, Samantha Lucia; Werk, Jessica
The diffuse gaseous atmosphere surrounding the star-filled inner region of a galaxy is known as its circumgalactic medium (CGM). Galaxies draw gas from the CGM to fuel star formation; internal processes within the galaxies, in turn produce feedback that ejects gas and material back out into the CGM. This constant cycling of gas allows the CGM to maintain a record of how a galaxy builds its star population and holds key information on how galaxies begin to quench. By investigating this record, we can tell a richer story of a galaxy's past and make strides in predicting its future. In this dissertation I describe the work I have done using observations and analytical models focused on the metal ion C IV to answer two main questions. First, is there a correlation between the pc-scale physics of black hole growth and the global, kpc-scale gas flows of the CGM in observations, and how does this compare to simulations? Garza et al. (2024) suggest that C iv column density does not vary with SMBH mass; however, sSFR is highly correlated with the ionization content of the CGM. This raises the question: what do C IV observations reveal about the ionization state, temperature, and processes responsible for this ionization mechanism in the CGM? Garza et al. (2025) investigates whether a dichotomy exists for C IV between star-forming and passive galaxies (9.5 ≤ log10 M*/M⊙ ≤ 11.2). Results are statistically consistent (≥ 99%) with dichotomy findings for C iv in sub-L* galaxies (Bordoloi et al., 2014) and O VI in L* galaxies (Tumlinson et al., 2011). This suggests that C IV is more O VI-like than "low-ion-like," indicating it traces gas maintained by star formation and/or feedback, unlike other low-ionization state gas (e.g., H I, Si II, C III). In a follow-up paper, we find that C IV and O VI are kinematically coincident but the relationship between these intermediate to high ions and lower ions is not straightforward. To explore the origins of C IV and O VI, we employed a simple cooling flow model under collisional ionization (PI) and find that O VI is very consistent with the predictions for an expected temperature range, while C IV is not consistent and has column densities on average 2.5 times higher than the predictions. Since C IV has a lower ionization energy than O vi, it is possible that C vi has contributions from both the warm/hot phase and from the cool photoionized (PI) phase. Any successful model of the CGM which incorporates multiphase conditions, must result in ionized gas containing C IV and O VI that is kinematically coexist, but "avoids" existing at the same temperature and density. Ultimately, these analyses help us understand the complex relationships governing galaxy star formation rates, feedback from their supermassive black holes, and the contents of their diffuse gaseous halos.
A Galaxy of Binaries: The impact of binary interactions on gravitational-wave sources, asteroseismic pulsations and stellar kinematics
(2025-10-02) Wagg, Tom; Bellm, Eric C
Massive stars are extremely impactful across a range of astrophysics, providing essential feedback for galaxy evolution and producing transients such as supernovae and gravitational-wave mergers. The vast majority of these stars are formed in binaries and higher-order multiple systems, a large fraction of which will interact during their lifetimes and significantly alter their subsequent evolution. Despite the importance of massive stars, and the prevalence of massive binary stars, many aspects of binary stellar evolution remain uncertain across several orders of In this dissertation I describe my work in exploring the impact of binary interactions on a variety of massive stellar populations, both through rapid population synthesis and detailed 1D stellar evolution models. In particular, I consider the sensitivity of these results to major uncertainties in binary physics, with a view towards placing constraints on binary physics parameters. The culmination of this work is not only a series of scientific results and predictions, but also two new open-source codes (LEGWORK and cogsworth) that enable future community-driven investigations into these matters. In Chapter 1, I introduce the importance of massive binary stars, and outline the major uncertainties that remain, as well as some methodologies for addressing these uncertainties. After this introduction, I consider the population of gravitational-wave sources that will be detectable by the future spaced-based detector LISA (Chapters 2 & 3). Our new open-source code, LEGWORK, provides the community with a reliable resource for calculating the detectability of LISA sources and computing their evolution due to gravitational-wave emission. We use LEGWORK to explore how the rates and demographics of the galactic population of black hole and neutron star binaries are sensitive to different aspects of binary From here, I move from gravitational-waves to gravity-mode pulsations and the asteroseismology of massive stars that have accreted material from a companion (Chapter 4). With our proof-of-principle analysis, we established for the first time that mass transfer can leave a significant imprint on the asteroseismic signals of accretor stars. We showed that the rejuvenation of the convective core as a result of mass transfer leaves an imprint on the chemical composition gradient in the star. This change in gradient influences all pulsations that are sensitive to that region of the star, shifting their pulsation periods. We showed that typical asteroseismic methods for estimating the mass and age of the star (which assume single star evolution) can be significantly erroneous for accretor stars. In the subsequent chapters, I focus on the new techniques I developed for predicting the extrinsic positions and kinematics of massive binaries, in addition to their intrinsic properties such as rates, masses and orbital periods. I present a new open-source code, cogsworth, which can be used to perform self-consistent population synthesis and galactic dynamics simulation (Chapter 5). We used cogsworth to demonstrate how the positions of massive stellar populations can inform our understanding of binary interactions. From here, I examine how ejection velocities of stars are relatively insensitive to the strength of supernova natal kicks and compare the prescriptions for these ejections across three different population synthesis codes (Chapter 6). In Chapter 7, I use cogsworth to explore how binary interactions impact the timing and location of core-collapse supernovae. Current models for supernova feedback in hydrodynamical simulations assume that all massive stars are formed as single stars. We show that binary interactions result in both late-time and spatially-displaced supernovae. We find that more than a quarter of supernovae occur after the time of the final single star supernova, while 13% explode more than 100pc from their parent star cluster. We assessed the robustness of these predictions to a plethora of variations in binary physics, initial conditions and galaxy parameters and found these results are surprisingly insensitive to uncertainties in the parameters we considered. Given that these supernovae could have a significantly different impact on galactic feedback, we developed a new analytic model for core-collapse supernova feedback. This model includes physically-motivated metallicity-dependent transitions and reproduces the timing and velocity distributions of supernovae to within 1% and 4% respectively. Our model can be used in future hydrodynamical simulations to better account for binary evolution. This may be particularly relevant for high redshift galaxies in which the spatial extent of the galaxy is reduced, while the low-metallicity environment produces even later and more distributed supernovae. Finally in Chapter 8, I discuss the future directions of this research and consider the potential avenues for constraining binary evolution that this thesis has enabled. In Chapter 9, I summarise this thesis, concluding with the hope that the findings I presented, as well as the new open-source codes that we released, will drive forward the field of massive binary evolution.
Accelerating and enabling discovery in the decade of astronomical surveys
(2025-08-01) Bektesevic, Dino; Conolly, Andrew
With the advent of a new generation of astronomical surveys such as the Legacy Survey of Space and Time (LSST) from the Rubin Observatory astronomers will have access to a wealth of data. If we are to fully exploit the data these surveys will generate, we will need to develop novel algorithmic approaches for analyzing astronomical images and tools to scale these algorithms to petabytes of data. In this thesis I focus on these two aspects scaling novel algorithms to extract more science from Rubin data than was previously possible, and developing a novel approach to the analysis and classification of lightcurves. The challenges in scaling to petabytes of data are multi-faceted. The Vera C. Rubin Science Pipelines are a collection of algorithms and a workflow management functionality intended to be used to process data taken by the Rubin Observatory's Legacy Survey of Space (LSST). In the first chapter of this thesis I describe how we implemented an Amazon Web Services and Google Computing Services compliant cloud service backend for the Rubin Middleware components that enable executing the Rubin Science Pipelines on cloud resources. I demonstrate how for short-term projects with a large in-going dataset and a small out-going data volume of results the cloud is almost always cost effective. Analysis results may be retrieved much sooner by allocating more resources, than they would when allocating less compute resources, at the same cost. In chapter 2 I demonstrate how new algorithms can be used to improve on the amount of science delivered by processing Rubin-like data on Rubin-like scales. Kernel Based Moving Object Detection package (KBMOD) is a tool developed to perform searches for moving objects on collections of images using a shift-and-stack method along linear trajectories. \citet{Smotherman2024} demonstrated it can detect objects below the SNR of a single exposure. In this chapter I discuss work required to improve the performance of KBMOD and execute it on all of DEEP's data (~200TB, equivalent to 10 nights of Rubin data), while simultaneously being able to increase the range of searched angles by a 100\% and the range of searched velocities by an additional 38\% compared to the previous search. Compared to Smotherman et al (2024), we achieve a 10\% higher peak detection efficiency but a 0.3 magnitude lower limiting magnitude at which 50\% of the objects were recovered. We identify the higher filtering threshold values, chosen due to a large number of estimated returned results, as the key culprit for the loss of limiting magnitude. In the final chapter of the thesis I develop a new approach to time series classification. By applying ideas inherited from differential geometry on Riemannian manifolds I demonstrate how it's possible to construct a measure of distance between two curves based on nothing more than their shape. I consider two distance measures Square Root Velocity and varifold fidelity measures. The latter is robust different light curve parameterizations. Multiple classification schemes are constructed based on these distances including agglomerative (hierachical clustering), fitting a sum of Gaussians, and a K-Means like algorithm to find the generalized means (Frechet means). Classification accuracy for a high SNR dataset was 96.58\%, 95.9\% and 98.93\% for the sums of Gaussians, agglomerative clustering and K-Means approach respectively. Validation of the approach on the PLAsTICC lightcurves dataset was less successful, achieving a top classification accuracy of 77.03\%. Two key reasons for the drop in classification accuracy are identified: heteroskedastic uncertainties in the data, and reparameterizations of curves.
Driving Discovery in Astronomy Using Scalable Computing and Fast Algorithms
(2025-05-12) Stetzler, Steven; Jurić, Mario
The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will produce and publicly release an imaging and catalog dataset larger than any before it. At a total data volume of 500 PB, it will only be possible to realize the full scientific potential of this survey if tools and algorithms are available that can scale to the size of the dataset. This thesis contributes to this effort through three key advancements. First, I demonstrate how cloud-based science platforms can be used to access and analyze large catalogs using a distributed computing framework. The distributed computing tools utilized are user-friendly and accessible, allowing astronomers to scale their workflows to the entire LSST catalog. Second, I illustrate how the LSST Science Pipelines--a set of tools and algorithms for processing astronomical images--can be applied to a wide-field imaging survey, verifying their scalability and applicability in processing the LSST imaging dataset. I also provide tools that enable users and research groups to perform their own image processing campaigns. Finally, I introduce an optimized version of the shift-and-stack algorithm, enhancing its efficiency for detecting faint solar system objects in multi-epoch imaging surveys. This improved algorithm provides speedup relative to a naive implementation. This optimization lays the foundation for application of shift-and-stack to the entire LSST dataset, particularly in the search for faint inner solar system objects.
Cuentos de las Estrellas: Understanding the Properties and Impacts of Stellar Flares Across Time and Wavelength
(2024-10-16) Tovar Mendoza, Guadalupe; Davenport, James
The era of new space and ground based telescopes is providing us with more detailed exoplanet observations and advancing the search for life beyond our own solar system. These observations are providing opportunities to both study the makeup of these extrasolar worlds and also search for signs of habitability. However, stellar variability remains a limiting factor for planet detection and characterization. We see stellar contamination present in almost all exoplanet observations, especially around active M-type stars. Although it is seen as a source of contamination for most observations, stellar activity provides us with key insights into the larger star-planet environment. In order to understand and model the detailed properties of exoplanets we must understand the host stellar environment. Specifically, the signals generated by stellar magnetic activity need to be properly characterized and understood in order to gain a clear picture of the astrobiological implications stellar activity has on extrasolar worlds. To address these challenges, I have focused my thesis work on developing tools and leading studies that help us gain a better understanding of the properties and impacts of stellar flares across time and wavelength regimes. By leveraging existing datasets such as Spitzer, Kepler, TESS, and Gaia I have studied how stellar activity impacts our search for life on other worlds beyond our solar system. In the first part of this thesis I generate new tools for the community to use to model and understand photometric stellar activity in detail. Specifically, I use Kepler data to study the morphology or shape of stellar flares. I derive a continuous and analytical model to describe the shape of flare events in photometric observations. I test the flare template on datasets of various cadences and wavelengths and find the model is versatile enough to fit flare events in all of the tested cases. Furthermore, this flare template has already been used by other teams to model flares in both Kepler and TESS data \citep{Barclay2023TheC}. In the second part of this dissertation I study the energetics of flare events at infrared wavelengths. Even though flare emission peaks at near-ultraviolet and blue optical wavelengths, they emit energy across the entire electromagnetic spectrum. Flares in the infrared are only just beginning to be studied in detail as JWST is revealing they are a source of contamination for exoplanet studies. We use archival Spitzer observations of flares to constrain flare temperatures and understand their implications on exoplanet atmospheres. I turn to the active planet hosting star, TRAPPIST-1, to study how flare temperatures vary at infrared observations and in turn affect the exoplanets in orbit around the star. I also compare our results to the latest JWST observations of TRAPPIST-1. The third part of this thesis looks to the future to understand what upcoming flagship missions like the Nancy Grace Roman Space Telescope will provide for flare science. The final part of this thesis work explores how flare energetics change as a function of stellar mass and how the results impact the exoplanets around those stars. The resulting code and models developed for this thesis will be made open source to serve as tools for the greater astronomy and astrobiology communities.
Searching for Signs of Habitability and Life in the Era of Extremely Large Telescopes
(2024-10-16) Currie, Miles Harrison; Meadows, Victoria S
We are entering an exciting era for astrobiology, with terrestrial exoplanet characterization studies now underway with the JWST and the next generation of ground-based Extremely Large Telescopes (ELTs) expected to be online by the end of the decade. However, the prospects for searching for signs of habitability and life with the upcoming 30 m class ELTs are not yet thoroughly explored. Specifically, it is not well understood the extent to which the high resolution spectroscopy and high contrast imaging capabilities of the ELTs can be leveraged to characterize terrestrial exoplanet atmospheres. Previous theoretical studies on ELT capabilities have focused on the detectability of the biosignature gas O2 in Earth-twin atmospheres, but have not rigorously considered the environmental context gained by detecting other molecules. Models that produce atmospheres that are photochemically self-consistent with their host stars can be used to show how additional molecules reveal more about the planet and its processes, either strengthening the interpretation of O2 as a biosignature, or ruling in or out biosignature false positive mechanisms that can generate abiotic O2. Furthermore, our ability to use future ELT observations to observationally constrain the composition of nearby transiting and non-transiting terrestrial exoplanets is uncertain, and could provide independent avenues for determining the origin of O2 in particular. The goal of this dissertation is to explore and define the terrestrial exoplanet characterization capabilities of the upcoming ELTs, which can complement and support JWST and the future Habitable Worlds Observatory (HWO) NASA flagship mission. To that end, this work provides recommendations and observational protocols that will enhance and maximize the science of the ELTs to search for signs of habitability and life in terrestrial exoplanet atmospheres, laying the foundation for ground-based terrestrial exoplanet science in the near term, and space-based characterization studies in the future. In this work, we develop and apply techniques for analyzing simulated high-resolution ground-based spectra and retrieving molecular abundances in simulated ELT data. Our approach extends well beyond current approaches for characterizing terrestrial exoplanet atmospheres with the ELTs by considering the detectability of a suite of molecular species that can help constrain the origin of atmospheric O2, and provide further environmental context for a potential biosignature detection. We simulated ELT detectability of atmospheric molecules for a range of different inhabited and uninhabited terrestrial atmosphere types, as well as a sub-Neptune atmosphere, for planets orbiting M dwarf host stars. We found that CH4, CO2, H2O, and CO are all potentially detectable for both transiting and non-transiting terrestrial exoplanets, and that two biosignature pairs (O2/CH4 and CO2/CH4) may be detectable for nearby Earth-like worlds in ~10 hours of observing for Proxima Centauri b, the nearest non-transiting target. Furthermore, we may be able to discriminate biosignature false positive environments using the direct imaging capabilities of the ELTs by detecting CO, an indicator gas for several false positive cases, in as little as 10 hours for nearby targets. We could also identify false positives by searching for signs of significant abiotic O2 buildup via H2O photolysis, which may be possible in <100 hours of observing time. Discriminating the atmospheres of non-transiting planets as either terrestrial- or sub-Neptune-like may also be possible via spectral characterization in ~1 hour of observing with the ELTs to detect absorption from hydrogen-bearing species such as NH3. We also explore our ability to measure the abundance of O2 in habitable Proxima Centauri b atmospheres using atmospheric retrieval methods for high-resolution cross-correlation spectroscopy for the first time, and we find that we may be able to measure Earth-like O2 abundances or lower (<=21% O2) in 100 hours of observing the O2 A-band; however, retrieving high O2 abundances in post-ocean loss scenarios with thick atmospheres may be challenging due to significant saturation of the O2 A-band. Considering other O2 bands that are less prone to saturation or searching for the spectral features of O2--O2 collisionally-induced absorption may instead be used to identify this scenario.This dissertation proposes future ELT observing protocols and observational strategies for characterizing terrestrial exoplanets. The tools we have developed will continue to be relevant in the preparation for the analysis of the first ELT transit transmission and reflected light observations of nearby terrestrial exoplanets, as well as for the development of future ground- and space-based instrumentation and science strategies. The code and methodology developed in this work are available to the exoplanet and astrobiology communities. The goal of this dissertation is to explore and define the terrestrial exoplanet characterization capabilities of the upcoming ELTs, which can complement and support JWST and the future Habitable Worlds Observatory (HWO) NASA flagship mission. To that end, this work provides recommendations and observational protocols that will enhance and maximize the science of the ELTs to search for signs of habitability and life in terrestrial exoplanet atmospheres, laying the foundation for ground-based terrestrial exoplanet science in the near term, and space-based characterization studies in the future. In this work, we develop and apply techniques for analyzing simulated high-resolution ground-based spectra and retrieving molecular abundances in simulated ELT data. Our approach extends well beyond current approaches for characterizing terrestrial exoplanet atmospheres with the ELTs by considering a suite of molecular species that can help constrain the origin of atmospheric O2, or provide further environmental context for a potential biosignature detection, for a range of different inhabited and uninhabited terrestrial atmosphere types, as well as a sub-Neptune atmosphere, for planets orbiting M dwarf host stars. We found that CH4, CO2, H2O, and CO are all potentially detectable for both transiting and non-transiting terrestrial exoplanets, and that two biosignature pairs (O2/CH4 and CO2/CH4) may be detectable for nearby Earth-like worlds in ~10 hours of observing for nearest target Proxima Centauri b. Furthermore, we may be able to discriminate biosignature false positive environments using the direct imaging capabilities of the ELTs by detecting CO, a false positive indicator gas for several cases, in as little as 10 hours for nearby targets, or by searching for signs of significant abiotic O2 buildup via H2O photolysis, which may be possible in <100 hours of observing time. Discriminating the atmospheres of non-transiting planets as either terrestrial- or sub-Neptune-like may also be possible via spectral characterization in ~1 hour of observing with the ELTs. We also explore our ability to measure the abundance of O2 in habitable Proxima Centauri b atmospheres using atmospheric retrieval methods for high-resolution cross-correlation spectroscopy for the first time, and we find that we may be able to measure Earth-like O2 abundances or lower (<=21% O2) in 100 hours of observing the O2 A-band; however, retrieving high O2 abundances in post-ocean loss scenarios with thick atmospheres may be challenging due to significant saturation of the O2 A-band--- considering other O2 bands that are less prone to saturation or searching for the spectral features of O2--O2 collisionally-induced absorption may instead be used to identify this scenario.This dissertation proposes future ELT observing protocols and observational strategies for characterizing terrestrial exoplanets. The tools we have developed will continue to be relevant in the preparation for the analysis of the first ELT transit transmission and reflected light observations of nearby terrestrial exoplanets, as well as for the development of future ground- and space-based instrumentation and science strategies. The code and methodology developed in this work are available to the exoplanet and astrobiology communities.
Sub-kpc Scale Analysis of Stellar Chemical Abundance and Star Formation Distributions in Nearby Galaxies
(2024-10-16) McKay, Myles Anthony Aaron Alexander; Williams, Benjamin
Nearby galaxies provide critical insights into the complex chemical and stellar evolution of galaxies over cosmic time. High-quality resolved measurements provide insight into the intrinsic details of evolution when investigating the stellar population and chemical abundance in the bulge and disk regions. Furthermore, this allows us to compare the radial distribution properties of different galaxies to examine their recent evolution, especially among distinct galaxies. In this work, we utilize integral field unit spectroscopy (IFS) and high-resolution stellar photometry to examine the distribution of nearby galaxies. We first investigate the radial distribution of stellar population, star formation, and oxygen abundance in centrally star-forming galaxies with photometrically red disks, termed BreakBRD galaxies, compared to the parent sample using the MaNGA IFS survey. We find that the BreakBRD galaxies exhibit a significant positive stellar population age profile but show no significant variation in stellar mass surface density, star formation surface density, or oxygen abundance. We estimate the RGB metallicity gradient of M31's northern and southern halves from the PHAT and PHAST surveys, respectively, and find a shallow negative gradient in both halves. We interpret this result as indicating that the older RGB stellar population is well mixed throughout the disk. Additionally, we have developed a photometry pipeline for future flagship surveys and used high-quality MaNGA data with a high-performance deep learning model to test prediction accuracy for recovering the ionization source.
Studying Global and Local Star Formation Processes with Young Massive Stars in NGC 6946
(2024-10-16) Tran, Debby; Williams, Benjamin; Levesque, Emily
I analyze resolved near-ultraviolet (NUV) stellar photometry of star-forming galaxy NGC 6946 (The Fireworks Galaxy) taken with the Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) with F275W and F336W filters to put new constraints on the relationships between the spatial distribution of ages, formation rate, masses, colors, and luminosity for massive stars and massive star clusters. In Chapter 2, we observe a correlation between age and density in the spiral arms and test diagnostics for star formation rate (SFR) and whether they are consistent with the unusually high observed rate of supernovae (10 in the past century). In Chapter 3, I modify a computer-vision algorithm to automate cluster candidate finding and produce a cluster and association candidate catalog. I measure the luminosity, effective radius, and color of each of the candidates. With the catalog obtained in Chapter 3, in Chapter 4, I constrain the cluster mass-radius relation and mass function of clusters. We measure the age of the clusters and determine the cluster formation efficiency of NGC 6946. We will publicly release this new clustering algorithm to the community and this observational data enabling detailed comparisons to numerical simulations. In Chapter 5, my future work, I discuss my progress and plans to measure an additional 13 bands ($\sim$200 visits) of archival optical and infrared (IR) HST WFC3 and Advanced Camera for Surveys (ACS) photometry of individual stars and clusters covering some areas in NGC 6946. With the planned stellar photometry catalog, I will fit spectral energy distributions (SEDs) for over 8 million stars in NGC 6946 to investigate how local environments and large scale mechanisms influence the formation of stars.
Planetesimal Accretion in the Solar System and Beyond: An Application to Systems of Tightly-Packed Inner Planets
(2023-09-27) Wallace, Spencer Clark; Quinn, Thomas R
Planetesimals are the smallest gravitationally bound objects to play a role in the planet formation process. In a bottom-up fashion, these bodies are thought to collide and grow to form protoplanets, which are roughly Mars-sized objects, and then eventually coalesce into larger worlds. Throughout this process, some planetesimals are left behind and can persist for billions of years after the formation of a planetary system. This is believed to be the principal source of asteroid and Kuiper belt objects, among many other small body populations in the present-day solar system. In addition to providing clues about the process of planet formation, planetesimals can also transport planets themselves, through weak but numerous gravitational interactions. Planetesimals around other stars sometimes collide and generate dust, producing a distinct observational marker that can be used to infer their presence. In this thesis, I use state of the art N-body simulations to understand the dynamics that govern planetesimal interactions and growth. For the first time, I follow this growth process using bodies with masses comparable to those predicted by planetesimal formation models. Upon doing so, I show that certain dynamical mechanisms involving mean-motion resonances only operate with sufficiently high resolution, and the effects of these mechanisms place a number of constraints on the planet formation process, including tracing the initial sizes of planetesimal formation and using collisionally-generated dust to infer the orbital properties of unseen planets in nearby disks. I also use these resolution capabilities to directly follow the growth of a system of terrestrial planets, starting from planetesimals. In doing so, I assess the viability of an in-situ formation model for systems of tightly-packed inner planets (STIPs), which appear to be a common outcome of planet formation. I also use these simulation results to train a neural network to generate a larger set of post-planetesimal accretion phase initial conditions, which I leverage to construct a statistical sample of simulated planetary systems for comparison with observations.
Empirical Approaches to the Near-Infrared Tip of the Red Giant Branch
(2023-09-27) Durbin, Meredith; Dalcanton, Julianne J
The infrared tip of the red giant branch (IR-TRGB) is a powerful tool for measuring distances to galaxies in the local Universe. However, establishing its absolute "anchor" with sufficient precision and accuracy has proved challenging due to lingering sources of systematic uncertainty in both theoretical predictions and empirical calibrations. Here I describe three studies aimed at improving observational constraints on the IR-TRGB. First, I develop a computational method for self-consistently measuring TRGB magnitudes and colors, and the covariance thereof, in multiwavelength stellar photometry catalogs. Traditional detection methods either marginalize over color, or else rely on assuming a fidicial color dependence, both of which may result in different stellar populations dominating the TRGB signal at different wavelengths. Next, I explore two complementary approaches to reconciling observations of the IR-TRGB as measured with ground- and space-based instruments respectively. The former are impacted by absorption features in Earth's atmosphere, where the latter are not. Furthermore, there is an extremely limited range of magnitudes at which current facilities can achieve the requisite data quality from both locations. To this end, I derive transformation equations between respective photometric systems using synthetic photometry of observed stellar spectra, and then present initial results from a three-year observing campaign designed to directly compare space- and ground-based observations of bright RGB stars in the Magellanic Clouds.
The Detection and Characterization of Terrestrial Exoplanets and Exoplanetary Satellites Via Their Transits
(2023-09-27) Gordon, Tyler A.; Agol, Eric
In recent decades the observation of planetary transits has emerged as the most successful method of exoplanet detection and characterization. Transits give us access to regions of parameter space that are inaccessible to RV and direct imaging -- notably low-mass planets in the habitable zones of dwarf stars are best observed via their transits. Followup observations of planetary transits can, in principle, constrain molecular signatures in rocky exoplanet atmospheres via transit transmission spectroscopy, measure the densities of planets via transit timing effects, and even potentially reveal the presence of exomoon companions. Despite the success of the transit method, challenges remain with regards to objects with shallow and/or infrequent transits. In this dissertation I detail some of those challenges and describe the progress that I have made towards mitigating them. Along the way I will describe a number of projects on subjects ranging from the characterization of stellar variability to modeling planetary interiors, all of which relate back to the basic theme of characterizing small, rocky transiting exoplanets. In Chapter 2 I review Gaussian process methods for modeling stellar variability, and demonstrate the utility of these methods by applying a one-dimensional GP model to measure stellar rotation periods in the K2 sample. In Chapter 3 I will extend this one-dimensional GP model to two dimensions in order to capture the wavelength-dependence of starspots and granulation. I then apply this model to simulated transit observations in order to demonstrate improved inference over the one-dimensional case. In Chapter 4 I summarize some additional applications of the Gaussian process framework including Fisher Information analysis, outlier detection and modeling, and non-stationary Gaussian processes. In Chapter 5 I review the subject of transiting exomoons and detail the development of an exomoon transit model that is intended to be combined with the stellar variability model from Chapter 2 in order to enable the detection of transiting exomoons. Finally, in chapter 6 I describe the prospects for exomoon detection in the near future with JWST.
Algorithms, Models, and Methods for SDSS-V Wide-field Robotic Fiber Spectroscopy
(2023-04-17) Sayres, Conor; Tuttle, Sarah
The field of astronomy has entered an age of big data, and this being driven by dedicated telescopes and instruments built for the sole purpose of conducting broad surveys to catalog the night sky. The highly successful space-based Gaia mission has already cataloged the broadband colors, 3D locations, and trajectories for nearly 2 billion astronomical sources. The much anticipated Legacy Survey of Space and Time (LSST) is projected to catalog an order of magnitude more objects. Imaging surveys like these are providing an unprecedented number of sources available for spectroscopic followup. Spectroscopy reveals a wealth of astrophysical information that imaging alone cannot, but spectroscopy is an inherently slow process. To spectroscopically observe even a small fraction of what is currently available, observations need to be performed as quickly and efficiently as possible. Robotic fiber positioners are a relatively new technology designed to speed up spectroscopic data collection. The fifth iteration of the Sloan Digital Sky Survey (SDSS-V) has built a pair of instruments each with 500 robots carrying optical fibers for multi-object spectroscopy. The first instrument operates from Apache Point Observatory (APO) in New Mexico and saw first light in December 2021. The second instrument operates from Las Campanas Observatory (LCO) in Chile and saw first light in August 2022. These instruments are designed to perform a spectroscopic survey of 6 million sources in a survey duration of five years. Operationally these instruments are complex, and they are required to perform at very high precision. This work describes the mathematics, algorithms, analysis, and general calibration strategies we have developed to make these instruments operational. A robot's fundamental task is to position a 120 $\mu$m diameter optical fiber in the focal plane of the telescope to capture light from an astronomical source. A software package {\tt coordio} was developed to perform the calculations for determining where an astronomical source will land in the focal plane of the telescope and how a robot should be moved to collect light from that object. This package defines a series of coordinate systems and transforms and provides a computational backbone for many pieces of SDSS-V's survey operations and infrastructure codes. SDSS-V has chosen a spatially-agressive layout for the robotic fiber positioner array in which the physical workspace for a robot heavily overlaps with its neighbors. This layout grants more sky coverage to each robot but introduces a high risk of collision between robots during reconfiguration. A software package {\tt kaiju} was developed to compute safe paths for every robot while moving from one spectroscopic target to the next. This path planning algorithm was an important success for the project, and it allowed the instrument to realize its full potential in using every available robot in every spectroscopic exposure. When instrument assembly was completed, a period of lab testing and calibration was performed. A lab test camera was used to measure positions of back-lit optical fibers as robots were moved through many reconfigurations. From this process we derive a kinematic model unique to each robot for use in {\tt coordio} routines to accurately predict fiber locations. This period also served to test and tune {\tt kaiju} path planning parameters while operating the robot array at full scale, ensuring proper operation prior to shipping the instrument to the telescope. After lab calibration, each instrument was installed at the telescope where a Fiber View Camera (FVC) is used to measure and adjust fiber positions during array reconfiguration. A series of camera distortion models were derived to reach sufficient FVC measurement accuracy. On-sky instrument commissioning consisted of spectroscopic observations of Gaia sources using a telescope dither technique, which allowed us to estimate on-sky fiber position errors. Analysis of these data informed additional layers of instrument calibration which improved overall fiber positioning accuracy. The instrument at APO was commissioned first, and is currently placing robotic fibers with an RMS error of 21 $\mu$m relative to astrophysical sources in the focal plane. Spectroscopic targets are generally well centered within the 120 $\mu$m fiber aperture, and normal survey operations are proceeding. We expect that ongoing efforts will further improve survey efficiencies and fiber positioning performance with an end goal of limiting fiber placement error to $\sim$17 $\mu$m. The instrument at LCO is currently in the commissioning and science verification phase, but initial indications suggest that fiber throughput will be sufficient for achieving SDSS-V science goals. SDSS-V is the third project worldwide to successfully deploy a robotic fiber positioner instrument to conduct a multi-year spectroscopic survey, and several other projects of comparable scale are nearing deployment. Throughout the process of designing, building, and using these instruments, the we have developed a number of strategies and techniques that are directly applicable to contemporary instruments today. Robotic fiber positioner arrays will likely continue to grow in importance, and proposals for building instruments with tens of thousands of robots on large aperture telescopes are already in place. The solutions derived from today's robotic multi-object spectrosopic surveys will directly influence the feasibility and design of future projects.
The Characterization and Discovery of Solar System Small Bodies in Modern Astronomical Surveys
(2023-01-21) Moeyens, Joachim; Juric, Mario
Starting in 2024, the Vera C. Rubin Observatory will conduct a comprehensive survey of the night sky named the Legacy Survey of Space and Time (LSST). LSST will observe the Southern sky every three nights continuously for ten years and is predicted to discover six million new minor planets. Claims in the literature have suggested that optically-derived diameters from such surveys are only accurate to within 50% compared to IR-based estimates. Using “Asteroid Thermal Modeling” (ATM), we show that optical measurements can be used to constrain asteroid diameters with 0.4% bias and scatter of only 17%. LSST will discover minor planets by observing “tracklets”: intranight linkages of two or more observations. The requirement to observe tracklets places a strong constraint on cadence and makes datasets not constructed with such a cadence unsuitable for discovery searches. We present “Tracklet-less Heliocentric Orbit Recovery” (THOR), a cadence-independent discovery algorithm that does not require tracklets. We apply THOR to two weeks of observations from the Zwicky Transient Facility (ZTF) and show that it can recover 1.5-2x as many asteroids as its tracklet-based counterparts. We describe our efforts to deploy THOR on the Asteroid Discovery, Analysis, and Mapping (ADAM) platform – the beginnings of a cloud-based discovery service which we name ADAM::THOR. We use the prototype service to search 0.2% of the exposures contained in the NOIRLab Source Catalog and discover 104 new minor planets.
Mapping the Circumgalactic Medium: Connecting Galaxies to their Environment
(2022-09-23) Wilde, Matthew; Werk, Jessica
The work presented in this dissertation provides the groundwork for understanding key unresolved questions in the study of galaxy evolution: the circumgalactic medium (CGM) and the role of environment. I present a new survey that has the power to drastically improve our understanding of the CGM and use it to estimate the size of the CGM as a function of galactic mass. In addition, a novel method to reconstruct the cosmic web using \textit{Physarum polycephalum} slime mold is presented and applied to the SDSS surveys. This cosmic web reconstruction was released publicly with SDSS DR17 and will allow exciting connections between galaxies and their environments to be illuminated, to distances greater than other previous catalogs.
Sifting Through the Static: Exploring the Space Beyond Neptune with Digital Tracking
(2022-09-23) Smotherman, Hayden; Connolly, Andrew
Trans-Neptunian Objects (TNOs) provide a window into the history of the Solar System, but they can be challenging to observe due to their distance from the Sun and relatively low brightness. Digital tracking helps address these challenges by algorithmically searching many possible TNO trajectories in a stack of images, enabling detection of TNOs too faint to detectin single images. Here we report the detection and characterization of 86 classical TNOs, 5 detached TNOs, 6 resonant TNOs, and 2 scattering TNOs that we could not link to any other known objects. We report measurements of semi-major axis, eccentricity, inclination, longitude of ascending node, argument of pericenter, and time of pericenter passage for these 99 objects. We also report values for the absolute magnitude H in the VR band with a largest measured H value of H=9.63. These objects are dynamically classified using 10 Myr Rebound orbital integrations. Additionally, we report the detection of 75 moving objects with short ~4 day arcs that we could not link to any other known objects and place constraints on the barycentric distance, inclination, and longitude of ascending node of these objects. We describe extensions to the Kernel-Based Moving Object Detection (KBMOD) software that helped enable these detections, including an in-line graphics processing unit (GPU) filter, a convolutional neural network (CNN) stamp filter, and an astrometric and photometric post-processing tool. These tools enable KBMOD to take advantage of difference images and help ready KBMOD for deployment on future big data surveys such as LSST.
The Physical Connection between Cosmic Gas Flows, Supermassive Black Holes Growth, and Galaxy Evolution
(2022-09-23) Sanchez, Natalie Nicole; Werk, Jessica
The circumgalactic medium (CGM) represents a key interface in the processes of galactic evolution. Here, the gas which enters galaxies through mergers and filaments and the gas expelled from a disk through stellar and black hole feedback intersect, maintaining a reservoir that will shape a galaxy throughout its lifetime. However, due to the diffuse and difficult-to- observe nature of this gaseous region, the degree to which galactic processes impact it are still uncertain, making the CGM a natural laboratory for testing the impact of different feedback models. The CGM of Milky Way-mass galaxies are the best targets for these analyses as these galaxies lie at the turnover mass during which galaxies switch from being dominated by stellar processes and become dominated by supermassive black hole (SMBH) or active galactic nucleus (AGN) processes.My focus of my thesis work is in exploring the impact of supermassive black hole (SMBH) feedback on the evolution of Milky Way-mass (MW-mass) galaxies in hydrodynamic sim- ulations. We use simulations from the N-body+Smoothed particle hydrodynamics code, ChaNGa, and include a 25 Mpc cosmological volume, Romulus25, and a suite of ”genet- ically modified” (GM) galaxies. These GM galaxies originate from nearly identical initial conditions resulting in minor modifications to their accretion histories that maintain the large scale structure and final halo mass of the original simulation. We find that (1) the SMBH propagates metals from the disk out into CGM, (2) the mass of metals retained by the galaxy depends on its deviation from the M-sigma relation, and (3) black hole accretion histories can be influenced by larger scale galaxy accretion physics, which work in tandem to quench star formation.
Galactic Gas Flows from Halo to Disk: Kinematics and Tomography in the Milky Way’s Low-Velocity Circumgalactic Medium
(2022-09-23) Bish, Hannah V.; Werk, Jessica
The evolution of galaxies is closely linked to the exchange of gas between their disk and the circumgalactic medium (CGM) – the massive, extended, diffuse halo of gas in which galaxies are embedded. Recent advances in high-resolution spectroscopy have enabled observers to firmly establish the key role played by the CGM in the life cycle of galaxies: it is the hiding place of at least half of all galactic baryons, acting as a massive reservoir that replenishes the supply of fuel for star formation via gas accretion onto the disk. However, this nearly-invisible halo gas is challenging to observe, and we are still missing a complete picture of its distribution, kinematics, and multiphase structure. In this thesis, I use the Milky Way as a case study to shed light on the nature of cool and warm CGM gas flows, taking advantage of the abundance of quasar and stellar sightlines which probe the Galactic CGM. In particular, I focus on the behavior of low-velocity gas, which is often overlooked by CGM studies because it is difficult to measure in isolation. I show that local CGM gas is predominantly inflowing, place constraints on the inflowing cloud sizes, and determine that these clouds lie close to the disk. I use a novel spectral differencing technique to correct for foreground absorption along sightlines through the Galactic halo, and present the first unobscured measurements of the Milky Way’s extended low-velocity CGM. The results demonstrate that either the warm CGM does not have a spherical morphology, as is often assumed for star-forming galaxies, or that the Milky Way is not a typical star-forming galaxy. Finally, I find that inflow velocities are higher for warmer gas, suggesting a picture in which warm accreting gas slows down and cools as it approaches the disk. The mass accretion rates of these inflows indicate that a significant fraction of star-formation fuel may accrete onto the disk at low velocities.
Small Samples No More: Probing the Evolution of Massive Stars
(2021-10-29) Dorn-Wallenstein, Trevor Z; Levesque, Emily M
Evolved massive stars --- the post-main sequence descendants of stars with initial masses higher than roughly 8 solar masses --- are rare yet critically important objects. As residents of their host galaxies, they inject radiation and matter into their surroundings on short timescales before exploding as supernovae. Individually, they are fascinating astrophysical laboratories in which many of the unknowns of stellar evolution coalesce. Due to their rarity, these multitudinous unknowns remain under-constrained. In this work, I attempt to understand evolved massive stars using a variety of techniques that have only recently begun to be applied to these interesting objects. My studies of populations of young stars reveal that the massive star binary fraction can be inferred using only simple demographic statistics. However, these methods can only be used given large numbers of well-classified stars, and I show that even using advanced machine learning techniques, existing data are insufficient to classify these stars. Finally, I demonstrate the immense potential of using asteroseismology to probe the interiors of evolved massive stars, and discover a new class of pulsating supergiant.
Simulating Multiphase Galactic Halos: The Impact of Cosmic Rays Through an Observational Lens
(2021-08-26) Butsky, Iryna Sotolongo; Quinn, Thomas R.; Werk, Jessica K.
All galaxies evolve embedded in a vast gaseous halo --- the circumgalactic medium (CGM) --- that dwarfs the mass and spatial extent of the stars in the galactic disk. Recent ultraviolet (UV) absorption studies reveal that the CGM has a rich, multiphase structure shaped by metal-rich galactic winds that expel the byproducts of stellar evolution and pristine inflows from the intergalactic medium. Galaxy evolution is intricately tied to the state of its CGM, as galaxies depend on a consistent supply of cool gas in order to form new stars. While current state-of-the-art simulations excel at reproducing galactic disk properties, most struggle to simultaneously reproduce the observed abundance, properties, and kinematic structure of multiphase CGM gas.In this thesis, I argue that the missing piece is likely a nonthermal component in the form of cosmic rays. I implement cosmic-ray physics into the {\sc enzo} and {\sc ChaNGa} simulation codes and use a combination of idealized, isolated-galaxy, and cosmological-zoom-in simulations to demonstrate the role of cosmic rays in driving galactic outflows and shaping the multiphase CGM. Throughout this work, I demonstrate how uncertainties in existing cosmic-ray transport models can lead to qualitatively different CGM structures, and motivate the need for more robust models of cosmic-ray hydrodynamics. Additionally, I explore more direct techniques for comparing simulations against observations. Using synthetic spectroscopy, I demonstrate how cosmic rays can naturally explain the observed kinematic alignment of multiphase CGM gas. I then use synthetic observations to demonstrate the prevalence and observational signatures of cool and warm gas in the halos around the most massive galaxies, which are traditionally studied with X-ray emission from their hot, dense inner regions.
Accreting Compact Objects in Technicolor: Multiwavelength Characterization of High Mass X-ray Binaries in the Local Group
(2021-08-26) Lazzarini, Margaret; Williams, Benjamin F.
High mass X-ray binaries (HMXBS) are systems that contain a compact object (neutron star or black hole) that accretes mass from a massive stellar companion. HMXBs are highly observable due to their bright X-ray luminosities, making them an observational window into the complex process of massive binary stellar evolution. In this thesis I present work I have completed over the past six years to measure the population demographics of HMXBs in Local Group galaxies M31 and the Small Magellanic Cloud (SMC) and the spatially resolved recent star formation history (SFH) of M33. In M31, I used observations from the Hubble Space Telescope (HST), the Chandra X-ray Observatory, and the Nuclear Spectroscopic Telescope Array (NuSTAR) to characterize the population of HMXBs in that galaxy and measure the overall HMXB production rate. In the SMC, I used NuSTAR observations to characterize the accreting compact objects in the detected HMXB systems. In M33 I use multi-band photometry to measure the spatially resolved recent SFH of M33 using color-magnitude diagram fitting. I discuss the results of my work in M31 and the SMC in the context of massive binary stellar evolution and previous studies of HMXB populations in the context of their host galaxies. For the spatially resolved recent SFH of M33, I discuss our measurements and their implications for the timescale of M33's large-scale morphological evolution.

Browse

Recent Submissions