Talks & Events
Ph.D. Thesis Defenses: 2022
Past Ph.D. Thesis Defenses
The Detection and Characterization of Exoplanets Around M Dwarf Stars Using TESS
The ongoing Transiting Exoplanet Survey Satellite (TESS) mission is an all-sky survey designed to detect planets around bright, nearby stars by providing high-cadence, precision photometry for hundreds of thousands of targets. TESS is optimized to detect planets around M dwarf stars, and my work focuses on the detection and characterization of such worlds. As the most common stellar type, with strong planet signals in both radial velocity and transit, M dwarf stars are ideal targets for exoplanet searches. But M dwarf transit searches and planet characterization are complicated by stellar activity in their light curves. Herein, I describe my research on detecting and characterizing planets orbiting M dwarf stars, particularly in the presence of spot modulation and flares. I begin by investigating the properties of the TOI-700 planets, including TOI-700 d, the first Habitable Zone, Earth-sized planet discovered with TESS. I inspect 11 Sectors of TESS observations and fit the transits of each of the 3 known planets in the system to determine their radii. Next, I study a more active star: the pre-Main Sequence M dwarf, AU Mic. AU Mic exhibits significant variability in its light curve in the form of spot modulation and flares. Both of these features complicate the transit fits for AU Mic b and c, but by fitting the activity and transits simultaneously, I am able to construct accurate transit models, and therefore derive precise planet radii and transit times. Finally, I model the optical flares of Proxima Centauri observed by TESS in order to conduct a transit search across the entire Habitable Zone of the star and show no evidence for transits of planet b or any other planets down to the size of Mars.
Planet-Planet Interactions in Exoplanet Systems
The gravitational interactions of planets in multiplanetary systems can have effects ranging from tiny orbital alterations to severe instability and everything in between. In this talk, I will discuss three applications of planet-planet interactions. First, I investigate a commonly used analytical approximation and its shortcomings when applied to planetary systems like those observed by the Kepler mission when used to calculate the rate at which the longitude of the ascending node changes. Secondly, I illustrate how the slight change in period ratio over time for planet pairs both in and out of mean-motion resonance can lead to observable sculpting of the time-averaged period ratio for a population of planets and how the strength of this sculpting depends on the eccentricity of the planets involved. Lastly, I endeavor to quantify how the presence of two giant planets affects the potential habitability of an Earth-like planet. I assess the relative habitability for each giant planet configuration---that is, the integrated habitability probability compared to a system with only an Earth-like planet. By varying the properties of the two giant planets (mass, semi-major axis, eccentricity, and inclination), I identify correlations between various parameters, identify particularly habitable and uninhabitable configurations, and demonstrate how the interplay of mean-motion resonance, secular resonances, and other dynamical effects must be taken into account when considering habitability.
Stellar Mass Assembly in Galaxy Clusters and High-Redshift Gravitationally Lensed Galaxies
A major challenge in the field of extragalactic astrophysics is understanding when the most massive galaxies form the bulk of their stars, and determining the specific pathways they take to assemble stellar mass across a wide range of redshifts and environments. In this thesis, I describe my work to characterize stellar populations in massive galaxies at two epochs — redshifts ~0.5 and ~5.
I use spectra of galaxies in massive South Pole Telescope galaxy clusters to address the question: on what timescales do galaxies that end up in clusters form their stars, and does the cluster sample matter when studying these properties? This mass-limited cluster sample across redshifts 0.3 < z < 1.5 allows me to constrain star formation histories and formation redshifts of 900 quiescent galaxies in clusters, as a function of cluster environment and mass. This study explores mass-dependent evolution in cluster quiescent galaxies and characterizes galaxy evolution across a descendent-antecedent cluster sample.
On the other ‘end’ of the redshift scale, I describe the discovery and characterization of high-redshift lensed galaxies by the COOL-LAMPS collaboration, including COOL J1241+2219 (CJ1241), a lensed galaxy at z = 5.04 that is the brightest galaxy known at z > 5 (at AB magnitude z~20.5). Using ground-based spectrophotometric data and SED fitting analyses, we find CJ1241 to be an intrinsically luminous and massive star-forming galaxy near the epoch of reionization. With anticipated multi-wavelength spectroscopic data, including from an approved JWST Cycle 1 Program (GO 2566, PI: Khullar), I describe the anticipated improvements in constraints on old stellar populations, dust and metallicity in CJ1241. I also show first results aimed at comparing CJ1241 and other COOL-LAMPS discovered lensed massive galaxies at z>3 with their potential descendents — quiescent massive lensed galaxies at lower redshifts. Finally, I describe efforts to create efficient machine learning-based frameworks — specifically using simulation-based inference (SBI) — to calculate posterior distributions of key galaxy parameters, and motivate future efforts to study stellar mass assembly in massive galaxies across different epochs.
Probing Cosmic Reionization with Quasar Proximity Zones
In the first billion years of the Universe, the first galaxies and quasars formed and their ionizing photons brought about a major phase transition of the IGM from a mostly neutral to a mostly ionized state. The exploration of this crucial epoch, called cosmic reionization, is entering a golden era with the exponential progress in computational ability and with JWST and 30m-class telescopes on the horizon. The regions around the first quasars, called quasar proximity zones, are among the first targets that JWST will observe. They are unique since they are thought to trace the densest environments where many galaxies reside. Besides, the quasar spectra carry rich information about the IGM during reionization. In this talk, I will show multiple aspects of what we can learn from the quasar proximity zones. First, I will show how to interpret the size of observed proximity zones and how to use the absorption features to recover the density and further constrain cosmology and quasar properties. Second, I will talk about my suite of quasar proximity zone simulations and show how galaxies form in this biased field with strong radiation. This study will help us interpret future JWST data and answer key questions about the quasar environment and radiative feedback. I will conclude with an outlook of synergizing JWST and ground-based observations of quasar proximity zones to learn more about reionization.
Particle Acceleration, Propagation, and Detection: A Journey from the Kinetic Structure of Plasma Physics to Particle Transport on Cosmic Scales
The origin of Ultra-High-Energy Cosmic Rays (UHECRs) and the highest-energy astrophysical neutrinos remains as one of the most prominent unresolved questions in astrophysics. We can shed light on such phenomena employing a thorough bottom-up approach to understand the spectra of UHECRs, neutrinos, and eventually x/gamma-rays from Active Galactic Nucleus (AGN) jets. In this respect, I will initially discuss an original theory of particle acceleration in AGN jets, i.e., the espresso mechanism, that we back by propagating protons and heavier elements in relativistic 3D MHD simulations of AGN jets accounting self-consistently for i) particle injection, ii) particle acceleration, iii) spectra of UHECRs, iv) effects of losses on UHECRs, and v) the resulting neutrino spectral features. Moving from the global scale of jets to the kinetic scales of the plasma, I will also present the first steps in understanding asymmetric reconnection in the relativistic regime using Particle-in-Cell (PIC) simulations. Considering the turbulent nature of AGNs, asymmetric reconnection can potentially be the main driver of nonthermal lepton acceleration, and thus nonthermal radiation, important to modeling UHECR losses and neutrino production. I complement these studies by propagating UHECRs in turbulent magnetic fields over large distances to examine their impact on the delay incurred during propagation. These propagation considerations have potentially similar predictive powers for galactic cosmic rays (CRs). The confinement time of galactic CRs could also be measured in a more direct manner using CR isotope ratios. In this respect, I have been heavily involved with the High Energy Light Isotope eXperiment or HELIX experiment to obtain an observationally motivated value for the confinement time of CRs in the galaxy. HELIX is a magnet spectrometer designed to make measurements of the composition of light CR isotopes. This NASA funded experiment is outfitted with a suite of modern, high-precision particle detectors designed specifically to make measurements of significant isotopic abundance ratios in the energy range ~0.2-~10~GeV/n, a range that is not accessible to any current or planned instrument.
Shedding light on the Low-Surface-Brightness Universe with Galaxy Surveys and Machine Learning
Low-surface-brightness galaxies (LSBGs), conventionally defined as galaxies with central surface brightness at least one magnitude less than that of the ambient dark sky, have remained largely elusive in past wide-field surveys. At the same time, observational and theoretical arguments point towards an LSBG-dominated universe. Current and upcoming deep and wide surveys are expected to illuminate the LSB regime. At the same time, the massive amount of data they are going to produce requires the development of novel, automated analysis techniques, with modern machine learning (ML) algorithms presenting a promising solution to this problem. In this work, I will first discuss the discovery and analysis of a large catalog of LSBGs from the Dark Energy Survey data. Afterward, I will describe the development of a number of ML-based tools that can automate the data analysis in future surveys, from using object detection methods in order to remove spurious light reflections in astronomical images, to automatically measuring galaxy profile parameters, with uncertainty quantification using Bayesian neural networks.
Seeking Solutions for the Hubble Tension
As the standard paradigm for cosmology, LCDM model is still challenged by more and more precise modern observations. One of the most significant challenges is so-called Hubble tension, the discrepancy between the locally measured Hubble constant $H_0$ and its value inferred from the early Universe Cosmic Microwave Background (CMB) measurements assuming $Lambda$CDM model. I will provide some thoughts and examples as the possible solutions for the Hubble tension based on my work with collaborators during my PhD, including the Acoustic Dark Energy model, the interaction between dark energy and dark matter, and early modified gravity. The future directions will also be discussed.
The Tip of the Red Giant Branch and its Application to Measurements of the Hubble Constant
In the Carnegie Chicago Hubble Program (CCHP), our collaboration used the Tip of the Red Giant Branch (TRGB) to calibrate the Type Ia supernova (SN Ia) distance scale and determine a new value of the Hubble constant (H0). Our TRGB-derived value is significantly less in tension with LCDM than that derived using Cepheid variable stars, thereby raising concerns of underestimated systematic errors and weakening the evidence for the so-called Hubble Tension and new physics beyond LCDM. Throughout my PhD, I have developed new methodologies for estimating TRGB distances and associated uncertainties. While doing so I have measured TRGB distances to eleven SN Ia Host galaxies that formed the basis of the CCHP’s H0 determination.
In my dissertation paper on calibrating the TRGB luminosity, I introduced a novel procedure for accurately selecting TRGB stars in the Magellanic Clouds and made the most precise TRGB measurement in that system to date, in turn resolving a recent debate over the brightness of the TRGB in the Clouds. I have continued to improve the TRGB calibration using brand new Hubble Space Telescope (HST) imaging (PI: Hoyt) aimed at the stellar halo of the megamaser host galaxy NGC 4258. These new calibration data will reduce uncertainties in the TRGB zero point to the systematic floor of the maser distance, or 1.5%.
Looking ahead to the method’s use with JWST, I demonstrated in a study of the Large Magellanic Cloud (LMC) that, when observed in the infrared (IR), the TRGB is sufficiently precise that it can be used to discern 2% variations in line-of-sight distance across the LMC, despite the IR application of the method requiring an additional correction that is a function of the colors of the TRGB stars. The TRGB is several times brighter in the IR and with JWST the method promises to revolutionize distance ladder measurements of H0.