Astronomy Theses and Dissertations

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ABOVE THE CLOUDS: 1-D MODELING OF OBSERVATIONS OF TIDALLY LOCKED EXTRASOLAR WORLDS
(2019) Afrin Badhan, Mahmuda; Deming, L. Drake; Domagal-Goldman, Shawn D.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Unique and exotic planetary environments give us an opportunity to understand how planetary systems form and evolve over their lifetime, by placing our planetary system in the context of vastly different extrasolar systems. With orbital separations a fraction of the Mercury-Sun distance, these close-in planets provide us with valuable insights regarding interactions between the stellar and planetary atmospheres. Further, observational biases actually allow such planets to be the first to be observed via transit spectroscopy. Observed spectrophotometric signatures from transit measurements can reveal spectrally active species in a planet’s atmosphere. Present observational technologies can also shed light on the atmosphere’s structure and dynamics. Future missions will allow us to constrain these properties with unprecedented accuracy, and are also being designed to observe increasingly smaller, cooler and less extreme planets. The eventual goal, after all, is to identify a world like our own. To interpret the observations with any certainty, however, we must build robust atmospheric models that sufficiently factor both physical and chemical processes expected in those atmospheres. 3-D climate modeling has shown that tidally-locked Earth-like planets, at the inner edge of M dwarf habitable zones, may retain water-vapor-rich stratospheres. However, flaring M dwarfs have strong UV activity, which may photodisassociate H2O. Using synthetical stellar UV within a 1-D photochemical model, I assess whether water vapor loss driven by high stellar UV would affect its detectability in JWST/MIRI transmission spectroscopy. I pseudo-couple a 3-D climate model to our 1-D model to achieve this. In a follow-up study, I also compute 125 additional atmospheric states by varying the Earth-like planet’s orbital distance (thus moistness) and methane production rates. I check for and quantify the simultaneous presence of detectable ozone and methane in an otherwise abiotic anoxic atmosphere. I have also implemented techniques to robustly quantify atmospheric properties of hot Jupiters from data-driven retrievals and built a versatile template for hot Jupiter atmospheres within our 1-D photochemical modeling tool, which was previously only valid for cool rocky worlds. I sketch out a plan for using this work towards mapping non-equilibrated (non-LTE) emissions from methane in the upper atmospheres of observable giants.
Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback
(2012) Park, KwangHo; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis focuses on radiation-regulated gas accretion onto black holes (BHs) from galactic scales emphasizing the role of thermal and radiation pressure in limiting gas supply to the BH. Assuming quasi-spherical symmetry, we explore how the gas accretion depends on free parameters such as radiative efficiency, BH mass, ambient gas density/temperature, and the spectral index of the radiation. Our numerical simulations show an oscillatory behavior of the accretion rate, and thus the luminosity from the BH. We present a model for the feedback loop and provide analytical relationships for the average/maximum accretion rate and the period of the accretion bursts. The thermal structure inside the str sphere is a key factor for the regulation process, while with increasing ambient gas density and mass of BHs eventually the accretion rate becomes limited by radiation pressure. The period of the luminosity bursts is proportional to the average size of the ionized hot bubble, but we discover that there are two distinct modes of oscillations with very different duty cycles that are governed by different depletion processes of the gas inside the ionized bubble. We also study how angular momentum of the gas affects the accretion process. In the second part of the thesis, we study the growth rate and luminosity of BHs in motion with respect to their surrounding medium. Contrary to the case without radiation feedback, we find that the accretion rate increases with increasing BH velocity, v, reaching a maximum value at v ~ 20-30 km/s, before decreasing as v^{-3}. The increase of the accretion rate with v is produced by the formation of a D-type (density) ionization front (I-front) preceded by a standing bow-shock that reduces the downstream gas velocity to nearly sub-sonic values. Interestingly, there is a range of densities and velocities in which the dense shell downstream of the bow-shock is unstable; its central part is destroyed and reformed intermittingly, producing a periodic accretion rate with peak values about 10 times the mean. This effect can significantly increase the detectability of accreting intermediate mass BHs from the interstellar medium (ISM) in nearby galaxies. We find that the maximum accretion rate for a moving BH is larger than that of a stationary BH of the same mass, accreting from the same medium, if the medium temperature is T<10^4 K. This result could have an important impact on our understanding of the growth of seed BHs in the multi-phase medium of the first galaxies and for building and early X-ray background that may affect the formation of the first galaxies and the reionization process.
Accretion Physics Through the Lens of the Observer: Connecting Fundamental Theory with Variability from Black Holes
(2018) Hogg, James Andrew; Reynolds, Christopher S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Variability is a generic feature of accretion onto black holes. In both X-ray binaries and active galactic nuclei, variability is observed on nearly all accessible timescales and across the entire electromagnetic spectrum. On different timescales and at different wavelengths it has unique signatures that can be used to characterize the accretion processes generating the emission and probe the accretion disks, which would otherwise be impossible. Despite having been observed for over fifty years, interpreting this variability is difficult. Simple phenomenological models have been used to explain the behaviors and geometries of the observed accretion disk, but they have yet to be rigorously tested in a full magnetohydrodynamic framework. In this dissertation we use high-resolution numerical models to investigate: (1) ``propagating fluctuations" in mass accretion rate that give rise to the nonlinear signatures of accretion on viscous timescales, (2) the dynamics of truncated accretion disks which are invoked to explain the spectral variation of outbursting X-ray binaries and the bifurcation of AGN accretion states, and (3) the large-scale magnetic dynamo behavior in thick and thin accretion disks. We find that the structured variability readily seen in the light curves from accreting black holes (i.e. log-normal flux distributions, linear relations between the RMS and the flux, and radial coherence) quickly and naturally grows from the MRI-driven turbulence and that these properties translate into photometric variability. For the first time, we identify the large-scale magnetic dynamo as the source of the low-frequency modulations of the disk stress that cause this structure. We introduce a bistable cooling law into hydrodynamic and magnetohydrodynamic simulations to study the manifestation of a truncated accretion disk in each regime. We find that rather than a truncation edge, the transition is better described by a ``truncation zone" when the angular momentum transport and heating is governed by MRI-driven turbulence instead of a true viscosity. Additionally, we find that the hot gas in the simulation buoyantly rises in a gentle outflow and eventually fills the entire volume, instead of simply being confined to the innermost region. The outflow interacts with the disk body and enhances the magnetic stresses, which could produce stronger quasiperiodic variability. Finally, we conduct an investigation of the large-scale magnetic dynamo using a suite of four global magnetohydrodynamic disk simulations with scaleheight ratios of $h/r=\{0.05, 0.1, 0.2, 0.4\}$. Most notably, the organization that is prevalent in accretion disk simulations and described as a ``butterfly pattern" does not occur when $h/r \ge 0.2$, despite the dynamo action still operating efficiently.
ACTIVE GALACTIC NUCLEUS FEEDBACK IN GIANTS AND DWARFS
(2022) Liu, Weizhe; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Feedback from Active Galactic Nucleus (AGN) may play a critical role in the coevolution of galaxies and supermassive black holes (SMBH). Specifically, AGN feedback may quench star formation, suppress baryon-to-dark matter mass ratio, shape galaxy morphology, impact the circumgalactic (CGM)/intergalactic medium (IGM), and regulate SMBH accretion. One of the most important forms of AGN feedback is powerful, quasar/AGN-driven outflows. However, the physical details of these outflows, including their driving mechanism and spatial extent, are still not well constrained. In addition, while these outflows are believed to be effective in massive galaxies, their impact on dwarf galaxies (M⋆ ~10^9.5 M⊙) remains largely unknown. To answer these open questions, my thesis focuses on AGN feedback via quasar/AGN-driven outflows in both ultraluminous infrared galaxies (ULIRGs) and dwarf galaxies with four projects.In the first half of my thesis, I examine the outflows in nearby ULIRGs with two objectives: In Chapter 2, I present a dedicated investigation of the highly ionized, likely pc-scale quasar/AGN winds in a sample of 21 nearby ULIRGs through HST/COS far-ultraviolet (FUV) spectroscopy. Blueshifted Lyα emission is prevalent in the sample, which is probably closely related to the outflowing gas and AGN activity in these objects. Additionally, the Lyα escape fractions tend to be slightly larger in objects with stronger AGN and larger outflow velocities. Highly ionized O VI and N V outflows are detected in a coherently selected, AGN-dominated ULIRG sample for the first time. Together with the results from a matched quasar sample, these outflows show higher incidence rates and larger EW and velocities in X-ray weak sources and sources with high X-ray absorbing column densities, implying that these outflows are radiatively-driven; In Chapter 3, I describe a deep, Chandra imaging spectroscopy study of the nearby ULIRG Mrk 273. The data have revealed a ∼40 kpc×40 kpc X-ray nebula, which is relatively hot and has a super-solar α/Fe abundance ratio. This nebula is most likely heated and metal-enriched by outflows over time. Additionally, the existence of a dual AGN is strongly suggested by the data, and extended 1–3 keV emission are detected, likely related to the AGN photoionized gas and/or outflowing gas. In the second half of my thesis, I turn to look at the AGN-driven outflows in dwarf galaxies: In Chapter 4, I report the results from a dedicated optical integral field spectroscopic study of a sample of eight dwarf galaxies with known AGN and suspected outflows. Fast, kpc-scale outflows are detected in seven of them. The outflows show 50-percentile (median) velocity of up to ∼240 km s^−1 and 80-percentile line width reaching ∼1200 km s^−1, in clear contrast with the more quiescent kinematics of the host gas and stellar components. The kinematics and energetics of these outflows suggest that they are primarily driven by the AGN. A small but non-negligible portion of the outflowing material likely escapes the main body of the host galaxy and contributes to the enrichment of the circumgalactic medium. The impact of these outflows on their dwarf host galaxies is similar to those taking place in the more luminous AGN with massive hosts in the low-redshift universe. In Chapter 5, I discuss the results from a pilot HST/COS spectroscopy program to examine three objects studied in Chapter 4. Blueshifted absorption features tracing fast outflows are detected in two of the three objects. For object J0954+47, the outflow is detected in multiple ions and is much faster than those in star-forming galaxies with similar star formation rates. The outflow velocity exceeds the escape velocity of this system, suggesting that a large fraction of the outflowing gas may escape. The outflow carries significant amount of mass, momentum and kinetic energy, which may transport material out of the galaxy more efficiently than the gas consumption by star formation. The ratio of kinetic energy outflow rate to AGN luminosity of this outflow is at least comparable to the expectation from simulations of AGN feedback.Finally, in Chapter 6, I summarize the main results of the whole thesis, and briefly highlight several future works that may lead to a more comprehensive understanding of AGN feedback in ULIRGs and dwarf galaxies.
An Analysis of the Environment and Gas Content of Luminous Infrared Galaxies
(2010) Zauderer, Bevin Ashley; Vogel, Stuart N; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Luminous and ultraluminous infrared galaxies (U/LIRGs) represent a population among the most extreme in our universe, emitting an extraordinary amount of energy at infrared wavelengths from dust heated by prolific star formation and/or an active galactic nucleus (AGN). We present three investigations of U/LIRGs to better understand their global environment, their interstellar medium properties, and their nuclear region where molecular gas feeds a starburst or AGN. To study the global environment, we compute the spatial cluster-galaxy amplitude, B_gc, for 76 z < 0.3 ULIRGs. We find the environment of ULIRGs is similar to galaxies in the field. Comparing our results with other galactic populations, we conclude that ULIRGs might be a phase in the lives of AGNs and QSOs, but not all moderate-luminosity QSOs necessarily pass through a ULIRG phase. To study the interstellar medium properties, we observe HI and other spectral lines in 77 U/LIRGs with the Arecibo telescope. We detect HI in emission or absorption in 61 of 77 galaxies, 52 being new detections. We compute the implied gas mass for galaxies with emission, and optical depths and column densities for the seven sources with absorption detections. To study the molecular gas in the nuclear region of LIRG Arp 193, sub-arcsecond scale angular resolution is required and a method of atmospheric phase correction imperative. We present results of a large experiment observing bright quasars to test the limitations of the Combined Array for Research in Millimeter Astronomy's Paired Antenna Calibration System (C-PACS) for atmospheric phase correction. We conclude that C-PACS improves imaging capabilities when the atmospheric calibrator is nearby (≤ 6°), bright (> 1 Jy), and at moderately high elevation (> 45°). We map Arp 193 in 12CO(2-1) with CARMA, achieving 0.18'' x 0.12'' (~65 pc) resolution, and demonstrating an improvement with C-PACS. We compute a molecular gas mass of 2 x 109 M_sun and find ~20% of the total mass is in the form of molecular gas out to a radius of 750 pc. In the inner 150 pc of the nucleus, N(H₂) > 1025 cm-2.
ANALYZING THE STAR FORMATION EFFICIENCY AND PHYSICAL CONDITIONS OF THE MOLECULAR GAS IN NEARBY GALAXIES
(2023) Villanueva, Vicente; Bolatto, Alberto D; Vogel, Stuart N; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Star formation activity plays a key role in driving galaxy evolution, and it depends on the amount of cold gas available (as traced by CO emission) and on the physical conditions and dynamical state of this gas. This work focuses on investigating the star formation efficiency of the gas, both molecular and total, as a function of local and global galaxy properties. The galaxy samples studied in this thesis are representative of the nearby universe, and we use a combination of interferometric CO observations and integral field unit optical spectroscopy for most of our analyses. First, we show that in a sample of galaxies dominated by ``field galaxies'' the disk scale lengths for the molecular and stellar components are very similar, reflecting the close relation between CO emission and star formation activity. Our analysis of the radial dependence of the star formation efficiency of the total gas on morphological, structural, and dynamical properties of the galaxies shows that there is a smooth, continuous exponential decline with increasing radius (mostly driven by the increased contribution of atomic gas), and a systematic increase in the average gas efficiency from early to late type galaxies. Our results also show a morphological dependence of the efficiency per orbital time, which may reflect star formation quenching due to the presence of a bulge. Next, we test the impact of environmental processes on galaxies immersed in the rich environment of the Virgo cluster. We show that in these galaxies the CO emission is more centrally concentrated than the stars, unlike what we saw in the field. Moreover, in the central regions of galaxies with an increasing level of perturbation (as determined by the morphology and kinematics of their atomic gas emission), the mean molecular-to-atomic gas ratio increases while the star formation efficiency of the molecular gas in the same region decreases. This demonstrates that the cluster environment not only affects the outskirts of galaxy disks and their atomic gas, but deeply changes the distribution and efficiency of the centrally located molecular gas component. Finally, we study the onset of star formation cessation in galaxies (``quenching'') by investigating a complete sample of galaxies spanning from the main sequence (normal star forming objects) to the green valley (galaxies which are starting to quench) to the red cloud (galaxies that are mostly quiescent, that is, ``red and dead'' objects). We find that the star formation activity and the molecular gas-to-stellar mass ratio track each other. We also note that green valley galaxies have lower molecular star formation efficiencies than galaxies on the main sequence. On average, we find that within the bulges of green valley galaxies the molecular gas star formation efficiency is lower than in main sequence galaxies. Also in green valley galaxies, we find that the molecular gas to stellar ratio, the molecular gas star formation efficiency, and the specific star formation rate all increase with increasing distance to the center. Our results suggest that gas depletion or removal does not fully explain the star-formation quenching in galaxies transiting through the green valley, and that a reduction in star formation efficiency is also required during this stage. This is reminiscent of the so-called ``morphological quenching.''
APPLICATIONS OF ADVANCED STATISTICAL METHODS IN THE PAN-STARRS1 MEDIUM-DEEP SURVEY
(2015) Kumar, Sidharth; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The application of advanced statistical methods to astrophysical problems is desirable for reasons of time-efficiency, and robustness. A data-driven approach, when combined with physical insights, can expedite solutions to difficult problems, where data is aplenty, however, physical insights may be nebulous. This may be either due to the parametric complexities of the models assumed, or the inherent complexity in the behavior of the astrophysical system itself. In this thesis we demonstrate that, via the application of a variety of statistical tools to the Pan- STARRS1 medium-deep survey data, we solve two important classification problems faced by the survey. The Pan-STARRS1 (PS1) Survey is unique in terms of its temporal, spa- tial, and wavelength coverage, permitting extensive studies on known astrophysical sources such as active galactic nuclei (AGN) and supernovae (SNe), as well as ex- otic ones, such as tidal disruption events and recoiling supermassive black hole binaries. The Medium-Deep (MD) survey in particular offers a time resolution onthe order of a few days over 10 distinct 8 sq. deg. fields, or over 80 sq. deg. of sky, and with the technique of difference imaging, enables the detailed study of stochastic variations and explosive transients associated with extragalactic sources. In the first of two parts of this thesis, I outline a novel method for the light-curve characterization of Pan-STARRS1 Medium-Deep Survey (PS1 MDS) extragalactic sources into stochastic variables (SV) and burst-like (BL) transients, using multi- band difference-imaging time-series data. Using a combination of Bayesian leave- out-one-cross-validation and corrected-Akaike information criteria to model time- series in the four PS1 photometric bands g P 1 , r P 1 , i P 1 , and z P 1 , we use a k-means clustering decision algorithm to classify sources as bursting or stocastically variable with over 91% purity, based on spectroscopically confirmed AGN and SN verification samples. The performance of our classifier is comparable to the best among existing methods in terms of purity. We use our method to classify 4361 difference image sources with galaxy hosts in the PS1 MD fields as BL or SV, and then together with their host galaxy offsets, create a robust sample of AGN and SNe. From these variability-selected samples, we derive photometry and variability based priors that can be used in future survey data streams for near real-time classification. In the second part, I discuss the applications of a genetic algorithm optimized support vector machines or GA-SVM, machine learning classifier and regression tool, we developed to solve two important problems in astronomical surveys; a. star-galaxy classification where we show as proof of concept, the efficient separation of 11000 stars and galaxies in the MD fields using 32 photometric parameters de- rived from the PS1 MD stack [1]; and b. photometric redshift regression, where asproof of concept we predict with high accuracy, the photometric redshifts of 5000 galaxies in the COSMOS survey, based on 25 photometric parameters derived from the survey. We show that our GA-SVM method is more efficient as compared to ex- isting methods for star-galaxy classification, and more robust than existing methods for photometric redshift estimation.
An Assessment of Professional Development for Astronomy and Physics Faculty: Expanding Our Vision of How to Support Faculty's Learning About Teaching
(2016) Olmstead, Alice Rose; Turpen, Chandra A; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this thesis, we will explore approaches to faculty instructional change in astronomy and physics. We primarily focus on professional development (PD) workshops, which are a central mechanism used within our community to help faculty improve their teaching. Although workshops serve a critical role for promoting more equitable instruction, we rarely assess them through careful consideration of how they engage faculty. To encourage a shift towards more reflective, research-informed PD, we developed the Real-Time Professional Development Observation Tool (R-PDOT), to document the form and focus of faculty's engagement during workshops. We then analyze video-recordings of faculty's interactions during the Physics and Astronomy New Faculty Workshop, focusing on instances where faculty might engage in pedagogical sense-making. Finally, we consider insights gained from our own local, team-based effort to improve a course sequence for astronomy majors. We conclude with recommendations for PD leaders and researchers.
ATMOSPHERIC CHARACTERIZATION OF GIANT EXOPLANETS IN EXTREME ENVIRONMENTS
(2017) Wilkins, Ashlee Noelle; Deming, Leo D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The study of planets around other stars has entered a science-rich era of characterization, in which detailed information about individual planets can be inferred from observations beyond discovery and confirmation, which only yield bulk properties like mass or radius. Characterization probes more revealing quantities such as chemical abundances, albedo, and temperature/pressure profiles, allowing us to address larger questions of planet formation mechanisms, planetary evolution, and, eventually, presence of biosignature gases. The primary method for characterization of close-in planets is transit spectroscopy. My dissertation comprises transiting exoplanet case studies using the Hubble Space Telescopes Wide-Field Camera-3 (HST/WFC3) as a tool of exoplanet characterization in a near-infrared band dominated by broad water absorption. Much of my efforts went toward a characterization of the WFC3 systematic effects that must be mitigated to extract the incredibly small (tens to 200 parts per million) signals. The case study subjects in this dissertation are CoRoT-2b (in emission), WASP-18b (in transmission and emission), and HATS-7b (in transmission), along with some partial/preliminary analyses of HAT-p-3b and HD 149026b (both in transmission). I also present an analysis of transit timing of WASP-18b with HST and other observatories as another clue to its evolution as a close-in, extremely massive planet purported to be spiraling in to its host star. The five planets range from super Neptunes to Super-Jupiter in size/mass. The observability of such planets – i.e. giants across a continuum of mass/size in extreme local environments close to their respective host stars, – is a unique opportunity to probe planet formation and evolution, as well as atmospheric structures in a high-irradiation environment. This genre of observations reveal insights into aerosols in the atmosphere; clouds and/or hazes can significantly impact atmospheric chemistry and observational signatures, and the community must better understand the phenomenon of aerosols in advance of the next generation of space observatories, including JWST and WFIRST. In conducting these case studies as part of larger collaborations and HST observing campaigns, my work aids in the advancement of exoplanet atmosphere characterization from single, planetby-planet, case studies, to an understanding of the large, hot, gaseous planets as a population.
Benchmarking Charge Exchange Theory in the Dawning Era of Space-Borne High-Resolution X-ray Spectrometers
(2017) Betancourt-Martinez, Gabriele; Reynolds, Christopher; Porter, Frederick S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Charge Exchange (CX) is a process in which a highly charged ion captures one or more electrons from a neutral atom or molecule into an excited state during a close interaction. The electron's subsequent radiative cascade to the ground state produces diagnostic line emission in the X-ray band. CX with solar wind ions occurs frequently in the solar system, and CX may also occur astrophysically. In order to properly identify CX in astrophysical spectra and make use of its diagnostic properties, we must be able to model the emission. Theoretical treatments of CX are often computationally expensive, experimental benchmarks at high resolution are fairly scarce, and there is often poor agreement between the two. This dissertation seeks to build a better understanding of the mechanics and spectral signatures of CX through high-resolution experimental data paired with theoretical calculations of CX. Chapter 1 outlines the necessary ingredients for modeling and identifying CX spectra, describes several astrophysical environments in which CX has been observed or postulated to occur, and presents some of the challenges we are facing in identifying and understanding this emission. Chapter 2 describes the theoretical and computational tools used in this work. Chapter 3 discusses the experimental tools and facilities we use, namely an Electron Beam Ion Trap (EBIT) and an X-ray microcalorimeter. Chapter 4 presents experimental K-shell data that highlights both the subtle nature of the CX interaction and the difficulty in including those nuances in spectral synthesis codes. Chapter 5 presents the first high-resolution L-shell CX spectra of Ne-like Ni and describes what we can learn from these results. In Chapter 6, we take these data a step further and present a pipeline to calculate relative state-selective capture cross sections, previously only available from theoretical modeling. We then compare some of our results to theory. In Chapter 7, we discuss several future steps for our work.
BETTII: A pathfinder for high angular resolution observations of star-forming regions in the far-infrared
(2016) Rizzo, Maxime Jean; Mundy, Lee G; Rinehart, Stephen A; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this thesis, we study clustered star formation in nearby star clusters and discuss how high angular resolution observations in the far-infrared regime could help us understand these important regions of stellar birth. We use the increased angular resolution from the FORCAST instrument on the SOFIA airborne observatory to study 10 nearby star-forming regions, and discuss the physical properties of sources in these regions that we can infer from radiative transfer modeling using these new observations. We discuss the design of BETTII, a pathfinder balloon-borne interferometer which will provide significantly better angular resolution in the far-infrared regime, and pave the way for future space-borne observatories. We elaborate on the details of BETTII's core technique, called Double-Fourier interferometry, and how to accurately compute the sensitivity of instruments which use this technique. Finally, we show our design and implementation results of the control and attitude estimation system for the BETTII payload, which poses unique challenges as an interferometer on a balloon platform.
Black Hole Dynamics and Gravitational Radiation in Galactic Nuclei
(2009) Lauburg, Vanessa; Miller, Michael C.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this dissertation, we present new channels for the production of gravitational radiation sources: mergers of black holes in the nuclear star clusters found in many small galaxies, and mergers and tidal separations of black hole binaries in galaxies that host supermassive black holes. Mergers between stellar-mass black holes will be key sources of gravitational radiation for ground-based detectors. However, the rates of these events are highly uncertain, because we can not observe these binaries electromagnetically. In this work, we show that the nuclear star clusters found in the centers of small galaxies are conducive environments for black hole mergers. These clusters have large escape velocities, high stellar densities, and large numbers of black holes that will have multiple close encounters, which often lead to mergers. We present simulations of the three-body dynamics of black holes in this environment and estimate that, if many nuclear star clusters do not have supermassive black holes, tens of events per year will be detectable with Advanced LIGO. Larger galaxies that host supermassive black holes can produce extreme-mass ratio inspiral (EMRI) events, which are important sources for the future space-based detector, LISA. Here, we show that tidal separation of black hole binaries by supermassive black holes will produce a distinct class of EMRIs with near-zero eccentricities, and we estimate that rates from tidal separation could be comparable to or larger than those from the traditionally-discussed two-body capture formation scenario. Before tidal separation can occur, a binary encounters multiple stars as it sinks through the nucleus toward the supermassive black hole. In this region, velocities are high, and interactions with stars can destroy binaries through ionization. We investigate wide ranges in initial mass function and internal energy of the binaries, and find that tidal separations, mergers, and ionizations are all likely outcomes for binaries near the galactic center. Tidally separated binaries will contribute to the LISA detection rate, and mergers will produce tens of events per year for Advanced LIGO. We show, therefore, that galactic nuclei are promising hosts of gravitational wave sources for both LISA and LIGO.
BROADBAND OBSERVATIONS OF GAMMA-RAY BURSTS AND FAST RADIO BURSTS: ENERGETICS, AFTERGLOWS, AND PHYSICAL ORIGINS
(2021) Cunningham, Virginia Anne; Cenko, S. Bradley; Vogel, Stuart; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Long gamma-ray bursts (GRBs) are produced during the deaths of massive stars. They are the most powerful explosions known in the Universe and release most of their energy via a narrow cone of emission. The long-lived afterglows of the brightest GRBs detected by the \textit{Fermi} Large Area Telescope (LAT) are visible from radio to gamma-rays, and this relative abundance of broadband data makes them excellent tools for constraining theoretical models regarding their origins. Here, we use our sample of bright GRBs to test emission models beyond the canonical on-axis, top-hat jet model which has historically been applied throughout the literature. We demonstrate that many GRBs are likely to produce emission via a structured jet. We also find that derived physical parameters are highly dependent upon the fraction, $\xi$, of electrons which contribute to the synchrotron emission. Our findings for $\xi$ are contrary to what is generally assumed during GRB modeling ($\xi=1.0$), but consistent with theoretical simulations which predict lower values. Lower predictions for $\xi$ would impact our current understanding of GRBs, implying denser environments and higher energetics than commonly assumed. Fast radio bursts (FRBs) are extremely bright, short-duration pulses at radio frequencies that were only confirmed as true astrophysical sources a decade ago. Although the field has experienced major leaps in recent years, many questions regarding their progenitors and emission processes remain. The identification of counterparts at higher energies is critical to understanding the physical origins of FRBs. Here, we report on an archival search of previously identified FRBs with the \textit{Fermi} Gamma-ray Burst Monitor (GBM), the \textit{Fermi}-LAT, and the \textit{Swift} Burst Alert Telescope (BAT). We find no significant X-ray or gamma-ray counterparts but report upper limits on the high-energy fluence, $f_{\gamma}$, for each FRB in our sample. We also report lower limits on the ratio of radio to high-energy fluence ($\nicefrac{f_{r}}{f_{\gamma}}$). We discuss the implications of our results on several FRB progenitor theories, including pulsar-like analogs and magnetar flares.
Case Studies in AGN Feedback
(2022) Smith, Robyn N; Reynolds, Christopher S; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Galaxies in which the central supermassive black hole (SMBH) is actively accreting are referred to as active galactic nuclei (AGN) and are believed to play a crucial role in the evolution of both individual and clusters of galaxies. Empirically, the mass of the host galaxy and the mass of the SMBH are positively correlated. This is somewhat surprising given that the gravitational sphere of influence of the SMBH is orders or magnitude smaller than the host galaxy. The SMBH is believed to undergo periods of activity during which it is capable of powering galactic-scale outflows which in turn modulate star formation and therefore the overall mass of the host galaxy. Such processes are broadly referred to as feedback.Clusters of galaxies are the largest gravitationally bound systems in the universe. The intracluster medium (ICM) in relaxed clusters is strongly centrally peaked and suffi- ciently dense that it is expected to cool rapidly (in cosmological terms). Such cooling should create streams of cool gas flowing to the brightest cluster galaxy (BCG) which in turn should fuel high rates of star formation. Little evidence of either has been found giving rise to the ‘cooling flow problem’. AGN are again invoked to explain the absence of this cooling flow. The BCGs hosting AGN, often with powerful radio jets, are believed to inject energy into the ICM at a rate which can counteract the cooling. This cyclical nature of balancing the cooling is another form of AGN feedback. In this thesis, we present case studies of three AGN which provide unique insight into these feedback processes. Chapter 2 presents evidence for a relativistic X-ray driven outflow on accretion disk scales in an ultraluminous infrared galaxy known to host a galactic-scale molecular outflow. The observational properties which make a galaxy an ideal candidate for detection of large-scale outflows are intrinsically at odd with the properties which are ideal for detecting small-scale outflows. IRASF05189-2524, the subject of Chapter 2, is one of only a handful of galaxies for which positive detection of outflows on both small- and large-scale exist. Next, we turn our attention to AGN in BCGs and the cooling flow problem. Chapter 3 presents new Chandra observations of NGC 1275, the BCG in the famous Perseus Cluster. The high-cadence observing campaign finds X-ray variability on short intraweek timescales. The inclusion of archival observations reveals a general ‘harder when brighter’ trend. Examination of multiwavelength light curves finds a strongly correlated optical and γ-ray flare in late 2015 in which the optical emission leads the γ-ray emission by ~5 days. This robust (> 3σ) result is the first strong evidence of correlated emission with a time delay and is lends support to the idea that the γ-ray emission is produced by synchrotron self-Compton upscattering. In Chapter 4, we present new Chandra observations of the rare radio-quiet BCG quasar H1821+643. It is one of only two examples in the nearby universe of a highly luminous quasar with minimal radio jet activity at the center of a galaxy cluster. Despite observational challenges, we produce the first high-resolution spectrum of the quasar well-separated from the ICM in ~20 years. Our short-cadence observing campaign again reveals rapid variation on timescales corresponding to the light crossing time of the accretion disk. Although the flux varies, the spectrum is remarkably constant when compared to observations from previous decades. The result of this thesis is to add to the existing body of knowledge of AGN feedback on both galaxy and galaxy cluster scales. These three AGN presented various observing challenges which required a combination of non-standard observational techniques and data reduction methods in order to maximize results with current X-ray instrumentation.
Characterizing young debris disks through far-infrared and optical observations
(2014) Donaldson, Jessica Kate; Hamilton, Douglas P; Roberge, Aki; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Circumstellar disks are the environments where extrasolar planets are born. Debris disks in particular are the last stage of circumstellar disk evolution, the youngest of which may harbor still-forming terrestrial planets. This dissertation focuses on examining the properties of dust grains in the youngest debris disks as a proxy to study the unseen parent planetesimal population that produces the dust in destructive collisions. The parent planetesimals are important to understanding the late stages of terrestrial planets because they can deliver volatile material, such as water, to young terrestrial planets. We used the Herschel Space Observatory to study young debris disks (ages 10-30 Myr) in the far-infrared where the thermal emission from the dust grains is brightest. We constructed spectral energy distributions (SEDs) of 24 debris disks and fit them with our debris disk models to constrain dust parameters such as temperature, dust location, and grain size. We also looked for correlations between the stellar and disk parameters and we found a trend between the disk temperature and stellar temperature, which we fit as a power-law. One bright, well studied disk in our sample, HD32297, has a well populated SED, allowing us to fit it with a more detailed model to determine dust grain composition. The HD32297 disk has also been imaged in scattered light, so we used the image to constrain the dust location before fitting the SED. We found the dust grains are composed of a highly porous and icy material, similar to cometary grains. This suggests there are icy comets in this system that could deliver water to any terrestrial planets in the disk. We followed up this system by observing it with the Hubble Space Telescope to get simultaneous spatial and spectral data of the disk. These data let us look for compositional changes with disk radius. We found the disk has a very red color at optical wavelengths in the innermost radius we probed (110 AU). This could indicate the presence of organic material, or it could be a property of the scattering phase function of large grains. Further analysis of this data is ongoing.
Circumstellar Material on and off the Main Sequence: Methods and tools for constraining the composition of gas and dust around stars
(2020) Steele, Amy; Vogel, Stuart; Debes, John; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Circumstellar (C-S) dust is present around stars throughout their entire life cycles. About 20-50\% of sun-like stars on the main sequence (MS) have belts of rocky material like the Solar System's asteroid and Kuiper Belts. Observations of white dwarfs (WDs), the endpoints of stellar evolution for stars 0.8 to 8 times the mass of our sun, reveal that 20-50\% of them have accreted metals typically found in rocky bodies like the Earth. Between the MS and WD stages, stars expand, engulfing any nearby companions, and lose mass, destabilizing their planetary system. So, this ``pollution" is unexpected because the inner regions of system near a WD should be barren. The focus of this thesis is on the observation and characterization of C-S gas and dust around stars of increasing age, with the goal of studying the evolution of the material in planetary systems. Are we seeing unchanged Earth-like bodies survive long enough to pollute a WD? What are the stellar systems like along the way? To address these questions, we explore several case studies. We present submillimeter observations with the Submillimeter Array (SMA), the Combined Array for Research in Millimeter-wave Astronomy (CARMA), and the Atacama Large Millimeter/submillimeter Array (ALMA) at $\sim$2'' resolution that spatially resolve the C-S dust or debris disks around five nearby ($d\sim$50 pc) young solar analogues. We perform an MCMC analysis to fit for basic structural parameters, including the inner radius and width of the debris ring, the total mass of the disk, and the characteristic dust grain size. We find that the cold outer belts around the solar analogues in our sample generally exhibit properties consistent with scaled-up versions of the Solar System's Kuiper Belt. The composition of the dust is consistent with an astronomical silicate. Dust around post-MS stars is expected as a result of stellar evolution, but if a star has not yet reached a stage of predicted mass loss, then the dust is unexpected. We analyze a set of post-MS stars with excess IR emission, using the most recent \textit{Gaia} data release (DR2) to investigate the stellar age, and \textit{Herschel} observations at far-IR and submillimeter wavelengths to constrain the thermally emitting dust. We find that all but one of the 20 stars are post-MS. For the stars detected at submillimeter wavelengths, we find that their spectral indices, $\alpha$ are more shallow than the $\alpha$ that would be expected of grains in the interstellar medium (ISM), pointing toward grain growth in these systems. A fraction of the sample presents characteristics (fast rotation, enhanced lithium abundance, and excess IR emission) that point toward an external source causing the dust excess. One explanation for this trifecta of properties is that these stars have recently engulfed a planet. Due to the relatively fast gravitational settling times of heavy elements in a WD atmosphere, the presence of those heavy elements is linked to the accretion of planetesimals perturbed by one or more outer planets to within the white dwarf's tidal disruption radius. We present an updated high signal-to-noise ratio spectrum, a new Keck HIRES spectrum, an updated white dwarf atmosphere analysis, and a self-consistent model of the C-S gas around white dwarf WD 1124-293. We constrain the abundances of Ca, Mg, Fe, and a number of other elements in both the photosphere of the white dwarf and the circumstellar disk. We find the location of the gas is approximately one hundred white dwarf radii, the C-S and photospheric compositions are consistent, the gas is not isothermal, and the amount of C-S Ca has not changed in two decades. We also demonstrate for the first time how the radiative transfer code Cloudy can be used to model C-S gas viewed in absorption around a polluted white dwarfs. Modeling the abundances of C-S gas around polluted white dwarfs with Cloudy provides a new method to measure the instantaneous composition of the material sublimating from the polluting planetesimals. Expanding the work with WD 1124-293, we use Cloudy to predict the physical conditions of C-S gas around model WD photospheres with temperatures ranging from 9000 K to 24,000 K. The models can be used to determine the gas temperature with distance from the star, predict optical depths, constrain the hydrogen number density, and possibly constrain the location of the gas. We compare our models to a small sample of WDs with emission features, and show how the equivalent width can be useful as a diagnostic tool in these systems. We find, unsurprisingly, that it is difficult to determine the composition of dust around stars on and off the MS. However, the modeling of gas around WDs offers an opportunity to determine the composition of planetesimals in evolved stellar systems. We find that the compositions of C-S gas around polluted WDs are consistent with the bulk Earth, but recommend further modeling. The radiative transfer code, Cloudy can and should be used to search for trends among these planetary systems, to provide unprecedented insight to the composition of extrasolar planetesimals that have survived the evolution of their host stars.
A Comprehensive Study of the Outskirts of Galaxy Clusters Using Suzaku
(2014) George, Jithin Varghese; Mushotzky, Richard F.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Galaxy clusters, which contain up to tens of thousands of galaxies and which are the largest virialized structures in the universe, serve as unique probes of cosmology. Most of their baryonic mass is in the form of hot gas that emits X-rays via thermal bremsstrahlung radiation. The study of this emission from the outer, least-relaxed portions of clusters yields valuable information about the hierarchical assembly of large scale structure. In this thesis, we report on our X-ray analysis of the outskirts of four clusters. For this purpose, we Suzaku data, which is well-suited to the study of the outsides of clusters. Accurate parameter estimates require reliable data and proper analysis, so we focus on the 0.7--7.0 keV range because other studies have shown that energies below or above this range are less reliable. A key component of our analysis is our careful modeling of the background emission as a thermal component plus a power law contribution. Our power law model uses a fixed slope of 1.4, which is consistent with other clusters. We constrain our thermal background component by fitting it to ROSAT data over the energy range 0.3--2.0~keV. Using this method, we extract the temperature, density, and surface brightness from the Suzaku data. These parameters are somewhat different from the values obtained using XMM-Newton data but are consistent with other measurements using Suzaku. We then deprojected these quantities to estimate the total mass, entropy, pressure, and baryonic fraction. We find an entropy that is consistent with the previously suggested `universal' entropy profile, but our pressure deviates from the `universal' profile. We discuss some possible reasons for this discrepancy. Consistent with previous observations but in contrast to what is expected from simulations, we infer that the outer parts of the clusters we study have baryon fractions in excess of the cosmic fraction. We suggest some explanations for this, focusing on clumping as a possibility. We then finish by discussing the role of our observations in cluster physics studies and by enumerating other avenues of exploration to attain a more complete picture of galaxy clusters.
Connecting Molecular Clouds to Clustered Star Formation using Interferometry
(2018) Dhabal, Arnab; Mundy, Lee G.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Stars are commonly formed in clusters in dense regions of interstellar medium called molecular clouds. In this thesis, we improve our understanding of the physics of star formation through multiple experiments involving interferometry. We use CARMA observations of filaments in Serpens and Perseus molecular clouds to study their morphology and kinematics using dense gas tracers. The observations are compared against predictions from simulations to explain how filaments form and evolve to form stars. Ammonia inversion transitions data is obtained from VLA observations of the NGC 1333 molecular cloud. From this data, we derive temperature, structural and kinematic information about the gas participating in star formation on scales from 2 parsec to 0.01 parsec, thereby connecting the large scale gas and dust structure to individual protostellar envelopes. These observations from ground-based arrays are complemented by the development of the Balloon Experimental Twin Telescope for Infra-red Interferometry (BETTII). This pioneering instrument performs Michelson interferometry along with Fourier Transform Spectroscopy, thereby providing sub-arcsecond angular resolution and spectroscopic capabilities at far-infrared wavelengths 30-100 microns. Using this capability, BETTII will study the dusty envelopes around protostars in clustered star forming regions. The instrument development is a component of the thesis with focus on the optics designing, evaluation and alignment for the completed and upcoming flights. We discuss how the optical system mitigates the challenges of phase control for such a balloon borne interferometer. Further, interferometric simulations of BETTII observations are carried out to investigate how well these observations can constrain the defining parameters of protostars.
CONNECTING THEORY AND OBSERVATIONS OF EXOPLANET ATMOSPHERES AND SURFACES AT THE INDIVIDUAL AND POPULATION LEVEL WITH JWST
(2024) Ih, Jegug; Kempton, Eliza M.-R.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Observing an exoplanet’s atmosphere via photometry and spectroscopy has provided the main window to understanding its properties and processes, as the atmospheric spectra encompass information about the chemistry, thermal structure, surfaces, as well as formation history and even biology. To this end, one key science goal of the James Webb Space Telescope (JWST) is to establish whether rocky planets around M dwarfs can host atmospheres or not. JWST offers unprecedented signal-to-noise and unlocks new parameter space regimes of planets available for characterizing not only the atmosphere but also the surface. This advancement in observing capability simultaneously poses novel challenges to atmospheric characterization. My dissertation addresses some of the new challenges to atmospheric retrievals in the era of JWST and the characterization of rocky planets. Firstly, I quantified the effects of wavelength-correlated systematics on atmospheric retrievals. Wavelength-correlated noise can occur due to instrumental systematics or stellar effects and the merging of discrete data sets. I investigated the effect of correlated noise and constrained the potential biases incurred in the retrieved posteriors by performing retrievals on multiple noise instances of synthetic data. The study found that correlated noise allows for overfitting the spectrum, thereby yielding a better goodness of fit on average but degrading the overall accuracy of retrievals by roughly 1σ. In particular, correlated noise can manifest as an apparent non-Rayleigh slope in the optical range, leading to an incorrect estimate of cloud/haze parameters. Finally, I show that while correlated noise cannot be reliably distinguished with Hubble Space Telescope observations, inferring its presence and strength may be possible with JWST. Secondly, I studied the how the choice in parameterization of the atmospheric composition can influence the posterior when performing retrieval analyses on terrestrial planet atmospheres, for which the mean molecular weight is not known a priori. By performing self-retrievals and varying the parameterization, I found that the centered log-ratio transform, commonly used for this application, tends to overestimate the abundances of spectroscopically active gases when inactive ones are present. Over multiple noise instances, I found that no one parameterization method always outperforms others. Comparing the Bayesian evidences from retrievals on multiple noise instances, I found that for a given spectrum, the choice in parameterization can affect the Bayes factor of whether a molecule should be included. Alongside astrophysical effects, this remains a fundamental challenge to atmospheric retrievals for small planet and can addressed by more observations. Finally, I constrained the atmospheric thickness and characterized the surface from the first JWST measurement of thermal emission from a rocky exoplanet, TRAPPIST-1 b. I compared TRAPPIST-1 b’s measured secondary eclipse depth to predictions from a suite of self-consistent radiative-convective equilibrium models. I found that plausible atmospheres (i.e., those that contain at least 100 ppm CO2) with surface pressures greater than 0.3 bar are ruled out at 3σ, regardless of the choice of background atmosphere, and a Mars-like thin atmosphere with surface pressure 6.5 mbar composed entirely of CO2 is also ruled out at 3σ. I modelled the emission spectra for bare-rock planets of various compositions and found that a basaltic surface best matches the measured eclipse depth to within 1σ.
THE COOL SIDE OF GALACTIC WINDS: EXPLORATION WITH HERSCHEL-PACS AND SPITZER-IRS
(2020) Stone, Myra; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Galactic-scale outflows driven by starbursts and/or active galactic nuclei (AGN) are key ingredients to theoretical models and numerical simulations of galaxy assembly and evolution. The feedback induced by the presence of these outflows (or winds) may affect the evolution and formation of a galaxy by regulating the amount of cold, dense gas responsible for star formation and black hole accretion. We present the results from a systematic search for galactic-scale, molecular (OH 119 μm) outflows in a sample of 52 Local Volume (d < 50 Mpc) Burst Alert Telescope detected active galactic nuclei (BAT AGN) with Herschel-PACS. We combine the results from our analysis of the BAT AGN with the published Herschel/PACS data of 43 nearby (z < 0.3) galaxy mergers, mostly ultraluminous infrared galaxies (ULIRGs) and QSOs. Our data show that both the starburst and AGN contribute to driving OH outflows, but the fastest OH winds require AGN with quasar-like luminosities. We also analyze Spitzer InfraRed Spectrograph (IRS) observations of the OH 35 μm feature in 15 nearby (z < 0.06) (ultra-)luminous infrared galaxies (U/LIRGs). The measured OH 35 μm equivalent widths are used to compute an average OH column density which is then compared to the hydrogen column density for a typical optical depth at 35 μm of ∼0.5 and gas-to-dust ratio of 125 to derive an OH−to−H abundance ratio of X_{OH} = 1.01 ± 0.15 × 10^{−6}. The OH 35 μm line profiles predicted from published radiative transfer models constrained by observa- tions of OH 65, 79, 84, and 119 μm in five objects are found to be consistent with the IRS OH 35 μm spectra. Finally, we analyze Herschel-PACS observations of five atomic fine-structure transition lines ([O I] 63 μm, [O III] 88 μm, [N II] 122 μm, [O I] 145 μm, and [C II] 158 μm) in seven nearby (d < 16 Mpc) galaxies with well-known galactic- scale outflows (Cen A, Circinus, M 82, NGC 253, NGC 1068, NGC 3079, and NGC 4945). With this suite of atomic emission lines, we investigate the cool neutral atomic (T ~ 10^3 K) and warm ionized (T ~ 10^4 K) gas phases within each outflow. The outflows in the Herschel data are spatially isolated from the galactic disk based on the kinematic signatures of the outflows. The spatial distribution and physical properties of the outflows detected in the Herschel data are compared with published results at other wavelengths. For completeness, an analysis of the molecular gas traced by OH 119 μm is also presented.