Astronomy Theses and Dissertations
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Item MULTISCALE RADIATION-MHD SIMULATIONS OF COMPACT STAR CLUSTERS(2023) He, ChongChong; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Star formation is a crucial process that lies at the center of many important topics in astrophysics: the nature of the first sources of radiation, the formation and evolution of galaxies, the synthesis of elements, and the formation of planets and life. Recent advances in computing technology have brought about unprecedented opportunities to deepen our understanding of this complex process. In this dissertation, I investigate the physics of star formation in galaxies and its role in shaping the galaxies and the Universe through numerical simulations.My exploration of star formation begins with a large set of simulations of star cluster formation from isolated turbulent Giant Molecular Clouds (GMCs) with stellar feedback using \ramses{}, a state-of-the-art radiation-magneto-hydrodynamic (radiation-MHD) code. While resolving the formation of individual stars, I have pushed the parameters (mass and density) of the simulated GMCs well beyond the limit explored in the literature. I establish physically motivated scaling relationships for the timescale and efficiency of star formation regulated by photoionization feedback. I show that this type of stellar feedback is efficient at dispersing dense molecular clouds before the onset of supernova explosions. I show that star formation in GMCs can be understood as a purely stochastic process, where instantaneous star formation follows a universal mass probability distribution, providing a definitive answer to the open question of the chronological order of low- and high-mass star formation. In a companion project, I publish the first study of the escape of ionizing photons from resolved stars in molecular clouds into the intercloud gas. I conclude that the sources of photons responsible for the epoch of reionization, one of the most important yet poorly understood stages in cosmic evolution, must have been very compact star clusters, or globular cluster progenitors, forming in dense environments different from today's galaxies. In follow-up work, I use a novel zoom-in adaptive-mesh-refinement method to simulate the formation and fragmentation of prestellar cores and resolve from GMC scales to circumstellar disk scales, achieving an unprecedented dynamic range of 18 orders of magnitude in volume in a set of radiation-MHD simulations. I show that massive stars form from the filamentary collapse of dense cores and grow to several times the core mass due to accretion from larger scales via circumstellar disks. This suggests a competitive accretion scenario of high-mass star formation, a problem that is not well understood. We find that large Keplerian disks can form in magnetically critical cores, suggesting that magnetic braking fails to prevent the formation of rotationally-supported disks, even in cores with mass-to-flux ratios close to critical. This is because the magnetic field is extremely turbulent and incoherent, reducing the effect of magnetic braking by roughly one order of magnitude compared to the perfectly aligned and coherent case, which proposes a solution to the ``magnetic braking catastrophe.''Item On the Dynamics of Binary Asteroids Applied to DART Mission Target (65803) Didymos(2022) Agrusa, Harrison Fitzgerald; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)NASA’s Double Asteroid Redirection Test (DART) mission will be the first full-scale demonstration of a kinetic impactor for planetary defense. On September 26, 2022, the DART spacecraft is expected to impact Dimorphos, the secondary component of the Didymos binary asteroid system. The DART impact will reduce Dimorphos’s relative orbital velocity, shrinking both its semimajor axis and orbit period. The mutual orbit period will then be measured us- ing ground- and space-based observations in order to deduce the momentum transfer efficiency, which is an important parameter in planetary defense that has never been measured experimentally at a realistic scale. This thesis comprises a set of studies on the spin and orbital dynamics of the Didymos system conducted in support of the DART mission. Owing to the close proximity of Didymos and Dimorphos and their irregular shapes, the mutual dynamics are non-Keplerian and exhibit a high degree of spin-orbit coupling, which often requires the use of specialized numerical methods to model the system. First, we conducted a benchmarking and sensitivity study to identify the best simulation codes for future DART-supported studies and to understand how small perturbations in the initial conditions can affect the resulting dynamical evolution of the system. Then, we demonstrated that Dimorphos can enter a wide range of post-impact spin states, including possible chaotic non-principal axis rotation, depending on its shape and the amount of momentum transferred by the DART impact. We then explored the implications of an excited spin state, including the possibility of ongoing granular motion on Dimorphos’s surface resulting from the orbital perturbation induced by the DART impact. This thesis is focused predominantly on the dynamics of the Didymos binary. However, there are many other binary systems among the near-Earth asteroid population with similar physical and dynamical properties, making the results presented here relevant to the NEA binary population in general.Item 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.Item From Nanometers to Light Years: Exploration of the Early Universe with Gamma-ray Bursts and Development of Photonic Spectrographs for Astronomy(2020) Gatkine, Pradip; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent space- and ground-based studies of the circumgalactic medium (CGM) around nearby galaxies have revealed the dynamic interplay between the galaxy ecosystem and surrounding CGM using bright background quasars. In this thesis, we extend this investigation to higher redshifts by using the bright afterglows of gamma-ray bursts (GRBs) as background sources probing the CGM of their own host galaxies. We compiled a sample of 27 high-resolution (R $>$ 6000) rest-frame UV spectra of GRB afterglows in a redshift range (2 $\lesssim$ z $\lesssim$ 6); we call this the `CGM-GRB sample'. We find stronger blue wings in high-ionization species (Si IV, C IV) compared to the low-ionization species (Si II, Fe II), indicative of the presence of ubiquitous warm outflows in the GRB hosts at high redshifts. Using kinematic models, we estimated typical values of CGM properties (for the sample) such as CGM mass (10$^{9.8}$ $\mathrm{M_{\odot}}$) and outflow launch velocity (300 km s$^{-1}$). Further, by comparing our results with previous C IV absorption studies, we find a possible CGM-galaxy co-evolution. Over the course of evolution of present-day galaxies with $\mathrm{M_{*}}$ $>$ $10^{10}$ $\mathrm{M_{\odot}}$, the ratio of C IV mass in the CGM to the stellar mass remains fairly uniform, such that log$\mathrm{(M_{C IV}/M_{*})} \sim -4.5$ within $\pm$0.5 dex from $z\sim4$ to $z\sim0$, suggesting CGM-galaxy co-evolution. Next, we embarked on a search for possible relations between the outflow properties and those of the host galaxies such as $\mathrm{M_*}$, star formation rate (SFR), and specific SFR (= SFR/$\mathrm{M_*}$). To estimate the total SFR, we first investigated the degree of dust obscuration in the massive GRB hosts in our sample by comparing radio- and UV-based star-formation rates. We inferred that the GRB hosts in our sample are not heavily dust obscured, and hence, their SFRs can be estimated reliably using the established dust-correction methods. For the outflow-galaxy correlations, we focused on three outflow properties $-$ outflow column density ($\mathrm{N_{out}}$), maximum outflow velocity ($\mathrm{V_{max}}$), and normalized maximum velocity ($\mathrm{V_{norm}}$ = $\mathrm{V_{max}/V_{circ, halo}}$, where $\mathrm{V_{circ, halo}}$ is the halo circular velocity). We observe clear trends of $\mathrm{N_{out}}$ and $\mathrm{V_{max}}$ with increasing SFR in high-ion-traced outflows. These correlations indicate that these high-ion outflows are driven by star formation at these redshifts (in the mass range $\mathrm{log(M_*/M_{\odot})}$ $\sim$ $9-11$). We also, for the first time, observe a strong ($>$ 3$\sigma$) trend of normalized velocity decreasing with halo mass and increasing with sSFR at high redshifts, suggesting that outflows from low-mass halos and high sSFR galaxies are most likely to escape and enrich the outer CGM and IGM with metals. Thus, we demonstrate GRB afterglows as a method to uncover CGM-galaxy co-evolution and outflow-galaxy correlations at high redshifts, which constitute an important piece of the galaxy growth puzzle and cosmic metal enrichment. Next, we set out to develop a new tool $-$ an on-chip photonic spectrograph $-$ which will eventually expand our investigation to the first galaxies in the universe ($z > 6$). Astrophotonics is the application of versatile photonic technologies to channel, manipulate, and disperse guided light to efficiently achieve various scientific objectives in astronomy in a miniaturized form factor. We used the concept of arrayed waveguide gratings (AWG) to develop an on-chip photonic spectrograph in the H band ($1.45-1.65$ $\mu m$) with a moderate resolving power of $\sim$1500, a peak throughput of $\sim$23\%, and a size of only 1.5 cm $\times$ 1.5 cm. Various practical aspects of implementing AWGs as astronomical spectrographs are also discussed, including a) the coupling of the light between the fibers and AWGs, b) cleaving at the output focal plane of the AWG to provide continuous wavelength coverage, and c) a multi-input AWG design to receive light from multiple single-mode fibers at a time and produce a combined spectrum. Finally, we built a cross-dispersion setup which will orthogonally separate the overlapping spectral orders in the AWG and thus image the full spectrum on the detector. The AWG will be incorporated with this setup in the near future to get the spectrograph ready for our first on-sky test. The work conducted in this thesis is a crucial stepping stone towards building a high-throughput, miniaturized spectrograph for the next generation of ground-, balloon-, and space-based telescopes. With the nano-scale fabrication on a chip, we are poised to unravel the mysteries of galaxies billions of light years away, making this thesis a truly \textit{`Nanometers to Light Years'} journey.Item Numerical Simulations of Granular Physics in the Solar System(2017) Ballouz, Ronald; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granular physics is a sub-discipline of physics that attempts to combine principles that have been developed for both solid-state physics and engineering (such as soil mechanics) with fluid dynamics in order to formulate a coherent theory for the description of granular materials, which are found in both terrestrial (e.g., earthquakes, landslides, and pharmaceuticals) and extra-terrestrial settings (e.g., asteroids surfaces, asteroid interiors, and planetary ring systems). In the case of our solar system, the growth of this sub-discipline has been key in helping to interpret the formation, structure, and evolution of both asteroids and planetary rings. It is difficult to develop a deterministic theory for granular materials due to the fact that granular systems are composed of a large number of elements that interact through a non-linear combination of various forces (mechanical, gravitational, and electrostatic, for example) leading to a high degree of stochasticity. Hence, we study these environments using an N-body code, pkdgrav, that is able to simulate the gravitational, collisional, and cohesive interactions of grains. Using pkdgrav, I have studied the size segregation on asteroid surfaces due to seismic shaking (the Brazil-nut effect), the interaction of the OSIRIS-REx asteroid sample-return mission sampling head, TAGSAM, with the surface of the asteroid Bennu, the collisional disruptions of rubble-pile asteroids, and the formation of structure in Saturn's rings. In all of these scenarios, I have found that the evolution of a granular system depends sensitively on the intrinsic properties of the individual grains (size, shape, sand surface roughness). For example, through our simulations, we have been able to determine relationships between regolith properties and the amount of surface penetration a spacecraft achieves upon landing. Furthermore, we have demonstrated that this relationship also depends on the strength of the local gravity. By comparing our numerical results to laboratory experiments and observations by spacecraft we can begin to understand which microscopic properties (i.e., grain properties) control the macroscopic properties of the system. For example, we can compare the mechanical response of a spacecraft to landing or Cassini observations of Saturn's ring to understand how the penetration depth of a spacecraft or the complex optical depth structure of a ring system depends on the size and surface properties of the grains in those systems.Item The Cosmic Near-Infrared Background: From the Dark Ages to the Present(2014) Helgason, Kari; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Cosmic Infrared Background (CIB) is made up of the collective light from galaxies and quasars built-up over the entire cosmic history. It plays an important role in characterizing the evolution of galaxies and contains information on other sources inaccessible to direct detection. In this dissertation, I seek to understand current CIB measurements in terms of all sources emitting since the era of the first stars. First, I model the CIB arising from known galaxy populations using 233 measured UV, optical and NIR luminosity functions from a variety of surveys spanning a wide range of redshifts. Our empirical approach, in conjunction with a halo model describing the clustering of galaxies, allows us to compute the fluctuations of the unresolved CIB and compare to current measurements. I find that fluctuations from known galaxy populations are unable to account for the large scale CIB clustering signal seen by current space observatories, and this discrepancy continues to diverge out to larger angular scales. This suggests that known galaxy populations are not responsible for the bulk of the fluctuation signal seen in the measurements and favors a new population of faint and highly clustered sources. I also empirically reconstruct the evolving extragalactic background light from galaxies and derive the associated opacity of the universe to high energy photons out to z~4. Covering the whole range from UV to mid-IR (0.15-25 micron), I provide for the first time a robust empirical calculation of the photon-photon optical depth out to several TeV. In the absence of significant contributions to the cosmic diffuse background from unknown populations, such as the putative first stars and black holes, the universe appears to be largely transparent to gamma-rays at all Fermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already at z~0.2. In addition, I study contributions from extragalactic populations to a recently discovered cross-correlation signal of the CIB fluctuations with the Cosmic X-ray Background (CXB). I model the X-ray emission from AGN, normal galaxies and hot gas residing in virialized structures, calculating their CXB contribution and spatial coherence with all infrared emitting counterparts. At small angular scales the coherence between the CIB and the CXB can be explained by galaxies and AGN. However, at large angular scales I find the net contribution from these populations only to account for a fraction of the measured CIBxCXB signal. The discrepancy suggests that the signal originates from the same unknown source population producing the CIB clustering signal out to ~1 deg.