Astronomy
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Item THE FORMATION OF METAL-FREE POPULATION III STARS IN X-RAY AND LYMAN-WERNER RADIATION BACKGROUNDS(2024) Park, Jongwon; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal-free Population III (or Pop III) stars are instrumental in shaping the early universe, influencing the formation of the first galaxies. The formation of Pop III stars depends on the fraction of molecular hydrogen (H2), which is regulated by X-ray and Lyman-Werner (LW) radiation backgrounds. Therefore, gaining insight into the impact of these radiation backgrounds is essential for unraveling the mysteries surrounding Pop III star formation and their impacts on the first galaxies. In this dissertation, I investigate the interaction between X-ray/LW backgrounds and the formation of Pop III stars. To conduct this investigation, I employed the radiative hydrodynamics code RAMSES - RT. I implemented various physical processes governing Pop III star formation, such as primordial chemistry, radiation background, secondary ionization/heating, and self-shielding. Performing a grid of simulations covering a large parameter space of X-ray/LW intensity, I systematically explored the effects of radiation backgrounds on Pop III stars. I found that a moderate X-ray background boosts the H2 fraction in dark matter halos, facilitating Pop III star formation in low-mass halos. In contrast, a LW background dissociates H2 and prevents star formation in low-mass halos. This result suggests that the number of Pop III supernovae detected by the JWST is enhanced by an X-ray background. Furthermore, I discovered that an X-ray background reduces the characteristic mass and multiplicity of Pop III stars. This leads to a top-heavier initial mass function and may have a potential impact on galaxy formation. Moreover, I made further improvements to the simulations by incorporating radiative feedback from Pop III protostars. This study confirmed previous works that radiation from protostars suppresses their growth, thereby playing a significant role in determining the mass of Pop III stars theoretically. I also found that hierarchical binaries (binaries of binaries), eccentric orbits, and outward migration are common occurrences in Pop III star formation. Eccentric orbits induce variability of Pop III protostars and this is observable by the JWST when light is magnified through gravitational lensing. In a follow-up study, I investigated the origin of outward migration and found that the gas disks around the protostars accrete gas with high angular momentum and transfer the angular momentum to the binary stars through torques. This finding paves the wayfor studies of migration behaviors across different stellar populations. Finally, I explored the X-ray effects on the number of Pop III stars using cosmological simulations. Developing methods to calculate the intensity of the radiation background on the fly and realistically accounting for the X-ray feedback loop, I found that a weak X-ray background develops and this background ionizes the intergalactic medium, thereby moderately increasing the number density of Pop III stars (by a factor of ∼ 2). This rise in the number of Pop III stars due to X-ray radiation lowers the star formation rate of metal-enriched Pop II stars, highlighting the significance of the X-ray background in galaxy formation.This thesis covers various aspects of Pop III star formation and the effect of X-ray radiation backgrounds which has been overlooked by previous studies. It lays a foundation for future research aimed at connecting the theoretical understanding of Pop III star formation and observations targeting Pop III stars and the first galaxies.Item 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.Item Optical Time Domain and Radio Imaging Analyses of the Dynamic Hearts of AGN(2017) Smith, Krista Lynne; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Active galactic nuclei (AGN) are among the most extreme objects in the universe: galaxies with a central supermassive black hole feeding on gas from a hot accretion disk. Despite their potential as powerful tools to study topics ranging from relativity to cosmology, they remain quite mysterious. In the first portion of this thesis, we explore how an AGN may influence the formation of stars in its host galaxy. Using high-resolution 22 GHz radio imaging of an X-ray selected sample of radio-quiet AGN, we find that the far-infrared radio correlation for normal star forming galaxies remains valid within a few hundred parsecs of the central engine. Because the core flux is often spatially isolated from star formation, we can also determine that the radio emission in radio-quiet AGN is consistent with both coronal and disk-jet coupling models. Finally, we find that AGN with jet-like radio morphologies have suppressed star formation, possibly indicating ongoing feedback. The second portion of this thesis uses optical AGN light curves to study the physics of accretion. The Kepler spacecraft produces groundbreaking light curves, but its fixed field of view only contained a handful of known AGN. We conduct an X-ray survey of this field, yielding 93 unique X-ray sources identified by optical follow-up spectroscopy as a mixture of AGN and stars. For the AGN, we spectroscopically measure black hole masses and accretion rates. We then analyze a sample of 22 Kepler AGN light curves. We develop a customized pipeline for AGN science with Kepler, a necessary step since the initial data was optimized for the unique goal of exoplanet detection. The light curves display an astonishing variety of behaviors in a new regime of optical variability inaccessible with previous facilities. We find power spectral slopes inconsistent with the damped random walk model, characteristic variability timescales, correlations of variability properties with physical parameters, and bimodal flux distributions possibly consistent with passing obscuring material. We also conclude that this regime of optical variability is not produced by simple X-ray reprocessing. Finally, we explain how this work supports future robust accretion studies with upcoming large timing surveys.Item Molecular Gas and Star Formation at Low Metallicity in the Magellanic Clouds(2016) Jameson, Katherine Esther; Bolatto, Alberto D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Magellanic Clouds are two interacting, gas-rich, star-forming, low-mass, nearby satellite galaxies of the Milky Way that afford a unique view of low-metallicty star-forming regions, providing the nearest laboratories to study processes relevant to star formation in the early universe. We use the dust emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H2. On small (~ few pc) scales in the Small Magellanic Cloud (SMC), we study the effect of metallicity on the structure of photodissociation regions in the outskirts of molecular clouds using [CII] and [OI] spectroscopy combined with new ALMA 7-m array maps of 12CO and 13CO. We estimate the total amount of molecular gas using [CII] to trace H2 at low-Av and 12CO to trace H2 at high-Av. We find that most of the molecular gas is traced by [CII] emission and that metallicity only affects the relationship between 12CO emission and molecular gas through changes in Av. Using mid-infrared spectroscopy from Spitzer Space Telescope in the SMC, we model the H2 rotational line emission to estimate temperatures, column densities, and fractions of warm H2 gas (T>100 K). The temperatures and column densities of warm H2 gas are similar to nearby galaxies, but the SMC shows somewhat high fractions of warm H2. The properties of the warm H2 gas indicate that it is located in photodissociation regions that are more extended in the low metallicity environment of the SMC. We use dust-based molecular gas maps data to evaluate molecular depletion time scales as a function of spatial scale. We compare galaxy-scale analytic star formation models to our observations and find that successfully predicting the trends in the low metallicity environment needs the inclusion of a diffuse neutral medium. The analytic models, however, do not capture the scatter observed, which computer simulations suggest is driven primarily by the time-averaging effect of star formation rate tracers. The averaging of the scatter in the molecular gas depletion time as a function of scale size suggests that the drivers of the star formation process in these galaxies operate on large scales.Item Probing the Multiphase Interstellar Medium and Star Formation in Nearby Galaxies through Far-infrared Spectroscopy(2015) Herrera Camus, Rodrigo; Bolatto, Alberto; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a study of different aspects of the multi-phase interstellar medium (ISM) of nearby galaxies, including detailed analysis of the low-excitation ionized gas, the thermal pressure (Pth) of the neutral gas, the dust-to-gas mass ratio (DGR) in low-metallicity environments, and the use of far-infrared transitions as tracers of the star formation rate (SFR). We based our work on the largest sample to date of spatially-resolved, infrared observations of nearby galaxies drawn from the KINGFISH and ``Beyond the Peak'' surveys. We use deep infrared observations to study the DGR of the extremely metal-poor galaxy I Zw 18. We measure a DGR upper-limit of 8.1x10^{-5}. This value is a factor of ~8 lower than the expected DGR if a linear correlation between DGR and metallicity, as observed in spirals, were to hold. Based on the line ratio between the [NII] 122 and 205 um transitions, for 140 regions selected from 21 galaxies we measure electron densities of the singly-ionized gas in the ne~1-230 cm^{-3} range, with a median value of ne=30 cm^{-3}. We find that ne increases as a function of SFR and radiation field strength. We study the reliability of the [CII] and [NII] 122 and 205 um transitions as SFR tracers. In general, we find good correlations between the emission from these fine-structure lines and star formation activity. However, a decrease in the photoelectric heating efficiency in the case of the [CII] line, or collisional quenching of the [NII] lines, can cause calibrations based on these transitions to underestimate the SFR. Finally, for a sample of atomic-dominated regions selected from 31 galaxies, we use the [CII] and HI lines to measure the cooling rate per H atom and Pth of the cold, neutral gas. We find a \pt\ distribution that can be well described by a log-normal distribution with median Pth/k~5,500 K cm^{-3}. We find correlations of increasing Pth with radiation field intensity and SFR, which is consistent with the results from two-phase ISM models in pressure equilibrium.Item High-Resolution Imaging of Dense Gas Structure and Kinematics in Nearby Molecular Clouds with the CARMA Large Area Star Formation Survey(2015) Storm, Shaye; Mundy, Lee G; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis utilizes new observations of dense gas in molecular clouds to develop an empirical framework for how clouds form structures which evolve into young cores and stars. Previous observations show the general turbulent and hierarchical nature of clouds. However, current understanding of the star formation pathway is limited by existing data that do not combine angular resolution needed to resolve individual cores with area coverage required to capture entire star-forming regions and with tracers that can resolve gas motions. The original contributions of this thesis to astrophysical research are the creation and analysis of the largest-area high-angular-resolution maps of dense gas in molecular clouds to-date, and the development of a non-binary dendrogram algorithm to quantify the hierarchical nature and three-dimensional morphology of cloud structure. I first describe the CARMA Large Area Star Formation Survey, which provides spectrally imaged \NtwoH{}, \HCO{}, and HCN ($J=1\rightarrow0$) emission across diverse regions of the Perseus and Serpens Molecular Clouds. I then present a detailed analysis of the Barnard~1 and L1451 regions in Perseus. A non-binary dendrogram analysis of Barnard~1 \NtwoH{} emission and all L1451 emission shows that the most hierarchically complex gas corresponds with sub-regions actively forming young stars. I estimate the typical depth of molecular emission in each region using the spatial and kinematic properties of dendrogram-identified structures. Barnard~1 appears to be a sheet-like region at the largest scales with filamentary substructure, while the L1451 region is composed of more spatially distinct ellipsoidal structures. I then do a uniform comparison of the hierarchical structure and young stellar content of all five regions. The more evolved regions with the most young stellar objects (YSOs) and strongest emission have formed the most hierarchical levels. However, all regions show similar mean branching properties at each level, suggesting that dense gas fragmentation proceeds in a hierarchically similar way from earlier to later stages of star formation. Compared to the more evolved YSOs, the youngest YSOs are preferentially forming within leaves and at high-level locations in dendrogram hierarchies, indicating that dense gas in molecular clouds must reach a state of hierarchical complexity before young stars form efficiently.Item The Origins and Ionization Mechanisms of Halpha Filaments in the Cool Cores of Galaxy Groups and Clusters(2011) McDonald, Michael Adam; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a survey of 10 galaxy groups and 23 galaxy clusters aimed at explaining the presence of warm, ionized filaments in the cool cores of galaxy clusters. By combining deep, high spatial resolution H-alpha, far-UV, and X-ray data from the Maryland Magellan Tunable Filter, Hubble Space Telescope, and Chandra X-ray Observatory, respectively, we have assembled the most complete picture of these mysterious filaments to date. This extensive database has allowed us to shed new light on two critical questions: i) Where does the cool gas in these filaments come from? ii) What process or processes are responsible for ionizing the cool filaments. As a pilot project, we obtained high-resolution H-alpha and far-UV data for Abell1795 and find that the previously-discovered H-alpha filament is, in fact, two very thin, intertwined filaments extending 50 kpc and with a width < 700 pc. The clumpy UV morphology and UV/H-alpha flux ratios of these filaments suggest that they may consist of chains of star-forming regions. Based on these data we conclude that the H-alpha emission is a result of photoionization by young stars and that the cool gas filaments are a byproduct of the intracluster medium cooling onto a fast-moving central galaxy. When we consider the full sample of 23 galaxy clusters, we find several strong correlations between the X-ray and H-alpha data. In general, complex, extended H-alpha filaments are found in clusters with cool, low-entropy cores. Furthermore, the morphology of the warm gas is correlated with the soft X-ray morphology and the filaments are found to occupy regions where, locally, the intracluster medium is cooling fastest. Finally, we find a strong correlation between the mass of of gas cooling below X-ray temperatures and the mass of gas in the warm filaments. These results provide strong evidence that the ionized filaments are a result of highly-asymmetric runaway cooling in the intracluster medium. By extending this sample to include 10 galaxy groups we are able to probe more than 2 orders of magnitude in mass and cooling rate. We find that there is only a weak correlation between the presence of ionized filaments and the total mass of the system. Instead, the presence of ionized filaments appears to depend almost entirely on the core properties, specifically whether there is a cool, low-entropy core. We find that groups are, in general, cooling more efficiently that clusters, due to their lower starting temperature. This can only be the case if cool-core groups are experiencing exclusively weak AGN feedback, which we show is the case. Finally, using new far-UV data from the Hubble Space Telescope, we find that 12/15 systems with H-alpha emission are consistent with being photoionized by young stars. The three remaining systems are under-luminous in the far-UV for their H-alpha luminosity, suggesting an alternative ionization source such as fast shocks or significant internal reddening. When we supplement this sample with UV data from GALEX and IR data from Spitzer we find a correlation between the star formation rate and the ICM cooling rate for 32 systems. The inferred efficiency of stars forming out of the cooling ICM is 14(+18/-8)%, which is consistent with the Universal fraction of baryons in stars. These results suggest that, for the majority of cases, young star formation provides sufficient UV flux to ionize the cool filaments.Item Dust Structure and Composition Within Molecular Clouds and Cores(2007-10-02) Chapman, Nicholas; Mundy, Lee; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We observed three molecular clouds and four isolated cores at wavelengths from 3.6-24 microns. The clouds we observed were Ophiuchus, Perseus, and Serpens and the cores were L204C-2, L1152, L1155C-2, and L1228. Our goal was to use these deep infrared data to map changes in the extinction law and the dust properties throughout the observed regions. In our clouds, we found the lowest density regions have an IRAC extinction law similar to the one observed in the diffuse ISM. At higher extinctions, there is evidence for grain growth because the extinction law flattens compared to the diffuse ISM law and becomes more consistent with a model utilizing larger dust grains. In the densest regions of Serpens and Perseus, Ak > 2, it appears icy mantles are forming on the dust grains. We detected one low extinction region in Perseus with an anomalous extinction law that is not explained by current ideas about grain growth or the formation of ices onto dust grains. The extinction law in the cores shows only a slight flattening of the extinction law with increased extinction. Even at the lowest extinctions, the extinction law is more consistent with a dust model containing grain growth, rather than with the diffuse ISM. Two of the four cores have evidence for ices forming the densest regions. Molecular outflows appear to have an impact on the dust grains in two of our cores: L1152 and L1228. In both our clouds and cores, the extinction law at 24 microns is almost always higher than the value predicted by current dust models, but is consistent with other observations. We find some evidence for the 24 micron extinction law decreasing as the extinction increases. Overall, there are relatively few stars with detections >3 sigma at 24 microns. More observations are needed to understand the nature of the extinction law at this wavelength.