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IDENTIFYING PROTOCLUSTERS IN THE HIGH REDSHIFT UNIVERSE AND MAPPING THEIR EVOLUTION
Doctor of Philosophy, Case Western Reserve University, 2018, Astronomy
To investigate the growth and evolution of the earliest structures in the Universe, we identify more than 200 galaxy overdensities in the Candidate Cluster and Protocluster Cat- alog (CCPC). This compilation is produced by mining open astronomy data sets for over- densities of high redshift galaxies that are spectroscopically confirmed. At these redshifts, the Universe is only a few billion years old. This data mining approach yields a nearly ten fold increase in the number of known protoclusters in the literature. The CCPC also includes the highest redshift, spectroscopically confirmed protocluster at z = 6.56. For nearly 1500 galaxies contained in the CCPC between redshifts of 2.0 < z < 6.6, we find archival Spitzer images at 3.6 and 4.5 µm bandpasses. These Spitzer wavelengths serve as a proxy measurement for the stellar mass of the galaxies. The galaxies in protoclusters appear to be consistent with a passively evolving, older stellar population. We find no sta- tistically significant difference between protocluster and field galaxy populations. Galaxy formation models suggest that galaxies in dense environments should be more massive. Comparing the brightness distribution of the data at different epochs provides an evolution- ary track for how protocluster galaxies evolve. We compare the data to the predictions of a large-scale simulation, the Millennium Run. We analyze the simulated data with the same suite of algorithms and metrics as in the CCPC. The results of this exercise yield a number of significant discrepancies between the theoretical predictions and what is seen. The universe contains a much larger density of bright galaxies than what the model predicts. At z > 2, the brightest galaxies are older and more massive than anticipated by the model.

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

galaxy clusters, protoclusters, high redshift, astronomy, galaxy formation, galaxy evolution

Laser Guide Star Design Project for the USAF John Bryan State Park Quad Axis Observatory
Master of Science (MS), Wright State University, 2015, Physics
Atmospheric Turbulence has long remained one of the great unsolved problems in physics. Laser guide stars were invented in order for telescopes to overcome atmospheric turbulence while used in combination with adaptive optics. This study focuses on the design and implementation phase of a Rayleigh laser guide star for the John Bryan State Park Observatory, owned and operated by the United States Air Force. Atmospheric simulations, as well as optical modelling of proposed equipment, were completed to optimize the design of this laser guide star. In addition, a novel method for the implementation of the guide star onto this very unique four axis telescope platform is presented.

#### Committee:

Douglas Petkie, Ph.D. (Committee Chair); Elizabeth Beecher, Ph.D. (Committee Member); Tamara Payne, Ph.D. (Committee Member)

#### Subjects:

Astronomy; Atmosphere; Atmospheric Sciences; Physics

#### Keywords:

Atmospheric Science; Laser Guide Star; Rayleigh Laser Beacon; Turbulence; Astronomy; Telescopes;

Massive Stellar Clusters in the Disk of the Milky Way Galaxy
MS, University of Cincinnati, 2010, Arts and Sciences: Physics
This thesis outlines successful efforts for identifying and characterizing the stellar content of two Galactic disk star clusters using near-infrared observations. Astronomers have a great wealth of knowledge about Globular Clusters. They are easy to see as most lie outside the plane of the galaxy, in the halo. Extinction is low, stellar population is dense in the cluster, and they are fairly common. However, in the plane of the galaxy, relatively little is known of the open cluster population. Galactic disk open clusters, such as the two discussed in this thesis, are hidden behind gas, dust, and projected against a multitude of field stars. Through the use of near-infrared broad-band photometry and spectroscopy, the distance, age and approximate mass of two disk clusters has been determined.

#### Committee:

Margaret Hanson, PhD (Committee Chair); Michael Sitko, PhD (Committee Member); F Paul Esposito, PhD (Committee Member)

Astronomy

#### Keywords:

open cluster;milky way;massive cluster;galactic disk;near-infrared;photometry

Discovery of an Active Supermassive Black Hole in the Bulge-less Galaxy NGC 4561
Master of Science, The Ohio State University, 2012, Astronomy
We present XMM-Newton observations of the Chandra-detected nuclear X-ray source in NGC 4561. The hard X-ray spectrum can be described by a model composed of an absorbed power-law with Γ= 2.5-0.3+0.4 , and column density NH=1.9-0.2+0.1 × 1022 atoms cm-2. The absorption corrected luminosity of the source is L(0.2 - 10.0 keV) = 2.5× 1041 ergs s-1, with bolometric luminosity over 3×1042 ergs s-1. Based on the spectrum and the luminosity, we identify the nuclear X-ray source in NGC 4561 to be an AGN, with a black hole of mass about 2×104 MSUN;< MBH < 9×105 MSUN; radiating at ≥ 3% of the Eddington luminosity. The presence of a supermassive black hole at the center of this bulge-less galaxy shows that bulges are not necessary for the existence of a black hole. Observations such as these call into question several theoretical models of BH-galaxy formation and co-evolution. Several emission lines are clearly present in the soft X-ray spectrum, which can be well parameterized by an absorbed diffuse thermal plasma with non-solar abundances of some heavy elements. Similar soft X-ray emission is observed in spectra of Seyfert 2 galaxies and low luminosity AGNs, suggesting an origin in a circumnuclear starburst.

#### Subjects:

Astronomy

A Search for the Smallest Supermassive Black Holes
Doctor of Philosophy, The Ohio State University, 2009, Astronomy
Relations between supermassive black holes (SMBHs) and their hostgalaxies are now well known, but several questions remain: Do all galaxies harbor SMBHs? Do correlations between BH mass and host galaxy properties extend to lower mass BHs and all galaxy types? Is the galactic bulge or the dark matter halo the defining component for the nuclear BH? Answering these questions requires a study of low mass SMBHs, in particular those that reside in the latest-type spiral galaxies. While the presence of an active galactic nucleus (AGN) provides certain proof of the existence of an SMBH, galaxies that are not previously known to host AGNs may nevertheless have SMBHs at their centers. In other words, a galaxy may appear quiescent when in reality there is an accreting SMBH, if the accretion level is low enough. This thesis presents a search for such SMBHs by looking for the presence of low-level nuclear activity, as evidenced primarily by their x-ray emission, in a well-defined sample of nearby, optically quiescent spiral galaxies. This work demonstrates that traditional methods of identifying AGNs, developed over the course of studying luminous (> 1042 erg/s) AGNs, are inadequate for the population of low luminosity AGNs found in nearby galaxies, and develops the techniques that must be used instead. These techniques are then applied to an x-ray survey of nearby, face-on spiral galaxies. The survey includes new snapshot observations of 37 galaxies, which are combined with archival data for a further 18 galaxies. Where available, multi-wavelength data are used to help distinguish AGNs from other types of x-ray sources. These observations show the power of x-ray observations in detecting hidden AGNs, and also address the question of the prevalence of SMBHs in spiral galaxies that do not have bulges. This study has uncovered 14 previously unknown AGNs and strong AGN candidates, including two in galaxies of type Sd and Sdm. If the latter are confirmed as bona fide AGNs they will be only the fourth and fifth AGNs known to exist in bulgeless galaxies.

#### Committee:

Smita Mathur, PhD (Advisor); Paul Martini, PhD (Committee Member); Andrew Gould, PhD (Committee Member)

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

active galactic nuclei; spiral galaxies; late-type spirals

Cross-Correlation Cluster Cosmology
Doctor of Philosophy, The Ohio State University, 2013, Astronomy
The accelerated expansion of the Universe can be explained by either the cosmological constant Λ, a repulsive dark energy, or modifications to General Relativity (GR) on cosmological scales. The key to distinguishing these possibilities is to study the growth of large-scale structures over cosmic time, measured primarily by three methods: cosmic shear from weak lensing (WL), redshift-distortion (RSD) of galaxy clustering, and the traditional cluster cosmology using the abundance of galaxy clusters as function of mass. Although cosmic shear and RSD are commonly studied using galaxies, the two effects are much more enhanced around galaxy clusters. In this dissertation, I develop novel yet powerful techniques to combine cluster WL, galaxy infall kinematics, and cluster abundances to derive constraints on cosmological models and test of modified gravity (MG) theories. One of the main systematical uncertainties of cluster cosmology is in cluster mass calibration. Common mass observables, including galaxy richness, X-ray luminosity, and Sunyaev-Zel'dovich (SZ) decrements, correlate with cluster true mass with some scatter. Using MaxBCG cluster catalog derived from the Sloan Digital Sky Survey (SDSS) and assuming Λ-Cold Dark Matter (ΛCDM) model, I developed an alternative method for constraining cosmological parameters, i.e., the matter density Ωm and the clustering amplitude σ8, while simultaneously calibrating the scatter in the richness-mass relation, by combining the abundance of clusters as function of richness and the large-scale cluster WL profiles. We find σ8m/0.325)0.501=0.828±0.049, consistent with constraints from other MaxBCG studies that use WL measurements on small scales. The (Ωm8) constraint is consistent with and orthogonal to the one inferred from WMAP CMB data, reflecting agreement with the structure growth predicted by General Relativity for a ΛCDM cosmological model. A joint constraint assuming ΛCDM yields Ωm=0.298-0.020+0.019 and σ8=0.831-0.020+0.020. For these parameters and our best-fit scatter we obtain a tightly constrained mean richness-mass relation of MaxBCG clusters, N200=25.4(M/3.61 ×1014 h-1Msun)0.74, with a normalization uncertainty of 1.5%. Cluster WL is currently the only viable method to measure cluster mass profiles beyond the virial radius. I developed an novel method to infer the dynamical cluster mass profiles by extracting the characteristic infall velocities as function of radius, from measurements of the redshift-space cluster-galaxy cross-correlation function, ξcgs. Using the Millennium simulation, I calibrate the galaxy kinematic profiles around clusters as a two-component mixture model comprised of a virialized component with an isotropic velocity distribution and a infall component described by a skewed t-distribution with separate radial and tangential dispersions. By convolving with the real-space cluster-galaxy cross-correlation function ξcgs, I show that the galaxy infall kinematics (GIK) model provides accurate predictions for ξcgs, and I show that the measurement of ξcgs can be inverted to recover the GIK profiles for any given cluster sample. In particular, the inferred characteristic infall velocity profile is a promising tool for estimating the dynamical mass of clusters. As a first-cut experiment, I apply the GIK-modelling to ξcgs measured for rich galaxy groups derived from the SDSS main galaxy sample, and the inferred masses agree well with those estimated from virial scaling and weak lensing. To explain cosmic acceleration, most MG theories rely on an extra scalar field on top of the metric tensor field, mediating a "fifth" force on intermediate scales. However, photons do not respond to the scalar field, so there will be a mismatch between lensing and dynamical mass estimates if gravity is modified on relevant scales. Since GR has to be recovered in high density regions to evade stringent tests in the solar system and binary pulsars, MG theories generally invoke some "screening" mechanisms to either make the effective mass of the scalar field environment-dependent (Chameleon mechanism) or suppress the scalar field close to sources (Vainshtein mechanism). I investigate the impact of MG on the GIK profiles around massive clusters using two suites of MG simulations, namely, the f(R) and Galileon models, which employ the Chameleon and Vainshtein screening mechanisms, respectively. For clusters of M~1014h-1Msun at z~0.25, I find ~100-200 km/s enhancement in the characteristic infall velocity and ~50-100 km/s broadening in the radial and tangential velocity dispersions at r~5 Mpc compared to GR predictions. These deviations from GR are detectable via the GIK-modeling of ξcgs measured from existing and future galaxy redshift surveys. For future work, I will address the major source of systematic uncertainties in GIK calibration, the imperfect understanding of the impact of galaxy formation physics on GIK, by comparing GIK profiles measured from mock galaxy catalogs constructed using different Halo Occupation Distribution (HOD) prescriptions and semi-analytic models (SAMs) within large-volume, high-resolution cosmological simulations. By calibrating the GIK as function of halo mass in simulations of different cosmological parameters, I will be able to combine WL, GIK, and abundances of galaxy clusters as one of the most powerful diagnostics of theories of cosmic acceleration and modified gravity.

#### Committee:

David Weinberg (Advisor); Christopher Kochanek (Committee Member); Todd Thompson (Committee Member)

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

cosmology; large-scale structures; galaxy clusters; galaxy infall kinematics; modified gravity; dark energy; weak lensing; redshift-space distortion; cosmic acceleration;

The cosmological X-ray evolution of stars, AGN, and galaxies
Doctor of Philosophy, The Ohio State University, 2006, Physics
After reviewing the pillars of the concordance cosmology, which serves as the framework of this thesis, I describe how the accretion history of the Universe is revealed by thermal X-ray emission from the stellar sytems and nuclear regions of galaxies. I then go on to discuss how my collaborators and I have used this information to trace the formation of stars and the growth of supermassive black holes over the last ~ 8 Gyrs, i.e., from z ~ 1 to the present. It is of great importance for our understanding of galaxy evolution to determine whether there is a break or a continuum in these processes from the powerful quasars and starbursts of the past, to the seemingly dormant, "normal" galaxies more typical of the present epoch. To help settle the question, we combined optical data from the NOAO Deep Wide-Field Survey (NDWFS), X-ray data from the XBootes survey, and spectral information from the AGN and Galaxy Evolution Survey (AGES) in order to simultaneously obtain deep and wide coverage of the normal galaxy population. In this manner, my collaborators and I were able to bridge the gap in normal galaxy X-ray coverage between large-area local surveys and high redshift, small volume deep fields. Our findings suggest the accretion history of the Universe plays out as a continuum, with the growth of stellar populations and supermassive black holes steadily tailing off rather than abruptly ceasing as we move toward the faint end of the luminosity function.

#### Subjects:

Physics, Astronomy and Astrophysics

#### Keywords:

Cosmology; Accretion History of the Universe; Galaxy Evolution; X-rays; X-ray Binaries; Active Galactic Nuclei (AGN)

Sharpening The Tools of Gravitational Microlensing
Doctor of Philosophy, The Ohio State University, 2009, Astronomy

We attempt to identify all microlensing parallax events for which the parallax fit improves ΔΧ2 > 100 relative to a standard microlensing model. We outline a procedure to identify three types of discrete degeneracies and find many new degenerate solutions in 16 previously published and 6 unpublished events. The lens of event OGLE-2003-BLG-84 may be a Jupiter-mass free-floating planet candidate based on a weak 3σ detection of finite-source effects. These events were examined for xallarap, which can mimic parallax. We find that 23% of these events are strongly affected by xallarap.

The mid-IR flux ratios of the two images of the gravitationally lensed quasar HE 1104-1805 show no wavelength dependence to within 3% across 3.6-8.0 microns, no time dependence over 6 months and agree with the broad emission line flux ratios. This indicates that the mid-IR emission likely comes from scales large enough to be little affected by microlensing and that there is little differential extinction between the images. We measure a revised time-delay between these two images. We also observed uncorrelated variations of ~ 0.05 mag/yr-1 which we attribute to microlensing of the optical emission from the accretion disk. The optical colors have also changed significantly in the sense that image A is now redder than image B, rather than bluer as it was in 1993.

Based on the microlensing variability of the two-image gravitational lens HE 1104-1805 observed between 0.4 and 8 microns, we have measured the size and wavelength-dependent structure of the quasar accretion disk. Modeled as a power law in temperature, we measure a B-band half-light radius of R = 6.7(+6.2)(-3.2)x10^15 cm (68% CL) and a logarithmic slope of 0.61(+0.21)(-0.17) (68% CL) for our standard model with a logarithmic prior on the disk size. Both the scale and the slope are consistent with simple thin disk models. The observed fluxes favor a slightly shallower temperature profile.

Using 11-years of OGLE V-band photometry of Q2237+0305, we measure the transverse velocity of the lens galaxy and the mean mass of its stars. We measure the inclination i of the accretion disk to be cos i > 0.63 at 68% confidence. Very edge on (cos i < 0.34) solutions are ruled out at 95% confidence. For the first time, we fully include the random motions of the stars in the lens galaxy in the analysis of the light curves. We find the best fit transverse velocity of the lens galaxy is ~ 420 km/s to the Northeast. There is no strong correlation between the direction of motion of the lens galaxy and the orientation of the disk. The mean stellar mass is 0.37(+1.07)(-0.26) solar masses after including a well-defined velocity prior. We measure the V-band radius of the accretion disk. We also show for the first time that analyzing subsets of a microlensing light curve, in this case the first and second halves of the OGLE V-band light curve, give mutually consistent physical results.

#### Committee:

Christopher Kochanek, PhD (Advisor); Andrew Gould, PhD (Committee Member); Bradley Peterson, PhD (Committee Member)

Astronomy

#### Keywords:

gravitational lensing; microlensing; parallax; xallarap; quasar microlensing; accretion disk

Probing the Circumstellar Disks of Classical Be Stars with Optical and Near-Infrared Spectroscopy
Doctor of Philosophy, University of Toledo, 2009, Physics
Variable asymmetric double-peaked hydrogen emission line profiles in the optical in classical Be stars have been interpreted as evidence of one-armed density waves (or other density structures) within the circumstellar disks. Contemporaneous optical and IR spectroscopy can provide a means of exploring the density structure of any such density variations as a function of radius, since the infrared lines serve as a probe of the inner disk, while the Hα line probes a much larger region. While variability has been detected in these stars in the optical spectral region over both short (days to weeks) and longer (months) timescales, the variability of infrared line profiles has not been similarly investigated. Presented are contemporaneous Ritter Observatory (Hα) and IRTF SpeX (0.8-5.4 μm) spectroscopy of 33 classical Be stars observed between September 2005 and January 2008. The data illustrate a range of line profiles common in Be stars and show significant variability in both the optical and the near-infrared. IR hydrogen line profile variability is found to be similar both in character and variety to that observed in the optical. Several targets show asymmetric line profiles with reversed orientation between the optical and the IR hydrogen lines at least once. For at least one target, and possibly several more, this reversed asymmetry suggests a phase delay between quasi-periodic variation in the Hα and the IR hydrogen lines. For other targets no evidence of a phase delay is detected, indicating that a variety of disk morphologies may be present. A handful of targets are observed in the process of disk formation or loss; line strength in these targets appears to vary at the same time for all lines.

#### Committee:

Karen Bjorkman, PhD (Advisor); Nancy Morrison, PhD (Committee Member); Steven Federman, PhD (Committee Member); Thomas Megeath, PhD (Committee Member); Scott Lee, PhD (Committee Member); Carol Jones, PhD (Committee Member)

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

circumstellar disks; Be stars; near-infrared spectroscopy; IRTF; zeta Tauri

The Effect of Age and Metallicity on Be Circumstellar Disk Formation
Doctor of Philosophy, University of Toledo, 2005, Physics

While rapid rotation is likely the dominant mechanism which influences the development of classical Be circumstellar disks, recent observational and theoretical work suggest that evolutionary age and/or metallicity may also influence the onset of the Be phenomenon. We use a simple 2-color diagram photometric technique to identify the candidate Be population in 16 Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic clusters having a wide range of ages and metallicities. We detect an enhancement in the fractional early-type candidate Be star population relative to the fractional later-type candidate population in clusters whose early-type stars are near the end of their main sequence lifetimes, suggesting the Be phenomenon is enhanced with evolutionary age. Furthermore, in contrast to the suggestion of Fabregat & Torrejon (2000) that the Be phenomenon should begin at least 10 Myr after the zero-age-main-sequence, we detect a substantial number of candidate Be stars in clusters as young as 5 Myr. Follow-up photo-polarimetric observations of these young candidates reveal many are true classical Be stars, indicating that a significant number of zero-age-main-sequence stars must be rotating close to their critical breakup velocities. The improved statistics offered by our study also reveal clear evidence of an enhancement of the Be phenomenon in low metallicity environments.

It is commonly assumed in the literature that all B-type objects detected as excess H alpha emitters via 2-color diagrams are ''Be stars''. We explore the nature of many of these candidate Be stars with additional photo-polarimetric observations, and find that ~25% of these objects exhibit properties which aren't consistent with those expected from classical Be stars. We also find that the prevalence of polarization Balmer jumps in Be stars located in low metallicity environments is lower than that typically observed for Galactic Be stars. One interpretation of this result is that disk systems in low metallicity environments have fundamentally different properties, i.e. smaller disks and/or lower disk temperatures, than their Galactic counterparts. We also detect evidence of cluster-wide alignment of Be circumstellar disks in 2 LMC clusters.

#### Subjects:

Physics, Astronomy and Astrophysics

#### Keywords:

Large Magellenic Cloud; Small Magellanic Cloud; Be star; polarization

Photoluminescence by Interstellar Dust
Doctor of Philosophy, University of Toledo, 2005, Physics

In this dissertation, we report on our study of interstellar dust through the process of photoluminescence (PL). We present the discovery of a new band of dust PL, blue luminescence (BL) with λpeak ~ 370 nm in the proto-planetary nebula known as the Red Rectangle (RR). We attribute this to fluorescence by small, 3-4-ringed polycyclic aromatic hydrocarbon (PAH) molecules. Further analysis reveals additional independent evidence for the presence of small PAHs in this nebula. Detection of BL using long-slit spectroscopic observations in other ordinary reflection nebulae suggests that the BL carrier is an ubiquitous component of the ISM and is not restricted to the particular environment of the RR. We present the spatial distribution of the BL in these nebulae and find that the BL is spatially correlated with IR emission structures attributed to aromatic emission features (AEFs), attributed to PAHs.

The carrier of the dust-associated photoluminescence process causing the extended red emission (ERE), known now for over twenty five years, remains unidentified. We constrain the character of the ERE carrier by determining the wavelengths of the radiation that initiates the ERE – λ < 118 nm. We note that under interstellar conditions most PAH molecules are ionized to the di-cation stage by photons with E > 10.5 eV and that the electronic energy level structure of PAH di-cations is consistent with fluorescence in the wavelength band of the ERE.

In the last few chapters of the dissertation we present first results from ongoing work: i) Using narrow-band imaging, we present the optical detection of the circumbinary disk of the RR in the light of the BL, and show that the morphology of the BL and ERE emissions in the RR nebula are almost mutually exclusive. It is very suggestive to attribute them to different ionization stages of the same family of carriers such as PAH molecules. ii) We also present a pure spectrum of the BL free of scattered light, resolved into seven molecular emission bands, superimposed upon a broad continuum. The relative intensity of the component bands varies with position within the nebula, suggesting an origin in a set of several related molecular species, most likely small PAHs.

#### Subjects:

Physics, Astronomy and Astrophysics

#### Keywords:

photoluminescence; polycyclic aromatic hydrocarbons; fluorescence; interstellar dust

Deep R-Band Surface Photometry of NGC891
BA, Oberlin College, 1996, Physics and Astronomy

Understanding how galaxies evolve is a challenge for astronomers, given the very long timescales for such evolution to occur. Fortunately, the structure of a galaxy encodes a fossil record of its evolution. In particular, by decomposing this structure into a number of independent components, we can compare the results to theoretical predictions and begin to trace the history of mass accretion and star formation that have shaped the stellar systems we see today.

In this thesis, I present deep optical R-band surface photometry of the nearby edge-on spiral galaxy NGC891. This galaxy bears many similarities to the Milky Way, and thus exploring its structure can improve our understanding of our own Galaxy's evolution. Using a method developed by collaborators to carefully exposure-correct the image data and remove contributions of foreground stars, I am able to detect diffuse stellar light from NGC891 to a level 8 magnitudes (or 1500 times) fainter than the night sky background. Using one-dimensional surface brightness profiles extracted perpendicular to the galactic disk, I detect two vertically extended stellar disk components: a "thin disk" with scale height 0.6 kpc; and a "thick disk" with scale height of 1.7 kpc. The thin disk is considerably more extended than the corresponding component in the Milky Way, perhaps because of heating from the more massive molecular cloud layer in NGC891. The origin of the extended thick disk is not completely understood at this time and requires further two-dimensional modeling. Such modeling will also help constrain the presence of an extended stellar halo, which is not detected in this work.

#### Subjects:

Astronomy; Astrophysics; Physics

#### Keywords:

disk galaxies;galactic evolution;galactic halos;galactic structure;NGC891;

Black hole scaling relationships: new results from reverberation mapping and Hubble Space Telescope imaging
Doctor of Philosophy, The Ohio State University, 2007, Astronomy

We investigate the results of reverberation-mapping mass measurements and two-dimensional (2-D) decompositions of high-resolution host galaxy images in the context of black hole scaling relationships. We present new monitoring data leading to an improved black hole mass measurements for the nearby Seyfert galaxy NGC 4151. We also present new monitoring data for the well-studied NGC 5548, which show the active galactic nucleus (AGN) in the lowest luminosity state during a monitoring campaign. We show that the mass derived from this low state is consistent with previous mass measurements, and that the virial relationship previously discovered between the width of the broad lines and their time lags relative to the continuum flux still holds.

We also present complete, 2-D galaxy decompositions for 29 of the 36 objects in the reverberation-mapped sample. We create PSF-subtracted images of the host galaxies and measure the starlight contribution to the luminosity measurements from the original monitoring programs. Accounting for this contribution, we revisit the radius – luminosity relationship for AGNs and find a reduced scatter and a slope of approximately 0.5 that holds over 5 decades in luminosity. This slope is consistent with the naive expectation that all AGNs are simply scaled models of each other.

Finally, we use the galaxy model parameters to investigate several black hole mass scaling relationships with host galaxy properties, namely the black hole mass – bulge luminosity relationship, the black hole mass – bulge mass relationship, and the black hole mass – Sérsic index relationship. We find tight correlations for the black hole mass – bulge luminosity and black hole mass – bulge mass relationships, but these results only marginally agree with previous studies. Rather, they point to an increased black hole mass fraction at lower bulge luminosities and masses than was previously seen. Additionally, we find no evidence for the black hole mass – Sérsic index relationship in this sample of objects. We discuss several differences between the methods employed by various studies and how these may lead to the observed discrepancies in the results.

#### Subjects:

Physics, Astronomy and Astrophysics

Topics in the Physics and Astrophysics of Neutron Stars
Doctor of Philosophy (PhD), Ohio University, 2010, Physics and Astronomy (Arts and Sciences)

In this dissertation, four topics related to the physics and astrophysics of neutron stars are studied. Two first topics deal with microscopical physics processes in the star outer crust and the last two with macroscopical properties of a star, such as mass and radius.

In the first topic, the thermodynamical and transport properties of a dilute gas in which particles interact through a delta-shell potential are investigated. Through variations of a single parameter related to the strength and size of the delta-shell potential, the scattering length and effective range that determine the low-energy elastic scattering cross sections can be varied over wide ranges including the case of the unitary limit (infinite scattering length). It is found that the coefficients of shear viscosity, thermal conductivity and diffusion all decrease when the scattering length becomes very large and also when resonances occur as the temperature is increased. The calculated ratios of the shear viscosity to entropy density as a function of temperature for various interaction strengths (and therefore scattering lengths) were found to lie well above the recently suggested minimal value of (4π)-1 /kB. A new result is the value of (4/5) for the dimensionless ratio of the energy density times the diffusion coefficient to viscosity for a dilute gas in the unitary limit. Whether or not this ratio changes upon the inclusion of more than two-body interactions is an interesting avenue for future investigations. These investigations shed pedagogical light on the issue of the thermal and transport properties of an interacting system in the unitary limit, of much current interest in both atomic physics and nuclear physics in which very long scattering lengths feature prominently at very low energies.

In the second topic, the shear viscosity of a Yukawa liquid, a model for the outer crust of a neutron star, is calculated in both the classical and quantum regimes. Results of semi-analytic calculations in both regimes are presented for various temperatures and densities, and compared with those of classical molecular dynamical simulations performed for the same system by collaborators from Indiana University. For heavy-ion plasmas, as energetically favored in the outer crust of a neutron star, excellent agreement was found between the results of semi-analytic calculations and those of molecular dynamical simulations. However, in the case of light-ion plasmas, substantial differences were found between the results of quantum and classical cases, which underscores the importance of incorporating quantum effects in molecular dynamical simulations, even in the dilute limit.

In the third topic, first steps are taken to reconstruct the uncertain high-density nuclear equation of state from the measured masses and radii of several individual stars. Inherent errors of the measurements are incorporated into the analysis. A new inversion procedure of the Tolman-Oppenheimer-Volkov stellar structure equation is developed so that a model independent dense matter of equation can be derived from observations. Successful tests of the inversion procedure emphasize the need to determine the masses and especially the radii of several individual stars. The aim here is to provide a benchmark equation of state for theoretical advances to be made.

The fourth topic is concerned with the emerging field of gravitational-wave detections and its ability to shed light on the dense matter equation of state. In an external tidal gravitational field, as for example in binary star configurations, each star deforms and acquires a quadrupole moment. The quadrupole polarizability given by the coefficient of proportionality between the induced moment and the field called the tidal Love number after the English mathematician Love. By calculating Love numbers for several model equations of state, connections between the underlying equation of state, star structure and the tidal Love numbers of normal neutron stars and self-bound strange quark matter stars are established. It is shown that the measurement of the Love number from the gravitational signals produced from inspiraling binaries can distinguish between normal and self-bound structures of neutron stars as they are characterized by distinctly different magnitudes of Love numbers.

#### Committee:

Madappa Prakash, Prof. (Advisor); Markus Bottcher, Prof. (Committee Member); Daniel Phillips, Prof. (Committee Member); David Drabold, Prof. (Committee Member); Klaus Himmeldirk, Prof. (Committee Member)

#### Subjects:

Astronomy; Astrophysics; Nuclear Physics; Particle Physics; Physics

#### Keywords:

astrophysics of compact objects; nuclear matter; transport properties; viscosity; tidal Love numbers; equation of state; unitary gas; statistical and thermal physics of dilute system, delta-shell gas with large scattering length; quark matter stars

OVI Absorbers in SDSS Spectra
Doctor of Philosophy, The Ohio State University, 2008, Astronomy

We conducted a systematic search for signatures of the Intergalactic Medium in Quasar spectra of the Sloan Digital Sky Survey, focusing on intervening absorbers via detection of their OVI doublet. We present a search algorithm, and criteria for distinguishing candidates from spurious Lyman lines. In addition, we compare our findings with simulations of the Lyα forest to estimate the detectability of OVI over various redshift intervals. We obtain a sample of 1866 candidates with equivalent widths > 0.05 A in 855 AGN spectra (out of 3702 objects with redshifts in the range for OVI detection). We divide the sample into 3 groups according to the likelihood of being real and potential for follow-up observations. The best group is comprised of 145 candidates, 69 of these reside at a velocity separation > 5000 km/s from the QSO, and can be classified tentatively as intervening absorbers. Most of these have not been picked up by earlier absorption line detection algorithms. This sample increases the number of OVI absorbers at redshifts beyond 2.7 substantially. We propose to obtain observations of some candidates with high signal-to-noise and resolution to better constrain the physical state of the absorbers.

We then focus on 387 sightlines with a S/N > 5.0, allowing for the detections above a rest-frame EW > 0.19 A for the 1032 A component. Accounting for random interlopers, we derive for the first time a lower limit for the redshift number density for z > 2.8. With extensive Monte Carlo simulations we quantify losses of absorbers due to blending with Lyα forest lines, and estimate the success rate of retrieving individual candidates. This allows us to derive the ‘incompleteness corrected’ redshift number density.

We place a secure lower limit for the contribution of OVI to the mass density at the redshifts probed here. We show that the strong lines account for 65% of the mass in the OVI absorbers; weak absorbers, dominant in line number density, do not contribute significantly.

Making conservative assumptions about the ionisation fraction, we derive the mean metallicity of the gas, which is in good agreement with other studies. These results demonstrate that large spectroscopic datasets as SDSS play an important role in QSO absorption line studies, despite their relatively low resolution.

Lastly, we performed a stacking analysis whereby we shift individual spectra to the rest-frame of the absorber, and derive a mean spectrum. Besides further validating the reality of candidates, we use stacked spectra for a variety of purposes. First, we judge effects of additional cuts like a minimal strength for CIV absorption, and produce cleaner subsamples, increasing the strength of proposals for high-resolution studies. Secondly, the stack itself contains information about the gas probed in our search. We have begun to measure absorption features for an OVI+CIV selected subsample, and compared them with multiple transitions found in a CIV-only selected sample. One of the first results is that we probe gas with large OVI column densities lacking many of the lower ionisation lines frequently seen in CIV-selected systems.

#### Committee:

Smita Mathur, Professor (Advisor); Patrick S. Osmer, Professor (Committee Member); David H. Weinberg, Professor (Committee Member); Nathan Rosenstein, Professor (Committee Member)

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

IGM; QSO Absorption lines

Self-Gravitating Eccentric Disk Models for the Double Nucleus of Μ31
Doctor of Philosophy (PhD), Ohio University, 2004, Physics (Arts and Sciences)

We present new dynamical models of weakly self-gravitating, finite dispersion eccentric stellar disks around central black holes for the double nucleus of Μ31. The disk is fixed in a frame rotating at constant precession speed, and is populated by stars on quasi-periodic orbits whose parents are numerically integrated periodic orbits in the total potential. A distribution of quasi-periodic orbits about a given parent is approximated by a distribution of Kepler orbits dispersed in eccentricity and orientation, using an approximate phase-space distribution function written in terms of the integrals of motion in the Kepler problem. We use these models, along with an optimization routine, to fit available published kinematics and photometry in the inner 2 arcseconds of the nucleus. A grid of 24 best-fit models is computed to accurately constrain the mass of the central black hole and nuclear disk parameters. We find that the supermassive black hole in Μ31 has mass Μ BH = 5.62 ± 0.66 × 10 7 Μ , which is consistent with the observed correlation between the central black hole mass and the velocity dispersion of its host spheroid. Our models precess rapidly, at Ω = 36.5 ± 4.2 km/s‾ 1 /pc‾ 1 , and possess a characteristic radial eccentricity distribution, which gives rise to multi-modal line of sight velocity distributions along lines of sight near the black hole. These features can be used as sensitive discriminants of disk structure.

#### Subjects:

Physics, Astronomy and Astrophysics

#### Keywords:

Black Holes; Galaxy Nuclei; Galactic Dynamics; Stellar Dynamics

Quasar Structure from Microlensing in Gravitationally Lensed Quasars
Doctor of Philosophy, The Ohio State University, 2008, Astronomy

I analyze microlensing in gravitationally lensed quasars to yield measurements of the structure of their continuum emission regions. I first describe our lensed quasar monitoring program and RETROCAM, the auxiliary port camera I built for the 2.4m Hiltner telescope to monitor lensed quasars.

I describe the application of our Monte Carlo microlensing analysis technique to SDSS 0924+0219, a system with a highly anomalous optical flux ratio. For an inclination angle cos(i)=0.5, I find an optical scale radius log[r_s/cm] = 14.8±0.4. I extrapolate the best-fitting light curves into the future to find a roughly 45% probability that the anomalous image (D) will brighten by at least an order of magnitude during the next decade.

I expand our method to make simultaneous estimates of the time delays and structure of HE1104-1805 and QJ0158-4325, two doubly-imaged quasars with microlensing and intrinsic variability on comparable time scales. For HE1104-1805, I find a time delay of Delta t_{AB} = 162±6 days and estimate a scale radius of log[r_s/cm] = 15.7±0.5 at 200 nm in the rest frame. I am unable to measure a time delay for QJ0158-4325, but the scale radius is log[r_s/cm] = 14.9±0.3 at 300 nm in the rest frame.

I then apply our Monte Carlo microlensing analysis technique to the optical light curves of 11 lensed quasar systems to show that quasar accretion disk sizes at 250 nm are strongly correlated with black hole mass. The resulting scaling relation is consistent with the expectation from thin disk theory, but it implies that black holes radiate with relatively low efficiency. These sizes are also larger, by a factor of approximately 3, than the size needed to produce the observed 800 nm quasar flux by thermal radiation from a thin disk with the same temperature profile.

Finally, I analyze the microlensing of the X-ray and optical emission of the lensed quasar PG 1115+080. I find that the size of the X-ray emission region is approximately 1.3 dex smaller than that of the optical emission, and I find a weak trend supporting models with low stellar mass fractions near the lensed images.

#### Committee:

Christopher Kochanek, PhD (Advisor); Andrew Gould, PhD (Committee Member); Darren DePoy, PhD (Committee Member)

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

accretion, accretion disks; dark matter; gravitational lensing; quasars: general

Searching for Solar-Type Hypervelocity Stars
Bachelor of Science (BS), Ohio University, 2013, Astrophysics
Hypervelocity stars (HVSs) are thought to be produced via three-body interactions between a binary star system and a supermassive black hole. HVSs are powerful tools studying the structure of the Galactic dark matter halo, and the mass function and population of stars near the Galactic Center. To date, there are 21 known HVSs, almost all of which are assumed to be massive early-type (OBA) main sequence stars. Current detection methods are unable to find low-mass, solar-type HVSs. We use a new method for finding solar-type HVSs which selects candidates on the basis of their proper motions, requiring their velocity vectors to be consistent with an origin in the Galactic Center. Using intermediate-resolution optical spectra for 115 potential HVSs, we measure the three-dimensional galactic rest-frame velocity and position. We have identified 10 strong HVS candidates whose galactic rest-frame velocity is larger than 400 km/s and total velocity vector is consistent with galactocentric origin and 50 candidates whose galactic rest-frame velocity is larger than 400 km/s. Follow-up observations of these candidates are necessary to confirm their distance, and total velocity vector. We have shown it is possible to search for low-mass HVSs using a proper motion survey and Advancements in photometry, astrometry and distance measurements, like the GAIA mission, will pave the way for similar surveys

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

Galaxy: center -- Galaxy: kinematics and dynamics

A 6-Year Study of Long Period Variable Stars in the Globular Cluster NGC 6388
Master of Science (MS), Bowling Green State University, 2017, Physics
We present the results of a 6-year observing campaign conducted using the PROMPT-5 telescope to detect, identify and characterize long-period variable (LPV) stars in the metal-rich globular cluster NGC 6388. LPV stars are asymptotic giant branch stars that exist on a Mira-to-semi-regular-to-irregular continuum in terms of the regularity of their variable behavior. The long time-baseline was found to be fruitful in characterizing that aspect of LPV pulsation that cannot be captured in shorter campaigns. The ISIS image subtraction package is used for variability detection and the production of flux difference photometric data. We have also manually recovered known variable stars with use of their published celestial coordinates. Newly detected sources of variability were ranked on the basis of a variability index. An attempt was also made to quantitatively characterize the positions of LPV stars on the regularity spectrum with a newly defined regularity index, with partial success in the case of known variables. The known and new variables are discussed and characterized and their cluster membership determined with the use of a color-magnitude diagram and the variable’s projected distance from the cluster center. Out of 72 variables studied, two of which were found to be duplicated in the literature, 56 are known variables that were detected or recovered. The LPVs detected number to 26 and 16 known and new, respectively. Almost all LPVs’ periods are determined using the period-dispersion minimization method. A list of suspected variables, which require higher-resolution observations to confirm, amounts to an additional 22 stars. A period-luminosity relation is determined using the available literature data and our own data.

#### Committee:

Andrew Layden, Dr. (Advisor); John Laird, Dr. (Committee Member); Dale Smith, Dr. (Committee Member)

#### Subjects:

Astronomy; Astrophysics; Physics

#### Keywords:

LPV; Mira; Semiregular; Irregular; long period variables; variable star; globular cluster; flux difference photometry; astronomy; AGB star; NGC 6388; ISIS; image subtraction;

Master of Science (MS), Wright State University, 2017, Physics
Atmospheric turbulence has afflicted accurate observations of celestial bodies since man first gazed upon the stars. In this past century, the technology of adaptive optics was invented to help compensate for the optical distortions that atmospheric turbulence causes. As part of that technology, artificial guide stars, wave front sensors, deformable mirrors, and other optical components were developed to correct these wave aberrations. The purpose of this study focuses on the modeling and configuration of an adaptive optics system that is appropriate for the John Bryan Observatory Quad Axis Telescope System (JBO-Q), which is funded by the United States Air Force. Scaling law modeling of site-specific atmospheric parameters using numerical weather data and laser propagation theory was used determination and optimization of some critical system specifications and threshold parameters for this baseline model.

#### Committee:

Jerry Clark, Ph.D. (Advisor); Jason Schmidt, Ph.D. (Committee Member); Elizabeth Beecher, Ph.D. (Committee Member)

#### Subjects:

Astronomy; Atmosphere; Atmospheric Sciences; Engineering; Optics; Physics

#### Keywords:

Adaptive Optics; John Bryan Observatory; Atmospheric Turbulence Compensation; LIDAR; Air Force Research Lab; Fried Parameter; Greenwood Frequency; Rytov Number; Baseline specifications for AO system

Improving the Sensitivity of a Pulsar Timing Array: Correcting for Interstellar Scattering Delays
BA, Oberlin College, 2017, Physics and Astronomy

The NANOGrav collaboration aims to detect low frequency gravitational waves by measuring the arrival times of radio signals from pulsars. A confirmation of such a gravitational wave signal requires timing tens of pulsars with a precision of better than 100 nanoseconds for around 10 – 25 years. A crucial component of the success of pulsar timing relies on understanding how the interstellar medium affects timing accuracy. Current pulsar timing models account only for the large-scale dispersion delays from the ISM. As a result, the relatively small-scale propagation effects caused by scattering are partially absorbed into the dispersion delay component of the model.

In this thesis we developed a model that accounted for both dispersion delay and scattering delay. In addition to the two observable quantities used in the NANOGrav model, we also included the slopes of those two observables. We then simulated data describing motion of a pulsar through the interstellar medium over 11 years. We used a weighted linear least squares formalism to solve the system of four equations and two parameters at every epoch of measurement in order to remove the effects of dispersion and scattering from the data as fully as possible. This model was successful at removing these effects.

#### Subjects:

Astronomy; Astrophysics; Physics

#### Keywords:

Interstellar medium; pulsars; gravitational waves; data analysis; gaussian process regression; dispersion; scattering

Microwave Spectroscopy: From the Lab to the Stars
Doctor of Philosophy, The Ohio State University, 2017, Physics
In the past few decades, microwave spectroscopy has proven to be a very valuable and versatile tool. The important scientific contributions made in this work as well as the variety in the types of problems explored serves as a prime example of such. In the field of chemical sensing, much interest has been drawn towards utilizing microwave spectroscopy to distinguish molecular species. The submillimeter (SMM) spectral region is attractive in terms of being able to obtain a higher degree of specificity compared to that of infrared or optical. This is due to the relatively narrow Doppler line widths that microwave spectral lines exhibit. In this work, we developed a technique called Cavity Based Medium Resolution Spectroscopy (CBMRS), where we used a Fabry-Perot cavity to measure the absolute absorption spectra of large molecules. The spectra of large molecules usually contain broad \enquote{quasi-continuum} features, and measuring these features pose a significant challenge in traditional Doppler limited spectroscopy. This is due to the fact that standing waves that are present in a system are not always discernible from broad spectral features. In CBMRS, we calculated an absorption coefficient at a spectral point by measuring the changes in the widths of the cavity resonances when the cavity was loaded with analyte. This technique allows us to measure an absorption spectrum regardless of the power emitted at the source, the sensitivity of the detector, or the efficiency of coupling power into the cavity. We also successfully implemented CBMRS to function as a chemical sensor by measuring the spectra of of a mixture of gases and identifying the concentrations of the constituent components of the mixture. The SMM region has attracted much interest in astronomy as well. For astronomers interested in finding new molecular species in the interstellar medium (ISM), the SMM proves to be quite favorable. This is due to the fact that microwave radiation can more easily penetrate interstellar dust due to its longer wavelength. Also, the blackbody emission of these clouds tend to be in the temperature range such that the peaks emission falls in the SMM. Studies along these lines have been made possible by the rapid growth in the accessibility of the SMM region over the past couple of decades. Telescopes operating in this regime have also significantly improved in terms of spectral coverage and sensitivity. However, greater sensitivity of telescopes has also brought about a significant challenge. Many of the lines that can be observed belong to a group of molecules called \enquote{astrophysical weeds}. These molecules are abundant but have lines that are difficult to assign due to perturbed rotational transitions and low-lying vibrational states. This has left the quantum mechanical catalogs incomplete. In this work, we used line list catalogs generated in the laboratory to assign lines belonging to astrophysical weed molecules by analyzing intensity-calibrated spectra over a range of temperatures. Using the line list catalogs, we identify lines belonging to these weed molecules in the astrophysical data and determine the physical conditions in which the molecules are found. Then, by using these determined physical conditions, we can predict the spectra of these molecules in the interstellar clouds. This allows astronomers to focus on the remaining features in the astrophysical data to explore further as potential candidates for undiscovered species.

#### Committee:

Frank De Lucia (Advisor); Douglass Schumacher (Committee Member); Gregory Lafyatis (Committee Member); Robert Perry (Committee Member)

#### Subjects:

Astronomy; Astrophysics; Physics

#### Keywords:

Microwave Spectroscopy; Physics

Analyzing the Information Content in Gravitational Shadows
Doctor of Philosophy, The Ohio State University, 2016, Physics
In cosmology, we study the universe through the light we observe from distant sources. For this dissertation, I aim to describe my work in helping advance our understanding of the universe through the development of tools for the Dark Energy Survey and related work in the field of weak lensing. I start with an introduction to cosmology and the theory behind our understanding of the Universe. This introduces the idea of Dark Energy and Dark Matter, which together with General Relativity explains the universe as we know it. Following the introduction, I cover various observational probes in cosmology, many of which are used in the Dark Energy Survey. These probes naturally break down into two camps: probes that primarily measure the evolution of distance scales in the universe, and probes that relate to the growth of structure, i.e. the development of galaxies, galaxy clusters, and the clustering of matter. On large scales and at early times the evolution of the universe can be adequately described with linear perturbation theory; similarly, in the non-linear regime methods for predicting the number density and formation history of Dark Matter haloes. The trouble lies in making predictions in the non-linear regime and accurately describing the shapes of the Dark Matter haloes. For this, we rely on simulations of the universe, and I describe the current simulation tools used in cosmology today. Further, I explain how one extracts Fisher information from these simulations and detail a correction factor that I use in my own predictions. From this, I illustrate the details of the Dark Energy Survey and my work on the Dark Energy Camera (DECam). This optical survey will image 5000 square degrees of the Southern Hemisphere in 5 different filter bands and aims to measure the properties of approximately 300 million galaxies. The tools I developed for DECam are used in the daily operations of the telescope both for the Dark Energy Survey and other observations made by community observers. Finally, I describe the work I have done running simulations for an analysis of the Fisher information available in the 1-point weak lensing convergence probability distribution. Two point measurements of the weak lensing field are the standard techniques for measuring cosmological information. However, I demonstrate that the 1-point weak lensing information supplements this by breaking degeneracies in the cosmological parameters. In $\Omega_m$-$\sigma_8$ parameter plane I show that adding the 1-point statistics can provide an improvement in constraining area by greater than a factor of 2.

#### Committee:

Klaus Honscheid (Advisor); David Weinberg (Committee Member); Amy Connolly (Committee Member); Thomas Humanic (Committee Member); Rene Anand (Committee Member)

#### Subjects:

Astronomy; Astrophysics; Physics

#### Keywords:

cosmology; weak lensing; Dark Energy Survey

A Spatially Resolved Survey of the Mid-Infrared Aromatic Features in Nearby Galaxies
Doctor of Philosophy, University of Toledo, 2016, Physics
Mid-infrared spectra of almost all astronomical objects reveal a rich array of emission features largely attributed to the IR florescence of polycyclic aromatic hydrocarbons (PAHs). These features can account for a significant portion of the total infrared luminosity but exhibit a dependence on their local environment, including metalicity, that is not yet well understood. We present deep 5 to 38 um spectroscopic data taken with the SL and LL IRS instrument of the Spitzer space telescope of three high radial abundance gradient nearby galaxies: NGC5457, NGC0628, and NGC2403. Available data are subdivided into 13" extraction regions in order to maximize the resolution of our analysis while still presenting a reasonable signal to noise. We measure the strength of the PAH features in each sub-region by performing a multi-dimensional fi t of the underlying continuum due to aggregate dust temperatures, PAH features, and electronic transitions using the IDL routine PAHFIT. We also present Herschel PACS 70,100,160, GALEX FUV and NUV data on our targeted regions. We show for the first time that the well known correlation of total PAH intensity to total Infrared intensity and oxygen abundance on galactic scales also hold on sub kpc scales within these galaxies. We examine the impact that the local abundance, intensity and hardness of the radiation field has on PAH intensity and the physical conditions implied by PAH band ratios and that all play a significant role.

#### Committee:

J.D. Smith, Dr. (Advisor); Chandar Rupali, Dr. (Committee Member); Lawrence Anderson-Huang, Dr. (Committee Member); Jacques Amar, Dr. (Committee Member); Karin Sandstrom, Dr. (Committee Member)

#### Subjects:

Astronomy; Astrophysics; Physics

#### Keywords:

infrared; spectroscopy; PAH; PAHFIT

Lighting the dark molecular gas and a Bok globule
Doctor of Philosophy, University of Toledo, 2016, Physics
Stars are the building blocks of galaxies. The gas present in galaxies is the primary fuel for star formation. Galaxy evolution depends on the amount of gas present in the interstellar medium (ISM). Stars are born mainly from molecular gas in the GMCs. Robust knowledge of the molecular hydrogen H2 gas distribution is necessary to understand star formation in galaxies. Since H2 is not readily observable in the cold interstellar medium (ISM), the molecular gas content has traditionally been inferred using indirect tracers like carbon-monoxide (CO), dust emission, gamma ray interactions, and star formation efficiency. Physical processes resulting in enhancement and reduction of these indirect tracers can result in misleading estimates of molecular gas masses. My dissertation work is based on devising a new temperature power law distribution model for H2, a direct tracer, to calculate the total molecular gas mass in galaxies. The model parameters are estimated using mid infrared (MIR) H$_{2}$ rotational line fluxes obtained from IRS-Spitzer (Infrared Spectrograph- Spitzer) instrument and the model is extrapolated to a suitable lower temperature to recover the total molecular gas mass. The power law model is able to recover the dark molecular gas, undetected by CO, in galaxies at metallicity as low as one-tenth of our Milky Way value. I have applied the power law model in U/LIRGs and shocks of Stephan's Quintet to understand molecular gas properties, where shocks play an important role in exciting H2. Comparing the molecular gas content derived through our power law model can be useful in studying the application of our model in mergers. The parameters derived by our model is useful in understanding variation in molecular gas properties in shock regions of Stephan's Quintet. Low mass stars are formed in small isolated dense cores known as Bok globules. Multiple star formation events are seen in a Bok globule. In my thesis I also studied a Bok globule, B207, and determined the physical properties and future evolutionary stage of the cloud. My thesis spans studying ISM properties in galaxies from kpc to sub-pc scales. Using the power law model in the coming era of James Webb Space Telescope (JWST) with the high sensitivity MIR Instrument (MIRI) spectrograph we will be able to understand the properties of molecular gas at low and high redshifts.

#### Subjects:

Astronomy; Astrophysics

#### Keywords:

H2 lines, Infrared, Molecular gas, IRS-Spitzer, dark molecular gas, Bok globules, coreshine, scattering, dust emission, LDN 1489