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<p> Dwarf galaxies present interesting observational challenges for the studies of various galaxy properties: despite their abundance and proximity to the Milky Way, they typically have very low surface brightnesses and small physical sizes. Until now, only the extreme variety of dwarfs &mdash; those undergoing strong bouts of star formation &mdash; have been observed in the FIR, due to observational difficulties. However, this population does not represent the majority of dwarfs, which have only moderate star formation rates and extremely low metallicity (the fraction of heavy elements to hydrogen). The advent of the Herschel Space Telescope, with its superior resolution and sensitivity over previous generations of telescopes, has made it possible to measure FIR spectral lines and broadband continuum in normal dwarf galaxies, expanding the scope of studies beyond the brighter, but more extreme, varieties. </p><p> The general goal of my research was to study the conditions in the interstellar media (ISM) of typical dwarf galaxies. The LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, TheHI Nearby Galaxy Survey) project aims to unravel many mysteries of nearby dwarfs using a suite of multi-wavelength data, and the new additions from <i>Herschel</i> help provide insight into the physics of these systems. I reduced and analyzed FIR fine-structure spectral line data for the LITTLE THINGS sample to study the different phases of the ISM, as well as FIR photometry data to access the dust properties and infrared continuum emission in these systems. The FIR spectral lines are diagnostics for the conditions in the ISM of galaxies, telling us about heating efficiency, the fraction of gas that resides in photodissociation regions (PDRs), abundance of highly ionized gas from massive stars, and other physical descriptions. The photometric continuum observations enable the modeling of interstellar dust properties &ndash; dust plays an important role in shielding and cooling molecular clouds which form stars, as well as heating via the photoelectric effect. I also utilized neutral hydrogen data to probe the neutral medium in relation to the FIR, as well as optical and UV data to characterize star formation and the emission of starlight.</p>

The strong radio emission from Sgr A*, an object located at the dynamical center of the Milky Way, has been attributed to accretion of interstellar gas by a supermassive compact object or dense cluster of objects. We show that any dynamically stable cluster of objects cannot compress the ambient magnetic field or heat the accreting gas sufficiently to reproduce the spectrum of Sgr A*, reaffirming the paradigm that Sgr A* is a single supermassive black hole. We investigate how such a black hole would interact with its surroundings and attempt to determine observational consequences of this interaction. The complexity of the gas, dust, and stellar dynamics of the central parsec of the Galaxy complicates this problem, however. Focusing our attention on the black hole itself but being constrained by observations of the surrounding gas and stars, we have constructed models of the accretion process. We examine two types of accretion models. The first, involving a cold, massive, fossilized accretion disk, is found to generate too much infrared radiation as infalling gas impacts the disk. The second model is spherical accretion, in which the radio emission from Sgr A* is dominated by magnetic bremsstrahlung. Such a model requires accurate emissivities for a wide range of temperatures and field strengths. In this work, we derive the magnetic bremsstrahlung emissivities and apply them to the spherical accretion model, yielding a spectrum that is fully consistent with the radio emission from Sgr A*. We empirically determine a magnetic field profile that suggests the presence of other phenomena, such as a central magnetic dynamo. In addition, the model predicts that the observed high energy emission from the Galactic Center region is not dominated by emission from the central black hole and its environs.

This dissertation clarifies two aspects of neutron-star formation and evolution: the formation of neutron stars through the accretion-induced collapse of white dwarfs and the common envelope evolution of neutron stars. In both cases, we utilize a 1-D lagrangean and 2-D SPH hydrodynamics codes equipped with a broad range of physics including neutrino emission, absorption, and transport, general relativity and dense equations of state. These results are then applied to a Monte-Carlo population synthesis code to study the effects of kicks placed upon neutron stars near the time of their formation. By comparing these results with the current observational data, we find that a bimodal kick distribution of neutron-star kicks is required.

The magnetic field dynamo in the inner part of accretion disks around supermassive black holes in AGNs may be an important mechanism for the generation of magnetic fields in galaxies and in extragalactic space. We consider dynamo with the necessary helicity generation produced by star-disk collisions. Gas heated by a star passing through the disk is buoyant and form rising and expanding plume of plasma. Due to Coriolis forces the flow produced by plumes have coherent helicity. This helicity is the source of alpha effect in the alpha-O dynamo in differentially rotating accretion disk. We apply the mean field dynamo theory to the ensemble of plumes produced by star-disk collisions. We demonstrate the existence of the dynamo and evaluate the growth rate of magnetic field. The estimate of the nonlinear saturated state of the dynamo gives the magnetic field exceeding equipartition with the thermal energy in the accretion disk. Thus, star-disk collision dynamo can be important in generating dynamically significant magnetic fields, which could alter the disk structure and be the source of the energy flow in extragalactic jets. We present results of numerical simulations of the kinematic dynamo produced by star-disk collisions. It was found that for about one star-disk collision per period of rotation of the inner edge of an accretion disk, the typical value of the threshold magnetic Reynolds number is of the order of 100. The generated mean magnetic field has predominantly even parity. We also present theoretical consideration of magnetic dynamo in New Mexico dynamo experiment, which is currently under construction. The experiment utilizes Couette flow and driven jets of liquid sodium to simulate astrophysical alpha-O dynamos in the laboratory. We perform numerical simulations with ideally conducting boundary and evaluate the changes, which vacuum boundary conditions introduce in our numerical results. We also develop the theory of the MHD Ekman boundary layer in differentially rotating conducting fluid. The Ekman layer is formed at the end plates in the experiment. We show that the Ekman layer does not influence the generation of the large scale magnetic field in the experiment.

The Yarkovsky effect is a change in a body's orbit caused by its reaction to the momentum carried away by the thermal photons that it emits. This effect may play a key role in the orbital evolution of asteroids and near-Earth objects. To evaluate the Yarkovsky acceleration under a wide range of conditions, I have developed a three-dimensional finite-difference solution to the heat equation. This approach employs neither the linearized boundary conditions, the plane-parallel heat flow approximation, nor the assumption of fast rotation used in earlier approaches (Rubincam, 1998; Vokrouhlickỳ and Farinella, 1998). Thus it can be used to explore a wide range of orbital elements and physical properties that had not been previously accessible. I use the finite-difference approach to compute Yarkovsky perturbations for homogeneous, spherical stony bodies with 1-, 10- and 100-m diameters. For a 1-m scale body rotating with a 5-h period, the semimajor axis can change as much as 1 AU in 1 Myr and the eccentricity can change as much as 0.1 in 1 Myr. These rates are much faster than any found previously because those treatments were not valid for very eccentric orbits. For rotation periods expected to be more typical of such small bodies, these rates would be considerably slower. Nevertheless, there is no data concerning rotation rates for small bodies so these fast rates may be relevant. Yarkovsky drift rates are computed for models of specific near-Earth asteroids, demonstrating that the shape of a body is important in computing its precise Yarkovsky effect. Such calculations may be useful for assessing observable Yarkovsky perturbations and in predicting and mitigating NEA hazards. The approach presented in this dissertation is the only current one with the potential to rigorously treat bodies with arbitrary shapes.

We present the first results from the F̲aint I̲nfra-R̲ed E̲xtragalactic S̲urvey (FIRES) of the Rubble Deep Field (HDF) South. Using a combination of very deep ground-based near infrared (NIR) data with the WFPC2 Hubble Space Telescope data, we constructed a K(s)-band selected sample to K(s,AB) ≤ 26.0. To interpret this data, we developed a new photometric redshift technique and tested it using spectroscopic redshifts in the HDF-N and HDF-S. Our accuracy was Δz/(1 + z) ≈ 0.07 for z < 6. We derived realistic error estimates in z(phot) by accounting both for template mismatch and for the dependence of the redshift uncertainty on the photometric errors. We estimated the rest-frame optical luminosities from an initial NIR data set and found 90 times more galaxies at 2 < z < 3.5 and Lʳᵉˢᵗ(B) > 5 x 10¹⁰ h⁻² L(⊙,B) than are expected from local luminosity functions. This discrepancy can be explained if L*, B increases by a factor of 2.4-3.2 with respect to locally determined values. Using all available NIR data in the HDF-S, we then derived the rest-frame colors (U - B)(rest), (B - V)(rest), and (U - V)(rest) of all galaxies with K(s,AB) < 25. Eight of the 12 rest-frame optically reddest galaxies at 2 < z(phot) < 3.2 would have been missed by the U-dropout selection criteria. Three of the galaxies at z > 2 have strong rest-frame optical breaks with colors corresponding to those of present day Sbc's. Using theoretical relations between the color and stellar mass-to-light ratio M/L , we estimated the M/L and stellar mass M . Using these estimates, we found that the most massive galaxies at any redshift are those with the reddest rest-frame colors and those that would be missed by the U-dropout technique. We also found that the stellar mass budget at z < 3.2 has significant contributions from galaxies redder than local Scd's. There are, however, large uncertainties in the M/L analysis and we have a relatively small field. Confirmation of these results will require additional modeling, observations over a larger area, and extensive spectroscopic follow up. We found an intrinsically bright (Lʳᵉˢᵗ(V) =5.10x10¹⁰ h⁻² L(⊙,V)) U-dropout galaxy in the HDF-S with z(spec) = 2.793. This galaxy has an extended ring-like morphology, and a co-moving diameter of ≳ 9.4 h⁻¹ Kpc for a Ω(M) = 0.3, Ω(Λ) = 0.7 cosmology. The light profile appears more centrally concentrated and symmetric at longer wavelengths and this object may have an older population superimposed on a star-forming disk.

<p> Mergers of a neutron star and a black hole are interesting because of the dual complexity of the black hole's strong gravity and the neutron star's nuclear-density fluid. Mergers can yield short-lived nuclear accretion disks, emitting copious neutrinos. This radiation may change the thermodynamic state of the disk itself, may drive an ultrarelativistic jet of electrons and positrons, may oscillate in its flavor content, may irradiate surrounding matter, playing a role in nucleosynthesis, and may be detected directly. </p><p> In this thesis I present a model of such a merger, its remnant accretion disk, and its neutrino emission. In particular, we evolve a neutron star&mdash;black hole merger through &sim;100 ms, solving the full general relativistic hydrodynamics equations, from inspiral through merger and accretion epochs. We treat the neutrinos approximately, using a leakage framework, which accounts for local energy losses and composition drift in the fluid due to escaping neutrinos. We use geodesic ray tracing on a late time slice of the model to calculate the full spatial-, angular-, and energy-dependence of the neutrino distribution function around the accretion disk. This distribution then serves in a computation of the energy available to form a jet via neutrino-antineutrino annihilation in the disk funnel. In this scenario, we find that enough energy is deposited to drive a jet of short-gamma-ray-burst-energy by neutrino processes alone.</p>

I present moderate resolution spectra for 39 Quasi-Stellar Objects (QSOs) at z ≈ 2 obtained at the Multiple Mirror Telescope (MMT). These are combined with spectra of comparable resolution of 60 QSOs from the literature with z > 1.7 to investigate the distribution of Lyman α (Ly-α) forest absorption lines in redshift and equivalent width. I find γ = 1.88 ± 0.22 for lines stronger than a rest equivalent width of 0.32 Å, where γ is the line redshift distribution parameter, in good agreement with some previous studies. These spectra are used to measure J(ν₀), the mean intensity of the extragalactic background radiation at the Lyman limit, using the proximity effect signature. I find J(ν₀) = 7.0⁺³·⁴₋₄.₄ x 10⁻²² ergs s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ at 1.7 < z < 3.8. A sample of 151 QSO spectra from the Faint Object Spectrograph on the Hubble Space Telescope are used to measure J(ν₀) at low redshift. I find J(ν₀) = 6.5⁺³⁸₋₁.₆ x 10⁻²³ ergs s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ at z < 1, and J(ν₀) = 1.0⁺³·⁸₋₀.₂ x 10⁻²² ergs s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ at z > 1, indicating that J(ν₀) is evolving over 0.03 < z < 3.8. This work confirms that the evolution of the number density of Ly-alpha lines is driven by a decrease in the ionizing background from z ∼ 2 to z ∼ 0 as well as by the growth of structure in the intergalactic medium and the formation of galaxies from intergalactic gas. These measurements of J(ν₀) are in reasonable agreement with the predictions of models based on the integrated quasar luminosity function. I present simulated Ly-α forest spectra created using the lognormal approximation to the linear and mildly non-linear evolution of the density and velocity fields. The model spectra give a mean Ly-α forest flux decrement of 0.128 at < z >= 2.07, while the MMT data show < D >= 0.129. The photoionization effects of quasars placed in the simulated density fields on the surrounding intergalactic medium are incorporated into the synthetic spectra. This reasonably reproduces the proximity effect signature seen in the data, a 2-3σ deficit of absorption lines within 2h⁻¹ Mpc of quasars. I find that maximum likelihood methods reliably estimate the ionization rate from the UV background radiation if quasars do not preferentially occupy regions of high overdensity. I analyze the extent to which the clustering of mass around quasars and uncertainty in quasar redshifts will bias the measurement of the ionizing background. In both cases, the ionization rates are overestimated by a factor of ∼3.