Post-T Tauri stars (PTTSs) are low-mass, pre-MS stars which have ceased accreting, and are not necessarily near star-forming molecular clouds. Historically, they have been difficult to identify due to their benign spectroscopic signatures. With recent all-sky X-ray surveys and proper motion catalogs, it is now possible to find PTTSs in large numbers. The nearest PTTSs will be important targets for future imaging surveys characterizing dust disks and planetary systems around young solar analogs. The goal of this work is to systematically identify samples of PTTSs, investigate the evolution of circumstellar disks, to infer the fossil star-formation history of molecular clouds, and to estimate kinematic distances to young stars lacking trigonometric parallaxes. We present the results of a spectroscopic survey which identified 110 PTTS members of the nearest OB association (Sco-Cen). We find that 2/3rds of the low-mass star-formation in each OB subgroup occurred in <5 Myr, and that only ∼1% of solar-type stars with mean age ∼13 Myr shows signs of accretion from a circumstellar disk. In order to assess how long circumstellar material is detectable around PTTSs, we conducted a 10 μm imaging survey of post-T Tauri members of the ∼30-Myr-old Tuc-Hor association. The goal was to find evidence of either remnant accretion disks or dusty debris disks with orbital radii of ≲10 AU. Combined with data from other surveys, we conclude that mid-IR emission from warm dust grains in the terrestrial planet zones around young stars become undetectable compared to the stellar photosphere for nearly all stars by age ∼20 Myr. Lastly, we present a technique for calculating distances isolated young field stars that currently lack trigonometric parallax measurements. The technique is a generalization of the classical cluster parallax method, but can handle anisotropic velocity dispersions and non-zero Oort parameters. Distances and isochronal ages are estimated for a subsample of PTTSs included in the Formation and Evolution of Planetary Systems (FEPS) Spitzer Space Telescope (SST) Legacy Science program. The techniques developed in this thesis will allow one to efficiently conduct a systematic survey to identify the nearest, youngest stars to the Sun using existing databases.

The heliosphere, the volume inflated by the solar wind, is impenetrable to inter-stellar plasma, except for high-energy galactic cosmic rays. Neutral components of the local interstellar medium (LISM), however, do enter at the speed of Sun's relative motion to LISM. On their journey through the heliosphere, interstellar neutrals are subject to ionization by solar wind protons and electrons, solar photons, and also the gravitational pull of the Sun. Once ionized, the newly created ions, called "pickup ions" (PUIs), are swept out by the solar wind toward the termination shock (TS). Helium atoms, having a higher ionization potential, penetrate deeper into the solar system, with their trajectories gravitational focused to form a cone of high concentration of LISM He in the wake of the Sun's motion through LISM. This He cone provides a rich source of He⁺ PUIs. Recent theories suggest that PUIs are the seed particles for the detected anomalous cosmic rays (ACRs)--singly-charged ions highly enriched in He, N, O, and Ne, and with energies < 50 MeV/nuc after solar modulation. To reach such high energies, it would be necessary that the PUIs be pre-accelerated before reaching the TS, where the ions are finally accelerated to become ACRs. This thesis investigates two aspects of the PUIs life cycle by using the data gathered by Solar Wind Composition Spectrometer (SWICS) on the Advance Composition Explorer (ACE). The first is a study of pre-acceleration of He⁺ PUIs up to 100 KeV, by shocks generated by coronal mass ejecta. Our result implies that quasi-parallel shocks are more efficient at accelerating He⁺ PUIs in this energy interval than quasi-perpendicular shocks. The second is a study of time-variability of the gravitationally focused He cone. Our results suggest that the most likely cause of observed variability is due to the changing ionization rate of He over the solar cycle.

Supermassive black holes are ubiquitous in nearby galaxies. The strong correlations between black hole masses and their host galaxy bulges suggest they are intimately connected. To understand their coevolution we study quasars where both quantities can be probed out to high redshifts. To overcome the well known obstacles in studying quasar hosts at z > 1, we study 28 gravitationally lensed host galaxies, located at 1 ≤ zs ≤ 4.5, which are stretched out into arcs and Einstein rings. Applying two new algorithms, GALFIT and LENSFIT, to images obtained in the HST NICMOS F160W filter, we clearly resolve the host galaxies. Many have evidences of multiple components, interaction, offset galaxy components, or bulges and disks. The host galaxies at z > 1 are mostly brighter than L*V galaxies today, but would become fainter than L*V today after accounting for passive evolution. Furthermore, they have modest sizes (Re ≲ 6 kpc), and the profiles of the hosts are roughly equally split between bulge dominated and disk dominated. Due to these evidences, the quasar hosts may not be fully evolved early-type galaxies undergoing passive evolution if they evolve into L*V galaxies today. Moreover, comparing the hosts of radio-loud quasars and radio-quiet quasars, there is not a significant difference in their luminosities. Finally, we study the bulge luminosities (L(bulge)) and black hole masses (M(BH)) at z ≈ 1 and z ≈ 2, finding that the hosts at z > 2 already lie near the same L(bulge) vs. M(BH) relationship as for z = 0 normal galaxies . Accounting for an early-type galaxy evolution, they would fade below the relationship at present day. Therefore, the hosts at z ≈ 2 must undergo a stellar mass buildup by a factor of 3-5, if they evolve into early-type galaxies. This implies their M(BH)/M(bulge) ratio at z ≈ 2 is a factor of 3 higher than today. On the other hand, the L(bulge)-M(BH) relation for hosts at z ≈ 1 is consistent with early-type galaxies undergoing fading. Thus the parent population of the hosts at z = 2 may not be not the same as those at z = 1; the z = 2 hosts may either be progenitors of more typical late-type galaxies today, or must undergo significant stellar mass buildup.

In this thesis, we study the relationship between the angular momentum and shape of galactic disks, satellite galaxies, dark matter halos, and large scale structure using N-body simulations in the context of current models of galaxy formation. In warped galactic disks, the angular momenta of the inner and outer disk are misaligned. We have calculated the torques a misaligned halo imparts on an embedded galactic disk. N-body simulations of disks subject to torques of this strength indicate that the disk tilts in response and develops a trailing warp of the same magnitude as the Milky Way warp. We have investigated whether the warp of the Milky Way's disk is caused by nearby satellite galaxies. The misaligned warp angular momentum is anti-aligned with the orbital angular momentum of the Sgr dSph, and is of the same magnitude. This suggests that Sgr is responsible for the warp. However, N-body simulations of such disk-satellite interactions indicate that the warps excited by Sgr with its current mass and orbit are much smaller than the warp of the Milky Way. The alignment of the shapes and angular momenta of dark matter halos and the large scale environment has been studied in cosmological dark matter simulations. We have analyzed several late snapshots of such a simulation and found rotation of the triaxial figure of the halos. The figure rotates about the minor axis in most cases, at a rate that follows a log-normal distribution centred on Ωp = 0.148 h km s⁻¹ kpc⁻¹. Halos have triaxial shapes that become more spherical at larger radii. The principal axes of individual halos show strong internal alignment, as does the angular momentum, which is usually oriented along the minor axis. This alignment is not perfect, and the median misalignment is large enough to cause galactic warps. The minor axes of halos tend to point perpendicular to filaments and sheets. Major axes show a weaker tendency to point along filaments. These alignments are much stronger for higher mass halos. The angular momenta of galaxy mass halos tend to point along filaments and sheets, while those of group mass halos point perpendicular to the surrounding mass distribution.

Io's mountains are cataloged in order to investigate their formation. Of the 101 mountains imaged with sufficient coverage and resolution for further analysis, 4 are volcanoes, and 97 are tectonic massifs. Of the 97 tectonic mountains, ≥40 abut paterae (volcanic or volcano-tectonic depressions). This juxtaposition is unlikely to be coincidental as the probability of it occurring by chance is ∼1.08%. The observed mountain-patera association may be due to orogenic faults acting as conduits for magma ascent, thus fueling patera formation near mountains. As resurfacing buries a shell of material from Io's surface to the base of the lithosphere, its effective radius is reduced and it heats up. The volume change due to subsidence and thermal expansion is calculated as a function of lithospheric thickness. Conservation of volume dictates that this material is uplifted at Io's surface. By estimating the total volume of mountains, Io's average lithospheric thickness is constrained to ≥12 km. A kinematic analysis of Nefertiti Corona, Venus, reveals that the corona's interior moved east as a relatively coherent thrust sheet with most deformation occurring on the distal margin. Additionally, an en-echelon array indicates a history of semi-brittle deformation on the northern side of Nefertiti's tectonic annulus. Regional heating from the thermal diapir that formed Nefertiti probably reduced the crustal viscosity and enabled the semi-brittle deformation. The "Odessa Craters" in the Channeled Scabland of eastern Washington State are basaltic ring structures (BRSs) 50-500 m in diameter that are comprised of discontinuous, concentric outcrops of subvertically-jointed basalt and autointrusive basaltic dikes. It is postulated that they formed as follows: phreatovolcanic activity disrupted a relatively thin, active lava flow forming rootless cones; the lava flow inflated around the cones; tensile stresses caused concentric fracturing; dikes exploited the fractures and fed lava to the surface; and subsequent erosive floods excavated the structures. A second population of BRSs near Tokio Station, WA, are morphologically analogous to quasi-circular structures in Athabasca Valles, Mars (a region that is geologically similar to the Channeled Scabland). If the martian features formed as BRSs, then they indicate local water-lava interactions and at least two floods through Athabasca Valles.

We present a study of a sample of starburst galaxies. The systems chosen for study range in age and strength. We have obtained high-quality infrared spectroscopy and imaging of 18 starburst galaxies and 4 early-type galaxies as a comparison sample of an old stellar population. The spectra consist of high-resolution data (R ∼ 3000) in the H (1.65μm) and K (2.2μm) bands and, in a few cases, the J (1.25μm) band; most of the J-band spectra were obtained at a resolution of 800. We obtained broadband images of all the galaxies in the J, H, and K bands, as well as narrowband images of several of the galaxies in the (1.0)S(1) line of H₂ and in Brγ. We use these data and data from the literature to constrain models of the nuclear starbursts for these galaxies. The high sensitivity and resolution of the spectra plus the use of a long slit allows us to derive kinematic properties of the nuclear region in addition to other constraints on the starburst population, such as ionizing flux and CO index. We compare the properties of these galaxies and suggest that a burst of star formation can account for the galaxy properties and that the range of properties observed is derived from a range of burst ages and strengths. Our high-quality data allow us to examine the properties of the (Fe II) and H₂ emission regions. We measure, for the first time, the density in the (Fe II) -emitting region of a starburst galaxy and find that this density is consistent with the origin of this emission in supernova remnants. We find, in contrast to earlier studies, that fluorescence plays a large role in the H₂ emission of some starburst galaxies.

A new λ = 1.3 mm polarimeter, Cyclops, was constructed to make observations of dust continuum emission from star formation regions. The polarization of the inner arcminute of DR 21 was mapped with Cyclops. The polarization percentage and position angle are remarkably constant, indicating a uniform magnetic field throughout the cloud. Turbulent gas motions are a more significant source of support against self gravity in the cloud core than thermal pressure or magnetic fields. The polarization toward the cloud core increases slightly from λ = 100 μm to λ = 2 mm and is consistent with the standard dust composition of silicates and graphite. A small continuum polarization survey of cloud cores with embedded protostars was made with Cyclops and combined with observations from the literature. There is no clear tendency for any preferred alignment of cloud core elongations with respect to magnetic field lines, especially for the bright, high mass star forming regions. This confirms that the massive cloud cores are magnetically supercritical. The magnetic field lines appear randomly oriented with respect to the local Galactic plane position angles, implying that the random component of the Galactic magnetic field dominates the spiral component in this sample. Three-σ upper limits of 0.4%, 1.2%, and 1.2% were placed on the polarization of the HCO⁺ J = 1-0 emission line from the DR 21 and Mon R2 molecular outflows, and the CS J = 2-1 line from the IRAS 16293-2422 molecular outflow, respectively. These polarizations are an order of magnitude lower than predicted by theoretical models. In the case of DR 21, the lack of polarization is probably due to a disordered magnetic field in clumpy, turbulent gas, although multiple scattering may also diminish the polarization. CS J = 2-1 polarizations of 0.9% ± 0.1% and 5.1% ± 1.5% were observed from the envelopes of the evolved stars IRC+10216 and CRL 2688, respectively. An anisotropic optical depth to escape of infrared photons from the central star, perhaps caused by a toroidal dust distribution, could generate the IRC+10216 polarization.

The relationship between the radio and optical properties of quasars and the connection between these properties and the quasar host galaxies are investigated. Radio data have been analyzed for 359 of the 1055 quasars in the Large Bright Quasar Survey (LBQS). A major result of this work is that the radio-loud fraction is mostly insensitive to redshift and quasar optical luminosity, remaining at ≈ 10% over the absolute magnitude range -28 ≤ M(B) ≤ -23 and from redshifts z = 0.2 to z ∼ 5. Two deviations from these flat distributions occur at z ∼ 1, where there is a modest increase in the radio-loud fraction, and for absolute magnitudes brighter than M(B) = -28, where the fraction climbs to 20-30%. The rise in radio-loud fraction at z ∼ 1 is reproduced by a model based on the optical and radio quasar luminosity functions. The increase at high optical luminosities is consistent with the existence of two radio emission mechanisms, one correlated with optical luminosity, the other independent. The nearly flat distributions in the LBQS differ markedly from those of the optically selected Palomar-Green Bright Quasar Survey and the X-ray selected Extended Medium Sensitivity Survey. A subset of 16 LBQS quasars was imaged with the Hubble Space Telescope to study the dependence of radio and optical luminosity on the absolute magnitudes and morphologies of the host galaxies. There is no distinction in host galaxy magnitude between radio-loud and radio-quiet quasars, assuming they are all of the same galaxy type. The magnitudes of the hosts are ≳ L*, and the optical luminosities of the hosts and nuclear components are positively correlated. Many of the host galaxies have small axial ratios, which may indicate that they are inclined disk systems; or else they have bright elongated features which are visible while the bulk of the underlying lower surface brightness components of the host galaxy are not.

A geologic study of lithospheric displacements on the Jovian moon Europa reveals lateral motions and plate flexure. Tectonics are governed by the rotation rate, nature of the lithosphere and underlying decoupling layer, the nature and causes of lateral displacements, plus mechanisms for creating and consuming surface area, and for restoring lithospheric rigidity. Astypalaea Linea is an 810-km-long strike-slip fault near the south pole, with 42 km of right-lateral offset, and includes a large pull-apart. Considering scale and contaminants, the lithosphere may have a tensile strength of ∼2.5 bars. The fault probably formed as a crack due to stresses from nonsynchronous rotation and diurnal tides, and was displaced by "walking" due to diurnal tides. Adjacent regional structures record earlier episodes of strike-slip. Wedge-shaped bands in the antijovian fracture zone are reconstructed, confirming the occurrence of block rotation and episodic dilation. A band on the leading side of the satellite is also reconstructed. Whether these bands formed under the influence of the same stress patterns which caused Astypalaea Linea is unclear; regional structures in the antijovian region suggest deformation by distributed shear. Dilation has also occurred across at least one ridge representative of a type independently interpreted as dilational based on ridge morphology. Other ridges apparently flex the underlying lithospheric plate downward. The lithosphere is inferred from flexural parameters at one locality to be 0.25-3.5 km thick. New lithosphere forms by ratchet-type spreading at bands and some ridges. Surface area may be removed by chaos formation or other processes. A global time marker based on a shift in ridge size is used to show that displacement was probably long-lived as well as widespread. During displacement, lithospheric plates were rigid and integral despite the appearance of cracks, perhaps due to annealing processes. Tides are the primary driving force for Europan tectonics and have produced a complex geologic history, consistent with Greenberg et al. (1997). A subsurface ocean, maintained by tidal heating, probably existed at the time of the displacements, which are relatively recent, and may well exist today.

We present detailed studies of the kinematics and star formation properties of low surface brightness (LSB) galaxies. A total of five giant LSB galaxies including the prototype, Malin 1, were imaged in the 21-cm line of H sc I to provide the first glimpse into the kinematics of these systems. We find that these are some of the first examples to be uncovered of galaxies that are both massive and dark matter dominated. We also find that most of the galaxies have gas surface densities that lie below the critical density for star formation at all radii, consistent with their lack of star formation. In a couple of cases, though, the gas exceeds the critical density where there is no star formation implying a higher gas velocity dispersion or strong flaring of the gas disk. Long-slit optical spectroscopy and broadband CCD imaging are presented for a total of 71 LSB galaxies. We find that these galaxies follow a Fisher-Tully relation with a slope that is in good agreement with the slopes found for other samples of LSB and HSB galaxies. We interpret both the optical and H sc I rotation curves in terms of mass models consisting of a halo only or a stellar component plus a halo of either an isothermal form or an NFW halo of the form described by Navarro et al. (1996) and find that the rotation curves are generally best modeled by low mass-to-light ratio stellar components. By modeling the rotation curves with only an NFW halo and comparing the results with the predictions of cosmological simulations we find that these data are marginally consistent at best with Standard Cold Dark Matter and generally favor lower density models. However, we also find that when including the stellar component, the NFW halo gives a significantly worse fit than an isothermal halo in many cases. This suggests that the inner parts of galactic halos may not be well described by the NFW halo profile.