Dust obscuration studies along quasar sight lines using simulated galaxies

We use the results of a set of three-dimensional SPH-Treecode simulations which model the formation and early evolution of disk galaxies, including the generation of heavy elements by star formation, to investigate the effects of dust absorption in quasar absorption line systems. Using a simple prescription for the production of dust, we have compared the column density, zinc abundance and optical depth properties of our models to the known properties of Damped Lyman alpha systems. We find that a significant fraction of our model galaxy disks have a higher column density than any observed DLA system. We are also able to show that such parts of the disk tend to be optically thick, implying that any background quasar would be obscured through much of the disk. This would produce the selection effect against the denser absorption systems thought to be present in observations.Comment: 7 pages, 8 figures, to be published in MNRA

Microwave Transmissivity of Sub-Wavelength Metallic Structures

The use of patterned metallic surfaces for the control of the transmission of microwave radiation has been reinvigorated in recent years due to the success and interest in metamaterial research. These metallic periodic structures, commonly referred to as frequency selective screens (FSSs), allow responses to be tailored according to the geometry of the metallic structure as opposed to the material composition. A consequence of the presence of a metallic corrugation is the possible excitation of surfaces waves (commonly referred to as surface plasmon polaritons at visible frequencies). Surface waves can be utilised to achieve further control of the transmission properties of a structure. In this thesis several highly original metallic structures are investigated which use FSS and surface wave concepts. These structures exhibit interesting and previously unexplained transmission behaviour. The experimental chapters within this thesis are divided into two areas. The first three experimental chapters (4-6) present original investigations into the excitation of diffractively coupled surface waves on metallic hole/patch arrays and their role in the enhanced transmission/reflection of microwave radiation. The importance of metallic connectivity within arrays is highlighted through measurements of the metallic filling fraction dependence on the transmission properties of regular periodic and random arrays. The last two experimental chapters (7-8) contain investigations into the transmission properties of two novel resonant cavities. The structure studied in chapter 7 provides a mechanism for remarkably enhanced microwave transmission on resonance through an otherwise opaque continuous thin metal film. The second resonant cavity structure in chapter 8 uses a resonant array of metallic crosses to form a ‘resonant mirror’ Fabry-Perot cavity. These resonant FSSs exhibit a frequency dependent transmission/reflection and phase response thus producing an interesting series of modes which have very different properties to those supported by a non-resonant mirror etalon.EPSRCDst

The Chemical Evolution of the Universe I: High Column Density Absorbers

We construct a simple, robust model of the chemical evolution of galaxies from high to low redshift, and apply it to published observations of damped Lyman-alpha quasar absorption line systems (DLAs). The elementary model assumes quiescent star formation and isolated galaxies (no interactions, mergers or gas flows). We consider the influence of dust and chemical gradients in the galaxies, and hence explore the selection effects in quasar surveys. We fit individual DLA systems to predict some observable properties of the absorbing galaxies, and also indicate the expected redshift behaviour of chemical element ratios involving nucleosynthetic time delays. Despite its simplicity, our `monolithic collapse' model gives a good account of the distribution and evolution of the metallicity and column density of DLAs, and of the evolution of the global star formation rate and gas density below redshifts z 3. However, from the comparison of DLA observations with our model, it is clear that star formation rates at higher redshifts (z>3) are enhanced. Galaxy interactions and mergers, and gas flows very probably play a major role.Comment: 36 pages, 11 figures; accepted by MNRA

Smilansky's model of irreversible quantum graphs, II: the point spectrum

In the model suggested by Smilansky one studies an operator describing the interaction between a quantum graph and a system of K one-dimensional oscillators attached at different points of the graph. This paper is a continuation of our investigation of the case K>1. For the sake of simplicity we consider K=2, but our argument applies to the general situation. In this second paper we apply the variational approach to the study of the point spectrum.Comment: 18 page

Outflows in Infrared-Luminous Starbursts at z < 0.5. I. Sample, NaI D Spectra, and Profile Fitting

We have conducted a spectroscopic survey of 78 starbursting infrared-luminous galaxies at redshifts up to z = 0.5. We use moderate-resolution spectroscopy of the NaI D interstellar absorption feature to directly probe the neutral phase of outflowing gas in these galaxies. Over half of our sample are ultraluminous infrared galaxies that are classified as starbursts; the rest have infrared luminosities in the range log(L_IR/L_sun) = 10.2 - 12.0. The sample selection, observations, and data reduction are described here. The absorption-line spectra of each galaxy are presented. We also discuss the theory behind absorption-line fitting in the case of a partially-covered, blended absorption doublet observed at moderate-to-high resolution, a topic neglected in the literature. A detailed analysis of these data is presented in a companion paper.Comment: 59 pages, 18 figures in AASTeX preprint style; to appear in September issue of ApJ

Modeling the radial abundance distribution of the transition galaxy ngc 1313

NGC 1313 is the most massive disk galaxy showing a flat radial abundance distribution in its interstellar gas, a behavior generally observed in magellanic and irregular galaxies. We have attempted to reproduce this flat abundance distribution using a multiphase chemical evolution model, which has been previously used sucessfully to depict other spiral galaxies along the Hubble morphological sequence. We found that it is not possible to reproduce the flat radial abundance distribution in NGC 1313, and at the same time, be consistent with observed radial distributions of other key parameters such the surface gas density and star formation profiles. We conclude that a more complicated galactic evolution model including radial flows, and possibly mass loss due to supernova explosions and winds, is necessary to explain the apparent chemical uniformity of the disk of NGC 1313Comment: 14 paginas, 4 figures, to be published in ApJ, apri

High Carbon in I Zwicky 18: New Results from Hubble Space Telescope Spectroscopy

We present new measurements of the gas-phase C/O abundance ratio in both the NW and SE components of the extremely metal-poor dwarf irregular galaxy I Zw 18, based on ultraviolet spectroscopy of the two H II regions using the Faint Object Spectrograph on the Hubble Space Telescope. We determine values of log C/O = -0.63 +/- 0.10 for the NW component and log C/O = -0.56 +/- 0.09 for the SE component. In comparison, log C/O = -0.37 in the sun, while log C/O = -0.85 +/- 0.07 in the three most metal-poor irregular galaxies measured by Garnett et al. (1995a). Our measurements show that C/O in I Zw 18 is significantly higher than in other comparably metal-poor irregular galaxies, and above predictions for the expected C/O from massive star nucleosynthesis. These results suggest that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass carbon producing stars; this idea is supported by stellar photometry of I Zw 18 and its companion, which demonstrate that the current bursts of massive stars were not the first. Despite its very low metallicity, it is likely that I Zw 18 is not a ``primeval'' galaxy.Comment: 14 pages including 4 figures; uses aaspp4.sty. Accepted for publication in ApJ. Postscript version also available by e-mail request to author at [email protected]

The SCUBA Local Universe Galaxy Survey I: First Measurements of the Submillimetre Luminosity and Dust Mass Functions

We have used SCUBA to observe a complete sample of 104 galaxies selected at 60 microns from the IRAS BGS and we present here the 850 micron measurements. Fitting the 60,100 and 850 micron fluxes with a single temperature dust model gives the sample mean temperature T=36 K and beta = 1.3. We do not rule out the possibility of dust which is colder than this, if a 20 K component was present then our dust masses would increase by factor 1.5-3. We present the first measurements of the luminosity and dust mass functions, which were well fitted by Schechter functions (unlike those 60 microns). We have correlated many global galaxy properties with the submillimetre and find that there is a tendancy for less optically luminous galaxies to contain warmer dust and have greater star formation efficiencies (cf. Young 1999). The average gas-to-dust ratio for the sample is 581 +/- 43 (using both atomic and molecular hydrogen), significantly higher than the Galactic value of 160. We believe this discrepancy is due to a cold dust component at T < 20 K. There is a suprisingly tight correlation between dust mass and the mass of molecular hydrogen as estimated from CO measurements, with an intrinsic scatter of ~50%.Comment: 24 pages, 15 figures, 8 tables, accepted for publication in MNRA