DSpace Collection:

Quasi-stars and the Schönberg–Chandrasekhar limit

Mon, 02 Jul 2012 23:00:00 GMT

Title: Quasi-stars and the Schönberg–Chandrasekhar limit Authors: Ball, Warrick Heinz Abstract: The mechanism by which the supermassive black holes that power bright quasars at high redshift form remains unknown. One possibility is that, if fragmentation is prevented, the monolithic collapse of a massive protogalactic disc proceeds via a cascade of triaxial instabilities and leads to the formation of a quasi-star: a growing black hole, initially of typical stellar-mass, embedded in a hydrostatic giant-like envelope. Quasi-stars are the main object of study in this dissertation. Their envelopes satisfy the equations of stellar structure so the Cambridge STARS code is modified to model them. Analysis of the models leads to an extension of the classical Schönberg–Chandrasekhar limit and an exploration of the implications of this extension for the evolution of main-sequence stars into giants. In Chapter 1, I introduce the problem posed by the supermassive black holes that power high-redshift quasars. I discuss potential solutions and describe the conditions under which a quasi-star might form. In Chapter 2, I outline the Cambridge STARS code and the modifications that are made to model quasi-star envelopes. In Chapter 3, I present models of quasi-stars where the base of the envelope is located at the Bondi radius of the black hole. The black holes in these models are subject to a robust upper fractional mass limit of about one tenth. In addition, the final black hole mass is sensitive to the choice of the inner boundary radius of the envelope. In Chapter 4, I construct alternative models of quasi-stars by drawing from work on convection- and advection-dominated accretion flows around black holes. To improve the accuracy of my models, I incorporate corrections owing to special and general relativity into a variant of the STARS code that includes rotation. The evolution of these quasi-stars is qualitatively different from those described in Chapter 3. Most notably, the core black holes are no longer subject to a fractional mass limit and ultimately accrete all of the material in their envelopes. In Chapter 5, I demonstrate that the fractional mass limit found in Chapter 3, for the black holes in quasi-stars, is in essence the same as the Schönberg–Chandrasekhar limit. The analysis demonstrates how other similar limits are related and that limits exist under a wider range of circumstances than previously thought. A test is provided that determines whether a composite polytrope is at a fractional mass limit. In Chapter 6, I apply this test to realistic stellar models and find evidence that the existence of fractional mass limits is connected to the evolution of stars into the red giants.

Rotation and magnetism in massive stars

Mon, 02 Jul 2012 23:00:00 GMT

Title: Rotation and magnetism in massive stars Authors: Potter, Adrian Thomas Abstract: Rotation has a number of important effects on the evolution of stars. Apart from structural changes because of the centrifugal force, turbulent mixing and meridional circulation can dramatically affect a star's chemical evolution. This leads to changes in the surface temperature and luminosity as well as modifying its lifetime. Rotation decreases the surface gravity, causes enhanced mass loss and leads to surface abundance anomalies of various chemical isotopes all of which have been observed. The replication of these physical effects with simple stellar evolution models is very difficult and has resulted in the use of numerous different formulations to describe the physics. We have adapted the Cambridge stellar evolution code to incorporate a number of different physical models for rotation, including several treatments of angular momentum transport in convection zones. We compare detailed grids of stellar evolution models along with simulated stellar populations to identify the key differences between them. We then consider how these models relate to observed data. Models of rotationally-driven dynamos in stellar radiative zones have suggested that magnetohydrodynamic transport of angular momentum and chemical composition can dominate over the otherwise purely hydrodynamic processes. If this is the case then a proper consideration of the interaction between rotation and magnetic fields is essential. We have adapted our purely hydrodynamic model to include the evolution of the magnetic field with a pair of time-dependent advection--diffusion equations coupled with the equations for the evolution of the angular momentum distribution and stellar structure. This produces a much more complete, though still reasonably simple, model for the magnetic field evolution. We consider how the surface field strength varies during the main-sequence evolution and compare the surface enrichment of nitrogen for a simulated stellar population with observations. Strong magnetic fields are also observed at the end of the stellar lifetime. The surface magnetic field strength of white dwarfs is observed to vary from very little up to 10^9G. As well as considering the main-sequence evolution of magnetic fields we also look at how the strongest magnetic fields in white dwarfs may be generated by dynamo action during the common envelope phase of strongly interacting binary stars. The resulting magnetic field depends strongly on the electrical conductivity of the white dwarf, the lifetime of the convective envelope and the variability of the magnetic dynamo. We assess the various energy sources available and estimate necessary lifetimes of the common envelope.

Probing the Intergalactic Medium with high-redshift quasars

Tue, 08 Nov 2011 00:00:00 GMT

Title: Probing the Intergalactic Medium with high-redshift quasars Authors: Calverley, Alexander Peter Abstract: Clues about the timing of reionization and the nature of the ionizing sources responsible are imprinted in the ionization and thermal state of the IGM. In this thesis, I use high-resolution quasar spectra in conjunction with state-of-the-art hydrodynamical simulations to probe the IGM at high redshift, focusing on the ionization and thermal state of the gas. After reionization, the ionization state of the IGM is set by the intensity of the ultraviolet background (UVB), quantified by the hydrogen photoionization rate, Γ_bkg. At high redshifts this has been estimated by measuring the mean flux in the Lyα forest, and scaling Γ_bkg in simulations such that the simulated mean flux matches the observed value. In Chapter 3 I investigate whether the precision of these estimates can be improved by using the entire flux probability distribution function (PDF) instead of only the mean flux. Although I find it cannot improve the precision directly, the flux PDF can potentially be used to constrain other sources of error in observational estimates of Γ_bkg, and so may increase the precision indirectly. The ionizing output of a quasar will locally dominate over the UVB, and this leads to enhanced transmission bluewards of the quasar Lyα line, known as the proximity effect. In Chapter 4 I present the first measurements of Γ_bkg at z > 5 from the proximity effect. The UVB intensity declines smoothly with redshift over 4.6 < z < 6.4, implying a smooth evolution in the mean free path of ionizing photons. This suggests that reionization ends at z > 6.4. There is a drop in Γ_bkg by roughly a factor of five, which corresponds to a drop in the ionizing emissivity by about a factor of two. Such a redshift evolution in the emissivity cannot continue to much higher redshift without reionization failing to complete, which suggests that reionization cannot have ended much higher than z = 6.4. Estimates of Γ_bkg from the proximity effect and the mean flux are generally discrepant at z ∼ 2−4, with those from the proximity effect systematically higher. This is generally attributed to effects of the quasar environment. I investigate the significance of several environmental biases on proximity effect measurements at z ∼ 5−6 in Chapter 5. The biases are found to be small, and so the proximity effect is expected to give relatively unbiased estimates of Γ_bkg at z > 5, in contrast to lower redshifts. Photoionization heats the gas in the IGM, and so the thermal history of the IGM provides important constraints on reionization. The thermal state of the IGM is reflected in the level of small-scale structure in the Lyα forest. In Chapter 6 I quantify the small-scale structure using two independent statistics, the curvature and the peakiness, and convert these into a temperature by comparing with simulations. These are the first measurements of the temperature in the general IGM at z > 5. Both statistics show an increase in the temperature by a factor of roughly two from z = 4.4 to 5.6. This rise is sensitive, however, to any smoothing of the gas density distribution due to the thermal history spanning reionization. I find that this should only be a small effect, as otherwise the corrected temperatures at z ∼ 4−5 are implausibly low. The temperature evolution therefore suggests a late reionization. The temperatures at z ≥ 4.8 are well fit by an adiabatic cooling curve, for which reasonable peak temperatures at the end of reionization are reached at 6 ≤ z ≤ 7. The temperatures at z ∼ 4−5 are consistent with reionization being carried out by Pop II stars. In conclusion, the ionization and thermal state of the IGM at z ∼ 5−6 suggest a late hydrogen reionization, driven by star-forming galaxies and ending around 6.5 ≤ z ≤ 7. This is consistent with other recent lines of observational evidence, and supports theoretical models that infer a late reionization from the observed star formation rate history.

Constraining fundamental physics from cosmology

Mon, 10 Oct 2011 23:00:00 GMT

Title: Constraining fundamental physics from cosmology Authors: Bird, Simeon Abstract: I use mathematical models and numerical simulations to constrain cosmological inflation, the seeds of structure, and the mass of the neutrino. I revisit arguments that simple models of inflation with a small red tilt in the scalar power spectrum generically yield an observable tensor spectrum. I show that criteria for fine-tuning based upon the algebraic simplicity of the potential depend strongly upon the assumptions they incorporate about the potential. Furthermore, several models with algebraically simple potentials require carefully tuned initial field configurations. I demonstrate the existence of potentials with vanishingly small tensor amplitudes which are natural in terms of both their algebraic form and initial conditions. I thus argue that proposed experiments which make highly sensitive measurements of the tensor amplitude cannot definitively rule out the inflationary paradigm. The overshoot problem is the need to make the initial kinetic energy of the inflaton small enough to ensure slow-roll. I investigate claims that brane inflation solves the overshoot problem through microphysical restrictions on the phase space of initial conditions. By carrying out a comprehensive analysis of the parameter space allowed by the latest advances in brane inflation model-building, I find that the vast majority of the phase space of initial conditions is still dominated by overshoot trajectories. Current results from the Lyman-α forest assume that the primordial power spectrum of density perturbations follows a simple power-law form with running. I perform a large suite of numerical simulations, using them to calibrate a minimally parametric framework for describing the power spectrum. Combined with cross-validation this framework allows me to directly reconstruct the power spectrum shape, a consistency check on the standard model. I find no evidence for deviation from scale-invariance, but current Lyman-α data do not have sufficient statistical power to robustly probe the shape of the power spectrum at these scales. In contrast, the ongoing Baryon Oscillation Sky Survey will be able to do so with high precision. I perform an extensive suite of N-body simulations of the matter power spectrum, probing deep into the non-linear regime while incorporating massive neutrinos. I compare my results to the widely used HALOFIT approximation, and find that in the strongly nonlinear regime it significantly over-predicts the suppression due to the free-streaming of neutrinos. Most published constraints are not affected, as they have used HALOFIT only in the linear or mildly non-linear regime. However, my results are important for future galaxy and weak lensing surveys.

Protoplanetary disc evolution and dispersal

Mon, 10 Oct 2011 23:00:00 GMT

Title: Protoplanetary disc evolution and dispersal Authors: Owen, James Edward Abstract: In this thesis I have studied how discs around young stars evolve and disperse. In particular, I build models which combine viscous evolution with photoevaporation, as previous work suggests that photoevaporation can reproduce the observed disc evolution and dispersal time-scales. The main question this thesis attempts to address is: Can photoevaporation provide a dominant dispersal mechanism for the observed population of young stars? Photoevaporation arises from the heating that high energy (UV and X-ray) photons provide to the surface layers of a disc. Before I started this work, only photoevaporation from a pure EUV radiation field was described within a hydrodynamic framework. Therefore, I start by building a hydrodynamic solution to the pure X-ray photoevaporation problem, and then extend this solution to the entire high energy spectrum. This hydrodynamic model leads me to conclude that it is the X-ray radiation field that sets the mass-loss rates. These mass-loss rates scale linearly with X-ray luminosity, are independent of the underlying disc structure and explicitly independent of stellar mass. I build a radiation-hydrodynamic algorithm, based on previous work, to describe the process of X-ray heating in discs. I then use this algorithm to span the full range of observed parameter space, to fully solve the X-ray photoevaporation problem. I further extend the algorithm to roughly approximate the heating an FUV radiation field would have on the photoevaporative flow, as well as separately testing the effect an EUV radiation field will have. These numerical tests are in agreement with the hydrodynamic model derived. Specifically, it is the X-rays that are driving the photoevaporative flow from the inner disc. Armed with an accurate description of the photoevaporative mass-loss rates from young stars, I consider the evolution of a population of disc-bearing, young ($0.7$\msun) stars, in order to asses photoevaporation's role as a dispersal mechanism. This study shows that the observed spread in X-ray luminosity of young stars is sufficient to drive the dispersal of the entire population of discs, reproducing both the required time-scales and the required spread in observables (disc lifetime, accretion rate). I also show that a large fraction of the observed population of `transition' discs are consistent with being created through photoevaporation. Having shown photoevaporation can provide a dominant dispersal mechanism for a population of discs, I attempt to describe some direct observable consequences of photoevaporation, both through gas tracers and dust emission. During this work, the first direct evidence of a photoevaporative flow emerging from a young star was obtained from TW Hya, in the NeII 12.8$\mu$m line. Therefore, I discuss this result within the framework of the X-ray photoevaporation model. Furthermore, I suggest that emission from the photoevaporative flow is the origin of the unexplained, blue-shifted, OI 6300\AA~ line observed around all young stars that possess discs. I then described the properties of the dust particles that may be entrained within the photoevaporative flow. The total dust mass in the flow is found to be small compared to the disc, although such a region becomes observable once the disc presents as edge-on, obscuring the central star and hot inner disc. I discuss the emission from these regions and compare them to the sample of currently imaged edge-on discs. The presented photoevaporation model reproduces all of the current observations, and I discuss some predictions it makes with regard to future observations. Finally, photoevporation may have some intriguing consequences on planet formation and dust evolution that warrant further investigation.

Tracer populations in the local group

Mon, 18 Apr 2011 23:00:00 GMT

Title: Tracer populations in the local group Authors: Watkins, Laura Louise Abstract: So often in astronomy, an object is not considered for its individual merits, but for what we may learn from its properties regarding some larger population. The existence of dark matter is a prime example of this; we cannot see it directly but we can infer its presence by noting its effects on the stars orbiting within its potential. This thesis describes how various sets of tracer populations can be used to probe the properties of a variety of galaxies in the Local Group. I begin by describing the extraction of a variable catalogue from the Sloan Digital Sky Survey Stripe 82 dataset and then use the catalogue to select a high-quality set of RR Lyrae stars. Analysing the distribution of the RR Lyraes reveals three significant substructures in the Milky Way halo: the Hercules-Aquila Cloud and the Sagittarius Stream, which were already known to exist, and the Pisces Overdensity, which was previously undetected. It is a faint, extended structure found at ~80 kpc and is of unknown origin. Altogether, I find that nearly 80% of the RR Lyraes are associated with substructures, consistent with the theory that galaxy halos are predominantly, or even entirely, made up from disrupted satellites. I also investigate the density distribution of RR Lyraes in the halo, finding that it is best fit by a broken-power-law model, in good agreement with previous work. I go on to develop a set of tracer mass estimators that build on previous work which make use of actual (and not projected) distance and proper motion data, reflecting the amount and quality of data now available to us. I show that proper motion data is, in theory, very useful and can greatly increase the accuracy of the mass estimates; in practice, however, current analysis is hampered by the large errors inherent in the proper motion data. The results are also subject to mass-anisotropy degeneracy, which current data is not yet able to break. Nevertheless, I am able to estimate the mass of the Milky Way to be M = 2.7 +/- 0.5 x 10^12 Msun and the mass of M31 to be M = 1.5 +/- 0.4 x 10^12 Msun. Andromeda XII and Andromeda XIV are two M31 satellites that have been dubbed "extreme" and are thought to be on first infall into the M31 system. I modify the classical Timing Argument so that it can be applied to two external galaxies and then apply it to M31 and each of And XII and And XIV in turn to investigate the properties of their orbits. I then run a series of Monte Carlo simulations to investigate how likely such satellites are to exist and conclude that they are not as unusual as previously believed. Finally, I discuss three upcoming wide-field, all-sky surveys and their implications for the future of the study of the Local Group.

High redshift star-forming galaxies in absorption and emission

Mon, 11 Jul 2011 23:00:00 GMT

Title: High redshift star-forming galaxies in absorption and emission Authors: Quider, Anna Marie Abstract: Galaxies in the redshift range 1 < z < 3 existed during the most vigorous period of star formation in the history of the Universe. In the past 15 years, large rest-frame UV spectroscopic samples of z ∼ 3 star-forming galaxies have been assembled. However, this particular redshift range, the so-called Redshift Desert, has only begun to be characterized. Most studies involve low resolution, low signal-to-noise spectra because the small angular size (δ ≤ 1′′) and faintness (RAB = 24 − 25.5) of high redshift galaxies limit what can be accomplished with a reasonable investment of observing time, even using the world’s largest optical telescopes. One way to circumvent these two issues is to study gravitationally lensed galaxies. The magnification boost (up to a factor of 30×) and morphological distortion of a high redshift galaxy by an intervening mass concentration allow for the study of the high redshift Universe in unprecedented detail. I present a detailed analysis of the rest-UV spectrum of two gravitationally lensed galaxies: the ‘Cosmic Horseshoe’ (zsys = 2.38115) and the ‘Cosmic Eye’ (zsys = 3.07331). The characterization of the stellar populations and the interstellar gas geometry, kinematics, and composition which I achieve is a preview of the type of information that will be available for unlensed high redshift galaxies with the next generation of optical telescopes. I probe the lower redshift end of the Redshift Desert with a study of Fe ii and Mg ii features in the rest-frame near-UV spectrum of 96 star-forming galaxies in the redshift range 1 < z < 2. Stacked spectra are used to explore average outflow and line profile trends with stellar mass and reddening. I also investigate the phenomenon of emission filling of absorption lines which has implications for the line strength and velocity offset of interstellar absorption lines. Individual galaxies are used to assess the range of outflow velocities as well as the prevalence of emission filling in galaxies from this epoch. This is the first large scale study of fine-structure emission from Feii in high redshift galaxies, both in stacked and individual galaxy spectra. An alternative to investigating galaxies by collecting their light is to study them as seen in absorption against a cosmic backlight, such as a quasar. The Sloan Digital Sky Survey, an imaging and spectroscopic survey which covers about one-quarter of the night sky, has collected many thousands of quasar spectra. I search ∼ 44 600 of these spectra, up through Data Release 4, for Mg ii λλ2796,2803 absorption doublets. The final catalog includes ∼ 16700 Mgii absorption line systems in the redshift range 0.36 ≤ z ≤ 2.28. Measurements of the absorption redshift and rest equivalent widths of the Mg ii doublet as well as select metal lines are available in the catalog. This is the largest publicly available catalog of its kind and its combination of large size and well understood statistics make it ideal for precision studies of the low-ionization and neutral gas regions of galaxies. I conclude this thesis by suggesting several avenues for extending the studies of high redshift star-forming galaxies presented herein.

Cosmology with CMB and Large Scale Structure

Mon, 11 Jul 2011 23:00:00 GMT

Title: Cosmology with CMB and Large Scale Structure Authors: Ma, Yin-Zhe Abstract: Cosmology has become a precision science due to a wealth of new precise data from various astronomical observations. It is therefore important, from a methodological point of view, to develop new statistical and numerical tools to study the Cosmic Microwave Background (CMB) radiation and Large Scale Structure(LSS), in order to test different models of the Universe. This is the main aim of this thesis. The standard inflationary $\Lambda$-dominated Cold Dark Matter ($\Lambda$CDM) model is based on the premise that the Universe is statistically isotropic and homogeneous. This premise needs to be rigorously tested observationally. We study the angular correlation function $C(\theta)$ of the CMB sky using the WMAP 5-year data, and find that the low-multipoles can be reconstructed from the data outside the sky cut. We apply a Bayesian analysis and find that $S_{1/2}$ statistic ($S_{1/2}=\int [C(\theta)]^{2}d\cos \theta$, used by various investigators as a measure of correlations at large angular scales) cannot exclude the predictions of the $\Lambda$CDM model. We clarify some issues concerning estimation of correlations on large angular scales and their interpretation. To test for deviation from statistical isotropy, we develop a quadratic maximum likelihood estimator which we apply to simulated Planck maps. We show that the temperature maps from Planck mission should be able to constrain the amplitude of any spherical multipole of a scale-invariant quadrupole asymmetry at the $1\%$ level ($2\sigma$). In addition, polarization maps are also precise enough to provide complimentary constraints. We also develop a method to search for the direction of asymmetry, if any, in Planck maps. B-mode polarisation of the CMB provides another important test of models of the early Universe. Different classes of models, such as single-field inflation, loop quantum cosmology and cosmic strings give speculative but testable predictions. We find that the current ground-based experiments such as BICEP, already provided fairly tight constraints on these models. We investigate how these constraints might be improved with future observations (e.g. Planck, Spider). In addition to the CMB related research, this thesis investigates how peculiar velocity fields can be used to constrain theoretical models of LSS. It has been argued that there are large bulk flows on scales of $\gtrsim 50$ Mpc/h. If true, these results are in tension with the predictions of the $\Lambda$CDM model. We investigate a possible explanation for this result: the unsubtracted intrinsic dipole on the CMB sky may source this apparent flow, leading to the illusion of the tilted Universe. Under the assumption of a superhorizon isocurvature fluctuation, the constraints on the tilted velocity require that inflation lasts at least 6 e-folds longer (at the 95\% confidence interval) than that required to solve the horizon problem. Finally, we investigate Cosmic Mach Number (CMN), which quantifies the ratio between the mean velocity and the velocity dispersion of galaxies. We find that CMN is highly sensitive to the growth of structure on scales $(10,150)$ Mpc/h, and can therefore be used to test modified gravity models and neutrino masses. With future CMN data, it should be possible to constrain the growth factor of linear perturbation, as well as the sum of the neutrino mass to high accuracy.

Characterization and mitigation of radiation damage on the Gaia Astrometric Field

Tue, 11 Jan 2011 00:00:00 GMT

Title: Characterization and mitigation of radiation damage on the Gaia Astrometric Field Authors: Brown, Scott William Abstract: In November 2012, the European Space Agency (ESA) is planning to launch Gaia, a mission designed to measure with microarcsecond accuracy the astrometric properties of over a billion stars. Microarcsecond astrometry requires extremely accurate positional measurements of individual stellar transits on the focal plane, which can be disrupted by radiation-induced Charge Transfer Inefficiency (CTI). Gaia will suffer radiation damage, impacting on the science performance, which has led to a series of Radiation Campaigns (RCs) being carried out by industry to investigate these issues. The goal of this thesis is to rigorously assess these campaigns and facilitate how to deal with CTI in the data processing. We begin in Chapter 1 by giving an overview of astrometry and photometry, introducing the concept of stellar parallax, and establishing why observing from space is paramount for performing global, absolute astrometry. As demonstrated by Hipparcos, the concept is sound. After reviewing the Gaia payload and discussing how astrometric and photometric parameters are determined in practice, we introduce the issue of radiation-induced CTI and how it may be dealt with. The on board mitigating strategies are investigated in detail in Chapter 2. Here we analyse the effects of radiation damage as a function of magnitude with and without a diffuse optical background, charge injection and the use of gates, and also discover a number of calibration issues. Some of these issues are expected to be removed during flight testing, others will have to be dealt with as part of the data processing, e.g. CCD stitches and the charge injection tail. In Chapter 3 we turn to look at the physical properties of a Gaia CCD. Using data from RC2 we probe the density of traps (i.e. damaged sites) in each pixel and, for the first time, measure the Full Well Capacity of the Supplementary Buried Channel, a part of every Gaia pixel that constrains the passage of faint signals away from the bulk of traps throughout the rest of the pixel. The Data Processing and Analysis Consortium (DPAC) is currently adopting a ’forward modelling’ approach to calibrate radiation damage in the data processing. This incorporates a Charge Distortion Model (CDM), which is investigated in Chapter 4. We find that although the CDM performs well there are a number of degeneracies in the model parameters, which may be probed further by better experimental data and a more realistic model. Another way of assessing the performance of a CDM is explored in Chapter 5. Using a Monte Carlo approach we test how well the CDM can extract accurate image parameters. It is found that the CDM must be highly robust to achieve a moderate degree of accuracy and that the fitting is limited by assigning finite window sizes to the image shapes. Finally, in Chapter 6 we summarise our findings on the campaign analyses, the on-board mitigating strategies and on how well we are currently able to handle radiation damage in the data processing.

Multi-dimensional analysis of the chemical and physical properties of spiral galaxies

Tue, 09 Feb 2010 00:00:00 GMT

Title: Multi-dimensional analysis of the chemical and physical properties of spiral galaxies Authors: Rosales Ortega, Fernando Fabián Abstract: The PPAK Integral Field Spectroscopy (IFS) Nearby Galaxies Survey: PINGS, a 2-dimensional spectroscopic mosaicking of 17 nearby disk galaxies in the optical wavelength range. This project represents the first attempt to obtain continuous coverage spectra of the whole surface of a galaxy in the nearby universe. The final data set comprises more than 50000 individual spectra, covering in total an observed area of nearly 80 arcmin square. The powerful capabilities of wide-field 2D spectroscopic studies are proven. The chemical composition of the whole surface of a spiral galaxy is characterised for the first time as a function not only of radius, but of the intrinsic morphology of the galaxy, allowing a more realistic determination of their physical properties. The methodology, analysis and results of this dissertation will hopefully contribute in a significant way to understand the nature of the physical and chemical properties of the gas phase in spiral galaxies. Description: The emergence of a new generation of instrumentation in astrophysics, which provide spatially-resolved spectra over a large 2-dimensional (2D) field of view, offers the opportunity to perform emission-line surveys based on samples of hundreds of spectra in a 2D context, enabling us to test, confirm, and extend the previous body of results from small-sample studies based on typical long-slit spectroscopy, while at the same time opening up a new frontier of studying the 2D structure of physical and chemical properties of the disks of nearby spiral galaxies. The project developed in this dissertation represents the first endeavour to obtain full 2D coverage of the disks of a sample of spiral galaxies in the nearby universe, by the application of the Integral Field Spectroscopy (IFS) technique. The semi-continuous coverage spectra provided by this spectral imaging technique allows to study the small and intermediate linear scale variation in line emission and the gas chemistry in the whole surface of a spiral galaxy. The PPAK IFS Nearby Galaxies Survey: PINGS, was a carefully devised observational project, designed to construct 2D spectroscopic mosaics of 17 nearby galaxies in the optical wavelength range. The sample includes different galaxy types, including normal, lopsided, interacting and barred spirals with a good range of galactic properties and star forming environments, with multi-wavelength public data. The spectroscopic data set comprises more than 50000 individual spectra, covering an observed area of nearly 100 arcmin^2, an observed surface without precedents by an IFS study. All sources of errors and uncertainties during the reduction process of the IFS observations are assessed very carefully. This methodology contributed not only to improve the standard reduction pipeline procedure for the particularly used instrument, improvements that can be applied to any similar integral-field observation and/or data reduction, but to defining a self-consistent methodology in terms of observation, data reduction and analysis for the kind of IFS surveys presented in this dissertation, as well as providing a whole new set of IFS visualization and analysis software made available for the public domain.

Lucky exposures: diffraction limited astronomical imaging through the atmosphere

Tue, 25 Nov 2003 00:00:00 GMT

Title: Lucky exposures: diffraction limited astronomical imaging through the atmosphere Authors: Tubbs, Robert Nigel Abstract: The resolution of astronomical imaging from large optical telescopes is usually limited by the blurring effects of refractive index fluctuations in the Earth’s atmosphere. By taking a large number of short exposure images through the atmosphere, and then selecting, re-centring and co-adding the best images this resolution limit can be overcome. This approach has significant benefits over other techniques for high-resolution optical imaging from the ground. In particular the reference stars used for our method (the Lucky Exposures technique) can generally be fainter than those required for the natural guide star adaptive optics approach or those required for other speckle imaging techniques. The low complexity and low instrumentation costs associated with the Lucky Exposures method make it appealing for medium-sized astronomical observatories. The method can provide essentially diffraction-limited I-band imaging from well-figured ground-based telescopes as large as 2.5 m diameter. The faint limiting magnitude and large isoplanatic patch size for the Lucky Exposures technique at the Nordic Optical Telescope means that 25% of the night sky is within range of a suitable reference star for I-band imaging. Typically the 1%—10% of exposures with the highest Strehl ratios are selected. When these exposures are shifted and added together, field stars in the resulting images have Strehl ratios as high as 0.26 and full width at half maximum flux (FWHM) as small as 90 milliarcseconds. Within the selected exposures the isoplanatic patch is found to be up to 60 arcseconds in diameter at 810 nm wavelength. Images within globular clusters and of multiple stars from the Nordic Optical Telescope using reference stars as faint as I 16 are presented. A new generation of CCDs (Marconi L3Vision CCDs) were used in these observations, allowing extremely low noise high frame-rate imaging with both fine pixel sampling and a relatively wide field of view. The theoretical performance of these CCDs is compared with the experimental results obtained.

Studying galaxy formation through Lyman alpha in emission and absorption

Tue, 12 Jan 2010 00:00:00 GMT

Title: Studying galaxy formation through Lyman alpha in emission and absorption Authors: Barnes, Luke Andrew Abstract: Galaxy formation is one of the central problems of Physical Cosmology. Neutral hydrogen plays an important role, linking the collapse of cooling gas into haloes with the formation of stars. Lyman alpha, hydrogen’s strongest spectral line, can directly probe neutral hydrogen in the high redshift Universe. Lyα can be observed in absorption in Damped Lyman Alpha systems (DLAs): high Hi column density regions that dominate the neutral gas content of the Universe between z ∼ 0 − 5. Lyα in emission is an important signature of early, starforming galaxies. Both populations, however, present significant theoretical challenges. As part of my thesis, I have developed a Monte Carlo Lyα radiative transfer code to investigate models of early galaxies. Rauch et al. (2008) performed an ultra-deep spectroscopic survey and discovered a new population of very faint, spatially extended Lyα emitters, which they claimed to be the long-sought host galaxies of DLAs at z ∼ 3. I show here that a simple analytical model, which reproduces the incidence rate and kinematics of DLAs in the context of [Lambda]CDM models for structure formation, also reproduces the size distribution of the faint Lyα emitters for plausible parameters, which supports their identification as DLA host galaxies. The model suggests that galaxies in haloes with vc ~ 100−150 km s−1 account for the majority of DLA host galaxies, and that these galaxies at z ~ 3 are the building blocks of typical present-day galaxies like our Milky Way. I further use my newly developed Lyα code to perform detailed 1D radiative transfer calculations, investigating the spatial and spectral distribution of Lyα emission due to star formation at the centre of DLAs, and its dependence on the spatial and velocity structure of the gas. The modelling reproduces the observed properties of both DLAs and the faint Lyα emitters, including the velocity width and column density distribution of DLAs and the large observed spatial extent of the faint emitters. In the model, haloes hosting DLAs retain up to 20% of the cosmic baryon fraction in the form of neutral hydrogen. The scattering of Lyα photons at the observed radii, which can be as large as 50 kpc, requires the bulk velocity of the gas at the centre of the haloes to be moderate. I furthermore perform 3D Lyα radiative transfer simulations, building on numerical simulations of galaxy formation that include galactic winds and gas infall. The Lyα emission region is shown to be larger and smoother than the cross-section for damped absorption by ~ 50%, with Lyα photons scattered effectively by gas with column densities >~ 1017 cm−2. The spectra typically show two peaks, with the relative strength of the red (blue) peak being a reflection of the relative contribution of outflow (inflow) in the velocity profile. There is considerable variation in the observed line profile and spectral intensity with viewing angle. These more realistic models support many of the simplifying assumptions of my previous models, and have the potential to probe the important role of galactic winds in protogalaxies. The main conclusion is that the faint population of Lyα emitters are indeed the longsought host population of DLAs. Ultra-faint observations of Lyα emission have exceptional potential to directly probe the spatial distribution and kinematics of neutral hydrogen in early galaxies.