Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05

In dieser Arbeit wird ein schnelles Strahlungsmodell im sichtbaren Spektralbereich erstellt. Ausgehend vom von einem meteorologischen Modell berechneten Zustand der

Atmosphäre ergibt dieses Strahlungsmodell die Strahldichte am oberen Rand der Atmosph

äre, die ein Satellit messen würde, wenn die Atmosphäre tatsächlich so aussehen würde (synthetisches Satellitenbild). Der Vergleich mit einem tatsächlichen Satellitenbild gibt Hinweise über mögliche Fehler des meteorologischen Modells. Mehrere Forschungsrichtungen werden in dieser Arbeit untersucht. Das Endprogramm (pastat) basiert weitgehend auf Look-Up Tables. Eine Erweiterung der Eddington-Approximation bis auf die dritte Ordnung wird verwendet, um die azimutal gemittelte Strahldichte zu berechnen. Ein Ansatz mit 16 statistisch bestimmten Parametern bringt die notwendige azimutale Abhängigkeit.

Das Genauigkeitskriterium für das zu erstellende Strahlungsmodell ist ein relativer Fehler der Re�ektivität unter 10% oder ein absoluter Fehler unter 0.02. Die Erfolgsrate von pastat hängt stark vom Sonnenzenitwinkel ab. Der mittlere relative Fehler von pastat ist eine Gröÿenordnung höher als der von den meisten vorhandenen schnellen Strahlungsmodellen im Infraroten. Im Sichtbaren bringt der Mehrfachstreuterm der

Strahlungstransportgleichung nämlich eine erhebliche zusätzliche Erschwernis im Vergleich

zum Infraroten. Jedoch wird das Erfolgskriterium in fast allen Fällen von pastat eingehalten. Somit erlaubt dieses Verfahren, Modellfehler auf 10% Genauigkeit zu detektieren. pastat ist das erste schnelle Strahlungsmodell im sichtbaren Spektralbereich für wolkige Atmosphären.

The work presented here focuses on the investigation and further development of simple mass estimators for early-type galaxies which are suitable for large optical galaxy surveys with poor and/or noisy data. We consider simple and robust methods that provide an anisotropy-independent estimate of the galaxy mass relying on the stellar surface brightness and projected velocity dispersion profiles. Under reasonable assumptions a fundamental mass-anisotropy degeneracy can be circumvented without invoking any additional observational data, although at a special (characteristic) radius only, i.e these approaches do not recover the radial mass distribution. Reliable simple mass estimates at a single radius could be used

(i) to cross-calibrate other mass determination methods;

(ii) to estimate a non-thermal contribution to the total gas pressure when compared with the X-ray mass estimate at the same radius;

(iii) to evaluate a dark matter fraction when compared with the luminous mass estimate;

(iv) to derive the slope of the mass profile when combined with the mass estimate from strong lensing;

(v) or as a virial mass proxy.

Two simple mass estimators have been suggested recently - the local (Churazov et al. 2010) and the global (Wolf et al. 2010) methods - which evaluate mass at a particular radius and are claimed to be weakly dependent on the anisotropy of stellar orbits. One approach (Wolf et al. 2010) uses the total luminosity-weighted velocity dispersion and evaluates the mass at a deprojected half-light radius, i.e. relies on the global properties of a galaxy. In contrast, the Churazov et al. technique uses local properties: logarithmic slopes of the surface brightness and velocity dispersion profiles, and recovers the mass at a radius where the surface brightness declines as R^{-2} (see also Richstone and Tremaine 1984, Gerhard 1993).

To test the robustness and accuracy of the methods I applied them to analytic models and to simulated galaxies from a sample of cosmological zoom-simulations which are similar in properties to nearby early-type galaxies. Both local and global simple mass estimates are found to be in good agreement with the true mass at the corresponding characteristic radius. Particularly, for slowly rotating simulated galaxies the local method gives an almost unbiased mass-estimate (when averaged over the sample) with a modest RMS-scatter of 12% (Chapter 2). When applied to massive simulated galaxies with a roughly flat velocity dispersion profile, the global approach on average also provides the almost unbiased mass-estimate, although the RMS-scatter is slightly larger (14-20 %) than for the local estimator (Chapter 4). A noticeable scatter in the determination of the characteristic radius is also expected since the half-light radius depends on the radial range used for the analysis and applied methodology.

Next I tested the simple mass estimators on a sample of real early-type galaxies which had previously been analyzed in detail using state-of-the-art dynamical modeling. For this set of galaxies the simple mass estimates are in remarkable agreement with the results of the Schwarzschild modeling despite the fact that some of the considered galaxies are flattened and mildly rotating. When averaged over the sample the simple local method overestimates the best-fit mass from dynamical modeling by 10% with the RMS-scatter 13% between different galaxies. The bias is comparable to measurement uncertainties. Moreover, it is mainly driven by a single galaxy which has been found to be the most compact one in the sample. When this galaxy is excluded from the sample, the bias and the RMS-scatter are both reduced to 6%. The global estimator for the same sample gives the mean deviation 4% with the slightly larger RMS-scatter of 15% (Chapter 4).

Given the encouraging results of the tests I apply the local mass estimation method to a sample of five X-ray bright early-type galaxies observed with the 6-

The search for exo-planets is currently one of the most exiting and active topics in astronomy. Small and rocky planets are particularly the subject of intense research, since if they are suitably located from their host star, they may be warm and potentially habitable worlds. On the other hand, the discovery of giant planets in

short-period orbits provides important constraints on models that describe planet formation and orbital migration theories. Several

projects are dedicated to discover and characterize planets outside of our solar system. Among them, the Wide-Field Camera Transit Survey

(WTS) is a pioneer program aimed to search for extra-solar planets, that stands out for its particular aims and methodology. The WTS has

been in operation since August 2007 with observations from the United Kingdom Infrared Telescope, and represents the first survey that

searches for transiting planets in the near-infrared wavelengths; hence the WTS is designed to discover planets around M-dwarfs. The

survey was originally assigned about 200 nights, observing four fields that were selected seasonally (RA = 03, 07, 17 and 19h) during a year. The images from the survey are processed by a data reduction pipeline, which uses aperture photometry to construct the light curves. For the most complete field (19h-1145 epochs) in the

survey, we produce an alternative set of light curves by using the method of difference imaging, which is a photometric technique that has

shown important advantages when used in crowded fields. A quantitative comparison between the photometric precision achieved with both methods is carried out in this work. We remove systematic

effects using the sysrem algorithm, scale the error bars on the light curves, and perform a comparison of the corrected light curves. The results show that the aperture photometry light curves provide slightly better precision for objects with J < 16. However, difference photometry light curves present a significant improvement for fainter stars. In order to detect transits in the WTS light curves, we use a modified version of the box-fitting algorithm. The implementation on the detection algorithm performs a trapezoid-fit to the folded light curve. We show that the new fit is able to produce more accurate results than the box-fit model. We describe a set of selection criteria to search for transit candidates that include a parameter calculated by our detection

algorithm: the V-shape parameter, which has proven to be useful to automatically identify and remove eclipsing binaries from the survey. The criteria are optimized using Monte-Carlo simulations of artificial transit signals that are injected into the real WTS light curves and subsequently analyzed by our detection algorithm. We separately optimize the selection criteria for two different sets of light curves, one for F-G-K stars, and another for M-dwarfs. In order

to search for transiting planet candidates, the optimized selection criteria are applied to the aperture photometry and difference imaging

light curves. In this way, the best 200 transit candidates from a sample of ~ 475 000 sources are automatically selected. A visual

inspection of the folded light curves of these detections is carried out to eliminate clear false-positives or false-detections. Subsequently, several analysis steps are performed

on the 18 best detections, which allow us to classify these objects as transiting planet and eclipsing binary candidates. We report one

planet candidate orbiting a late G-type star, which is proposed for photometric follow-up. The independent analysis on the M-dwarf sample

provides no planet candidates around these stars. Therefore, the null detection hypothesis and upper limits on the occurrence rate of giant

planets around M-dwarfs with J < 17 mag presented in a prior study are confirmed. In this work, we extended the search for transiting planets to stars with J < 18 mag, which enables

us to impose a more st

Das Gebiet der Nichtgleichgewichtsdynamik stark korrelierter Quantensysteme beinhaltet eine Vielzahl interessanter Fragestellungen, erweist sich dabei allerdings oftmals als schwer zugänglich für gängige numerische und analytische mathematische Methoden. In den letzten Jahren hat sich durch die experimentelle Realisierung gut kontrollierbarer quantenmechanischer Systeme die Möglichkeit eröffnet, Experimente als Quantensimulatoren für das Verhalten komplexer Vielteilchensysteme zu benutzen. Ultrakalte Atome in optischen Gittern eignen sich hervorragend als Simulatoren für simple Festkörpersysteme, da sich sämtliche Parameter der zugrunde liegenden Hamiltonoperatoren präzise kontrollieren lassen und der Zustand der Systeme mit einer Vielzahl an Messmethoden untersucht werden kann.

In unseren Experimenten realisieren wir Bose-Hubbard Systeme durch ultrakalte 39K Atome in blau verstimmten optischen Gittern. Zusätzliche optische Dipolpotenziale und magnetische Feshbach-Resonanzen erlauben es uns dabei, die Parameter der Systeme zu jedem Zeitpunkt beliebig zu variieren. Dadurch sind die von uns erzeugten Systeme in besonderem Maße dazu geeignet, Nichtgleichgewichtseffekte zu untersuchen.

Unser Hauptaugenmerk liegt auf der Untersuchung der Expansionsdynamik wechselwirkender Atome in homogenen Gittern. Wir beginnen unsere Experimente mit einem Anfangszustand im tiefen Gitter, der aus lokalisierten Atomen auf maximal einfach besetzten Gitterplätzen besteht. Durch gleichzeitiges schnelles Verringern der Gittertiefe und der externen Potenziale werden die Atome in ein homogenes Gitter entlassen und die Zeitentwicklung ihrer Dichteverteilung wird durch Absorptionsabbildungen festgehalten.

Es zeigt sich, dass sowohl die Wechselwirkung zwischen den Atomen als auch die Dimensionalität der Gitter einen starken Einfluss auf die Dynamik haben. In allen integrablen Grenzfällen des Bose-Hubbard Modells verhalten sich die Atome ballistisch und expandieren mit hoher Geschwindigkeit, doch sobald sich das System außerhalb der integrablen Regime befindet verringert sich die Expansionsgeschwind-igkeit drastisch. Diese verringerte Geschwindigkeit geht einher mit der Ausbildung charakteristischer bimodaler Dichteverteilungen, die auf eine diffusive Dynamik schließen lassen. Für stark wechselwirkende Systeme können wir einen dimensionalitätsabhängigen Übergang zwischen ballistischer Dynamik im 1D hard-core-regime und diffusiver Dynamik im 2D Fall beobachten sowie eine starke Verringerung der Expansionsgeschwindigkeit, wenn der Anfangszustand des Systems mehrfach besetzte Gitterplätze enthält. Des Weiteren beobachten wir die Erzeugung solcher Mehrfachbesetzungen nach dem Entlassen der Atome, deren schnelle Entwicklung auf eine lokale Relaxationsdynamik hin zu quasistationären Werten deuten lässt.

Als Letztes untersuchen wir die Entwicklung der Quasiimpulsverteilung stark wechselwirkender expandierender Atome, die laut theoretischer Vorhersagen eine vorübergehende Quasikondensation zeigen sollen, bei der sich scharfe lokale Maxima in der Quasiimpulsverteilung bei endlichen Quasiimpulsen bilden. Wir beobachten die Entstehung nicht-thermischer Quasiimpulsverteilungen die Maxima an den vor-hergesagten Positionen zeigen. Allerdings sind die von uns beobachteten Maxima wesentlich breiter als die vorhergesagten und wir diskutieren eine Reihe möglicher Erklärungen für diese Verbreiterung sowie Vorschläge zur Verbesserung zukünftiger Experimente.

The majority of stars in the universe has formed in disc galaxies with masses similar to that of the Milky Way.

Ab-initio cosmological hydrodynamical simulations of the formation and evolution of galaxies in a Lambda Cold Dark Matter universe have long suffered from serious problems in correctly modelling the star-formation history and structure of disc galaxies.

We first use idealized semi-cosmological simulations to gain a better understanding of processes leading to problems in disc formation simulations. We add rotating spheres of hot gas to cosmological dark-matter-only simulations of individual haloes and follow the formation and evolution of galaxy discs from the cooling gas. The initial orientation of the baryonic angular momentum with respect to the halo has a major effect on disc formation. Despite the coherently rotating initial conditions, the orientations of the disc and the outer gas and the relative angle between the components can all change by more than 90 degrees over several billion years. Dominant discs with realistic structural and kinematical properties form preferentially if slow cooling times shift disc formation to later times, if the initial angular momentum

is aligned with the halo minor axis and if there is little reorientation of the disc.

We then present a new set of fully cosmological simulations with an updated multiphase smoothed

particle hydrodynamics galaxy formation code. The update includes improved treatment of metal-line cooling, metal production, turbulent diffusion of metals, kinetic and thermal supernova feedback and radiation pressure from massive young stars. We compare the models to a variety of observations at high and low redshifts

and find good agreement for morphologies, stellar-to-dark-matter mass ratios, star formation rates,

gas fractions and heavy element abundances. Agreement is better at redshift z=1 than at present day as discrepancies in star formation histories for the lowest and highest simulated galaxy masses become apparent at late times. 18 out of 19 of our model galaxies at z=0 contain stellar discs with kinematic disc fractions up to 65 %, higher than in any previous simulations.

We finally compare our model galaxies in detail with recent observations of the structural evolution of stellar galactic discs and the structure of z=0 gas discs. Stellar surface density profiles agree well with observations at z>1, but reveal too little central growth afterwards. This is likely connected to a lack of

bars in our simulations resulting from overly strong feedback. Discs at z=0 are too

extended by a factor \sim 2. The discs have diverse formation histories ranging from pure inside-out growth in systems with quiescent merger histories to continuous mass growth at all radii. Central mass growth in our models is driven by mergers and misaligned infall events, which leave signatures in the present day distributions of radii and element abundances as functions of stellar age. Gas discs agree well with observations in terms of sizes and profile shapes, but on average have overly high gas-to-stellar mass ratios. Our models agree well with the observed neutral hydrogen mass-size relation.

Despite significant progress, our models continue to suffer from various problems illustrating that we are still far away from capturing all relevant physical processes accurately.

Semiconducting single-walled carbon nanotubes (CNTs) exhibit a chirality depended band structure of a one-dimensional lattice. Due to the radiative recombination of excitons CNTs emit photoluminescence in the near and mid infrared ranges depending on the tube diameter. Excitons are subject to diffusion along the tube before radiative recombination. Thereby they probe sites that give rise to spin-flips or non-radiative decay, or, at cryogenic temperatures, they localize in zero-dimensional quantum dots at the minima of the local energy potential landscape. Thus, the optical spectroscopy of individual CNTs probes not only the intrinsic exciton dynamics, like diffusion and intrinsic life-time, but also disorder of the CNT lattice and its environment. Intrinsic and extrinsic inhomogeneities and impurities may give rise to photoluminescence quenching, brightening of dark exciton states or generation of charged exciton complexes.

In the framework of this thesis the physics of excitons in CNTs was investigated in two ways: On the one hand their environment was varied with an static electric field, on the other hand the CNTs were isolated from their environment. A comprehensive set of optical spectroscopy techniques was used to study individual CNTs at low temperatures. This included photoluminescence excitation, (time-resolved) photoluminescence, and photon correlation spectroscopy.

This work identified exciton localization as predominant feature of individual CNTs at cryogenic temperatures. CNTs on substrate exhibited asymmetric line shapes at low temperature and temperature dependent shifts on the PL energy. Moreover for constant temperature, PL energies were subject to spectral diffusion, which arose - in analogy to compound semiconductor quantum dots - from interaction with a few close charge fluctuators in the dielectric environment. In addition, evidence for exciton localization was provided by the non-classical photon emission statistics of cryogenic CNTs.

The main focus of this thesis was the study of individual CNTs in a static electric field. A metal-oxide-semiconductor device was used to probe for the transverse polarizability of excitons. In consequence, the PL energy of CNTs exhibited red-shifts as a quadratic function of the perpendicular electric field. However, a subclass of CNTs was characterized by satellite peaks in the emission profile. By their energy splitting they were assigned to PL emission from dark exciton states, e.g. triplet and k-momentum excitons, and resulted presumably from impurity induced symmetry breaking. As a function of the electric field, CNTs with a broken symmetry featured linear shifts of the PL energy of bright and triplet excitons. A third energy scale in the exciton fine structure was manifested by CNTs that exhibited the emergence of a satellite peak as a function of the electric field. These satellites were assigned to the PL of trions generated by doping of individual CNTs with charges from close oxide states. Presumably such close charge states played also an important role in the variation of the excitation spectra of individual CNTs, which was observed as a function of the applied electric field. This variation could be mediated by switching of charge states, which varied the localization potential of excitons.

Finally, the extrinsic effects of the surrounding dielectric medium were contrasted by the remarkable optical properties of as-grown suspended CNTs. Freely suspended CNTs featured isolated localized excitons with narrow linewidths, intrinsic exciton lifetime and a significantly increased quantum yield. Moreover, they lack signatures of spectral diffusion or intermittency even on the shortest timescales.