This thesis explores the evolution of galaxies from the onset of star formation shortly after the Big Bang until the present day. Particular emphasis lies on the redshift range z = 1.4 2.5, the so-called “redshift desert”, as it coincides with the peak epoch of cosmic star formation activity and mass assembly. Most of the information about galaxies and their evolution arrives in the form of their integrated light, i.e. the conglomeration of light emitted by stars of various ages and metallicities. In order to interpret the observed spectra and magnitudes, and to extract the physical parameters we therefore require models. This holds true in particular for galaxies too faint to target them spectroscopically, and for which redshifts and physical parameters derived from only their photometry is the only feasible way to study them in more detail. This thesis is concerned with such models, and describes how GALEV evolutionary synthesis models describe the spectral and chemical evolution of galaxies, accounting for gaseous emission and the increasing initial abundances of successive stellar generations, how they compare to observations and what we can learn from their application. Based on a large model grid, covering all observed galaxy evolution stages, I find that sub-solar metallicities have significant impact on the spectra of galaxies, and can lead to systematic errors and biases if not accounted for. A comparison of models with different metallicities furthermore reveals that photometric redshifts are also systematically biased if sub-solar metallicities are not properly accounted for. I also note that even a small mass-fractions of young stars can dominate the overall spectrum, leading to a large underestimation of the mass and age of the stellar population. The models explain not only the colour evolution of galaxies observed at a range of redshifts, but also their physical parameters. I show that with magnitudes in only a few bands we can successfully explain not only the masses of galaxies, but also their star formation rates and, where available from observations, their metallicities. If additional data are available, the grid of models can be used to refine colour selection criteria and to break degeneracies, e.g. between dust-reddened actively star-forming galaxies and intrinsically old, passively evolving galaxies. Using GAZELLE, a photometric redshift code that is purpose-tailored to harmonise with these models, I can extract accurate redshifts and a wealth of physical parameters from the largest ever sample of observed multi-wavelength photometry of galaxies. I then compare our findings with semi-analytical models that trace the evolution of individual galaxies based on cosmological simulations. In my sample I find a significant population of high-mass galaxies that is not accounted for by this class of models. Furthermore a small percentage of massive, yet starforming galaxies challenges our idea on how these galaxies form and evolve. In an appendix to this thesis I present a complementary approach to reconstruct the evolution of galaxies, using star clusters as tracers. I introduce a new technique to break the age-metallicity degeneracy and obtain individual ages and metallicities for a sample of globular clusters, revealing a merger of two Sb/Sc-type spirals 2 Gyrs ago in NGC 4570, a lenticular galaxy in the Virgo cluster. Also in the appendix I show that, at least in the studied galaxy Arp 78, the initial mass function conforms with our assumptions and does not change in low-density environments as recently predicted. Although studies of galaxy evolution are a major field in astronomy, there is still a lot more to be done to reveal the inner workings of these island universes, and this thesis also addresses how to continue and improve the work presented herein.

Eine der zentralen Fragestellungen der modernen Astrophysik ist es, unser Verständnis fuer die Bildung von Sternen und Sternhaufen in unserer Milchstrasse zu erweitern und zu vertiefen. Sterne entstehen in interstellaren Wolken aus molekularem Wasserstoffgas. In den vergangenen zwanzig bis dreißig Jahren ging man davon aus, dass der Prozess der Sternentstehung vor allem durch das Wechselspiel von gravitativer Anziehung und magnetischer Abstossung bestimmt ist. Neuere Erkenntnisse, sowohl von Seiten der Beobachtung als auch der Theorie, deuten darauf hin, dass nicht Magnetfelder, sondern Überschallturbulenz die Bildung von Sternen in galaktischen Molekülwolken bestimmt.<br /> <br /> Diese Arbeit fasst diese neuen Überlegungen zusammen, erweitert sie und formuliert eine neue Theorie der Sternentstehung die auf dem komplexen Wechselspiel von Eigengravitation des Wolkengases und der darin beobachteten Überschallturbulenz basiert. Die kinetische Energie des turbulenten Geschwindigkeitsfeldes ist typischerweise ausreichend, um interstellare Gaswolken auf großen Skalen gegen gravitative Kontraktion zu stabilisieren. Auf kleinen Skalen jedoch führt diese Turbulenz zu starken Dichtefluktuationen, wobei einige davon die lokale kritische Masse und Dichte für gravitativen Kollaps überschreiten koennen. Diese Regionen schockkomprimierten Gases sind es nun, aus denen sich die Sterne der Milchstrasse bilden. Die Effizienz und die Zeitskala der Sternentstehung hängt somit unmittelbar von den Eigenschaften der Turbulenz in interstellaren Gaswolken ab. Sterne bilden sich langsam und in Isolation, wenn der Widerstand des turbulenten Geschwindigkeitsfeldes gegen gravitativen Kollaps sehr stark ist. Überwiegt hingegen der Einfluss der Eigengravitation, dann bilden sich Sternen in dichten Gruppen oder Haufen sehr rasch und mit grosser Effizienz. <br /> <br /> Die Vorhersagungen dieser Theorie werden sowohl auf Skalen einzelner Sternentstehungsgebiete als auch auf Skalen der Scheibe unserer Milchstrasse als ganzes untersucht. Es zu erwarten, dass protostellare Kerne, d.h. die direkten Vorläufer von Sternen oder Doppelsternsystemen, eine hochgradig dynamische Zeitentwicklung aufweisen, und keineswegs quasi-statische Objekte sind, wie es in der Theorie der magnetisch moderierten Sternentstehung vorausgesetzt wird. So muss etwa die Massenanwachsrate junger Sterne starken zeitlichen Schwankungen unterworfen sein, was wiederum wichtige Konsequenzen für die statistische Verteilung der resultierenden Sternmassen hat. Auch auf galaktischen Skalen scheint die Wechselwirkung von Turbulenz und Gravitation maßgeblich. Der Prozess wird hier allerdings noch zusätzlich moduliert durch chemische Prozesse, die die Heizung und Kühlung des Gases bestimmen, und durch die differenzielle Rotation der galaktischen Scheibe. Als wichtigster Mechanismus zur Erzeugung der interstellaren Turbulenz lässt sich die Überlagerung vieler Supernova-Explosionen identifizieren, die das Sterben massiver Sterne begleiten und große Mengen an Energie und Impuls freisetzen. Insgesamt unterstützen die Beobachtungsbefunde auf allen Skalen das Bild der turbulenten, dynamischen Sternentstehung, so wie es in dieser Arbeit gezeichnet wird. / Understanding the formation of stars in galaxies is central to much of modern astrophysics. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift. Recently, however, both observational and numerical work has begun to suggest that supersonic interstellar turbulence rather than magnetic fields controls star formation. <br /> <br /> This review begins with a historical overview of the successes and problems of both the classical dynamical theory of star formation, and the standard theory of magnetostatic support from both observational and theoretical perspectives. We then present the outline of a new paradigm of star formation based on the interplay between supersonic turbulence and self-gravity. Supersonic turbulence can provide support against gravitational collapse on global scales, while at the same time it produces localized density enhancements that allow for collapse on small scales. The efficiency and timescale of stellar birth in Galactic gas clouds strongly depend on the properties of the interstellar turbulent velocity field, with slow, inefficient, isolated star formation being a hallmark of turbulent support, and fast, efficient, clustered star formation occurring in its absence. <br /> <br /> After discussing in detail various theoretical aspects of supersonic turbulence in compressible self-gravitating gaseous media relevant for star forming interstellar clouds, we explore the consequences of the new theory for both local star formation and galactic scale star formation. The theory predicts that individual star-forming cores are likely not quasi-static objects, but dynamically evolving. Accretion onto these objects will vary with time and depend on the properties of the surrounding turbulent flow. This has important consequences for the resulting stellar mass function. Star formation on scales of galaxies as a whole is expected to be controlled by the balance between gravity and turbulence, just like star formation on scales of individual interstellar gas clouds, but may be modulated by additional effects like cooling and differential rotation. The dominant mechanism for driving interstellar turbulence in star-forming regions of galactic disks appears to be supernovae explosions. In the outer disk of our Milky Way or in low-surface brightness galaxies the coupling of rotation to the gas through magnetic fields or gravity may become important.

In this thesis I investigate the dynamics and stellar populations of a sample of 28 edge-on early-type (S0--Sb) disk galaxies, 22 of which host a boxy or peanut-shaped bulge. I begin by constructing mass models of the galaxies based on their observed photometry and stellar kinematics. Subject to cosmologically motivated assumptions about the shape of dark haloes, I measure in a purely dynamical way their stellar and dark masses. I make a preliminary comparison between the dynamically determined stellar masses and those predicted by stellar population models. I then compare the Tully-Fisher (luminosity--velocity) relations of the spirals and S0s in the sample. I show that S0s are systematically fainter at a given rotational velocity, but the amount by which they are fainter is less than expected by models in which they are the products of truncation of star formation in spirals. This raises the possibility that S0s are smaller or more concentrated than spirals of the same mass. I then study the vertical structure of the boxy and peanut-shaped bulges of a subset of the sample. Among this sample of five galaxies, I find one example in which the stellar populations show no evidence that the bulge and the disk formed in different processes, and in which the bulge is in perfectly cylindrical rotation, i.e. its line-of-sight velocity does not change with height above the disk. This galaxy is probably a pure disk galaxy. However, even with this small sample, I also show that cylindrical rotation and homogeneous stellar populations are not ubiquitous properties of boxy and peanut-shaped bulges. Finally I analyse central and radial trends in the stellar populations of the bulges of full sample of 28 galaxies. I find that, at a given velocity dispersion, the central stellar populations of these barred early-type disk galaxies are identical to those of elliptical galaxies, which suggests that secular evolution does not dominate the centre of these galaxies. However, the radial metallicity gradients are shallower than those of ellipticals. This is qualitatively consistent with chemodynamical models of bar formation, in which radial inflow and outflow smears out pre-existing gradients.