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Understanding of solar and stellar brightness variability plays a crucial role in the studies of solar-stellar and solar-terrestrial connection. In particular, the modeling of solar brightness variations is important for understanding the role of the Sun in the climate variability, while studying the stellar variability allows to better constrain the physics of the stellar activity. Since the launch of the NIMBUS-7 mission in 1978 the solar brightness has been continuously monitored and has been found to vary on all time scales on which it has ever been measured. Although the number of solar brightness datasets has been increasing over the last years, the observations alone do not provide a sufficient means to understand the influence of solar radiation on climate due to large uncertainties and gaps in the available datasets. This calls for the devel- opment of solar brightness modeling in order to complement the observational data. The physics-based modeling of solar and stellar brightness variations relies on the spectra of magnetic features and surrounding quiet regions in the solar and stellar atmospheres. These spectra are provided by radiative transfer codes. Currently the radiative transfer codes oriented towards such modeling represent the stellar atmospheres as a one-dimensional structure. Development of 1D radiative transfer codes is a challenging task due to the Non-Local Thermodynamic Equillibrium (NLTE) coupling of matter and radia- tion. As a first approximation one can assume full coupling, i.e. Local Thermody- namic Equillibrium (LTE), but by now a substantial evidence has been acquired pointing to the inadequacy of this approximation. Hence, today one of the focal points of solar and stellar physics is the development of NLTE radiative transfer codes which is the main goal of this thesis. In Chapter 2 we present the NLTE Spectral SYnthesis (NESSY) code. The code was originally designed for modeling the spectra of hot stars with expand- ing atmospheres. This purpose predisposed the numerical scheme of the code to work in a way that is not efficient for spectrum synthesis under solar-like con- ditions in which the expansion is much less pronounced. The aim of our work was to adjust the code so that it can handle both expanding and non-expanding cases. Such an adjustment was a significant step on the way to make NESSY efficiently applicable to the synthesis of spectra emerging from all kinds of stars. It required a complete change of the algorithms of the code responsible for the NLTE calculations. The new version of the code is very well suited for spec- tral synthesis over broad spectral ranges which is required for modeling of solar and stellar brightness variations. In the following chapters we demonstrate the capabilities of the code in the exemplary case of the Sun. In Chapter 3 we apply the code to modeling the Center-to-Limb Variation(s) of solar brightness (CLV) which are a powerful diagnostic tool for constraining the models of solar atmosphere. They are also important for modeling of the solar brightness variations especially on the time scale of solar rotation. We compare the CLVs modeled with NESSY to those derived from the measurements of solar brightness variations during eclipses observed with the PREMOS instrument on- board the PICARD mission. We use the light curves of the three solar eclipses measured by the radiometers of PREMOS to derive CLVs in the UV, visible and IR parts of the solar spectrum. We show that in the visible and IR the modeled CLVs agree well with those derived from the eclipse observations which proves that NESSY can not only reproduce the full-disk solar spectrum, but also the distribution of brightness across the solar disk. In the UV the derived CLVs allow us to constrain the source of the so-called “missing opacities” and make a step toward the resolution of this well-known problem arising from the lack of laboratory measurements of lines constituting the UV solar spectrum within 160 nm - 320 nm range. Chapter 4 is devoted to the first ever physics-based reconstruction of the solar brightness variability in the radio. This reconstruction became possible thanks to the changes of the code described in Chapter 2. The NLTE effects are important for the formation of radio wavelengths and therefore with NESSY we could for the first time consistently model the brightness variability of the Sun in the radio. We can model both the variability from UV (where the LTE assumption also fails) to IR and in the radio without any empirical corrections. In Chapter 4 we take advantage of this capability and show how radio wavelengths can be used for reconstruction of of variability of the entire solar spectrum from UV to IR.

Most present-day galaxies with stellar masses of about 10^11 M⊙ show no ongoing star formation and are dense spheroids. Ten billion years ago, at a redshift of z~2, similarly massive galaxies were typically forming stars at rates of hundreds of solar masses per year. It is debated how star formation ceased, on which timescales, and how this “quenching” relates to the emergence of dense spheroids. This Ph.D. thesis presents results on the growth of galactic components in typical star-forming galaxies and delivers a major step forward in understanding how these galaxies stopped their star formation. The core of these results is based on observations, in particular with the Hubble Space Telescope and the Very Large Telescope in Chile. We have also developed and investigated numerical and analytical models to interpret our observations.

ISBN:978-3-86727-880-5

We present a photometric and spectroscopic study of galaxies at 0.5, Comment: 11 pages, 7 figures, accepted for publication in PASJ