Starburst galaxies undergo a phase of vigorous star formation. In these galaxies, gas is turned into stars at such a high rate, that the reservoir of available gas is quickly emptied. Numerous... Show moreStarburst galaxies undergo a phase of vigorous star formation. In these galaxies, gas is turned into stars at such a high rate, that the reservoir of available gas is quickly emptied. Numerous massive, bright star clusters are created from the giant molecular clouds. They form an ideal laboratory for the study of rapid star formation. In this thesis, a detailed study of active star-forming regions in the starburst galaxies NGC 4038/4039 and M83 is presented. Sincethe stars are hidden from view by large quantities of dust, we observe them at infrared wavelengths. We address the properties of the newly formed star clusters and the surrounding interstellar matter. Show less
The formation of complex organic molecules that consist of more than four atoms in space is one of the main questions in the field of astrochemistry and star formation. Although the exact formation... Show moreThe formation of complex organic molecules that consist of more than four atoms in space is one of the main questions in the field of astrochemistry and star formation. Although the exact formation mechanisms are not yet known, they are expected to form in thin ice layers on the surfaces of small interstellar dust grains through successive addition of H, C, N or O atoms to CO (carbon monoxide). In this thesis the formation of these molecules is studied in two different ways: simulation of interstellar ices analogues in the laboratory and observations of the same molecules after evaporation toward star forming regions. The laboratory experiments are high and ultra high vacuum setups in which ices of e.g. CO, CO2, HCOOH and CH3CHO are frozen out on an inert surface. The spectroscopy and the thermal behavior of pure and layered ices have been studied. Furthermore, the ices have been bombarded with H-atoms to test reactions schemes relevant for astronomical environments. In the second part of this thesis the same molecules have been observed with the single dish submillimeter telescopes the __James Clerk Maxwell Telescope__ at Hawaii and the Institut de Radioastronomie Millim_trique in Spain toward a sample star forming regions as well as with interferometer the SubMillimeter Array at Hawaii toward two sources. The relative abundances of molecules in different star forming regions measured with the single dish telescopes as well as the spatial extent of the emission detected with the interferometer has been used to determine the chemical relations between complex organics that have also been studied in the laboratory. Show less
In this thesis we study the dust around solar-type young stars. In particular, we focus on one specific species of dust, namely the Polycyclic Aromatic Hydrocarbons (PAHs), a family of large... Show moreIn this thesis we study the dust around solar-type young stars. In particular, we focus on one specific species of dust, namely the Polycyclic Aromatic Hydrocarbons (PAHs), a family of large molecules, or small grains, that are widely observed in nearby star-forming regions. We address the following questions. What happens to PAHs in the embedded phase of a forming star? Are PAHs present in low-mass young star systems? Does the PAH emission originate from the envelope or from the disk? What do they tell us about disk structure and evolution and grain growth? What can we say about the evolution of PAHs during star formation and their typical size? We present mid-infrared spectroscopy and imaging surveys combined with 3D radiative transfer models to constrain the presence and location of PAH emission toward embedded young stellar objects and circumstellar disks around young solar-type stars. PAHs are detected toward a small fraction (11-14%) of young solar-type stars with disks and toward a minority of embedded objects (<3%), with derived abundances of 10-100 times lower than standard interstellar values. A new class of disks with weak mid-IR continuum emission and very strong PAH features is found. Show less
From its origin at the center of a star to the edge, through the surrounding gas and dust in the distant galaxy, through the intergalactic medium, traveling billions of light years only to be... Show moreFrom its origin at the center of a star to the edge, through the surrounding gas and dust in the distant galaxy, through the intergalactic medium, traveling billions of light years only to be reflected by a mirror and captured by a detector; the little amount of light observed from galaxies in the early universe contains a wealth of historic information. This thesis concentrates on translating the luminosities and colors of distant galaxies to physical properties such as distance, mass, age of the stellar population, and dust content. Analyzing deep optical and infrared observations of distant (redshift z~2.5) galaxies, we learn that massive galaxies have on average redder colors than less massive galaxies, although simulations show that mass underestimates of star-forming galaxies are possible. Roughly half of the red galaxies at high redshift owe their color to an old, quiescent stellar population. The other half is still actively star-forming, but obscured by large columns of dust. The abundance of star-forming and quiescent galaxies is consistent with a model that assigns a key role to collisions between gas-rich disk galaxies involving quasar activity, and leaving red spheroids as remnants. However, this model does not reproduce the colors of dusty red galaxies. Show less
A key issue in astronomy today is understanding the star-formation and assembly history of massive galaxies. Stellar population studies show that the bulk of the stars in low-redshift massive... Show moreA key issue in astronomy today is understanding the star-formation and assembly history of massive galaxies. Stellar population studies show that the bulk of the stars in low-redshift massive galaxies is formed at z~2 or even higher. Furthermore, there are strong indications that about 50% of the massive, quiescent galaxies in the local universe was already in a quiescent phase when the universe was only half its current age. This raises the question of whether quiescent galaxies can be found at even earlier epochs, and when massive galaxies formed and assembled their stellar mass. In order to address these questions we have pushed current studies to higher redshift and conducted a near-infrared spectroscopic survey of massive galaxies at z~2.5. In total we obtained deep spectra with a wavelength coverage of 1-2.5 micron for a sample of 36 galaxies. A surprising result of our survey is that a significant fraction of the massive galaxies at z~2.5 is already in a quiescent phase. Furthermore, several massive galaxies host active galactic nuclei, which may be responsible for the quenching of star formation. This thesis introduces the survey, presents these main results, and discusses the resulting constraints on the formation history of massive galaxies. Show less
Radial velocity variations in K giants are observed on long (order hundreds of days) and short (order hours) time scales. Short term variations are most likely caused by solar-like oscillations,... Show moreRadial velocity variations in K giants are observed on long (order hundreds of days) and short (order hours) time scales. Short term variations are most likely caused by solar-like oscillations, stochastically excited in the turbulent atmosphere of the stars. Long term variations can be caused by either sub-stellar companions, pulsations or other mechanisms intrinsic to the star. The possible cause of observed long radial velocity variations in K giants is investigated with some different diagnostics. First, a strong correlation between surface gravity and the radial velocity amplitude is found. This indicates that it is likely that the radial velocity variations are induced by a mechanism intrinsic to the star. In addition, for a number of stars the radial velocity variations can be fit with a Keplerian orbit, indicating the possible presence of a sub-stellar companion. The orbital parameters of these inferred sub-stellar companions would be considerably different from those orbiting main sequence stars. But, there is growing evidence that more massive stars harbour more and more massive sub-stellar companions. In order to reveal the mechanism causing the radial velocity variations in individual stars, temperature and line depth variations at different epochs for a sub-sample of K giants are measured. Show less
Dark matter assembles to form massive halos, attracting gas particles that coalesce to form stars, and the photons produced by these sources are sent into the void. An accurate description of the... Show moreDark matter assembles to form massive halos, attracting gas particles that coalesce to form stars, and the photons produced by these sources are sent into the void. An accurate description of the transport of particles is therefore the essential ingredient for understanding how the Universe evolved into its present form. Of all these transport processes, the transfer of radiation is the most elusive: the non- locality and high-dimensionality of the problem currently put realistic simulations out of reach. In this thesis, we present a novel approach to this transport problem, combining methods from mathematics and statistical physics in a practical computer code. Its speed and versatility allow us, for the first time, to make realistic simulations in radiative gas dynamics and in cosmology. Because of the general mathematical/physical character of our methods, we expect them to be useful for transport phenomena in general. Show less
