Application of photometric redshifts on the correlation properties of galaxies and matter

Abstract

In the past years cosmology, the science of the universe as a whole, has seen tremendous progress. The Lambda-Cold-Dark-Matter scenario is widely accepted as the standard model of cosmology describing the evolution of the universe and its main constituents. Most cosmological parameters are known to a few percent accuracy now. This concordance cosmological model together with the basic theory of cosmological physics is presented in Chap. 1.<br /> Albeit, the main contributors to the energy density of the universe, Dark Matter (DM) and Dark Energy (DE), have not been observed so far in the laboratory. We can now predict the expansion history, the age, the energy density, etc. of the universe but we do not know the physical origin of the majority of the ingredients driving this cosmic evolution.<br /> There is hope that dark matter particles will be detected in the laboratory soon, either via passive detectors that measure the properties of these hypothetically weakly interacting massive particles (WIMPs) penetrating through the Earth, or actively via events created in the next generation of particle accelerators like the Large Hadron Collider.<br /> For DE, however, the situation is different. No concept was presented yet to detect this component, which is responsible for the accelerated expansion of the universe, in the laboratory. With its physical origin being completely unknown the only way to learn more about its properties, like e.g. its equation of state describing the time-evolution, will be from observational cosmology.<br /> Different methods are proposed to shed light on the nature of DE. One of them is the measurement of coherent distortions in the shape of galaxies due to the gravitational deflection of light by the large-scale structure of the universe, called cosmic shear. Another one is the detection of baryonic acoustic oscillations in the two-point correlation function of galaxies. The accuracy of both of these promising methods for constraining DE properties depends heavily on the measurement of redshifts, hence distances, of many million galaxies. This cannot be done in the traditional way by taking spectra for these large samples. Rather, approximate redshifts, called photometric redshifts, must be estimated from the colours of the galaxies.<br /> While the determination of cosmological parameters like the equation of state of DE is not the subject of this thesis, the photometric redshift technique is introduced and analysed in great detail in Chaps. 4 & 5. Understanding the efficacy of this tool and its shortcomings is essential for many large future survey projects tackling the questions above.<br /> Besides these purely cosmological questions which are hoped to be answered by the measurements of galaxy properties, it is the galaxy population itself we are interested in. In particular, we still have no precise picture about how galaxies form and how they evolve. The behaviour of the DM component which is the dominant driver of cosmological structure formation seems to be well understood through large N-body simulations, although we do not know about the nature of the DM particles. In contrast to this, the formation and evolution of galaxies involve mainly well-known baryonic physics. But the processes involved like star-formation, hydrodynamics, radiative feedback, etc. are so complicated that we are still far from a coherent description of galaxy formation and evolution.<br /> This ignorance is partly caused by the fact that we cannot observe a galaxy form and evolve directly because of the very long timescales for these processes. Due to the finite speed of light, looking at increasingly distant/redshifted galaxies means looking at younger objects. One main task to understand the physics of galaxy evolution is to identify which objects at an early cosmic epoch evolve into which type of galaxies observed today. Again, photometric redshift and similar techniques like the Lyman-break technique can be applied to select galaxies at different epochs. The properties of these samples, e.g. their clustering, can be studied and compared to numerical simulations. By doing so one gets insight into the relationship between the properties of luminous matter in form of galaxies and the underlying structure in form of DM halos. If this is done for several cosmic epochs, the evolution of galaxies can be understood in more detail because the evolution of the halo population is well-known from simulations.<br /> We contribute to the field of galaxy formation and evolution in this thesis by analysing the clustering properties of an unprecedented large sample of galaxies at redshift z~3. The selection of these Lyman-break galaxies (LBGs), the simulation of their properties, and the measurement of their two-point correlation function is described in Chap. 6. As a result we obtain estimates for the masses of the halos that host these galaxies, and these masses are compared to estimates at different redshifts from other studies to detect evolutionary trends in the galaxy-DM relationship.<br /> Neither the photometric redshift analyses nor the study of z~3 Lyman-break galaxies would be possible without high-quality imaging data from a modern multi-chip CCD camera. The general concepts of the complex processing of the raw data to reach scientifically exploitable images, also called data reduction, is presented in Chap. 2. These techniques are applied to a specific dataset, the optical data of the ESO Deep Public Survey (DPS), which forms the basis of most analyses presented in this thesis.