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Basu, Kaustuv (2004): CMB observations and the metal enrichment history of the universe. Dissertation, LMU München: Faculty of Physics
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Abstract

The main purpose of the work presented in this thesis is to investigate the phenomenon of resonant scattering of the Cosmic Microwave Background (CMB) photons by atoms and molecules. The fine-structure transitions of the various atoms and ions of Carbon, Nitrogen, Oxygen and other common metals have wavelengths in the far-infrared regions, which are particularly suitable for scattering the CMB photons at high redshifts ($2 \lesssim z \lesssim 30$). Since the CMB photons are released at redshifts $z\simeq 1100$, they must interact with all the intervening matter before reaching us at $z=0$. Therefore scattering of these photons in the far-IR fine-structure lines of various atoms and ions provide a plausible way to couple the radiation with the matter at those redshifts and to study the enrichment and ionization history of the universe. Moreover, rotational transitions of diatomic molecules like the CO have wavelengths extending into the sub-millimeter wavebands, and hence they can scatter the CMB photons at very low redshifts. Studying the very low density gas of nearby galaxies in CO lines can yield a definitive signature of resonant scattering of the CMB photons through a decrement in the background intensity of the microwave sky. Observation of this scattering signal from any object in the sky will tell us about its radial velocity in the CMB rest frame. In this work we first derive the detailed formalism for the scattering effect in presence of the peculiar motion of the scatterer. Then we investigate the possibility to detect individual objects at different redshifts through scattering and try to find applications for this effect. Our main example is the possibility to find the peculiar motions of nearby galaxies in the CMB rest frame through observation of the scattering signal, which we explore in detail. Next we discuss the density limits in which scattering effect can dominate over the line emission in individual objects. We describe three types of critical densities, and show that detection of single objects through scattering requires very low density, whereas observation of the integrated scattering signal coming from many unresolved objects in the sky will permit us to probe higher densities. We discuss this effect subsequently, as we compute the change in the angular fluctuations of the CMB sky temperature through resonant scattering. We found that the scattering signal gets strong enhancement due to a non-zero correlation existing between the density perturbations at the last scattering surface, where CMB anisotropies are generated, and at the epoch of scattering. This opens up a new way to study the ionization and enrichment history of the universe, and we investigate various enrichment scenarios and the temperature fluctuations that might be caused by them. The resulting signal is already within the sensitivity limits of some upcoming space- and ground-based CMB experiments, and we show upto what extent they shall be able to put constraints on different enrichment histories. Finally we analyze the effect of line and dust emission in the same frequency range that we used for the detection of scattering signal. These emissions are coming from very high density objects where active star formation is taking place, and due to the compactness of their size as well as absence of any velocity dependence the emission signal is significantly suppressed at large angular scales, where scattering will be dominant. We present some detailed analytic expressions for the scattering signal and also a method to solve for the detailed statistical balance equations in a multi-level system in the appendix.