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Multifrequency Tomography of the Reionization Epoch

Final Report Summary - TOMOGRAPHYEOR (Multifrequency Tomography of the Reionization Epoch)

The aim of the project was to study the reionization of the Universe caused by the appearance of the first stars within the history of structure formation. In this context, it is important to consider the processes of galaxy mergers, since they lead to enhanced star formation. Originally, the project contemplated the study of the contamination of the intergalactic medium with metals produced during star forming processes, and the use of the metal distribution as a probe of the reionization epoch. However, due to the relative importance of metal and transition lines with respect to the continuum stellar emission, it was also relevant to study the contribution of the continuum radiation of stars to the ionizing background. The joint radiation of all the star-forming processes that took place since the reionization of the Universe, observed in ultraviolet (UV), optical,
and near-infrared light, constitutes the extragalactic background light (EBL), and it is a very important probe of the star formation history of the Universe.

As stated before, star formation is highly enhanced during merging-galaxy processes, which increases largely the luminosity of the galaxies. Therefore, we studied the distribution of merging galaxies using a model for the distribution of dark matter halos and using different models for their merging history. Moreover, we modeled how star formation proceeds during a merging phase, based on results of numerical simulations in the literature.

In addition, we consider the details of realistic stellar spectra in the spectral energy distribution of a star-forming galaxy, and their importance on the observed EBL at optical and near-infrared wavelengths.

The EBL can be observed at all directions in the sky. We have computed the mean intensity of the EBL in the sky from our model, and compared it with current observations. We have studied which is the contribution that comes from different cosmic times (redshifts).

We have computed also the angular fluctuations of this radiation, and compare the spectrum of fluctuations with observations. The importance of our model is that it gives a simple physical explanation of the EBL, including a realistic description of the stellar emission in a star-forming galaxy, which have non-negligible effects on the derived predictions. We have taken into account how the light is affected by dust inside the galaxies, and how this would vary during different cosmic times.

Additionally, we have studied which is the contribution of evolved stars in the ionizing background at high redshifts, and the observed EBL. We drawn interest conclusions about the importance of the bright short-lived stages of a massive star after it leaves the main sequence on the spectral energy distribution of a merging galaxy in a young star-forming phase.

Finally, we have found that the most important contributions to the mean intensity and angular fluctuations of the EBL are coming from intermediate redshifts, and that the study of reionization epoch using this tool is very difficult due to the impossibility to disentangle the emission produced at different redshifts.