Contact Name: Donatella Romano
Abstract:
The reionisation of the all-pervading intergalactic medium (IGM) is a landmark event in the cosmic history of structure formation. Still, despite much recent progress, a coherent description of the thermal state and ionisation degree of the IGM, the repository of most of the baryons across the history of the Universe, remains elusive. Most of our understanding of IGM physics, and its implication for galaxy formation and metal enrichment, depends critically on the properties of the cosmic ionising background. While it is generally thought that the gas is kept ionised by the integrated UV emission from active galactic nuclei and star-forming galaxies, the relative contributions of these sources as a function of cosmic time are poorly known. While existing synthesis background models are successful in reproducing the main observational constraints, such as the HI ionisation rate vs redshift, the temperature evolution of the IGM, and the Thomson scattering opacity of the IGM, many uncertainties still exist, and background models are now facing hard challenges, when confronted to new observations both in the local Universe and at high redshifts. In this talk I will review our current understanding of the cosmic ionising background, I will discuss the main uncertainties affecting existing models, and sketch possible strategies aimed at improving their reliability.