The Thermal and Ionization state of the Universe in past 12.8 billion years

The cosmic evolution of large-scale structures along with the astrophysical processes of galaxy formation and evolution leads to the (re)ionization of the low-density intergalactic medium (IGM). The changes in the thermal and ionization state of the IGM can be used to measure the reionization history. While much of the current and future work is focused on understanding and detecting the HI reionization (z>6), the epoch of HeII reionization (4>z>2) is observationally more accessible with existing telescopes. However, the main challenges lie in modeling the reionization and understanding the observational systematics. In this talk, the speaker will discuss the modeling of reionization in the state-of-the-art cosmological hydrodynamical ( gadget-3) and radiative transfer (aton) simulations. The speaker will show their recent measurements of ionization and thermal parameters from the UV (HST-COS), optical (Keck, ESO) and infrared (MIKE) observations of Ly-alpha forest at 6>z>0. The implication of these measurements to reionization models, Galaxies/QSO properties and feedback processes will be discussed in the second part. Finally, the speaker will discuss the prospects of the upcoming space-based facility JWST, ground-based radio facility SKA and next-generation 30/40 m class optical telescopes TMT/E-ELT in measuring the reionization history and detecting the faint sources of reionization at z>6.