Probing Cosmic Reionization with Neutral Hydrogen 21cm Line

Keitaro Takahashi (Kumamoto University)

Cosmic reionization is an important era in the history of the universe, when intergalactic neutral hydrogen was ionized by radiation from high-redshift galaxies. So far, cosmic reionization has been observationally studied mainly by galaxy surveys and cosmic microwave background observation and it is well known that cosmic reionization was completed at around z~6. However, its beginning and evolution, and the source of ionizing photons are still unknown. These problems are deeply related to the formation and properties of first stars, first galaxies and first supermassive black holes so that cosmic reionization has attracted much attention of broad astronomical community. One of the most powerful probes of cosmic reionization is 21cm radiation from neutral hydrogen, from which we can obtain information on the spatial distribution of intergalactic neutral hydrogen and its time evolution. However, the 21cm signal has never been detected yet, due to the weakness of the signal, presence of huge foreground and difficulty of precise calibration. In this talk, I will review theoretical and observational studies of 21cm line radiation from cosmic reionization. Especially, I focus on several ongoing projects which are aiming to detect 21cm line radiation such as MWA, LOFAR and HERA, and a next-generation project SKA.