Radio astronomical probes of cosmic reionization and the first luminous sources: probing the 'twilight zone'
The epoch of reionization (EoR) corresponds to a 'cosmic phase transition', when the neutral intergalactic medium (IGM) becomes ionized by the first stars and/or AGN. While the discoveries of Gunn-Peterson (GP) absorption troughs in the spectra of the highest redshift QSOs, and large scale polarization of the CMB, have set the first hard constraints on the EoR, the redshift and process of reionization, and the nature of the first luminous objects, remain two of the paramount questions in cosmic structure formation. Moreover, the GP effect is such that observations of objects during this epoch will be difficult at wavelengths shorter than about 1 micron. Hence, cosmic reionization, and the formation of the first luminous objects, occurs in a 'twilight zone', observable only at radio through near-IR wavelengths. In this talk I explore studies of the EoR at meter through submillimeter wavelengths. I present recent observations of the dust, molecular gas, and star formation activity in the host galaxies of the highest redshift QSOs. These results have interesting implications on the timescale for metal and dust enrichment, on the possibility of co-eval formation of SMBHs and galaxies, and on the process of reionization. I then discuss future capabilities of low frequency radio astronomy to study the neutral IGM via the HI 21cm line, including imaging and power spectral analyses of large scale structure in emission, and absorption studies toward the first radio loud sources. I conclude with a summary of the VLA-VHF system to study cosmic Stromgren spheres associated with the highest redshift SDSS QSOs in the HI 21cm line at 190 MHz.