Signatures of Hard Ionizing Radiation from the First Galaxies
Aparna Venkatesan

The thermal and ionization history of the intergalactic medium (IGM) strongly affect galaxy evolution, the visibility of distant quasars and galaxies, and the anisotropy signals in the cosmic microwave background (CMB). I present some key results from investigating the joint effects of X-rays and hard UV radiation from the first stars and quasars on the hydrogen/helium reionization of the IGM and its heating history. X-rays have greater penetrating power relative to UV radiation, and can alter the ionization and thermal history of the IGM significantly, owing to secondary ionization effects. The 21 cm signature of X-ray versus UV ionization could be distinct, resulting from the inherently different spatial scales at which X-ray and UV ionization/heating manifest. Such differing scale-dependent contributions from X-ray and UV photons may therefore "blur" the 21 cm signature of reionization, and affect the interpretation of CMB and/or 21 cm data that aim to map the percolation of reionization from first-light sources. Last, I will share new calculations of the escape fraction of helium-ionizing radiation from the first galaxies, and the surprising role of X-rays in the escape fraction values for both hydrogen and helium. For sufficiently hard first-light sources, the helium ionization fronts closely track or even advance beyond that of hydrogen.