He II recombination lines from the first luminous objects

The hardness of the ionizing continuum from the first sources of UV radiati on plays a crucial role in the reionization of the intergalactic medium (IG M). While usual stellar populations have soft spectra, miniquasars or metal -free stars with high effective temperatures may emit hard photons, capable of doubly ionizing helium and increasing the IGM temperature. Absorption w ithin the source and in the intervening IGM will render the ionizing contin uum of high-redshift sources inaccessible to direct observation. Here we sh ow that He II recombination lines from the first luminous objects are poten tially detectable by the Next Generation Space Telescope. Together with mea surements of the H alpha emission line, this detection can be used to infer the ratio of He II to H I ionizing photons, Q = (N) over dot(ion)(HeII)/(N ) over dot(ion)(Ht). A measurement of this ratio would shed light on the na ture and emission mechanism of the first luminous sources, with important a strophysical consequences for the reheating and reionization of the IGM.