A method to infer the stellar population that dominated the UV background at the end of reionization

We present an observational test of the spectrum of the ionizing background at z ≃ 5; the test is sufficiently sensitive to determine whether Pop ii or Pop iii stars are the dominant source of ionizing radiation. The ionizing background at z ≃ 5 may reflect the nature of the sources responsible for the final overlap phase of reionization. We find that rest-frame extreme-UV He i absorption will be detectable in deep spectral observations of some rare z ≃ 5 quasars; the ratio of He i to H i absorption reflects the shape of the ionizing background in the photon energy range between 13.6 and 24.6 eV. Most z ≃ 5 quasars have too much H i absorption along their line of sight for He i absorption to be observed. However, based on current measurements of H i absorber statistics, we use Monte Carlo simulations to demonstrate that the Sloan Digital Sky Survey (SDSS) will discover a sufficient number of z ≃ 5 quasars to turn up a quasar suitable for measuring He i absorption (and we illustrate a selection method to identify that quasar). From simulated observations of a suitable z ≃ 5 quasar with a 10-meter telescope, we show that a constraint on the spectral slope of the ionizing background at that redshift can be obtained.