Looking for Population III stars with He ii line intensity mapping

Constraining the properties of Population III (Pop III) stars will be very challenging because they reside in small galaxies at high redshift which will be difficult to directly detect. In this paper, we suggest that intensity mapping may be a promising method to study Pop III stars. Intensity mapping is a technique proposed to measure large-scale fluctuations of galaxy line emission in three dimensions without resolving individual sources. This technique is well suited for observing many faint galaxies because it can measure their cumulative emission even if they cannot be directly detected. We focus on intensity mapping of He ii recombination lines. These lines are much stronger in Pop III stars than Pop II stars because the harder spectra of Pop III stars are expected to produce many He ii ionizing photons. Measuring the He ii 1640 angstrom intensity mapping signal, along with the signals from other lines such as Ly alpha, H alpha, and metal lines, could give constraints on the initial mass function (IMF) and star formation rate density of Pop III stars as a function of redshift. To demonstrate the feasibility of these observations, we estimate the strength of the Pop III He ii 1640 angstrom intensity mapping signal from z = 10-20. We show that at z a parts per thousand 10, the signal could be measured accurately by two different hypothetical future instruments, one which cross-correlates He ii 1640 angstrom with CO(1-0) line emission from galaxies and the other with 21 cm emission from the intergalactic medium.