Unique signatures of Population III stars in the global 21-cm signal

Abstract

We investigate the effects of Population III stars on the sky-averaged 21-cm\nbackground radiation, which traces the collective emission from all sources of\nultraviolet and X-ray photons before reionization is complete. While UV photons\nfrom PopIII stars can in principle shift the onset of radiative coupling of the\n21-cm transition -- and potentially reionization -- to early times, we find\nthat the remnants of PopIII stars are likely to have a more discernible impact\non the 21-cm signal than PopIII stars themselves. The X-rays from such sources\npreferentially heat the IGM at early times, which elongates the epoch of\nreheating and results in a more gradual transition from an absorption signal to\nemission. This gradual heating gives rise to broad, asymmetric wings in the\nabsorption signal, which stand in contrast to the relatively sharp, symmetric\nsignals that arise in models treating PopII sources only. A stronger signature\nof PopIII, in which the position of the absorption minimum becomes inconsistent\nwith PopII-only models, requires extreme star-forming events that may not be\nphysically plausible, lending further credence to predictions of relatively\nhigh frequency absorption troughs, $\\nu_{\\min} \\sim 100$ MHz. As a result,\nthough the trough location alone may not be enough to indicate the presence of\nPopIII, the asymmetric wings should arise even if only a few PopIII stars form\nin each halo before the transition to PopII star formation occurs, provided\nthat the PopIII IMF is sufficiently top-heavy and at least some PopIII stars\nform in binaries.\n