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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 478; no. 4; pp. 5604 - 5619
• Main Authors: Mirocha, Jordan, Mebane, Richard H, Furlanetto, Steven R, Singal, Krishma, Trinh, Donald
• Format: Journal Article
• Language: English
• Published: Oxford University Press 21-08-2018
• Subjects: intergalactic medium; galaxies: luminosity function, mass function; diffuse radiation; galaxies: high-redshift; dark ages, reionization, first stars
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sty1388

ABSTRACT
We investigate the effects of Population III stars on the sky-averaged 21-cm background radiation, which traces the collective emission from all sources of ultraviolet and X-ray photons before reionization is complete. While ultraviolet (UV) photons from Pop III stars can, in principle, shift the onset of radiative coupling of the 21-cm transition – and potentially reionization – to early times, we find that the remnants of Pop III stars are likely to have a more discernible impact on the 21-cm signal than Pop III stars themselves. The X-rays from such sources preferentially heat the intergalactic medium (IGM) at early times, which elongates the epoch of reheating and results in a more gradual transition from an absorption signal to emission. This gradual heating gives rise to broad, asymmetric wings in the absorption signal, which stand in contrast to the relatively sharp, symmetric signals that arise in models treating Pop II sources only. A stronger signature of Pop III, in which the position of the absorption minimum becomes inconsistent with Pop II-only models, requires extreme star-forming events that may not be physically plausible, lending further credence to predictions of relatively high-frequency absorption troughs, νmin  ∼ 100 MHz. As a result, though the trough location alone may not be enough to indicate the presence of Pop III, the asymmetric wings should arise even if only a few Pop III stars form in each halo before the transition to Pop II star formation occurs, provided that the Pop III IMF is sufficiently top-heavy and at least some Pop III stars form in binaries.