Unique Signatures of Population III Stars in the Global 21-cm Signal

Mon. Not. R. Astron. Soc. 000, ??–?? (2016) Printed 28 July 2017 (MN L A TEX style file v2.2) Unique Signatures of Population III Stars in the Global 21-cm Signal Jordan Mirocha 1∗ , Richard Mebane 1 , Steven R. Furlanetto 1 , Krishma Singal 2 , Donald Trinh 3 Department of Physics and Astronomy, University of California, Los Angeles, CA 90024, USA of Physics, Georgia Institute of Technology, Atlanta, GA 30332 3 Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA 2 School 28 July 2017 ABSTRACT We investigate the effects of Population III stars on the sky-averaged 21-cm back- ground radiation, which traces the collective emission from all sources of ultraviolet and X-ray photons before reionization is complete. While UV photons from PopIII 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 PopIII stars are likely to have a more discernible impact on the 21-cm signal than PopIII stars themselves. The X-rays from such sources preferentially heat the 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, asym- metric wings in the absorption signal, which stand in contrast to the relatively sharp, symmetric signals that arise in models treating PopII sources only. A stronger signa- ture of PopIII, in which the position of the absorption minimum becomes inconsistent with PopII-only models, requires extreme star-forming events that may not be phys- ically plausible, lending further credence to predictions of relatively high frequency absorption troughs, ν min ∼ 100 MHz. As a result, though the trough location alone may not be enough to indicate the presence of PopIII, the asymmetric wings should arise even if only a few PopIII stars form in each halo before the transition to PopII star formation occurs, provided that the PopIII IMF is sufficiently top-heavy and at least some PopIII stars form in binaries. Key words: galaxies: high-redshift – intergalactic medium – galaxies: luminosity function, mass function – dark ages, reionization, first stars – diffuse radiation. INTRODUCTION The formation of the first generations of stars in the Uni- verse has been a topic of great interest for several decades (for a recent review, see Bromm, 2013). These so-called Pop- ulation III (PopIII) stars by definition form out of chemi- cally pristine clouds, a fact which is expected to give rise to stellar initial mass functions (Bromm et al., 1999; Abel et al., 2002), atmospheres (Tumlinson & Shull, 2000; Bromm et al., 2001; Schaerer, 2002), and chemical yields (Heger & Woosley, 2002) that are distinct from stars today. Depend- ing on the efficiency with which such stars form in high- z dark matter halos and their longevity as a population, traces of their existence may be found in the reionization his- tory of the intergalactic medium (IGM) (e.g., Visbal et al., ∗ mirocha@astro.ucla.edu 2015; Miranda et al., 2017), chemical abundance patterns in metal-poor stars in the Milky Way (e.g., Jeon et al., 2017; Magg et al., 2017), and pair-instability supernova (PISN) rates at high redshift (e.g., Whalen et al., 2014). As a result, further investigation of the PopIII epoch continues not only due to a fundamental interest in the physical processes that govern star formation, but because progress in so many other areas may be inextricably linked to the lives and deaths of PopIII stars. Observationally, PopIII stars have remained elusive. This is not surprising, given that they are expected to form in low mass (∼ 10 5 -10 6 M ) dark matter halos at the high- est redshifts, and probably only in small numbers. There has been only one object detected at high-z with some evi- dence of PopIII-like stellar population (Sobral et al., 2015), though more ordinary explanations remain viable (Bowler et al., 2017). Similarly, while there have been a few claims