Abstract
We investigate the effects of Population III (Pop III) stars and their
remnants on the cosmological 21-cm global signal. By combining a semi-analytic
model of Pop III star formation with a global 21-cm simulation code, we
investigate how X-ray and radio emission from accreting Pop III black holes may
affect both the timing and depth of the 21-cm absorption feature that follows
the initial onset of star formation during the Cosmic Dawn. We compare our
results to the findings of the EDGES experiment, which has reported the first
detection of a cosmic 21-cm signal. In general, we find that our fiducial Pop
III models, which have peak star formation rate densities of $10^-4$
M$_ødot$ yr$^-1$ Mpc$^-3$ between $z 10$ and $z 15$, are able to
match the timing of the EDGES signal quite well, in contrast to models that
ignore Pop III stars. To match the unexpectedly large depth of the EDGES signal
without recourse to exotic physics, we vary the parameters of emission from
accreting black holes (formed as Pop III remnants) including the intrinsic
strength of X-ray and radio emission as well as the local column density of
neutral gas. We find that models with strong radio emission and relatively weak
X-ray emission can self-consistently match the EDGES signal, though this
solution requires fine-tuning. We are only able to produce signals with sharp
features similar to the EDGES signal if the Pop~III IMF is peaked narrowly
around $140 \, M_ødot$.
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