Shot noise and scatter in the star formation efficiency as a source of 21-cm fluctuations

The 21-cm signal from cosmic dawn and the epoch of reionization (EoR) probes
the characteristics of the high redshift galaxy population. Many of the
astrophysical properties of galaxies at high redshifts are currently
unconstrained due to the lack of observations. This creates a vast space of
possible astrophysical scenarios where the 21-cm signal needs to be modeled in
order to plan for, and eventually fit, future observations. This is done with
fast numerical methods which make simplifying approximations for the underlying
physical processes. In this work we quantify the effect of Poisson fluctuations
and scatter in the star formation efficiency; while Poisson fluctuations are
included in some works and not in others, scatter in the star formation
efficiency is usually neglected, and all galaxies of a given mass are assumed
to have the same properties. We show that both features can have a significant
effect on the 21-cm power spectrum, most importantly in scenarios where the
signal is dominated by massive galaxies. Scatter in the star formation
efficiency does not simply enhance the effect of Poisson fluctuations; for
example we show that the power spectrum shape at cosmic dawn has a feature
corresponding to the width of the galaxy brightness distribution. We also
discuss some of the consequences for 21-cm imaging, and the signature of
reduced correlation between the density and radiation fields.
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