High-redshift radio galaxies: a potential new source of 21-cm
  fluctuations
  Radio sources are expected to have formed at high redshifts, producing an
excess radiation background above the cosmic microwave background (CMB) at low
frequencies. Their effect on the redshifted 21-cm signal of neutral hydrogen is
usually neglected, as it is assumed that the associated background is small.
Recently, an excess radio background above the level of the CMB has been
proposed as one of the possible explanations for the unusually strong 21-cm
signal from redshift $z\sim 17$ reported by the EDGES collaboration. As a
result, the implications of a smooth and extremely strong excess radio
background on both the sky-averaged (global) 21-cm signal and its fluctuations
have been considered. Here we take into account the inhomogeneity of the radio
background created by a population of high-redshift galaxies, and show that it
adds a new type of 21-cm fluctuations to the well-known contributions of
density, velocity, Ly-${\alpha}$ coupling, heating and reionization. We find
that a population of high-redshift galaxies even with a moderately-enhanced
radio efficiency (unrelated to the EDGES result) can have a significant effect
on the 21-cm power spectrum and global signal in models with weak X-ray
heating. For models that can explain the EDGES data, we conduct a large
parameter survey to explore their signatures. We show that in such models the
21-cm power spectrum at $z\sim 17$ is enhanced by up to two orders of magnitude
compared to the CMB-only standard case, and the shape and time evolution of the
power spectrum is significantly modified by the radio fluctuations. These
fluctuations are within reach of upcoming radio interferometers. We also find
that these models can be significantly constrained by current and future
observations of radio sources.
