The imprint of the cosmic supermassive black hole growth history on the
  21 cm background radiation
  The redshifted 21 cm transition line of hydrogen tracks the thermal evolution
of the neutral intergalactic medium (IGM) at "cosmic dawn," during the
emergence of the first luminous astrophysical objects (~100 Myr after the Big
Bang) but before these objects ionized the IGM (~400-800 Myr after the Big
Bang). Because X-rays, in particular, are likely to be the chief energy courier
for heating the IGM, measurements of the 21 cm signature can be used to infer
knowledge about the first astrophysical X-ray sources. Using analytic arguments
and a numerical population synthesis algorithm, we argue that the progenitors
of supermassive black holes (SMBHs) should be the dominant source of hard
astrophysical X-rays---and thus the primary driver of IGM heating and the 21 cm
signature---at redshifts $z < 20$, if (i) they grow readily from the remnants
of Population III stars and (ii) produce X-rays in quantities comparable to
what is observed from active galactic nuclei and high-mass X-ray binaries. We
show that models satisfying these assumptions dominate over contributions to
IGM heating from stellar populations, and cause the 21 cm brightness
temperature to rise at $z > 20$. An absence of such a signature in the
forthcoming observational data would imply that SMBH formation occurred later
(e.g. via so-called direct collapse scenarios), that it was not a common
occurrence in early galaxies and protogalaxies, or that it produced far fewer
X-rays than empirical trends at lower redshifts, either due to intrinsic
dimness (radiative inefficiency) or Compton-thick obscuration close to the
source.
