The X-ray spectra of the first galaxies: 21cm signatures
  The cosmological 21cm signal is a physics-rich probe of the early Universe,
encoding information about both the ionization and the thermal history of the
intergalactic medium (IGM). The latter is likely governed by X-rays from
star-formation processes inside very high redshift (z > 15) galaxies. Due to
the strong dependence of the mean free path on the photon energy, the X-ray SED
can have a significant impact on the interferometric signal from the cosmic
dawn. Recent Chandra observations of nearby, star-forming galaxies show that
their SEDs are more complicated than is usually assumed in 21cm studies. In
particular, these galaxies have ubiquitous, sub-keV thermal emission from the
hot interstellar medium (ISM), which generally dominates the soft X-ray
luminosity (with energies < 1 keV, sufficiently low to significantly interact
with the IGM). Using illustrative soft and hard SEDs, we show that the IGM
temperature fluctuations in the early Universe would be substantially increased
if the X-ray spectra of the first galaxies were dominated by the hot ISM,
compared with X-ray binaries with harder spectra. The associated large-scale
power of the 21cm signal would be higher by roughly a factor of three. More
generally, we show that the peak in the redshift evolution of the large-scale
(k = 0.2 1/Mpc) 21cm power is a robust probe of the soft-band SED of the first
galaxies, and importantly, is not degenerate with their bolometric
luminosities. On the other hand, the redshift of the peak constrains the X-ray
luminosity and halo masses which host the first galaxies.
