The X-ray spectra of the first galaxies: 21 cm signatures

The cosmological 21 cm 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 a parts per thousand(3) 15) galaxies. Due to the strong dependence of the mean free path on the photon energy, the X-ray spectral energy distribution (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 21 cm 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 a parts per thousand(2)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 21 cm signal would be higher by a factor of similar to 3. More generally, we show that the peak in the redshift evolution of the large-scale (k similar to 0.2 Mpc(-1)) 21 cm 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.