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 \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ > $ } } 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 \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ < $ } } 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 a factor of \sim three .
More generally , we show that the peak in the redshift evolution of the large-scale ( k \sim 0.2 \mathrm { Mpc } ^ { -1 } ) 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 .