The epoch of first star formation and the state of the intergalactic medium ( IGM ) at that time are not directly observable with current telescopes .
The radiation from those early sources is now part of the Cosmic Infrared Background ( CIB ) and , as these sources ionize the gas around them , the IGM plasma would produce faint temperature anisotropies in the Cosmic Microwave Background ( CMB ) via the thermal Sunyaev-Zeldovich ( TSZ ) effect .
While these TSZ anisotropies are too faint to be detected , we show that the cross-correlation of maps of source-subtracted CIB fluctuations from Euclid , with suitably constructed microwave maps at different frequencies can probe the physical state of the gas during reionization and test/constrain models of the early CIB sources .
We identify the frequency-combined CMB-subtracted microwave maps from space and ground-based instruments to show that they can be cross-correlated with the forthcoming all-sky Euclid CIB maps to detect the cross-power at scales \sim 5 ^ { \prime } -60 ^ { \prime } with the signal/noise of up to S / N \sim 4 - 8 depending on the contribution to the Thomson optical depth during those pre-reionization epochs ( \Delta \tau \simeq 0.05 ) and the temperature of IGM ( up to \sim 10 ^ { 4 } K ) .
Such a measurement would offer a new window to explore emergence and physical properties of these first light sources .