Narrowing the discovery space of the cosmological 21-cm signal using multi-wavelength constraints

The cosmic 21-cm signal is a promising probe of the early Universe, owing to its sensitivity to the thermal state of the neutral intergalactic medium (IGM) and properties of the first luminous sources. Here, we constrain the 21-cm signal and infer IGM properties using the Population II galaxy parameters derived in a previous study through multi-wavelength synergies. This includes high-redshift UV luminosity functions (UVLFs) from Hubble Space Telescope (HST) and James Webb Space Telescope (JWST), cosmic X-ray and radio backgrounds (CXB and CRB), the SARAS 3 global 21-cm signal non-detection, and HERA 21-cm power spectrum upper limits. From CXB and HERA data, we infer the IGM kinetic temperature to be $T_{\text{K}}(z=15)\lesssim 7.7\text{ K}$, $2.5\text{ K}\lesssim T_{\text{K}}(z=10)\lesssim 66\text{ K}$, and $20\text{ K}\lesssim T_{\text{K}}(z=6)\lesssim 2078\text{ K}$ at 95% credible interval (C.I.). Similarly, CRB and HERA data limit the radio emission efficiency of galaxies, giving $T_{\text{rad}}(z=15)\lesssim 47\text{ K}$, $T_{\text{rad}}(z=10)\lesssim 51\text{ K}$, and $T_{\text{rad}}(z=6)\lesssim 101\text{ K}$. These constraints, strengthened by UVLFs from HST and JWST, enable the first $\mathit{\text{lower}}\mathit{\text{bound}}$ on the cosmic 21-cm signal. We infer an absorption trough of depth $-201\text{ mK}\lesssim T_{\text{21,min}}\lesssim -68\text{ mK}$ at $z_{\text{min}}\approx 10-16$, and a power spectrum of $8.7{\text{mK}}^2<{\mathrm{\Delta }}_{21}^2(z=15)<197{\text{mK}}^2$ at $k=0.35h{\text{Mpc}}^{-1}$. Our results highlight the power of multi-wavelength synergies in constraining the early Universe. While promising for upcoming 21-cm experiments, the results depend on our assumption of a redshift-independent X-ray and radio efficiency of galaxies, and the exclusion of a flexible model for Population III stars.