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. In this work, we constrain the 21-cm signal and infer IGM properties by leveraging multi-wavelength synergies. This builds on our earlier study, where we developed the synergy between high-redshift UV luminosity functions (UVLFs), cosmic X-ray and radio backgrounds (CXB and CRB), 21-cm global signal non-detection from SARAS~3, and 21-cm power spectrum upper limits from HERA, to constrain the astrophysical properties of Population II galaxies. Through a combination of CXB and HERA data, we find the IGM kinetic temperature to be TK(z=15) 7.7~K, 2.5~K TK(z=10) 66~K, and 20~K TK(z=6) 2078~K at 95% credible interval (C.I.). Similarly, CRB and HERA data place an upper limit on the radio emission efficiency of galaxies, giving Tᵣad(z=15) 47~K, Tᵣad(z=10) 51~K, and Tᵣad(z=6) 101~K. These constraints, strengthened by the inclusion of UVLFs from HST and JWST, allow us to place the first lower bound on the cosmic 21-cm signal. We infer an absorption trough of depth -201~mK T₂1,min -68~mK at zₘin≈10-16, and a power spectrum of 8.7~mK² < Δ₂₁²(z=15) < 197~mK² at k=0.35~hMpc⁻¹. Our results provide promising predictions for upcoming 21-cm experiments, and highlight the power of multi-wavelength synergies in constraining the early Universe.