The Global 21-cm Signal in the Context of the High-z Galaxy Luminosity Function

Abstract

Motivated by recent progress in studies of the high-$z$ Universe, we build a new model for the global 21-cm signal that is explicitly calibrated to measurements of the galaxy luminosity function (LF) and further tuned to match the Thomson scattering optical depth of the cosmic microwave background, $\tau_e$. Assuming that the $z \lesssim 8$ galaxy population can be smoothly extrapolated to higher redshifts, the recent decline in best-fit values of $\tau_e$ and the inefficient heating induced by X-ray binaries (HMXBs; the presumptive sources of the X-ray background at high-$z$) imply that the entirety of cosmic reionization and reheating occurs at redshifts $z \lesssim 12$. In contrast to past global 21-cm models, whose $z \sim 20$ ($\nu \sim 70$ MHz) absorption features and strong $\sim 25$ mK emission features were driven largely by the assumption of efficient early star-formation and X-ray heating, our new fiducial model peaks in absorption at $\nu \sim 110$ MHz at a depth of $\sim -160$ mK and has a negligible emission component. As a result, a strong emission signal would provide convincing evidence that HMXBs are not the only drivers of cosmic reheating. Shallow absorption troughs should accompany strong heating scenarios, but could also be caused by a low escape fraction of Lyman-Werner photons. Generating signals with troughs at $\nu \lesssim 95$ MHz requires a floor in the star-formation efficiency in halos below $\sim 10^{9} M_{\odot}$, which is equivalent to steepening the faint-end of the galaxy LF. These findings demonstrate that the global 21-cm signal is a powerful complement to current and future galaxy surveys and efforts to better understand the interstellar medium in high-$z$ galaxies.