The global 21-cm signal in the context of the high- z galaxy luminosity function

Abstract

We build a new model for the global 21-cm signal that is calibrated to measurements of the high-z galaxy luminosity function (LF) and further tuned to match the Thomson scattering optical depth of the cosmic microwave background, τe. Assuming that the z ≲ 8 galaxy population can be smoothly extrapolated to higher redshifts, the recent decline in best-fitting values of τe and the inefficient heating induced by X-ray binaries (the presumptive sources of the high-z X-ray background) imply that the entirety of cosmic reionization and reheating occurs at z ≲ 12. In contrast to past global 21-cm models, whose z ∼ 20 (ν ∼ 70 MHz) absorption features and strong ∼25 mK emission features were driven largely by the assumption of efficient early star formation and X-ray heating, our new models peak in absorption at ν ∼ 110 MHz at depths ∼−160 mK and have negligible emission components. Current uncertainties in the faint-end of the LF, binary populations in star-forming galaxies, and UV and X-ray escape fractions introduce ∼20 MHz (∼50 mK) deviations in the trough's frequency (amplitude), while emission signals remain weak (≲10 mK) and are confined to ν ≳ 140 MHz. These predictions, which are intentionally conservative, suggest that the detection of a 21-cm absorption minimum at frequencies below ∼90 MHz and/or emission signals stronger than ∼10mK at ν ≲ 140 MHz would provide strong evidence for 'new' sources at high redshifts, such as Population III stars and their remnants.

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