Interpreting the Global 21-cm Signal from High Redshifts. II. Parameter Estimation for Models of Galaxy Formation

TL;DR

This paper demonstrates that current and upcoming experiments can effectively constrain galaxy formation models using the global 21-cm signal, revealing key properties of early universe star formation and ionization processes.

Contribution

It introduces a method to constrain a simple galaxy formation model with four parameters using 21-cm observations, highlighting the potential of broad-band measurements for understanding early cosmic history.

Findings

Models can be constrained to 0.1 dex accuracy with detection of spectral features between 40-120 MHz.

Measurements can identify the halo mass threshold for star formation at z > 15.

Constraints on X-ray luminosity and ionizing radiation escape fraction can be extended to high redshifts.

Abstract

Following our previous work, which related generic features in the sky-averaged (global) 21-cm signal to properties of the intergalactic medium, we now investigate the prospects for constraining a simple galaxy formation model with current and near-future experiments. Markov-Chain Monte Carlo fits to our synthetic dataset, which includes a realistic galactic foreground, a plausible model for the signal, and noise consistent with 100 hours of integration by an ideal instrument, suggest that a simple four-parameter model that links the production rate of Lyman-$\alpha$, Lyman-continuum, and X-ray photons to the growth rate of dark matter halos can be well-constrained (to $\sim 0.1$ dex in each dimension) so long as all three spectral features expected to occur between $40 \lesssim \nu / \mathrm{MHz} \lesssim 120$ are detected. Several important conclusions follow naturally from this basic numerical result, namely that measurements of the global 21-cm signal can in principle (i) identify the characteristic halo mass threshold for star formation at all redshifts $z \gtrsim 15$, (ii) extend $z \lesssim 4$ upper limits on the normalization of the X-ray luminosity star-formation rate ($L_X$-SFR) relation out to $z \sim 20$, and (iii) provide joint constraints on stellar spectra and the escape fraction of ionizing radiation at $z \sim 12$. Though our approach is general, the importance of a broad-band measurement renders our findings most relevant to the proposed Dark Ages Radio Explorer, which will have a clean view of the global 21-cm signal from $\sim 40-120$ MHz from its vantage point above the radio-quiet, ionosphere-free lunar far-side.