Rapid and late cosmic reionization driven by massive galaxies: a joint analysis of constraints from 21-cm, Lyman line, and CMB data sets

ABSTRACT Observations of the Epoch of Reionization have the potential to answer long-standing questions of astrophysical interest regarding the nature of the first luminous sources and their effects on the intergalactic medium (IGM). We present astrophysical constraints from a Neural Density Estimation-Accelerated Bayesian joint analysis of constraints deriving from cosmic microwave background power spectrum measurements from Planck and South Pole Telescope, IGM neutral fraction measurements from Lyman line-based data sets, and 21-cm power spectrum upper limits from Hydrogen Epoch of Reionization Array, LOw Frequency ARray, and the Murchison Widefield Array. In the context of the model employed, the data are found to be consistent with galaxies forming from predominantly atomic-cooled hydrogen gas in dark matter haloes, with masses $M_\mathrm{h} \gtrsim 2.6 \times 10^{9}~{\rm M}_{\odot } ((1+z)/10)^{\frac{1}{2}}$ at 95 per cent credibility ($V_\mathrm{c} \gtrsim 50~\mathrm{km~s^{-1}}$) being the dominant galactic population driving reionization. These galaxies reionize the neutral hydrogen in the IGM over a narrow redshift interval ($\Delta z_\mathrm{re} < 1.8$ at 95 per cent credibility), with the mid-point of reionization constrained to $z_{50} = 7.16^{+0.15}_{-0.12}$. We also obtain a tight constraint on the optical depth to reionization, $\tau _{\mathrm{CMB}} = 0.052 \pm 0.002$, disfavouring high-$\tau _\mathrm{CMB}$ scenarios ($\tau _{\mathrm{CMB}} \sim 0.09$) proposed to ease Planck–Dark Energy Spectroscopic Instrument tensions. Given the parameter posteriors from our joint analysis, we find that the posterior predictive distribution of the global 21-cm signal is reduced in amplitude and shifted to lower redshifts relative to the model prior. We caution, however, that our inferences are model-dependent. Future work incorporating updated mass-dependent star formation efficiencies in atomic cooling haloes, informed by the latest UV luminosity function constraints from the James Webb Space Telescope, promises to refine these inferences further and enhance our understanding of cosmic reionization.

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