Signature of excess radio background in the 21-cm global signal and power spectrum

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Constraining the clustering and 21-cm signature of radio galaxies at cosmic dawn

The efficiency of radio emission is an important unknown parameter of early galaxies at cosmic dawn, as models with high efficiency have been shown to modify the cosmological 21-cm signal substantially, deepening the absorption trough and boosting the 21-cm power spectrum. Such models have been previously directly constrained by the overall extragalactic radio background as observed by ARCADE-2 and LWA-1. In this work, we constrain the clustering of high redshift radio sources by utilizing the observed upper limits on arcminute-scale anisotropy from the VLA at 4.9~GHz and ATCA at 8.7~GHz. Using a semi-numerical simulation of a plausible astrophysical model for illustration, we show that the clustering constraints on the radio efficiency are much stronger than those from the overall background intensity, by a factor that varies from 12 at redshift 7 to 30 at redshift 22. As a result, the predicted maximum depth of the global 21-cm signal is lowered by a factor of 5 (to 1700~mK), and the maximum 21-cm power spectrum peak at cosmic dawn is lowered by a factor of 24 (to $2\times 10^5$~mK$^2$). We conclude that the observed clustering is the strongest current direct constraint on such models, but strong early radio emission from galaxies remains viable for producing a strongly enhanced 21-cm signal from cosmic dawn.Comment: 8 pages, 3 figure

Mapping discrete galaxies at cosmic dawn with 21-centimeter observations

At cosmic dawn, the 21-centimeter signal from intergalactic hydrogen was driven by Lyman-$\alpha$ photons from some of the earliest stars, producing a spatial pattern that reflected the distribution of galaxies at that time. Due to the large foreground, it is thought that around redshift 20 it is only observationally feasible to detect 21-cm fluctuations statistically, yielding a limited, indirect probe of early galaxies. Here we show that 21-cm images at cosmic dawn should actually be dominated by large (tens of comoving megaparsecs), high contrast bubbles surrounding individual galaxies. We demonstrate this using a substantially upgraded semi-numerical simulation code that realistically captures the formation and 21-cm effects of the small galaxies expected during this era. Small number statistics associated with the rarity of early galaxies, combined with the multiple scattering of photons in the blue wing of the Lyman-$\alpha$ line, create the large bubbles and also enhance the 21-cm power spectrum by a factor of 2--7 and add to it a feature that measures the typical brightness of galaxies. These various signatures of discrete early galaxies are potentially detectable with planned experiments such as the Square Kilometer Array or the Hydrogen Epoch of Reionization Array, even if the early stars formed in dark matter halos with masses as low as $10^8\, M_\odot$, ten thousand times smaller than the Milky Way halo.Comment: First submitted for publication on Apr. 5th, 202