Abstract
ABSTRACT The ARCADE2 and LWA1 experiments have claimed an excess over the cosmic microwave background (CMB) at low radio frequencies. If the cosmological high-redshift contribution to this radio background is between 0.1 per cent and 22 per cent of the CMB at 1.42 GHz, it could explain the tentative EDGES low-band detection of the anomalously deep absorption in the 21-cm signal of neutral hydrogen. We use the upper limit on the 21-cm signal from the Epoch of Reionization (z = 9.1) based on 141 h of observations with LOFAR to evaluate the contribution of the high-redshift Universe to the detected radio background. Marginalizing over astrophysical properties of star-forming haloes, we find (at 95 per cent CL) that the cosmological radio background can be at most 9.6 per cent of the CMB at 1.42 GHz. This limit rules out strong contribution of the high-redshift Universe to the ARCADE2 and LWA1 measurements. Even though LOFAR places limit on the extra radio background, excess of 0.1–9.6 per cent over the CMB (at 1.42 GHz) is still allowed and could explain the EDGES low-band detection. We also constrain the thermal and ionization state of the gas at z = 9.1, and put limits on the properties of the first star-forming objects. We find that, in agreement with the limits from EDGES high-band data, LOFAR data constrain scenarios with inefficient X-ray sources, and cases where the Universe was ionized by stars in massive haloes only.
Highlights
Studies of the Epoch of Reionization (EoR) and Cosmic Dawn are key to understanding early galaxy formation and the evolution of the intergalactic medium (IGM; see e.g. reviews by Barkana & Loeb 2001; Furlanetto, Oh & Briggs 2006; Barkana 2018a; Mesinger 2019)
We use the upper limit on the 21-cm signal from the Epoch of Reionization (z = 9.1) based on 141 h of observations with LOFAR to evaluate the contribution of the high-redshift Universe to the detected radio background
We find that at z = 9.1 the bias varies as a function of the reionization parameters τ and Rmfp, while it has a very weak dependence on the rest of the parameters (Vc, f∗, fX, and Ar)
Summary
Studies of the Epoch of Reionization (EoR) and Cosmic Dawn are key to understanding early galaxy formation and the evolution of the intergalactic medium (IGM; see e.g. reviews by Barkana & Loeb 2001; Furlanetto, Oh & Briggs 2006; Barkana 2018a; Mesinger 2019). Weinberger, Haehnelt & Kulkarni 2019), while galaxy surveys provide independent constraints on star formation out to z ∼ 10 (e.g. see Behroozi et al 2019, and references therein) These observations do not constrain properties of the first population of star-forming objects such as their star formation efficiency, feedback mechanisms that regulated primordial star formation, and the properties of the first sources of heat (e.g. Xray binaries). These properties can be probed using low-frequency radio observations of the redshifted 21-cm signal of neutral hydrogen (e.g. Pober et al 2014; Greig, Mesinger & Pober 2016; Singh et al 2017; Monsalve et al 2018, 2019). Because the 21-cm signal is measured against the diffused radio background, usually assumed to be only the cosmic microwave background (CMB), this signal can be used to
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