The 21-cm signal from the Cosmic Dawn: metallicity dependence of high mass X-ray binaries





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X-rays from High-Mass X-ray Binaries (HMXBs) are likely the main source of heating of the intergalactic medium (IGM) during Cosmic Dawn (CD), before the completion of reionization. This Epoch of Heating (EoH; z∼10−15) should soon be detected via the redshifted 21-cm line from neutral hydrogen, allowing us to indirectly study the properties of HMXBs in the unseen, first galaxies. Low-redshift observations, as well as theoretical models, imply that the integrated X-ray luminosity to star formation rate of HMXBs (LX/SFR) should increase in metal-poor environments, typical of early galaxies. Here we study the impact of the metallicity (Z) dependence of LX/SFR during the EoH. For our fiducial models, galaxies with star formation rates of order 10⁻³−10⁻¹ M⊙ yr⁻¹ and metallicities of order 10⁻³−10⁻² Z⊙ are the dominant contributors to the X-ray background (XRB) during this period. Different LX/SFR-Z relations result in factors of ∼ 3 differences in these ranges, as well as in the mean IGM temperature and the large-scale 21-cm power, at a given redshift. We compute mock 21-cm observations adopting as a baseline a 1000h integration with the upcoming Square Kilometer Array (SKA), for two different LX/SFR-Z relations. We perform inference on these mock observations using the common simplification of a constant LX/SFR, finding that constant LX/SFR models can recover the IGM evolution of the more complicated LX/SFR-Z simulations only during the EoH. At z<10, where the typical galaxies are more polluted, constant LX/SFR models over-predict the XRB and its relative contribution to the early stages of the reionization.