2014/06/16 by Jordan Mirocha · 2 citations
Physics and Astronomy · #astro-ph.GA #astro-ph.HE
paper · pdf · doi:10.1093/mnras/stu1193
14 pages, 7 figures, accepted for publication in MNRAS
arxiv created 2014/06/16 · arxiv updated 2015/06/22
Evolution in the X-ray luminosity -- star formation rate (LX-SFR) relation could provide the first evidence of a top-heavy stellar initial mass function in the early universe, as the abundance of high-mass stars and binary systems are both expected to increase with decreasing metallicity. The sky-averaged (global) 21-cm signal has the potential to test this prediction via constraints on the thermal history of the intergalactic medium, since X-rays can most easily escape galaxies and heat gas on large scales. A significant complication in the interpretation of upcoming 21-cm measurements is the unknown spectrum of accreting black holes at high-z, which depends on the mass of accreting objects and poorly constrained processes such as how accretion disk photons are processed by the disk atmosphere and host galaxy interstellar medium. Using a novel approach to solving the cosmological radiative transfer equation (RTE), we show that reasonable changes in the characteristic BH mass affects the amplitude of the 21-cm signal's minimum at the ∼ 10-20 mK level --- comparable to errors induced by commonly used approximations to the RTE --- while modifications to the intrinsic disk spectrum due to Compton scattering (bound-free absorption) can shift the position of the minimum of the global signal by Δz ≈ 0.5 (Δz ≈ 2), and modify its amplitude by up to ≈ 10 mK (≈ 50 mK) for a given accretion history. Such deviations are larger than the uncertainties expected of current global 21-cm signal extraction algorithms, and could easily be confused with evolution in the LX-SFR relation.