The Effect of Cosmic Magnetic Fields on the Metagalactic Ionization Background inferred from the Lyman-α Forest

Abstract

The sources which reionized the Intergalactic Medium by redshift ~6 are still unknown. A severe constraint on the ionization process is the low emissivity required to maintain the ionization in the Lyman-α forest. Simulation-calibrated observations suggest a production rate of at most only a few photons per baryon. In this work, we present a new solution to this "photon-starvation" problem using a weak background of cosmic magnetic fields, which may be present as a consequence of early-Universe physics and subsequent magneto-hydrodynamical amplification. If present, such magnetic fields can induce density perturbations which are dominant on scales comparable to those probed by measurements of hydrogen-absorption lines at redshifts z~2-5. We show that a sub-nanoGauss magnetic field, coherent on scale ~1 Mpc with an almost scale-invariant spectrum, is sufficient to produce significant impact on the effective optical depth, the appearance of the Lyman-α forest on quasar spectra, the pixel-flux statistics and the power spectrum of transmitted flux. We also show that such magnetic-field signatures are effectively erased when the metagalactic photoionization rate is increased, hence relaxing the constraint on the cosmic photon budget available for reionization.