The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >= 5: evidence for a photon-starved and extended epoch of reionization

Abstract

Galaxies and quasars are thought to provide the bulk of the photons responsible for ionizing the hydrogen in the intergalactic medium (IGM). We use a large set of hydrodynamical simulations, combined with measurements of the Lyα opacity of the IGM taken from the literature, to obtain robust estimates of the photoionization rate per hydrogen atom at z= 5 and 6. We find the photoionization rate drops by a factor of 2 and 4, respectively, compared to our recent measurements at z= 2–4. The number of ionizing photons emitted by known sources at z= 5 and 6, based on an extrapolation of source numbers below the detection limit and standard assumptions for the relationship between the ionizing emissivity and observed luminosity density at 1500 Å, are in reasonable agreement with the photoionization rates inferred from the Lyα forest if the escape fraction of ionizing photons from galaxies is large (≳20 per cent). The expected number of ionizing photons from observed sources at these redshifts therefore appears sufficient to maintain the IGM in its highly ionized state. Claims to the contrary may be attributed to the adoption of an unduly high value for the clumping factor of ionized hydrogen. Using physically motivated assumptions for the mean free path of ionizing photons our measurements of the photoionization rate can be turned into an estimate of the ionizing emissivity. In comoving units the inferred ionizing emissivity is nearly constant over the redshift range 2–6 and corresponds to 1.5–3 photons emitted per hydrogen atom over a time interval corresponding to the age of the Universe at z= 6. This strongly suggests that the epoch of reionization was photon‐starved and extended. Completion of reionization at or before z= 6 requires either an emissivity which rises towards higher redshifts or one which remains constant but is dominated by sources with a rather hard spectral index. For standard assumptions, the ionizing emissivity required for completion of reionization at or before z= 6 lies at the upper end of recently reported values from searches for high‐redshift galaxies at z= 8–10.

Low-mass galaxy formation and the ionizing photon budget during reionization

“…We then discuss the physics driving the SFR function (Section 3) and the sSFR (Section 3.2). We show in Section 4 that our simulations support the paradigm of a 'PhotonStarved' Epoch of reionization (Bolton & Haehnelt 2007), in which the number of ionizing photons is only just sufficient to sustain global reionization. In Section 5, we demonstrate that there exists a faint galaxy population below observational limits that is consistent with extrapolating recent luminosity functions.…”

Reionization constraints on primordial magnetic fields

“…Models which are consistent with all available data sets usually require some efficient sources of ionizing photons at high redshifts, e.g., metal-free PopIII stars (Choudhury & Ferrara 2005; in addition to the usual PopII stars. These PopIII sources, however, are unlikely to contribute significantly to the photon budget at lower redshifts z 9 because of feedback effects, which is crucial in matching the low photoionization rate inferred from quasar absorption spectra z ∼ 6 (Bolton & Haehnelt 2007). One can thus conclude that in order to match the data sets, one requires a high ionizing emissivity at z ∼ 10 which should preferably decrease by z ∼ 6.…”

“…However, as reionization progresses, the radiative feedback effects become more important and regulates the formation of ionizing sources at z ∼ 10. This leads to a feedback-regulated, extended and photon-starved reionization which is required for good match with the data (Bolton & Haehnelt 2007;Mitra, Choudhury, & Ferrara 2012) 7 . It also follows that this τ el Figure 5.…”

Dark-ages reionization and galaxy formation simulation – X. The small contribution of quasars to reionization

“…Inhomogeneous recombinations are ignored, which can have a large impact (Sobacchi & Mesinger 2014). In this work, Nrec is set to be 0 as suggested by the high-redshift Lyα forest in the IGM (Bolton & Haehnelt 2007;McQuinn et al 2011). Expanding equation (28) gives…”

“…From the Lyα opacity, several measurements of the total emissivity (AGN and stars) at high redshift have been estimated (Bolton & Haehnelt 2007;McQuinn et al 2011;Becker & Bolton 2013). There are relatively large uncertainties in these measurements.…”