Concordance models of reionization: implications for faint galaxies and escape fraction evolution

Abstract

Recent observations have constrained the galaxy ultraviolet (UV) luminosity function up to z ∼ 10. However, these observations alone allow for a wide range of reionization scenarios due to uncertainties in the abundance of faint galaxies and the escape fraction of ionizing photons. We show that requiring continuity with post-reionization (z < 6) measurements, where the Lyα forest provides a complete probe of the cosmological emissivity of ionizing photons, significantly reduces the permitted parameter space. Models that are simultaneously consistent with the measured UV luminosity function, the Thomson optical depth to the microwave background and the Lyα forest data require either (1) extrapolation of the galaxy luminosity function down to very faint UV magnitudes M lim ∼ −10, corresponding roughly to the UV background suppression scale; (2) an increase of the escape fraction by a factor 10 from z = 4 (where the best fit is 4 per cent) to 9; or (3) more likely, a hybrid solution in which undetected galaxies contribute significantly and the escape fraction increases more modestly. Models in which star formation is strongly suppressed in low-mass, reionizationepoch haloes of mass up to M h ∼ 10 10 M (e.g. owing to a metallicity dependence) are only allowed for extreme assumptions for the redshift evolution of the escape fraction. However, variants of such models in which the suppression mass is reduced (e.g. assuming an earlier or higher metallicity floor) are in better agreement with the data. Interestingly, concordance scenarios satisfying the available data predict a consistent redshift of 50 per cent ionized fraction z reion (50 per cent) ∼ 10. On the other hand, the duration of reionization is sensitive to the relative contribution of bright versus faint galaxies, with scenarios dominated by faint galaxies predicting a more extended reionization event. Scenarios relying too heavily on highredshift dwarfs are disfavoured by kinetic Sunyaev-Zeldovich measurements, which prefer a short reionization history.

Cross-correlation of the cosmic 21-cm signal and Lyman α emitters during reionization

“…Kuhlen & Faucher-Giguere 2012). The resulting EoR morphologies are characterized by large cosmic HII patches, as seen in the top right panel of Fig.…”

Spatially adaptive radiation-hydrodynamical simulations of galaxy formation during cosmological reionization

“…Many works have argued that these difficulties may signal a significant role by physical processes that are currently not understood or unknown, and that are currently only poorly modelled or lacking altogether in cosmological simulations. Such processes include, among others, an evolving IGM escape fraction (e.g., Kuhlen & Faucher-Giguère 2012;Alvarez, Finlator, & Trenti 2012;Mitra, Ferrara, & Choudhury 2013;Ferrara & Loeb 2013) and the absorption of photons by Lyman limit systems (e.g., Bolton & Haehnelt 2013;Mesinger et al 2014; which our reference simulation begins to resolve). Some works have successfully matched the observational constraints by a careful calibration of parameters (e.g., Ciardi et al 2012).…”

Reionization and Galaxy Formation in Warm Dark Matter Cosmologies

“…In this work, we follow the approach of Kuhlen & Faucher-Giguere (2012) and combine the observational constraints on the H I photoionization rate (Γ H I ) from Wyithe & Bolton (2011) with the mean-free path of ionizing radiation (λ mfp ) from Songaila & Cowie (2010) to obtain the observational estimate ofṅ ion . We have used the fiducial values from Kuhlen & Faucher-Giguere (2012): γ=1 for the source spectral index and β=1.3 for the H I column density distribution. Varying these within allowed limits would only result in larger error bars, leaving our results unchanged (see Kuhlen & Faucher-Giguere 2012).…”

“…As seen, a constant f esc value severely over-predictsṅ ion compared to the observations (points) at z;5, 6 in all DM models. The observational results (and associated error bars) have been calculated following the approach of Kuhlen & Faucher-Giguere (2012), i.e., by combining the observational constraints on Γ H I from Wyithe & Bolton (2011) with the mean-free path for ionizing photons (λ mfp ) from Songaila & Cowie (2010). See Section 3.2 for details.…”

“…We have used the fiducial values from Kuhlen & Faucher-Giguere (2012): γ=1 for the source spectral index and β=1.3 for the H I column density distribution. Varying these within allowed limits would only result in larger error bars, leaving our results unchanged (see Kuhlen & Faucher-Giguere 2012).…”