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.

The impact of local stellar radiation on the H i column density distribution

“…Moreover, since our simulations also yield star formation histories that are in good agreement with observations (Schaye et al 2010), we used them to constrain the implied star formation rate weighted mean escape fraction that relates the predicted star formation rate density to the intensity of the UVB. We found that the average escape fraction in our simulations is 10 −2 − 10 −1 at z = 2 − 5, which agrees with previous constraints on the escape fraction from observations (e.g., Shapley et al 2006) and theoretical work (e.g., Schaye 2006;Gnedin et al 2008;Kuhlen & Faucher-Giguère 2012).…”

The impact of local stellar radiation on the H i column density distribution

“…Moreover, since our simulations also yield star formation histories that are in good agreement with observations (Schaye et al 2010), we used them to constrain the implied star formation rate weighted mean escape fraction that relates the predicted star formation rate density to the intensity of the UVB. We found that the average escape fraction in our simulations is 10 −2 − 10 −1 at z = 2 − 5, which agrees with previous constraints on the escape fraction from observations (e.g., Shapley et al 2006) and theoretical work (e.g., Schaye 2006;Gnedin et al 2008;Kuhlen & Faucher-Giguère 2012).…”

The Abundance of Star-Forming Galaxies in the Redshift Range 8.5-12: New Results From the 2012 Hubble Ultra Deep Field Campaign

“…Our estimate of the z ∼ 10 star formation rate densities are consistent with previous analyses aimed at explaining the measured Thomson optical depth (e.g. Robertson et al 2010a;Kuhlen & Faucher-Giguère 2012) and that required to produce the stellar masses of z < 8 sources observed by Spitzer (e.g., Stark et al 2012;Labbé et al 2012). Our results remain consistent with the simple picture for the evolving star formation rate density, stellar mass density, Thomson optical depth, and IGM ionization fraction presented in Robertson et al (2010a).…”

Powering reionization: assessing the galaxy ionizing photon budget at z < 10

“…Using a comprehensive set of SED models for two plausible Lyman continuum escape mechanisms -previously outlined in Zackrisson et al (2013) -we explore in detail the relationship between the UV continuum slope β and the number of ionizing photons produced per unit UV luminosity or star-formation (ξion and κion respectively). We find that the ionizing efficiencies assumed by several previous works (log 10 fescξion = 24.5 − 24.6: Robertson et al (2013); Kuhlen & Faucher-Giguère (2012)) are still consistent with the current β observations during the EoR. However, for both of the LyC escape models explored here, this assumption is close to the maximum efficiency which is still consistent with the fiducial UV slope typically considered at these redshifts (β = −2).…”

“…Applying the fiducial log 10 fescξion = 24.5 to the the latest observations of the luminosity and mass functions at z 4, we find that at z ∼ 6, the observed population can produce enough ionizing photons to maintain reionization assuming a clumping factor CHii = 3. At earlier times, we confirm previous results which found that galaxies from below our current observation limits are required to produce enough ionizing photons to maintain reionization at z ∼ 7 and beyond (Robertson et al 2010(Robertson et al , 2013Kuhlen & Faucher-Giguère 2012;Finkelstein et al 2012b).…”

The impact of local stellar radiation on the H i column density distribution

“…Moreover, since our simulations also yield star formation histories that are in good agreement with observations (Schaye et al 2010), we used them to constrain the implied star formation rate weighted mean escape fraction that relates the predicted star formation rate density to the intensity of the UVB. We found that the average escape fraction in our simulations is 10 −2 − 10 −1 at z = 2 − 5, which agrees with previous constraints on the escape fraction from observations (e.g., Shapley et al 2006) and theoretical work (e.g., Schaye 2006;Gnedin et al 2008;Kuhlen & Faucher-Giguère 2012).…”

The Abundance of Star-Forming Galaxies in the Redshift Range 8.5-12: New Results From the 2012 Hubble Ultra Deep Field Campaign

“…Our estimate of the z ∼ 10 star formation rate densities are consistent with previous analyses aimed at explaining the measured Thomson optical depth (e.g. Robertson et al 2010a;Kuhlen & Faucher-Giguère 2012) and that required to produce the stellar masses of z < 8 sources observed by Spitzer (e.g., Stark et al 2012;Labbé et al 2012). Our results remain consistent with the simple picture for the evolving star formation rate density, stellar mass density, Thomson optical depth, and IGM ionization fraction presented in Robertson et al (2010a).…”

Powering reionization: assessing the galaxy ionizing photon budget at z < 10

“…Using a comprehensive set of SED models for two plausible Lyman continuum escape mechanisms -previously outlined in Zackrisson et al (2013) -we explore in detail the relationship between the UV continuum slope β and the number of ionizing photons produced per unit UV luminosity or star-formation (ξion and κion respectively). We find that the ionizing efficiencies assumed by several previous works (log 10 fescξion = 24.5 − 24.6: Robertson et al (2013); Kuhlen & Faucher-Giguère (2012)) are still consistent with the current β observations during the EoR. However, for both of the LyC escape models explored here, this assumption is close to the maximum efficiency which is still consistent with the fiducial UV slope typically considered at these redshifts (β = −2).…”

“…Applying the fiducial log 10 fescξion = 24.5 to the the latest observations of the luminosity and mass functions at z 4, we find that at z ∼ 6, the observed population can produce enough ionizing photons to maintain reionization assuming a clumping factor CHii = 3. At earlier times, we confirm previous results which found that galaxies from below our current observation limits are required to produce enough ionizing photons to maintain reionization at z ∼ 7 and beyond (Robertson et al 2010(Robertson et al , 2013Kuhlen & Faucher-Giguère 2012;Finkelstein et al 2012b).…”

The impact of local stellar radiation on the H i column density distribution

“…Moreover, since our simulations also yield star formation histories that are in good agreement with observations (Schaye et al 2010), we used them to constrain the implied star formation rate weighted mean escape fraction that relates the predicted star formation rate density to the intensity of the UVB. We found that the average escape fraction in our simulations is 10 −2 − 10 −1 at z = 2 − 5, which agrees with previous constraints on the escape fraction from observations (e.g., Shapley et al 2006) and theoretical work (e.g., Schaye 2006;Gnedin et al 2008;Kuhlen & Faucher-Giguère 2012).…”

The Abundance of Star-Forming Galaxies in the Redshift Range 8.5-12: New Results From the 2012 Hubble Ultra Deep Field Campaign

“…Our estimate of the z ∼ 10 star formation rate densities are consistent with previous analyses aimed at explaining the measured Thomson optical depth (e.g. Robertson et al 2010a;Kuhlen & Faucher-Giguère 2012) and that required to produce the stellar masses of z < 8 sources observed by Spitzer (e.g., Stark et al 2012;Labbé et al 2012). Our results remain consistent with the simple picture for the evolving star formation rate density, stellar mass density, Thomson optical depth, and IGM ionization fraction presented in Robertson et al (2010a).…”

Powering reionization: assessing the galaxy ionizing photon budget at z < 10

“…Using a comprehensive set of SED models for two plausible Lyman continuum escape mechanisms -previously outlined in Zackrisson et al (2013) -we explore in detail the relationship between the UV continuum slope β and the number of ionizing photons produced per unit UV luminosity or star-formation (ξion and κion respectively). We find that the ionizing efficiencies assumed by several previous works (log 10 fescξion = 24.5 − 24.6: Robertson et al (2013); Kuhlen & Faucher-Giguère (2012)) are still consistent with the current β observations during the EoR. However, for both of the LyC escape models explored here, this assumption is close to the maximum efficiency which is still consistent with the fiducial UV slope typically considered at these redshifts (β = −2).…”

“…Applying the fiducial log 10 fescξion = 24.5 to the the latest observations of the luminosity and mass functions at z 4, we find that at z ∼ 6, the observed population can produce enough ionizing photons to maintain reionization assuming a clumping factor CHii = 3. At earlier times, we confirm previous results which found that galaxies from below our current observation limits are required to produce enough ionizing photons to maintain reionization at z ∼ 7 and beyond (Robertson et al 2010(Robertson et al , 2013Kuhlen & Faucher-Giguère 2012;Finkelstein et al 2012b).…”