Cosmic Reionization after Planck: The Derived Growth of the Ionizing Background now matches the Growth of the Galaxy UV Luminosity Density (original) (raw)
Related papers
On the evolution of the cosmic ionizing background
Monthly Notices of the Royal Astronomical Society, 2014
We study the observed cosmic ionizing background as a constraint on the nature of the sources responsible for the reionization of the Universe. In earlier work, we showed that extrapolations of the ultraviolet (UV) luminosity function (LF) of Lyman-break galaxies (LBGs) at fixed Lyman continuum photon escape fraction (f esc) are not able to reproduce the redshift evolution of this background. Here, we employ extrapolations of the high-z LFs to describe the contribution of LBGs to the ionizing photon rate, taking into account the smoothing of the baryonic perturbations, due to the background itself (i.e. the filtering mass), as well as a possible sharp increase of f esc in dwarf galaxies. Under the hypothesis of a dominant contribution of LBGs to cosmic reionization, our results suggest that sources fainter than the current observational limits should be characterized by f esc values of the order of ∼0.1-0.3 (larger than the current estimates for bright galaxies) to account for a z 6 reionization and the measured evolution of cosmic ionizing background, at the same time. The contribution to the background from quasars turns out to be relevant at z 3. Overall, our results support the case for dedicated observations of faint galaxies in the rest-frame UV, in order to better determine their physical properties. Observed f esc values outside our proposed range bear relevant consequences on the nature of the astrophysical sources responsible for cosmic reionization and/or its build-up process.
Powering reionization: assessing the galaxy ionizing photon budget at z < 10
Monthly Notices of the Royal Astronomical Society, 2015
We present a new analysis of the ionizing emissivity (Ṅ ion , s −1 Mpc −3) for galaxies during the epoch of reionization and their potential for completing and maintaining reionization. We use extensive SED modelling-incorporating two plausible mechanisms for the escape of Lyman continuum photon-to explore the range and evolution of ionizing efficiencies consistent with new results on galaxy colours (β) during this epoch. We estimateṄ ion for the latest observations of the luminosity and star-formation rate density at z < 10, outlining the range of emissivity histories consistent with our new model. Given the growing observational evidence for a UV colour-magnitude relation in high-redshift galaxies, we find that for any plausible evolution in galaxy properties, red (brighter) galaxies are less efficient at producing ionizing photons than their blue (fainter) counterparts. The assumption of a redshift and luminosity evolution in β leads to two important conclusions. Firstly, the ionizing efficiency of galaxies naturally increases with redshift. Secondly, for a luminosity dependent ionizing efficiency, we find that galaxies down to a rest-frame magnitude of M UV ≈ −15 alone can potentially produce sufficient numbers of ionizing photons to maintain reionization as early as z ∼ 8 for a clumping factor of C Hii 3.
Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of z~ 6 Quasars
We study the process of cosmic reionization and estimate the ionizing background in the intergalactic medium (IGM) using the Lyman series absorption in the spectra of the four quasars at 5.7 < z < 6.3 discovered by the Sloan Digital Sky Survey. We derive the redshift evolution of the ionizing background at high redshifts, using both semi-analytic techniques and cosmological simulations to model the density fluctuations in the IGM. The existence of the complete Lyα Gunn-Peterson trough in the spectrum of the z = 6.28 quasar SDSS 1030+0524 indicates a photoionization rate (Γ −12 in units of 10 −12 s −1 ) at z ∼ 6 lower than 0.08, at least a factor of 6 smaller than the value at z ∼ 3. The Lyβ and Lyγ Gunn-Peterson troughs give an even stronger limit Γ −12 ∼ < 0.02 due to their smaller oscillator strengths, indicating that the ionizing background in the IGM at z ∼ 6 is more than 20 times lower than that at z ∼ 3. Meanwhile, the volume-averaged neutral hydrogen fraction increases from 10 −5 at z ∼ 3 to > 10 −3 at z ∼ 6. At this redshift, the mass-averaged neutral hydrogen fraction is larger than 1%; the mildly overdense regions (δ > 3) are still mostly neutral and the comoving mean free path of ionizing photons is shorter than 8 Mpc. Comparison with simulations of cosmological reionization shows that the observed properties of the IGM at z ∼ 6 are typical of those in the era at the end of the overlap stage of reionization when the individual HII regions merge. Thus, z ∼ 6 marks the end of the reionization epoch. The redshift of reionization constrains the small scale power of the mass density fluctuations and the star forming efficiency of the first generation of objects.
Constraining the contribution of galaxies and active galactic nuclei to cosmic reionization
Monthly Notices of the Royal Astronomical Society, 2017
Understanding the detailed process of cosmic reionization is one of the remaining problems in astrophysics and cosmology. Here we construct a model of cosmic reionization that includes contributions from high-z galaxies and active galactic nuclei (AGNs) and calculate reionization and thermal histories with the model. To keep the model general and realistic, we vary the escape fraction of ionizing photons, f esc , and the faint-end slope of the AGN luminosity function at high redshifts, α hz , within constraints from the observed cosmic star formation history and observed bright-end UV luminosity functions at z ≤ 6. Additionally, we model the spectral energy distribution (SED) of AGNs, which depends on the Eddington ratio and the black hole mass. By comparing the computed reionization histories with the observed H I fractions and the optical depth for Thomson scattering from Planck, we find that α hz > −1.5 and f esc < 0.15 are favoured when we employ the bright-end luminosity function obtained by Giallongo et al. Our result suggests that an AGN-dominated model with an abundance of faint AGNs as large as the estimate by Giallongo et al. is allowed only if the contribution from high-z galaxies is almost negligible, while a galaxy-dominant model is also allowed. We also find that the shape of the SED has a significant impact on the thermal history. Therefore it is expected that measurements of the thermal state of the intergalactic medium (IGM) will provide useful information on the properties of ionizing sources.
The Astronomical Journal, 2002
We study the process of cosmic reionization and estimate the ionizing background in the intergalactic medium (IGM) using the Lyman series absorption in the spectra of the four quasars at 5.7 < z < 6.3 discovered by the Sloan Digital Sky Survey. We derive the redshift evolution of the ionizing background at high redshifts, using both semi-analytic techniques and cosmological simulations to model the density fluctuations in the IGM. The existence of the complete Lyα Gunn-Peterson trough in the spectrum of the z = 6.28 quasar SDSS 1030+0524 indicates a photoionization rate (Γ −12 in units of 10 −12 s −1) at z ∼ 6 lower than 0.08, at least a factor of 6 smaller than the value at z ∼ 3. The Lyβ and Lyγ Gunn-Peterson troughs give an even stronger limit Γ −12 ∼ < 0.02 due to their smaller oscillator strengths, indicating that the ionizing background in the IGM at z ∼ 6 is more than 20 times lower than that at z ∼ 3. Meanwhile, the volume-averaged neutral hydrogen fraction increases from 10 −5 at z ∼ 3 to > 10 −3 at z ∼ 6. At this redshift, the mass-averaged neutral hydrogen fraction is larger than 1%; the mildly overdense regions (δ > 3) are still mostly neutral and the comoving mean free path of ionizing photons is shorter than 8 Mpc. Comparison with simulations of cosmological reionization shows that the observed properties of the IGM at z ∼ 6 are typical of those in the era at the end of the overlap stage of reionization when the individual HII regions merge. Thus, z ∼ 6 marks the end of the reionization epoch. The redshift of reionization constrains the small scale power of the mass density fluctuations and the star forming efficiency of the first generation of objects.
On the relative contribution of high-redshift galaxies and active galactic nuclei to reionization
Monthly Notices of the Royal Astronomical Society, 2012
In this paper we discuss the contribution of different astrophysical sources to the ionization of neutral hydrogen at different redshifts. We critically revise the arguments in favour/against a substantial contribution of Active Galactic Nuclei (AGNs) and/or Lyman Break Galaxies (LBGs) to the reionization of the Universe at z > 5. We consider extrapolations of the high-z QSO and LBG luminosity functions and their redshift evolution as well as indirect constraints on the space density of lower luminosity Active Galactic Nuclei based on the galaxy stellar mass function. Since the hypothesis of a reionization due to LBGs alone requires a significant contribution of faint dwarf galaxies and a LyC photon escape fraction (f esc) of the order of ∼ 20%, in tension with present observational constraints, we examine under which hypothesis AGNs and LBGs may provide a combined relevant contribution to the reionization. We show that a relatively steep faint-end of the AGN luminosity function, consistent with present constraints, provides a relevant (although subdominant) contribution, thus allowing us to recover the required ionizing photon rates with f esc ∼ 5% up to z ∼ 7. At higher redshifts, we test the case for a luminosity-dependent f esc scenario and we conclude that, if the observed LBGs are indeed characterized by very low f esc , values of the order of f esc ∼ 70% are needed for objects below our detection threshold, for this galaxy population to provide a substantial contribution to reionization. Clearly, the study of the properties of faint sources (both AGNs and LBGs) is crucial.
Monthly Notices of the Royal Astronomical Society: Letters, 2012
The escape fraction, f esc , of ionizing photons from high-redshift galaxies is a key parameter to understand cosmic reionization and star formation history. Yet, in spite of many efforts, it remains largely uncertain. We propose a novel, semi-empirical approach based on a simultaneous match of the most recently determined luminosity functions of galaxies in the redshift range 6 ≤ z ≤ 10 with reionization models constrained by a large variety of experimental data. From this procedure, we obtain the evolution of the best-fitting values of f esc along with their 2σ limits. We find that, averaged over the galaxy population, (i) the escape fraction increases from f esc = 0.068 +0.054 −0.047 at z = 6 to f esc = 0.179 +0.331 −0.132 at z = 8 and (ii) at z = 10 we can only put a lower limit of f esc > 0.146. Thus, although errors are large, there is an indication of a 2.6 times increase of the average escape fraction from z = 6 to 8, which might partially release the 'starving reionization' problem.
THE GALAXY LUMINOSITY FUNCTION DURING THE REIONIZATION EPOCH
The Astrophysical Journal, 2010
The new Wide Field Camera 3/IR observations on the Hubble Ultra-Deep Field started investigating the properties of galaxies during the reionization epoch. To interpret these observations, we present a novel approach inspired by the conditional luminosity function method. We calibrate our model to observations at z = 6 and assume a non-evolving galaxy luminosity versus halo mass relation. We first compare model predictions against the luminosity function measured at z = 5 and z = 4. We then predict the luminosity function at z 7 under the sole assumption of evolution in the underlying dark-matter halo mass function. Our model is consistent with the observed z 7 galaxy number counts in the HUDF survey and suggests a possible steepening of the faint-end slope of the luminosity function: α(z 8) −1.9 compared to α = −1.74 at z = 6. Although we currently see only the brightest galaxies, a hidden population of lower luminosity objects (L/L * 10 −4 ) might provide 75% of the total reionizing flux. Assuming escape fraction f esc ∼ 0.2, clumping factor C ∼ 5, top heavy-IMF and low metallicity, galaxies below the detection limit produce complete reionization at z 8. For solar metallicity and normal stellar IMF, reionization finishes at z 6, but a smaller C/f esc is required for an optical depth consistent with the WMAP measurement. Our model highlights that the star formation rate in sub-L * galaxies has a quasi-linear relation to dark-matter halo mass, suggesting that radiative and mechanical feedback were less effective at z ≥ 6 than today.
The Astrophysical Journal, 2009
We detect three (plus one less certain) z 850 -dropout sources in two separate fields (HUDF and NICP34) of our UDF05 HST NICMOS images. These z ∼ 7 Lyman-Break Galaxy (LBG) candidates allow us to constrain the Luminosity Function (LF) of the star forming galaxy population at those epochs. By assuming a change in only M * and adopting a linear evolution in redshift, anchored to the measured values at z ∼ 6, the best-fit evolution coefficient is found to be 0.43 ± 0.19 mag per unit redshift (0.36 ± 0.18, if including all four candidates), which provides a value of M * (z = 7.2) = −19.7 ± 0.3. This implies a drop of the luminosity density in LBGs by a factor of ∼ 2 − 2.5 over the ∼ 170 Myr that separate z ∼ 6 and z ∼ 7, and a steady evolution for the LBG LF out to z ∼ 7, at the same rate that is observed throughout the z ∼ 3 to 6 period. This puts a strong constraint on the star-formation histories of z ∼ 6 galaxies, whose ensemble star-formation rate density must be lower by a factor 2 at ∼170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z ∼ 6 LBG population must form at redshifts well above z ∼ 7. The rate of ionizing photons produced by the LBG population decreases consistently with the decrease in the cosmic star formation rate density. Extrapolating this steady evolution of the LF out to higher redshifts, we estimate that galaxies would be able to reionize the universe by z ∼ 6, provided that the faint-end slope of the z > 7 LF steepens to α ∼ −1.9, and that faint galaxies, with luminosities below the current detection limits, contribute a substantial fraction of the required ionizing photons. This scenario gives however an integrated optical depth to electron scattering that is ∼ 2σ below the WMAP-5 measurement. Therefore, altogether, our results indicate that, should galaxies be the primary contributors to reionization, either the currently detected evolution of the galaxy population slows down at z 7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.
We study the evolution of the ionization state of the intergalactic medium (IGM) at the end of the reionization epoch using moderate resolution spectra of a sample of nineteen quasars at 5.74 < z em < 6.42 discovered in the Sloan Digital Sky Survey. Three methods are used to trace IGM properties: (a) the evolution of the Gunn-Peterson (GP) optical depth in the Lyα, β, and γ transitions; (b) the distribution of lengths of dark absorption gaps, and (c) the size of HII regions around luminous quasars. Using this large sample, we find that the evolution of the ionization state of the IGM accelerated at z > 5.7: the GP optical depth evolution changes from τ eff GP ∼ (1 + z) 4.3 to (1 + z) ∼ >11 , and the average length of dark gaps with τ > 3.5 increases from < 10 to > 80 comoving Mpc. The dispersion of IGM properties along different lines of sight also increases rapidly, implying fluctuations by a factor of ∼ > 4 in the UV background at