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

By J. Bolton, M. Haehnelt. 2007

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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 Lya 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 A, 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.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/80e5671df86dec5accba8d3e407e4356bd635dd5

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Citations in the corpus

THE RELATION BETWEEN STAR FORMATION RATE AND STELLAR MASS FOR GALAXIES AT 3.5 ⩽ z ⩽ 6.5 IN CANDELS (2014, background): “…the escape fraction must be fesc < 0.34, or fesc < 0.13 (2σ) at z ∼ 6 if the luminosity function extends to fainter galaxies than those observed, in order for the inferred ionization from galaxies to be consistent with the ionization background inferred from quasar spectra (Bolton & Haehnelt 2007).”
New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization (2013, methodology): “As discussed in Bolton et al. (2005) and Bolton & Haehnelt (2007), however, the calibration of H i ionization rate as a function of Lyα opacity will depend on cosmology.”
ARE ULTRA-FAINT GALAXIES AT z = 6–8 RESPONSIBLE FOR COSMIC REIONIZATION? COMBINED CONSTRAINTS FROM THE HUBBLE FRONTIER FIELDS CLUSTERS AND PARALLELS (2015, methodology): “The value of ξion is generally constrained using stellar population models of early galax- ies and the observed UV slope β of z > 6 galaxies (Bolton & Haehnelt 2007; Kuhlen & Faucher-Giguère 2012; Robertson et al. 2015).”
How neutral is the intergalactic medium surrounding the redshift z = 7.085 quasar ULAS J1120+0641? (2011, background): “At the luminosities typical of observed z ≃ 6 quasars, and assuming an optically bright phase of tq ∼ 10 7 yr, the size of a quasar H II region expanding into a partially neutral IGM and that of a proximity zone embedded in an already highly ionised IGM are comparable (Bolton & Haehnelt 2007a).”
Feedback-regulated star formation and escape of LyC photons from mini-haloes during reionization (2016, background): “…c.f., Madau & Haardt 2015); however, the timescale over which reionisation occurred and the mass range of haloes which provided the majority of the ionising photons are issues that remain unresolved (e.g., Bolton & Haehnelt 2007; Kuhlen & Faucher-Giguère 2012; Ahn et al. 2012; Wise et al. 2014).”
Keeping the Universe ionized: photoheating and the clumping factor of the high-redshift intergalactic medium (2008, background): “…2000; Gnedin 2000a; Haiman, Abel, & Madau 2001; Benson et al. 2001; Chiu, Fan, & Ostriker 2003; Iliev et al. 2007; Srbinovsky & Wyithe 2007; Kohler, Gnedin, & Hamilton 2007; Bolton & Haehnelt 2007; Maio et al. 2007; Furlanetto, Haiman, & Oh 2008) significantly lower clumping factors were derived.”
LOFAR: The LOw-Frequency ARray (2013, background): “Still the current constraints strongly suggest that the EoR roughly straddled the redshift range of z ∼ 6–12 (Komatsu et al. 2011; Fan, et al. 2003, 2006; Bolton & Haehnelt 2007; Theuns et al. 2002; Bolton et al. 2010; Oesch et al. 2010; Bunker et al. 2010).”
The physics of the intergalactic medium (2007, methodology): “Allowing for clouds photoionized by a QSO/AGN-dominated UV background, small enough to be Jeans stable and able to withstand thermal evaporation by the hot confining medium, while requiring the IGM to meet the Gunn-Peterson constraint, Sargent et al. (1980) found for the cloud parameters at z ≈ 2.4,”
Comparison of reionization models: radiative transfer simulations and approximate, seminumeric models (2010, background): “Taken at face value, these observations favor an extended reionization epoch (see also Cen 2003; Fan et al. 2006; Bolton & Haehnelt 2007b; Wyithe & Cen 2007).”
IONIZING PHOTON ESCAPE FRACTIONS FROM HIGH-REDSHIFT DWARF GALAXIES (2008, methodology): “When calibrating these models against WMAP observations (Spergel et al. 2007; Komatsu et al. 2008) and Lyα transmission from z ∼ 6 quasars (e.g. Bolton & Haehnelt 2007; Srbinovsky & Wyithe 2008), the values of ≥ 0.01 are usually required.”
Evolution of the Intergalactic Opacity: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity (2008, result): “Our results also support the picture of a “photon-starved” reionization similarly advocated by Bolton & Haehnelt (2007) based the z = 5 − 6 Lyα forest photoionization rate.”
THE GALAXY LUMINOSITY FUNCTION DURING THE REIONIZATION EPOCH (2010, methodology): “The LF evolution established from z ∼ 4 to z ∼ 6 (Bouwens et al. 2007) seems to continue into the dark ages, with progressively fewer bright sources (Bolton & Haehnelt 2007; Bouwens et al. 2008, 2010a; Oesch et al. 2009c).”
The birth of a galaxy – III. Propelling reionization with the faintest galaxies (2014, background): “But the ionizing emissivity measured at z = 4–6 through Lyα forest observations cannot account for this measured τ e, indicating that the end of reionization must be photon starved (Bolton & Haehnelt 2007) and that the emissivity must have been higher during reionization.”
NEW CONSTRAINTS ON COSMIC REIONIZATION FROM THE 2012 HUBBLE ULTRA DEEP FIELD CAMPAIGN (2013, methodology): “Nonetheless, the Bolton & Haehnelt (2007b) analysis shows that the ionizing background falls by about a factor of ten fromz ∼ 3 to toz ∼ 6 (though see McQuinn et al. 2011).”
21CMMC: an MCMC analysis tool enabling astrophysical parameter studies of the cosmic 21 cm signal (2015, result): “Finally, nrec ∼ 1 is consistent with the models of (Sobacchi & Mesinger 2014) which result in a ‘photon-starved’ end to reionization, consistent with emissivity estimates from the Lyα forest (e.g. Bolton & Haehnelt 2007; McQuinn et al. 2011).”
Conditions for Reionizing the Universe with a Low Galaxy Ionizing Photon Escape Fraction (2019, background): “We also did not include the upper limit on the integrated emissivity at z=6 from Bolton & Haehnelt (2007), as the value of log( -”
Concordance models of reionization: implications for faint galaxies and escape fraction evolution (2012, methodology): “…FG08, P09 5.0 0.52+0.35−0.21 13.9±3.6 4.3±2.6 (±2.6) B07, SC10 6.0 <0.19 7.0±2.0 <2.6 (< 2.6) B07, SC10 The HI photoionization rates measurements are taken from Faucher-Gigère et al. (2008a) (FG08) and Bolton & Haehnelt (2007) (B07); the mean free paths are taken from the fits of…”
Galaxies at z = 6 - 9 from the WFC3/IR imaging of the HUDF (2009, methodology): “We adopt the method of Bolton & Haehnelt (2007), converting the observed luminosity functions atz > 6 to ionising emissivities assuming a Salpeter IMF, a metallicity of 0.2 Z⊙, and an escape fraction for ionising photons of 20%.”