Measurements of the UV background at 4.6 < z < 6.4 using the quasar proximity effect

By Alexander P. Calverley, George D. Becker, M. Haehnelt, James S. Bolton KICCIoA Cambridge, Melbourne.. 2010

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We present measurements of the ionising ultraviolet background (UVB) at z ~ 5-6 using the quasar proximity effect. The fifteen quasars in our sample cover the range 4.6 5. The metagalactic hydrogen ionisation rate, Gamma_bkg, was determined by modelling the combined ionisation field from the quasar and the UVB in the proximity zone on a pixel-by-pixel basis. The optical depths in the spectra were corrected for the expected effect of the quasar until the mean flux in the proximity region equalled that in the average Ly-alpha forest, and from this we make a measurement of Gamma_bkg. A number of systematic effects were tested using synthetic spectra. Noise in the flux was found to be the largest source of bias at z ~ 5, while uncertainties in the mean transmitted Ly-alpha flux are responsible for the largest bias at z ~ 6. The impacts of large-scale overdensities and Lyman limit systems on Gamma_bkg were also investigated, but found to be small at z > 5. We find a decline in Gamma_bkg with redshift, from log(Gamma_bkg) = -12.15 $\pm$ 0.16 at z ~ 5 to log(Gamma_bkg) = -12.84 $\pm$ 0.18 at z ~ 6 (1 sigma errors). Compared to UVB measurements at lower redshifts, our measurements suggest a drop of a factor of five in the HI photoionisation rate between z ~ 4 and z ~ 6. The decline of Gamma_bkg appears to be gradual, and we find no evidence for a sudden change in the UVB at any redshift that would indicate a rapid change in the attenuation length of ionising photons. Combined with recent measurements of the evolution of the mean free path of ionising photons, our results imply decline in the emissivity of ionising photons by roughly a factor of two from z ~ 5 to 6, albeit with significant uncertainty due to the measurement errors in both Gamma_bkg and the mean free path.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/d82cb5ae7d3d7ded8b172112a73df3a069b0c8b1

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Citations in the corpus

New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization (2013, background): “The low measurement of Γ at z ∼ 6 has previously been interpreted as evidence for a low ionizing emissivity and a “photon-starved” mode of reionization (Bolton & Haehnelt 2007; Calverley et al. 2011).”
Concordance models of reionization: implications for faint galaxies and escape fraction evolution (2012, background): “Nevertheless, at z = 5 − 6, where some of these effects are mitigated, the proximity effect measurements of Calverley et al. (2011) are consistent with Bolton & Haehnelt (2007).”
The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time (2014, background): “At present, any such analysis is limited by quasar redshift uncertainty (σ r 1 Mpc), strong contaminating Lyman series absorption at lower redshifts, and the uncertainty in the likely evolving UV background at z > 5 (Calverley et al. 2011).”
Evolution of the AGN UV luminosity function from redshift 7.5 (2018, background): “The contribution of M1450 < −21 at this redshift is ∼ 3% relative to the measurements (Calverley et al. 2011; D’Aloisio et al. 2018; Davies et al. 2018).”
Cosmic Dawn (CoDa): the first radiation-hydrodynamics simulation of reionization and galaxy formation in the Local Universe (2015, background): “6, along with several observational constraints from Fan et al. (2006), Ouchi et al. (2010), Calverley et al. (2011), and the Planck CMB thomson optical depth τ (Planck Collaboration et al. 2016).”
Faint AGNs at z > 4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe (2015, methodology): “This is due to uncertainties on modelling the gas density and temperature distributions in the IGM at very low densities (Miralda-Escudé, Haehnelt, & Rees (2000)) and within the proximity region surrounding each QSO (Calverley et al. (2011)).”
Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six (2014, background): “Fan et al. noted a rapid increase in the mean Lyα opacity c© 0000 RAS at z > 5.7, however, which suggests a decline in the intensity of the ultraviolet background (UVB) near 1 Ryd (see also Bolton & Haehnelt 2007; Wyithe & Bolton 2011; Calverley et al. 2011).”