The Metagalactic Ionizing Radiation Field at Low Redshift

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Abstract
We compute the ionizing radiation field at low redshift, arising from Seyferts, QSOs, and starburst galaxies. This calculation combines recent Seyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGN database, and a new intergalactic opacity model based on Hubble Space Telescope and Keck Lyα absorber surveys. At z = 0 for AGNs only, our best estimate for the specific intensity at 1 ryd is I0 = 1.3 × 10-23 ergs cm-2 s-1 Hz-1 sr-1 , independent of H0, Ω0, and Λ. The one-sided ionizing photon flux is Φion ≈ 3400 photons cm-2 s-1, and the H I photoionization rate is Γ = 3.2 × 10-14 s-1, for αs = 1.8. We also derive ΓH I for z = 0–4. These error ranges reflect uncertainties in the spectral indexes for the ionizing EUV (αs = 1.8 ± 0.3) and the optical/UV (αUV = 0.86 ± 0.05), the IGM opacity model, the range of Seyfert luminosities (0.001L*–100L*), and the completeness of the luminosity functions. Our estimate is a factor of 3 lower than the most stringent upper limits on the ionizing background (Φion < 104 photons cm-2 s-1) obtained from Hα observations in external clouds, and it lies within the range implied by other indirect measures. Starburst galaxies with a sufficiently large Lyman continuum escape fraction, fesc ≥ 0.05, may provide a comparable background to AGNs, I0(z = 0) = 1.1 × 10-23 ergs cm-2 s-1 Hz-1 sr-1 . An additional component of the ionizing background of this magnitude would violate neither upper limits from Hα observations nor the acceptable range from other measurements.
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