The Metagalactic Ionizing Radiation Field at Low Redshift

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.