The Metagalactic Ionizing Radiation Field at Low Redshift

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 I_0=1.3^+0.8_-0.5x10^-23 ergs cm^-2 s^-1 Hz^-1 sr^-1 , independent of H_0, Ω_0, and Λ. The one-sided ionizing photon flux is Φ_ion~3400^+2100_-1300 photons cm^-2 s^-1, and the H I photoionization rate is Γ_Hi=3.2^+2.0_-1.2x10^-14 s^-1, for α_s=1.8. We also derive Γ_HI for z=0-4. These error ranges reflect uncertainties in the spectral indexes for the ionizing EUV (α_s=1.8+/-0.3) and the optical/UV (α_UV=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 (Φ_ion<10^4 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, <f_esc>>=0.05, may provide a comparable background to AGNs, I_0(z=0)=1.1^+1.5_-0.7x10^-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.