The Metagalactic Ionizing Radiation Field at Low Redshift

We compute the ionizing radiation field at low redshift, arising fromSeyferts, QSOs, and starburst galaxies. This calculation combines recentSeyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGNdatabase, and a new intergalactic opacity model based on Hubble Space Telescopeand Keck Ly-alpha absorber surveys. At z = 0 for AGN only, our best estimatefor the specific intensity at 1 Ryd is I_0 = 1.3 (+0.8/-0.5) x 10^-23ergs/cm^2/s/Hz/sr, independent of H_0, Omega_0, and Lambda. The one-sidedionizing photon flux is Phi_ion = 3400 (+2100/-1300) photons/cm^2/s, and the HI photoionization rate is Gamma_HI = 3.2 (+2.0/-1.2) x 10^-14 s^-1 for alpha_s= 1.8. We also derive Gamma_ HI for z = 0 - 4. These error ranges reflectuncertainties in the spectral indexes for the ionizing EUV (alpha_s = 1.8 +/-0.3) and the optical/UV (alpha_UV = 0.86 +/- 0.05), the IGM opacity model, therange of Seyfert luminosities (0.001 - 100 L*) and the completeness of theluminosity functions. Our estimate is a factor of three lower than the moststringent upper limits on the ionizing background (Phi_ion < 10^4photons/cm^2/s) obtained from H-alpha observations in external clouds, and itlies within the range implied by other indirect measures. Starburst galaxieswith a sufficiently large Lyman continuum escape fraction, f_ esc > 0.05, mayprovide a comparable background to AGN, I_0 (z=0) = 1.1 (+1.5/-0.7) x 10^{-23).An additional component of the ionizing background of this magnitude wouldviolate neither upper limits from H-alpha observations nor the acceptable rangefrom other measurements.Comment: 30 pages, 9 figures, accepted for Astronomical J. (Oct. 1999