A uniform analysis of the Ly alpha forest at z=0-5. II. Measuring the meanintensity of the extragalactic ionizing background using the proximity effect

Moderate-resolution data for 40 quasi-stellar objects (QSOs) at z approxima te to 2 were combined with spectra of comparable resolution of 59 QSOs with redshifts greater than 1.7 found in the literature to form a large, homoge neous sample of moderate-resolution (similar to1 Angstrom) QSO spectra. The se spectra were presented and the statistics of the Ly alpha forest were di scussed in Paper I. In this analysis, we demonstrate that a proximity effec t is present in the data; i.e., there exists a significant (5.5 sigma) defi cit of lines at z(abs) approximate to z(em). Within 1.5 h(-1) Mpc of the QS O emission redshift, the significance does depend on QSO luminosity, in acc ordance with the theory that this effect is caused by enhanced ionization o f hydrogen in the vicinity of the QSO from UV photons from the QSO itself. The photoionization model of Bajtlik, Duncan, & Ostriker permits an estimat e of the mean intensity of the extragalactic background radiation at the Ly man limit. We compare the results of this standard analysis with those obta ined using a maximum likelihood technique. If the spectrum of the backgroun d is assumed to be identical to that of each individual QSO, and if this ba ckground is assumed to be constant over the redshift range 1.7 < z < 3.8, t hen the best-fit value for J(nu (0)) is found to be 1.4(-0.5)(+1.1) x 10(-2 1) ergs s(-1) cm(-2) Hz(-1) sr(-1), using QSO redshifts based on the Ly alp ha emission line. Systemic QSO redshifts based on the [O III] lambda 5007 e mission line for 19 objects in our sample show an average redshift of simil ar to 400 km s(-1) with respect to Ly alpha emission. Using redshifts based on [O III] or Mg II for the 35 objects for which they are measured and add ing 400 km s(-1) to the remaining QSO Ly alpha redshifts gives a lower valu e of J(nu (0)), 7.0(-4.4)(+3.4) x 10(-22) ergs s(-1) cm(-2) Hz(-1) sr(-1). This value is in reasonable agreement with the predictions of various model s of the ionizing background based on the integrated quasar luminosity func tion. Allowing for the fact that individual QSOs have different spectral in dices that may also be different from that of the background, we use the st andard methods to solve for the H I photoionization rate, Gamma, and the pa rameters describing its evolution with redshift. The best-fit value for the H I ionization rate we derive is 1.9(-1.0)(+1.2) x 10(-12) s(-1), in good agreement with models of the background that incorporate QSOs only. Finally , we use simulated Ly alpha forest spectra including the proximity effect t o investigate curve-of-growth effects in the photoionization model used in the analysis. We find that the presence of lines on the saturated part of t he curve of growth could cause our estimates of the background intensity to be overestimated by a factor of 2-3. This large absorption-line sample and these techniques for measuring the background and understanding the system atics involved allow us to place what we believe are the firmest limits on the background at these redshifts.