A Uniform Analysis of the Lyα Forest at z  = 0–5. II. Measuring the Mean Intensity of the Extragalactic Ionizing Background Using the Proximity Effect

Abstract

Moderate resolution data for 40 quasi-stellar objects (QSOs) at z ≈ 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, homogeneous sample of moderate resolution (∼1Å) QSO spectra. These spectra were presented and the statistics of the Lyman α forest were discussed in Paper I. In this analysis, we demonstrate that a proximity effect is present in the data, ie. there exists a significant (5.5σ) deficit of lines at z abs ≈ z em . Within 1.5 h −1 Mpc of the QSO emission redshift, the significance does depend on QSO luminosity, in accordance with the theory that this effect is caused by enhanced ionization of hydrogen in the vicinity of the QSO from UV photons from the QSO itself. The photoionization model of Bajtlik, Duncan, & Ostriker permits an estimate of the mean intensity of the extragalactic background radiation at the Lyman limit. We compare the results of this standard analysis with those obtained using a maximum likelihood technique. If the spectrum of the background is assumed to be identical to that of each individual QSO, and if this background is assumed to be constant over the redshift range 1.7 < z < 3.8, then the best fit value for J(ν 0 ) is found to be 1.4 +1.1 −0.5 × 10 −21 ergs s −1 cm −2 Hz −1 sr −1 , using QSO redshifts based on the Lyα emission line. Systemic QSO redshifts based on the [OIII] λ5007 emission line for 19 objects in our sample show an average redshift of ∼400 km s −1 with respect to Lyα emission. Using redshifts based on [OIII] or Mg II for the 35 objects for which they are measured and adding 400 km s −1 to the remaining QSO Lyα redshifts gives a lower value of J(ν 0 ), 7.0 +3.4 −4.4 × 10 −22 ergs s −1 cm −2 Hz −1 sr −1 . This value is in reasonable agreement with the predictions of various models of the ionizing background based on the integrated QSO luminosity function. Allowing for the fact that individual QSOs have different spectral indicies which may also be different from that of the background, we use the standard methods to solve for the HI photoionization rate, Γ, and the parameters describing its evolution with redshift. The best fit value for the HI ionization rate we derive is 1.9 +1.2 −1.0 × 10 −12 s −1 , in good agreement with models of the background which incorporate QSOs only. Finally, we use simulated Lyman α forest spectra including the proximity effect to 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 the curve-of-growth could cause our estimates of the background intensity to be overestimated by a factor of two to three. This large absorption line sample and these techniques for measuring the background and understanding the systematics involved allow us to place what we believe are the firmest limits on the background at these redshifts.

Faint Active Galactic Nuclei and the Ionizing Background

“…An estimate of the ionizing rate can be obtained by determining the distance from the quasar at which the number of lines is equal to the background expectation. Over the years, measurements of J À21 (assuming UV $ 1:8) have taken values between $0.7 and $3 (Williger et al 1994;Bechtold 1994;Fernandez-Soto et al 1995;Cristiani et al 1995;Giallongo et al 1996;Cooke, Espey, & Carswell 1997;Scott et al 2000;Liske & Williger 2001). Since what is effectively measured is a ratio of proper distances, this analysis should be relatively independent of cosmological parameters.…”

“…Haardt & Madau (1996) and Fardal, Giroux, & Shull (1998) have tried to fit the whole set of proximity effect measurements with a Gaussian. Scott et al (2000) claim that their observations are well fitted by the following parameterization from Fardal et al (1998):…”

“…6 Note that because the Ly forest is very thin for zd1:7, it is difficult to make measurements of the ionizing background in this way at low redshift (e.g., Shull et al 1999 and references therein; although see Davé & Tripp 2001). Scott et al 2000), where we have used the conversion À À12 ¼ 2:64J À21 from Fardal et al (1998). We have also drawn the Gaussian fit to these data from Fardal et al (1998).…”

An Ionizing Ultraviolet Background Dominated by Massive Stars

“…QSOs and star-forming galaxies have long been considered the two prime candidates. It has been argued that QSOs may fall short of producing a metagalactic flux as high as that inferred from the proximity effect at high redshift (Bechtold 1994;Giallongo et al 1996;Cooke, Espey, & Carswell 1997;Scott et al 2000) since their space density declines rapidly at early epochs (Shapiro & Giroux 1987;Miralda-Escudé & Ostriker 1990;Meiksin & Madau 1993;Madau, Haardt, & Rees 1999). The detection of a numerous population of Lyman break galaxies (LBGs) at (Steidel et al 1996) makes the idea of massive z ≈ 3 stars dominating the UV background at early epochs quite plausible.…”

The nature of $\ion{N}{v}$ absorbers at high redshift

“…In a first approach we adopt the UV background from Haardt & Madau (2001) at the appropriate redshift scaled to J H i = 10 −21.15 erg s −1 cm −2 Hz −1 sr −1 at 1 Ryd (Scott et al 2000). This energy distribution consists of the radiation of QSOs and galaxies whose spectra are filtered while propagating through the IGM.…”