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

By Jennifer E. Scott, J. Bechtold, A. Dobrzycki, V. Kulkarni. 2000

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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 (∼1 Å) QSO spectra. These spectra were presented and the statistics of the Lyα forest were discussed in Paper I. In this analysis, we demonstrate that a proximity effect is present in the data; i.e., there exists a significant (5.5 σ) deficit of lines at zabs ≈ zem. 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(ν0) is found to be 1.4 × 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 [O III] 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 [O III] 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(ν0), 7.0 × 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 quasar luminosity function. Allowing for the fact that individual QSOs have different spectral indices that may also be different from that of the background, we use the standard methods to solve for the H I photoionization rate, Γ, and the parameters describing its evolution with redshift. The best-fit value for the H I ionization rate we derive is 1.9 × 10-12 s-1, in good agreement with models of the background that incorporate QSOs only. Finally, we use simulated Lyα 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 2-3. 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/1979d3bd1097b5efc75f3b0b427c7e5756b9c805

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Citations in the corpus

Evolution of the Intergalactic Opacity: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity (2008, background): “…evidence that this is the case is provided by the proximity effect, which is clearly detected at roughly the level predicted if the SED of quasars does not suffer a break at the Lyman limit from intrinsic absorption (e.g., Scott et al. 2000; Goncalves et al. 2007; Dall’Aglio et al. 2008).”
The effects of photoionization on galaxy formation – I. Model and results at z=0 (2001, methodology): “This is compared to observational estimates from the proximity effect in quasar spectra (e.g. Scott et al. 2000, and references therein), and upper limits from observational searches for Hα fluorescence from extragalactic Hi clouds at low redshift (Vogel et al. 1995) and for Lyα fluorescence from…”
An Observational Determination of the Bolometric Quasar Luminosity Function (2006, methodology): “We compare with several observational estimates of the total 12 from Scott et al. (2000), McDonald & Miralda-Escudé (2001, 2003), Tytler et al. (2004), Bolton et al. (2005), Rollinde et al. (2005), and Fan et al. (2006b) using various methods (for a review see Fan et al. 2006a).”
The Lyman alpha forest opacity and the metagalactic hydrogen ionization rate at z~ 2-4 (2004, methodology): “Constraints on Γ−12 from the proximity effect measurements of Scott et al. (2000) are shown by the hatched box, based on a set of 99 moderate resolution QSO spectra.”
Fluorescent Lyα Emission from the High-Redshift Intergalactic Medium (2005, background): “Even though on the low side, this is consistent with recent observational studies of the proximity effect, which give J(ν0) = 7.0 +3.4 −4.4 × 10−22 erg cm−2 s−1 sr−1Hz−1 and Γ = 1.9+1.2−1.0 × 10−12 s−1 (Scott et al. 2000).”
Quasars Probing Quasars. I. Optically Thick Absorbers near Luminous Quasars (2006, background): “Indeed, the decrease in the number of optically thin absorption lines ( log NH i < 17:2, hence LLP1) in the vicinity of quasars, known as the proximity effect (Bajtlik et al. 1988), provides a measurement of the UV background (Scott et al. 2000).”
Constraints on Warm Dark Matter from Cosmological Reionization (2001, background): “The measured value at zD 3 from the proximity e†ect is around i.e., the proper intensity is J \ 0.7] 10~21 ergsJ21\ 0.7,s~1 cm~2 Hz~1 sr~1 (Scott et al. 2000 ; see also Bajtlik, Duncan, & Ostriker 1988 for the basic measurement method).”
The observed ionization rate of the intergalactic medium and the ionizing emissivity at z≥ 5: evidence for a photon-starved and extended epoch of reionization (2007, background): “Measurements of −12 can also be made using the quasar proximity effect (e.g. Bajtlik, Duncan & Ostriker 1988; Giallongo et al. 1996; Scott et al. 2000), although these estimates can be subject to a bias introduced by the overdense regions in which quasars reside (Rollinde et al. 2005; Guimarães et…”
Probing reionization with Lyman α emission lines (2003, background): “This is the observed behaviour for any ionizing background with J21 1, the range observed at z < 5 (Scott et al. 2000).”
The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time (2014, background): “Such a proximity effect is observed in the optically thin Lyα forest (e.g. Scott et al. 2000; Dall’Aglio et al. 2008; Calverley et al. 2011).”
The physics of the intergalactic medium (2007, methodology): “…redshifts reflect the true distances to the QSOs, then the QSO flux producing the proximity effect will be overestimated using QSO redshifts based on Lyα or C IV, which in turn would result in an overestimate of the metagalactic ionization rate by as much as a factor of two (Scott et al., 2000).”
DIFFUSE Lyα EMITTING HALOS: A GENERIC PROPERTY OF HIGH-REDSHIFT STAR-FORMING GALAXIES (2011, background): “The radiation field intensity is usually expressed as Jν ≃ 2 − 10 × 10 −22 ergs s−1 cm−2 Hz−1 sr−1, where the quoted range indicates the dispersion among published observational or theoretical estimates (e.g., Shapley et al. 2006; Bolton et al. 2005; Scott et al. 2000; Faucher-Giguère et al. 2008).”
New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization (2013, background): “…2005; Kirkman et al. 2005; Bolton et al. 2005; Fan et al. 2006; Bolton & Haehnelt 2007; Faucher-Giguère et al. 2008a; Wyithe & Bolton 2011; Kuhlen & Faucher-Giguère 2012) and the quasar proximity effect (e.g., Bajtlik et al. 1988; Scott et al. 2000; Dall’Aglio et al. 2008; Calverley et al. 2011).”
The Distribution of Metallicity in the Intergalactic Medium at z ~ 2.5: O VI and C IV Absorption in the Spectra of Seven QSOs (2003, background): “It is also roughly consistent with observations of the proximity effect and X-Ray background at similar redshifts (Scott et al. 2000; Boyle et al. 1993).”
Implications of cosmological gamma-ray absorption - II. Modification of gamma-ray spectra (2003, result): “Comparing our results with the recent compilation of data from Scott et al. (2000) (upper panels Figure 1) shows that even the model with fesc = 1 lies below the data.”
The Direct Detection of Lyman Continuum Emission from Star-forming Galaxies at z~3 (2006, methodology): “It is possible to estimate the nature of the background from the quasar proximity effect (Bajtlik et al. 1988; Scott et al. 2000), and also from the evolution of optical depths of H I and He II in the Lyα forests of both those species, which are probed by the spectra of bright quasars (Shull et al.…”
Lyman-Continuum Emission from Galaxies at z ≃ 3.4 (2000, methodology): “There has been extensive speculation in the literature as to whether QSOs are sufficient to provide the implied value of Jν(912) at high redshift (e.g., Donahue & Shull 1987, Shapiro & Giroux 1987, Haardt & Madau 1996, Scott et al. 2000, Meiksin & Madau 1993, Madau, Haardt, & Rees 1999).”
Galaxies and Intergalactic Matter at Redshift z ~ 3: Overview (2002, background): “…high-redshift QSOs, a fact that presumably reflects a decrease in the hydrogen neutral fraction in regions where a quasar’s radiation overwhelms the ambient ionizing field (see, e.g., Weymann, Carswell, & Smith 1981; Murdoch et al. 1986; Bajtlik, Duncan, & Ostriker 1988; Scott et al. 2000).”