THE Lyman-alpha forest near 34 Quasi-Stellar Objects with z > 2.6

Jill Bechtold

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169 Scopus citations


Moderate-resolution spectra of 34 high-redshift QSOs were obtained at the Multiple Mirror Telescope and Palomar 5 m Telescope in order to study the properties of the Lyα forest absorption lines with zabs ≈ zem. For zabs ≪ zem, the evolution of the number of lines per unit redshift, dN/dz, is well described by the form (1 + z)γ, with γ = 1.89 ± 0.28 for lines with rest equivalent width greater than 0.32 Å. For lines stronger than 0.16 Å, γ = 1.32 ± 0.24. These results imply slower evolution in the mean free path for absorption for the clouds than previously thought. When this sample is combined with HST results for low redshift (Bahcall et al. 1993), a single γ is an acceptable fit to the data, with γ = 1.26 ± 0.13 for zabs = 0-4.1, although an inflection at z ≈ 1.5 may be present. In the high-redshift sample, there is a significant trend of decreasing γ with decreasing equivalent width limit, or, alternatively, a change in equivalent width distribution with redshift. There is no significant curvature in the log (dN/dz) versus log (1 + z) relation for the strong lines, but some steepening of γ at z ≈ 2.5 for the weak ones. While these findings for the weak lines are highly significant in a statistical sense, they may result from some subtle line-blending problem, rather than a real physical process. The lines with zabs ≈ zem are significantly depleted. The strength of the deficit of lines depends weakly on QSO Lyman limit luminosity, but not redshift. The strength of the deficit also depends on the rest equivalent width of the lines, with weak lines showing a relatively weaker effect. These findings support previous suggestions that the "proximity effect" results from enhanced photoionization by the QSO's EUV radiation, rather than some property of the QSO or intergalactic medium which evolves with redshift. The simple photoionization model of Bajtlik, Duncan, & Ostrīker describes the data well. If the ionizing background at the Lyman limit, Jv, is assumed to be constant with redshift for 1.6 < z < 4.1, and writing Jv = J-21 × 10-21 ergs cm-2 s-1 Hz-1 sr-1, then J-21 ≈ 3. This value is formally consistent with previous results, although about a factor of 3 larger. Hence, the factor by which QSOs fail to provide the EUV background at high redshifts (at least in this simple picture), is also increased. If the redshift dependence of Jv is parameterized as Jv ∝ (1 + z)j, then j is not well constrained owing to the small redshift interval sampled, and acceptable fits are obtained for -7 < j < 4. Additions to the simple model and the resulting uncertainties in these estimates of Jv are discussed. One serious uncertainty is the neutral fraction of the clouds. If the clouds are nearly neutral, then Jv must be implausibly low, J-21 ≈ 0.01, in order to account for the proximity effect observed in this sample. In addition, if the systemic redshifts of the QSOs are systematically underestimated, then J-21 may be up to a factor of ∼3 smaller. Given these uncertainties, the J-21 at z ≈ 3 derived from the proximity effect is in reasonable agreement with the expected contribution of luminous QSOs.

Original languageEnglish (US)
Pages (from-to)1-78
Number of pages78
JournalAstrophysical Journal, Supplement Series
Issue number1
StatePublished - Mar 1994


  • Quasars: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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