A direct precision measurement of the intergalactic Lyα opacityat 2 ≤ z ≤ 4.2

Claude André Faucher-Giguère*, Jason X. Prochaska, Adam Lidz, Lars Hernquist, Matias Zaldarriaga

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

179 Scopus citations


We measure the evolution of the intergalactic Lyα effective optical depth, τeff, over the redshift range 2 ≤ z ≤ 4.2 from a sample of 86 high-resolution, high-S/N quasar spectra obtained with the ESI and HIRES spectrographs on Keck and with the MIKE spectrograph on Magellan. This represents an improvement over previous analyses of the Lyα forest from high-resolution spectra in this redshift interval of a factor of 2 in the size of the data set alone. We pay particular attention to robust error estimation and extensively test for systematic effects. We find that our estimates of the quasar continuum levels in the Lya forest obtained by spline fitting are systematically biased low, with the magnitude of the bias increasing with redshift, but that this bias can be accounted for using mock spectra. The mean fractional error (ΔC/Ctrue) is <1% at z = 2, 4% at z = 3, and 12% at z = 4. Previous measurements of τeff at z S; 3 based on directly fitting the quasar continua in the Lyα forest, which have generally neglected this effect, are therefore likely biased low. We provide estimates of the level of absorption arising from metals in the Lya forest based on both direct and statistical metal removal results in the literature, finding that this contribution is ≈6%-9% at z = 3 and decreases monotonically with redshift. The high precision of our measurement, attaining 3% in redshift bins of width Δz = 0.2 around z = 3, indicates significant departures from the best-fit power-law redshift evolution [τeff = 0.0018(1 + z) 3.92, when metals are left in], particularly near z = 3.2. The observed downward departure is statistically consistent with a similar feature detected in a precision statistical measurement using SDSS spectra by Bernardi and coworkers using an independent approach.

Original languageEnglish (US)
Pages (from-to)831-855
Number of pages25
JournalAstrophysical Journal
Issue number2
StatePublished - Jul 10 2008


  • Cosmology: observations
  • Cosmology: theory
  • Methods: data analysis
  • Methods: numerical
  • Methods: statistical
  • Quasars: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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