arXiv:0709.2382v3 [astro-ph] 25 Jul 2008 Published in ApJ, 681, 831 Preprint typeset using L A T E X style emulateapj v. 08/22/09 A DIRECT PRECISION MEASUREMENT OF THE INTERGALACTIC LYMAN-α OPACITY AT 2 ≤ Z ≤ 4.2 1,2 Claude-Andr´e Faucher-Gigu`ere 3 , Jason X. Prochaska 4 , Adam Lidz 3 , Lars Hernquist 3 , Matias Zaldarriaga 3,5 Published in ApJ, 681, 831 ABSTRACT We directly measure the evolution of the intergalactic Lyman-α effective optical depth, τ eff , over the redshift range 2 ≤ z ≤ 4.2 from a sample of 86 high-resolution, high-signal-to-noise 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 two 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 Lyα 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/C true 〉 is < 1% at z = 2, 4% at z = 3, and 12% at z = 4. Previous measurements of τ eff at z  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 Lyα 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 (τ eff =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 Sloan Digital Sky Survey spectra by Bernardi and coworkers using an independent approach. Subject headings: Cosmology: observations, theory — methods: data analysis, statistical, numerical — quasars: absorption lines 1. INTRODUCTION The evolution of the intergalactic medium (IGM) as traced by the Lyman-α (Lyα) forest provides a powerful record of the thermal and radiative history of the Universe. This power owes to our ability to measure the Lyα opacity of the IGM as a function of redshift, from z = 0 to z  6, with high precision from quasar spectra (Press et al. 1993; Rauch et al. 1997; Songaila et al. 1999; McDonald & Miralda-Escud´e 2001; Meiksin & White 2003; Tytler et al. 2004a; Songaila 2004; Bolton et al. 2005; Kirkman et al. 2005; Jena et al. 2005; Fan et al. 2006b; Becker et al. 2007; Bolton & Haehnelt 2007), as well as to the relatively simple physics of the Lyα forest. In fact, fully and pseudo-hydrodynamical cosmological simulations, in which the forest arises from absorption by smooth den- sity fluctuations imposed on the warm photoionized IGM 1 Based, in part, on data obtained at the W.M. Keck Observa- tory, which is operated as a scientific partnership among the Cali- fornia Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. 2 Some of the data analyzed in this work were gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observa- tory, Chile. 3 Department of Astronomy, Harvard University, Cambridge, MA, 02138, USA; cgiguere@cfa.harvard.edu. 4 Department of Astronomy and Astrophysics, UCO/Lick Ob- servatory; University of California, 1156 High Street, Santa Cruz, CA, 95064, USA. 5 Jefferson Physical Laboratory, Harvard University, Cambridge, MA, 02138, USA. as a natural consequence of hierarchical structure for- mation within cold dark matter models (Cen et al. 1994; Zhang et al. 1995; Hernquist et al. 1996; Katz et al. 1996; Miralda-Escude et al. 1996; Theuns et al. 1998; Dav´e et al. 1999), have been remarkably successful at reproducing the properties of the absorption observed in high-resolution, high signal-to-noise quasar spectra (e.g., Lu et al. 1996; Kirkman & Tytler 1997; Kim et al. 2002a). This synergy between theory and observations make the Lyα forest a particularly compelling probe of the diffuse Universe. A number of observational results suggest changes in the thermal properties of the IGM over the redshift range probed by the Lyα forest, in particular near z = 3. These results fall into four classes: the mean Lyα opacity, the widths of the Lyα absorption lines, the strength of ab- sorption by metallic transitions, and ultra-violet obser- vations of the HeII forest. Bernardi et al. (2003) first claimed a statistical detection of a feature at z ≈ 3.2 in the evolution of the Lyα forest effective optical depth from an analysis of 1061 quasar spectra from the Sloan Digital Sky Survey (SDSS; York et al. 2000). These au- thors interpreted the observed reduction in the Lyα ef- fective optical depth near z =3.2 as evidence for the reionization of HeII (Theuns et al. 2002a). Ricotti et al. (2000) and Schaye et al. (2000) both found evidence from the Doppler widths of Lyα absorption lines for an up- ward jump in the temperature of the IGM and a change of its equation of state to nearly isothermal near z = 3. Songaila & Cowie (1996) and Songaila (1998) have ar- gued for a sudden hardening of the ionizing background