Abstract
We present a measurement of the Lyman α flux probability distribution function (PDF) obtained from a set of eight high-resolution quasar spectra with emission redshifts in the range 3.3 ≤z≤ 3.8. We carefully study the effect of metal absorption lines on the shape of the PDF. Metals have a larger impact on the PDF measurements at lower redshift, where there are relatively fewer Lyman α absorption lines. This may be explained by an increase in the number of metal lines that are blended with Lyman α absorption lines towards higher redshift, but may also be due to the presence of fewer metals in the intergalactic medium (IGM) at earlier times. We also provide a new measurement of the redshift evolution of the effective optical depth, τeff, at 2.8 ≤z≤ 3.6, and find no evidence for a deviation from a power-law evolution in the log (τeff)–log (1 +z) plane. The flux PDF measurements are furthermore of interest for studies of the thermal state of the IGM at z≃ 3. By comparing the PDF to state-of-the-art cosmological hydrodynamical simulations, we place constraints on the temperature of the IGM and compare our results with previous measurements of the PDF at lower redshift. At redshift z= 3, our new PDF measurements are consistent with an isothermal temperature–density relation, T=T0Δγ− 1, with a temperature at the mean density of T0= 19 250 ± 4800 K and a slope γ= 0.90 ± 0.21 (1σ uncertainties). In comparison, joint constraints with existing lower redshift PDF measurements at z < 3 favour an inverted temperature–density relation with T0= 17 900 ± 3500 K and γ= 0.70 ± 0.12, in broad agreement with previous analyses.
Highlights
The Lyman α forest corresponds to the large number of absorption features located blueward of the Lyman α emission line in the spectra of quasi-stellar objects (QSOs)
We present a measurement of the Lyman α flux probability distribution function (PDF) obtained from a set of eight high resolution quasar spectra with emission redshifts at 3.3 ≤ z ≤ 3.8
Once each spectrum has been normalised to the continuum, i.e. once the quantity F = S/C has been calculated for each pixel, where S and C are the observed flux and the estimated continuum, respectively, the flux probability distribution function can be measured
Summary
The Lyman α forest corresponds to the large number of absorption features located blueward of the Lyman α emission line in the spectra of quasi-stellar objects (QSOs). Croft et al 2002, Viel et al 2004, McDonald et al 2006) Another quantity thoroughly studied in the last few years is the flux probability distribution function (PDF), which is sensitive to the spatial distribution of dark matter, and to the thermal state of the intergalactic medium (IGM). Agreement between the data and simulations was obtained by adopting a power-law temperature-density relation, T = T0∆γ−1 (Hui & Gnedin 1997), where the low density IGM (∆ = ρ/ ρ ≤ 10) was close to isothermal (i.e. with γ ∼ 1) or inverted (γ < 1) This result implied that the low-density regions of the IGM may be considerably hotter and their thermal state more complicated than usually assumed at these redshifts
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