We present updated constraints on the free-streaming of warm dark matter (WDM) particles derived from an analysis of the Lyman-flux power spectrum measured from high-resolution spectra of 25 z > 4 quasars obtained with the Keck High Resolution Echelle Spectrometer and the Magellan Inamori Kyocera Echelle spectrograph. We utilize a new suite of high-resolution hydrodynamical simulations that explore WDM masses of 1, 2 and 4 keV (assuming the WDM consists of thermal relics), along with different physically motivated thermal histories. We carefully address different sources of systematic error that may affect our final results and perform an analysis of the Lyman-flux power with conservative error estimates. By using a method that samples the multidimensional astrophysical and cosmological parameter space, we obtain a lower limit m WDM * 3:3 keV (2) for warm dark matter particles in the form of early decoupled thermal relics. Adding the Sloan Digital Sky Survey Lyman-flux power spectrum does not improve this limit. Thermal relics of masses 1, 2 and 2.5 keV are disfavored by the data at about the 9, 4 and 3 C.L., respectively. Our analysis disfavors WDM models where there is a suppression in the linear matter power spectrum at (nonlinear) scales corresponding to k ¼ 10h=Mpc which deviates more than 10% from a Lambda cold dark matter model. Given this limit, the corresponding ''free-streaming mass'' below which the mass function may be suppressed is $2 Â 10 8 h À1 M . There is thus very little room for a contribution of the free-streaming of WDM to the solution of what has been termed the small scale crisis of cold dark matter.

We report the discovery of an extremely long (∼110 Mpc h −1 ) and dark (τ eff 7) Lyα trough extending down to z ≃ 5.5 towards the z em ≃ 6.0 quasar ULAS J0148+0600. We use these new data in combination with Lyα forest measurements from 42 quasars at 4.5 z em 6.4 to conduct an updated analysis of the line-of-sight variance in the intergalactic Lyα opacity over 4 z 6. We find that the scatter in transmission among lines of sight near z ∼ 6 significantly exceeds theoretical expectations for either a uniform ultraviolet background (UVB) or simple fluctuating UVB models in which the mean free path to ionizing photons is spatially invariant. The data, particularly near z ≃ 5.6-5.8, instead require fluctuations in the volumeweighted hydrogen neutral fraction that are a factor of 3 or more beyond those expected from density variations alone. We argue that these fluctuations are most likely driven by large-scale variations in the mean free path, consistent with expectations for the final stages of inhomogeneous hydrogen reionization. Even by z ≃ 5.6, however, a large fraction of the data are consistent with a uniform UVB, and by z ∼ 5 the data are fully consistent with opacity fluctuations arising solely from the density field. This suggests that while reionization may be ongoing at z ∼ 6, it has fully completed by z ∼ 5.

Galaxies and quasars are thought to provide the bulk of the photons responsible for ionizing the hydrogen in the intergalactic medium (IGM). We use a large set of hydrodynamical simulations, combined with measurements of the Lyα opacity of the IGM taken from the literature, to obtain robust estimates of the photoionization rate per hydrogen atom at z= 5 and 6. We find the photoionization rate drops by a factor of 2 and 4, respectively, compared to our recent measurements at z= 2–4. The number of ionizing photons emitted by known sources at z= 5 and 6, based on an extrapolation of source numbers below the detection limit and standard assumptions for the relationship between the ionizing emissivity and observed luminosity density at 1500 Å, are in reasonable agreement with the photoionization rates inferred from the Lyα forest if the escape fraction of ionizing photons from galaxies is large (≳20 per cent). The expected number of ionizing photons from observed sources at these redshifts therefore appears sufficient to maintain the IGM in its highly ionized state. Claims to the contrary may be attributed to the adoption of an unduly high value for the clumping factor of ionized hydrogen. Using physically motivated assumptions for the mean free path of ionizing photons our measurements of the photoionization rate can be turned into an estimate of the ionizing emissivity. In comoving units the inferred ionizing emissivity is nearly constant over the redshift range 2–6 and corresponds to 1.5–3 photons emitted per hydrogen atom over a time interval corresponding to the age of the Universe at z= 6. This strongly suggests that the epoch of reionization was photon‐starved and extended. Completion of reionization at or before z= 6 requires either an emissivity which rises towards higher redshifts or one which remains constant but is dominated by sources with a rather hard spectral index. For standard assumptions, the ionizing emissivity required for completion of reionization at or before z= 6 lies at the upper end of recently reported values from searches for high‐redshift galaxies at z= 8–10.

We present new measurements of the intensity of the ionizing ultraviolet background and the global emissivity of ionizing photons over 2 < z < 5. Our results are based on a suite of updated measurements of physical properties of the high-redshift intergalactic medium (IGM), including gas temperatures and the opacity of the IGM to Lyα and ionizing photons. Consistent with previous works, we find a relatively flat hydrogen photoionization rate over 2 < z < 5, although our measurements are roughly a factor of two higher than the 2008 values of Faucher-Giguère et al., due primarily to our lower gas temperatures. The ionizing emissivity we derive is also generally higher than other recent estimates due to a combination of lower gas temperatures, higher ionizing opacity, and an accounting of cosmological radiative transfer effects. We find evidence that the emissivity increases from z ∼ 3 to 5, reaching ∼5 ionizing photons per atom per gigayear at z = 4.75 for realistic galaxy spectra. We further find that galaxies must dominate the emissivity near 1 Ryd at z 4, and possibly at all redshifts z 2.4. Our results suggest that the globally-averaged ionizing "efficiency" of star-forming galaxies increases substantially with redshift over 3.2 z 4.75. This trend is consistent with the conclusion often drawn from reionization models that the ionizing efficiency of galaxies must be higher during reionization in order for galaxies to reionize the IGM by z = 6. Our emissivity values at z ∼ 5 suggest that ionizing photons may have been a factor of two more abundant during the final stages of reionization than previously indicated. The evolution of the ionizing emissivity over 2 < z < 5 suggests, moreover, that the steep decline in the photoionization rate from z ∼ 5 to 6 may indicate a rapid evolution in the mean free path at z > 5.

We present new measurements of the temperature of the intergalactic medium (IGM) derived from the Lyα forest over 2.0 ≤ z ≤ 4.8. The small-scale structure in the forest of 61 highresolution quasi-stellar object spectra is quantified using a new statistic, the curvature, and the conversion to temperature calibrated using a suite of hydrodynamic simulations. At each redshift, we focus on obtaining the temperature at an optimal overdensity probed by the Lyα forest, T (¯ ), where the temperature is nearly a one-to-one function of the curvature regardless of the slope of the temperature-density relation. The median 2σ statistical uncertainty in these measurements is 8 per cent, though there may be comparable systematic errors due to the unknown amount of Jeans smoothing in the IGM. We use our T (¯ ) results to infer the temperature at the mean density, T 0 . Even for a maximally steep temperature-density relation, T 0 must increase from ∼8000 K at z 4.4 to 12 000 K at z 2.8. This increase is not consistent with the monotonic decline in T 0 expected in the absence of He II reionization. We therefore interpret the observed rise in temperature as evidence of He II reionization beginning at z 4.4. The evolution of T 0 is consistent with an end to He II reionization at z ∼ 3, as suggested by opacity measurements of the He II Lyα forest, although the redshift at which T 0 peaks will depend somewhat on the evolution of the temperature-density relation. These new temperature measurements suggest that the heat input due to the reionization of He II dominates the thermal balance of the IGM over an extended period with z 1.