James S. Bolton

Warm dark matter as a solution to the small scale crisis: New constraints from high redshift Lyman-α forest data

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 (HIRES) and the Magellan Inamori Kyocera Echelle (MIKE) 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 multi-dimensional astrophysical and cosmological parameter space, we obtain a lower limit mWDM � 3.3 keV (2�) for warm dark matter particles in the form of early decoupled thermal relics. Adding the Sloan Digital Sky Survey (SDSS) Lyman-� flux power spectrum does not improve this limit. Thermal relics of masses 1 keV, 2 keV and 2.5 keV are disfavoured by the data at about the 9�, 4� and 3� C.L., respectively. Our analysis disfavours WDM models where there is a suppression in the linear matter power spectrum at (non-linear) scales corresponding to k = 10h/Mpc which

The observed ionization rate of the intergalactic medium and the ionizing emissivity at z ≥ 5: evidence for a photon-starved and extended epoch of reionization

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 Lya 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 A, 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.

The Lyman-alpha forest opacity and the metagalactic hydrogen ionization rate at z ~ 2-4

Estimates of the metagalactic hydrogen ionization rate from the Lya forest opacity in QSO absorption spectra depend on the complex density distribution of neutral hydrogen along the line of sight. We use a large suite of high-resolution hydrodynamical simulations to investigate in detail the dependence of such estimates on physical and numerical parameters in the context of ACDM models. Adopting fiducial values for cosmological parameters together with published values of the temperature of the IGM and the effective optical depth, the metagalactic ionization rates that reproduce the Lya effective optical depth at z = [2, 3, 4] are Γ HI = [1.3± 0.8 0.5 , 0.9 ± 0.3, 1.0± 0.5 0.3 ] × 10 -12 s -1 , respectively. The errors include estimates of uncertainties in the relevant physical parameters and the numerical accuracy of the simulations. We find the errors are dominated by the uncertainty in the temperature of the low-density IGM. The estimated metagalactic hydrogen ionization rate for the neutral hydrogen distribution in the current concordance ACDM model is more than four times the value inferred for that in an Einstein-de Sitter model of the same rms-density-fluctuation amplitude σ 8 . The estimated ionization rate is also more than double that expected from updated estimates of the emissivity of observed QSOs alone. A substantial contribution from galaxies appears to be required at all redshifts.

Detection of extended He ii reionization in the temperature evolution of the intergalactic medium

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 high-resolution 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(Δ[overbar]), 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(Δ[overbar]) 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 T0 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 Δz ≳ 1.

How cold is cold dark matter? Small-scales constraints from the flux power spectrum of the high-redshift Lyman-α forest

We present constraints on the mass of warm dark matter (WDM) particles derived from the Lyman-alpha flux power spectrum of 55 high-resolution HIRES spectra at 2.0 or approximately 1.2 keV (2sigma) if the WDM consists of early decoupled thermal relics and m(WDM) > or approximately 5.6 keV (2sigma) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-alpha flux power spectrum, we get m(WDM) > or approximately 4 keV and m(WDM) > or approximately 28 keV (2sigma) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor of 2.

Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six

We report the discovery of an extremely long (∼110 Mpc/h) and dark (τeff≳7) Lyα trough extending down to z≃5.5 towards the zem≃6.0 quasar ULAS J0148+0600. We use these new data in combination with Lyα forest measurements from 42 quasars at 4.5≤zem≤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 volume-weighted 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.

An improved measurement of the flux distribution of the Lyα forest in QSO absorption spectra: the effect of continuum fitting, metal contamination and noise properties

We have performed an extensive Voigt profile analysis of the neutral hydrogen (H i) and metal absorption present in a sample of 18 high-resolution, high signal-to-noise ratio quasi-stellar object (QSO) spectra observed with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. We use this analysis to separate the metal contribution from the H absorption and present an improved measurement of the flux probability distribution function (PDF) due to H i absorption alone at (z) = 2.07,2.52 and 2.94. The flux PDF is sensitive to the continuum fit in the normalized flux range 0.8 < F < 1.0 and to metal absorption at 0.2 < F < 0.8. Our new measurements of the flux PDF due to H i absorption alone are systematically lower at 0.2 < F < 0.8 by up to 30 per cent compared to the widely used measurement of McDonald et al., based on a significantly smaller sample of Keck High Resolution Echelle Spectrometer data. This discrepancy is probably due to a combination of our improved removal of the metal absorption and cosmic variance, since variations in the flux PDF between different lines-of-sight are large. The H effective optical depth τ eff Ht at 1.7 < z < 4 is best fit with a single power law, τ eff HI = (0.0023 ± 0.0007)(1 + z) 365±0.21 , in good agreement with previous measurements from comparable data. As also found previously, the effect of noise on the flux distribution is not significant in high-resolution, high signal-to-noise ratio data.

Possible evidence for an inverted temperature–density relation in the intergalactic medium from the flux distribution of the Lyα forest

We compare the improved measurement of the Lya forest flux probability distribution at 1.7 < z < 3.2 presented by Kim et al. to a large set of hydrodynamical simulations of the Lya forest with different cosmological parameters and thermal histories. The simulations are in good agreement with the observational data if the temperature-density relation for the low-density intergalactic medium (IGM), T = T 0 Δ γ-1 , is either close to isothermal or inverted (y < 1). Our results suggest that the voids in the IGM may be significantly hotter and the thermal state of the low-density IGM may be substantially more complex than is usually assumed at these redshifts. We discuss radiative transfer effects which alter the spectral shape of ionizing radiation during the epoch of He II reionization as a possible physical mechanism for achieving an inverted temperature-density relation at z ≃ 3.

The nature and evolution of the highly ionized near-zones in the absorption spectra of z~=6 quasars

We use state-of-the-art hydrodynamical simulations combined with a one-dimensional radiative transfer code to assess the extent to which the highly ionized regions observed close to z ≃ 6 quasars, which we refer to as near-zones, can constrain the ionization state of the surrounding intergalactic medium (IGM). We find the appearance in Lya absorption of a quasar H II ionization front expanding into a neutral IGM can be very similar to a classical proximity zone, produced by the enhancement in ionizing flux close to a quasar embedded in a highly ionized IGM. The observed sizes of these highly ionized near-zones and their redshift evolution can be reproduced for a wide range of IGM neutral hydrogen fractions for plausible values of the luminosity and lifetime of the quasars. The observed near-zone sizes at the highest observed redshifts are equally consistent with a significantly neutral and a highly ionized surrounding IGM. Stronger constraints on the IGM neutral hydrogen fraction can be obtained by considering the relative size of the near-zones in the Lya and Lyβ regions of a quasar spectrum. A large sample of high-quality quasar absorption spectra with accurate determinations of near-zone sizes and their redshift evolution in both the Lya and Lyβ regions should confirm or exclude the possibility that the Universe is predominantly neutral at the highest observed redshifts. The width of the discrete absorption features in these near-zones will contain important additional information on the ionization state and the previous thermal history of the IGM at these redshifts.

On the rapid demise of Ly α emitters at redshift z ≳ 7 due to the increasing incidence of optically thick absorption systems

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a emission over the redshift interval z=6-7. In combination with the strong damping wing extending redward of Ly-a in the spectrum of the bright z=7.085 quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in the intergalactic medium (IGM) is still substantially neutral at z~7. Current theoretical models imply HI fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly-a emitter population. We propose that such large neutral fractions are not necessary. Based on a hydrodynamical simulation which reproduces the absorption spectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Ly-a can rise rapidly in average regions of the Universe simply because of the increasing incidence of absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionisation is approached. Our simulations suggest these data do not require a large change in the IGM neutral fraction by several tens of per cent from z=6-7, but may instead be indicative of the rapid decrease in the typical mean free path for ionising photons expected during the final stages of reionisation.