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We present a new, uniform analysis of the H  i transmitted flux (F) and H  i column density (⁠|$N_{\mathrm{H\,{\small I}}}$|⁠) distribution in the low-density IGM as a function of redshift z for 0 <  z  < 3.6 using 55 HST /COS FUV (Δ z  = 7.2 at z  < 0.5), five HST /STIS + COS NUV (Δ z  = 1.3 at z  ∼ 1) and 24 VLT/UVES, and Keck/HIRES (Δ z  = 11.6 at 1.7 <  z  < 3.6) AGN spectra. We performed a consistent, uniform Voigt profile analysis to combine spectra taken with different instruments, to reduce systematics and to remove metal-line contamination. We confirm previously known conclusions on firmer quantitative grounds in particular by improving the measurements at z  ∼ 1. Two flux statistics at 0 <  F  < 1, the mean H  i flux and the flux probability distribution function (PDF), show that considerable evolution occurs from z  = 3.6 to z  = 1.5, after which it slows down to become effectively stable for z  < 0.5. However, there are large sightline variations. For the H  i column density distribution function (CDDF, f  ∝  |$N_{\rm H\,{\small I}}^{-\beta }$|⁠) at |$\log (N_{\mathrm{H\,{\small I}}}/1\, {\mathrm{cm}^{-2}})$|  ∈ [13.5, 16.0], β increases as z decreases from β = 1.60 at z  ∼ 3.4 to β = 1.82 at z ∼ 0.1. The CDDF shape at lower redshifts can be reproduced by a small amount of clockwise rotation of a higher- z CDDF with a slightly larger CDDF normalization. The absorption line number per z (d n /d z) shows a similar evolutionary break at z ∼ 1.5 as seen in the flux statistics. High- |$N_{\mathrm{H\,{\small I}}}$| absorbers evolve more rapidly than low- |$N_{\mathrm{H\,{\small I}}}$| absorbers to decrease in number or cross-section with time. The individual d n /d z shows a large scatter at a given z. The scatter increases towards lower z , possibly caused by a stronger clustering at lower z. [ABSTRACT FROM AUTHOR]

We characterize the properties of the intergalactic medium (IGM) around a sample of galaxies extracted from state-of-the-art hydrodynamical simulations of structure formation in a cosmological volume of 25 Mpc comoving at z ∼ 2. The simulations are based on two different subresolution schemes for star formation and supernova feedback: the Multi-Phase Particle Integrator (MUPPI) scheme and the Effective Model. We develop a quantitative and probabilistic analysis based on the apparent optical depth method of the properties of the absorbers as a function of impact parameter from their nearby galaxies: in such a way, we probe different environments from circumgalactic medium to low-density filaments. Absorbers’ properties are then compared with a spectroscopic observational data set obtained from high-resolution quasar spectra. Our main focus is on the $$N_{\rm C\,\small {IV}}$$ – $$N_{\rm H\,\small {I}}$$ relation around simulated galaxies: the results obtained with MUPPI and the Effective model are remarkably similar, with small differences only confined to regions at impact parameters b = [1 − 3] ×  r vir. Using $$\rm{C\,{\small IV}}$$ as a tracer of the metallicity, we obtain evidence that the observed metal absorption systems have the highest probability to be confined in a region of 150–400kpc around galaxies. Near-filament environments have instead metallicities too low to be probed by present-day telescopes, but could be probed by future spectroscopical studies. Finally, we compute $$\rm{C \,{\small IV}}$$ covering fractions which are in agreement with observational data. [ABSTRACT FROM AUTHOR]

At low densities, the standard ionization history of the intergalactic medium (IGM) predicts a decreasing temperature of the IGM with decreasing density once hydrogen (and helium) reionization is complete. Heating the high-redshift, low-density IGM above the temperature expected from photoheating is difficult, and previous claims of high/rising temperatures in lowdensity regions of the Universe based on the probability density function (PDF) of the opacity in Ly α forest data at 2 < z < 4 have been met with considerable scepticism, particularly since they appear to be in tension with other constraints on the temperature-density relation (TDR). We utilize here an ultrahigh signal-to-noise spectrum of the Quasi-stellar object HE0940-1050 and a novel technique to study the low opacity part of the PDF. We show that there is indeed evidence (at 90 per cent confidence level) that a significant volume fraction of the underdense regions at z ~ 3 has temperatures as high or higher than those at densities comparable to the mean and above. We further demonstrate that this conclusion is nevertheless consistent with measurements of a slope of the TDR in overdense regions that imply a decreasing temperature with decreasing density, as expected if photoheating of ionized hydrogen is the dominant heating process. We briefly discuss implications of our findings for the need to invoke either spatial temperature fluctuations, as expected during helium reionization, or additional processes that heat a significant volume fraction of the low-density IGM. [ABSTRACT FROM AUTHOR]

In this work, we investigate the abundance and distribution of metals in the intergalactic medium (IGM) at 〈z〉 ≃ 2.8 through the analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar HE0940-1050. In the CIV forest, our deep spectrum is sensitive at 3σ to lines with column density down to log NCIV ≃ 11.4 and in 60 per cent of the considered redshift range down to ≃11.1. In our sample, all H I lines with log NHI ≥ 14.8 show an associated C IV absorption. In the range 14.0 ≤ log NHI < 14.8, 43 per cent of H I lines has an associated C IV absorption. At log NHI < 14.0, the detection rates drop to <10 per cent, possibly due to our sensitivity limits and not to an actual variation of the gas abundance properties. In the range log NHI ≥ 14, we observe a fraction of H I lines with detected C IV a factor of 2 larger than the fraction of H I lines lying in the circumgalactic medium (CGM) of relatively bright Lyman-break galaxies hosted by dark matter haloes with 〈M〉 ~ 1012 M☉. The comparison of our results with the output of a grid of photoionization models and of two cosmological simulations implies that the volume filling factor of the IGM gas enriched to a metallicity logZ/Z☉≳-3log⁡Z/Z☉≳-3 should be of the order of ~10-13 per cent. In conclusion, our results favour a scenario in which metals are found also outside the CGM of bright star-forming galaxies, possibly due to pollution by lower mass objects and/or to an early enrichment by the first sources. [ABSTRACT FROM AUTHOR]

From the Voigt profile fitting analysis of 183 intervening CIVCIV systems at 1.7 < z < 3.3 in 23 high-quality UVES/VLT and HIRES/Keck QSO spectra, we find that a majority of CIVCIV systems (~75 per cent) display a well-characterized scaling relation between integrated column densities of HIHI and CIVCIV with a negligible redshift evolution, when column densities of all the HIHI and CIVCIV components are integrated within a given ±150?km?s-1 range centred at the CIVCIV flux minimum. The integrated CIVCIV column density NCIV,sys increases with NHI,sys at logNHI,sys∈[14,16] and logNCIV,sys∈[11.8,14.0], then becomes almost independent of NHI,sys at logNHI,sys=16, with a large scatter: at logNHI,sys∈[14,22], logNCIV,sys=[C1/logNHI,sys+C2]+C3, with C1 = -1.90 ± 0.55, C2 = -14.11 ± 0.19 and C3 = 14.76 ± 0.17, respectively. The steep (flat) part is dominated by SiIVSiIV-free (SiIVSiIV-enriched) CIVCIV systems. Extrapolating the NHI,sys-NCIV,sys relation implies that most absorbers with logNHI≤14 are virtually CIVCIV-free. The NHI,sys-NCIV,sys relation does not hold for individual components, clumps or the integrated velocity range less than ±100?km?s-1. This is expected if the line-of-sight extent of CIVCIV is smaller than HIHI and NCIV,sys decreases more rapidly than NHI,sys at the larger impact parameter, regardless of the location of the HIHI+CIVCIV gas in intergalactic medium filaments or in intervening galactic haloes. [ABSTRACT FROM AUTHOR]

We present the Voigt profile analysis of 132 intervening CIV+CIII components associated with optically thinHI absorbers at 2.1= 1.0 ± 0.3 with a negligible redshift evolution. For 54 CIV components tied (aligned) with HI at logNHI [12.2, 16.0] and logNCIV [11.8, 13.8], the gas temperature Tb estimated from absorption line widths is well approximated to a Gaussian peaking at log Tb ~ 4.4 ± 0.3 for log Tb [3.5, 5.5], with a negligible non-thermal contribution. For 32 of 54 tied HI+CIV pairs, also tied with CIII at logNCIII [11.7, 13.8], we ran both photoionization equilibrium (PIE) and non-PIE (using a fixed temperature Tb) CLOUDY models for the Haardt-Madau QSOs+galaxies 2012 UV background. We find evidence of bimodality in observed and derived physical properties. High-metallicity branch absorbers have a carbon abundance [C/H]temp =-1.0, a line-of-sight length Ltemp = 20 kpc and a total (neutral and ionized) hydrogen volume density log nH,temp [-4.5, -3.3] and log Tb [3.9, 4.5]. Low-metallicity branch absorbers have [C/H]temp = -1.0, Ltemp [20, 480] kpc and log nH,temp [-5.2, -4.3] and log Tb ∼ 4.5. High-metallicity branch absorbers seem to be originated from extended discs, inner haloes or outflowing gas of intervening galaxies, while low-metallicity absorbers are produced by galactic haloes or the surrounding intergalactic medium filament. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the thermal history of the intergalactic medium (IGM) in the redshift interval z = 1.7-3.2 by studying the small-scale fluctuations in the Lyman α forest transmitted flux. We apply a wavelet filtering technique to 18 high-resolution quasar spectra obtained with the Ultraviolet and Visual Echelle Spectrograph, and compare these data to synthetic spectra drawn from a suite of hydrodynamical simulations in which the IGM thermal state and cosmological parameters are varied. From the wavelet analysis we obtain estimates of the IGM thermal state that are in good agreement with other recent, independent wavelet-based measurements. We also perform a reanalysis of the same data set using the Lyman α forest flux probability distribution function (PDF), which has previously been used to measure the IGM temperature-density relation. This provides an important consistency test for measurements of the IGM thermal state, as it enables a direct comparison of the constraints obtained using these two different methodologies. We find the constraints obtained from wavelets and the flux PDF are formally consistent with each other, although in agreement with previous studies, the flux PDF constraints favour an isothermal or inverted IGM temperature-density relation. We also perform a joint analysis by combining our wavelet and flux PDF measurements, constraining the IGM thermal state at z = 2.1 to have a temperature at mean density of T0/[103 K] = 17.3 ± 1.9 and a power-law temperature-density relation exponent γ = 1.1 ± 0.1 (1σ). Our results are consistent with previous observations that indicate there may be additional sources of heating in the IGM at z < 4. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We compare the improved measurement of the Lyα forest flux probability distribution at 1.7 < z < 3.2 presented by Kim et al. to a large set of hydrodynamical simulations of the Lyα 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), , is either close to isothermal or inverted (γ < 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. [ABSTRACT FROM AUTHOR]

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 i absorption and present an improved measurement of the flux probability distribution function (PDF) due to H i absorption alone at , 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 i effective optical depth τeffH i at 1.7 < z < 4 is best fit with a single power law, , 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. [ABSTRACT FROM AUTHOR]

We measure the matter power spectrum from 31 Lyα spectra spanning the redshift range of 1.6–3.6. The optical depth, τ, for Lyα absorption of the intergalactic medium is obtained from the flux using the inversion method of Nusser & Haehnelt. The optical depth is converted to density by using a simple power-law relation, . The non-linear 1D power spectrum of the gas density is then inferred with a method that makes simultaneous use of the one- and two-point statistics of the flux and compared against theoretical models with a likelihood analysis. A cold dark matter model with standard cosmological parameters fits the data well. The power-spectrum amplitude is measured to be (assuming a flat Universe), , with α varying in the range of 1.56–1.8 with redshift. Enforcing the same cosmological parameters in all four redshift bins, the likelihood analysis suggests some evolution in the temperature–density relation and the thermal smoothing length of the gas. The inferred evolution is consistent with that expected if reionization of He ii occurred at . A joint analysis with the Wilkinson Microwave Anisotropy Probe results together with a prior on the Hubble constant as suggested by the Hubble Space Telescope key project data, yields values of Ωm and σ8 that are consistent with the cosmological concordance model. We also perform a further inversion to obtain the linear 3D power spectrum of the matter density fluctuations. [ABSTRACT FROM AUTHOR]

The temperature of the low-density intergalactic medium (IGM) is set by the balance between adiabatic cooling resulting from the expansion of the Universe , and photoheating by the ultraviolet (UV) background. We have analysed the Lyα forest of 11 high-resolution quasar spectra using wavelets, and find strong evidence of a marked jump in the temperature at the mean density, T[sub 0], of 60±14 per cent over the redshift interval z=[3.5,3.1], which we attribute to reionization of He ii. The jump can be seen in all three of our spectra that straddle redshift 3.3, at a significance of &ges;99 per cent. Below z∼3.1, our results are consistent with a smooth cooling down of the universe, as expected when adiabatic expansion dominates over photoheating by a UV background from quasi-stellar objects (QSOs) and galaxies. We find no evidence of thermal fluctuations on scales &ges;5000 km s[sup -1] larger than 50 per cent, which could be detected by our method, suggesting that the IGM follows a reasonably well-defined temperature–density relation. We demonstrate that the mean wavelet amplitude ∝1/T[sub 0], and calibrate the relation with hydrodynamical simulations. We find T[sub 0]&ges;1.2×10[sup 4] K at z&ges;3.6. Such high temperatures suggest that H i reionization occurred relatively recently. [ABSTRACT FROM AUTHOR]

Presents a corrected reprint of an article on the power spectrum of the flux distribution in the Lyman alpha forest of a large sample of UVES QSO Absorption Spectra, previously published in "Mon. Not. R. Astron. Soc.".

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. [ABSTRACT FROM AUTHOR]