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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 discuss the feasibility of the detection of the 21 cm forest in the diffuse intergalactic medium (IGM) with the radio telescope LOFAR. The optical depth to the 21 cm line has been derived using simulations of reionization which include detailed radiative transfer of ionizing photons. We find that the spectra from reionization models with similar total comoving hydrogen ionizing emissivity but different frequency distribution look remarkably similar. Thus, unless the reionization histories are very different from each other (e.g. a predominance of UV versus X-ray heating) we do not expect to distinguish them by means of observations of the 21 cm forest. Because the presence of a strong X-ray background would make the detection of the 21 cm line absorption impossible, the lack of absorption could be used as a probe of the presence/intensity of the X-ray background and the thermal history of the Universe. Along a random line of sight LOFAR could detect a global suppression of the spectrum from z ≳ 12, when the IGM is still mostly neutral and cold, in contrast with the more well-defined, albeit broad, absorption features visible at lower redshift. Sharp, strong absorption features associated with rare, high-density pockets of gas could also be detected at z ∼ 7 along preferential lines of sight. [ABSTRACT FROM PUBLISHER]

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 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]

Theoretical models that can be used to predict the range of mainlobe widths and the probability distribution of the peak sidelobe levels of two-dimensionally sparse arrays are presented here. The arrays are considered to comprise microphones that are randomly positioned on a segmented grid of a given size. First, approximate expressions for the mean and variance of the squared magnitude of the aperture smoothing function are formulated for the random arrays considered in the present study. By using the variance function, the mean value and the lower end of the range i.e., the first 1 per cent of the mainlobe width distribution, can be predicted with reasonable accuracy. To predict the probability distribution of the peak sidelobe levels, distributions of levels were modelled by using a Weibull distribution at each peak in the sidelobe region of the mean squared magnitude of the aperture smoothing function. The two parameters of the Weibull distribution were estimated from the means and variances of the levels at the corresponding locations. Next, the probability distribution of the peak sidelobe levels were identified by following a procedure in which the peak sideload level was determined as the maximum among a finite number of independent random sidelobe levels. It was found that the model obtained from that approach predicts the probability density function of the peak sidelobe level distribution reasonably well for the various combinations of the two different numbers of microphones and the various grid sizes tested in the present study. The application of these models to the design of random, sparse arrays having specified performance levels is discussed. [Copyright &y& Elsevier]

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]