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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]

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]

We investigate the influence of high-mass X-ray binaries (HMXBs) on their high-redshift environments. Using a one-dimensional radiative transfer code, we predict the ionization and temperature profiles surrounding a coeval stellar population, composed of main-sequence stars and HMXBs, at various times after its formation. We consider both uniform density surroundings, and a cluster embedded in a 108 M⊙ Navarro-Frenk-White (NFW) halo. HMXBs in a constant density environment produce negligible enhanced ionization because of their high-energy spectral energy distributions and short lifetimes. In this case, HMXBs only marginally contribute to the local heating rate. For NFW profiles, radiation from main-sequence stars cannot prevent the initially ionized volume from recombining since it is unable to penetrate the high-density galactic core. However, HMXB photons stall recombinations behind the front, keeping it partially ionized for longer. The increased electron density in these partially ionized regions promotes further cooling, resulting in lower intergalactic medium (IGM) temperatures. In the context of this starburst model, we have shown that HMXBs do not make a major contribution to reionization or IGM heating. However, X-ray escape fractions are high in both density profile cases. Continuous star formation may result in the build up of X-rays over time, reducing the ionization time-scale and potentially leading to low level ionization of the distant IGM. [ABSTRACT FROM AUTHOR]