The thermal history of the intergalactic medium down to redshift z=1.5: a new curvature measurement

According to the photoheating model of the intergalactic medium (IGM), He ii reionization is expected to affect its thermal evolution. Evidence for additional energy injection into the IGM has been found at 3 ≲ z ≲ 4, though the evidence for the subsequent fall-off below z ∼ 2.8 is weaker and depends on the slope of the temperature–density relation, γ. Here we present, for the first time, an extension of the IGM temperature measurements down to the atmospheric cut-off of the H i Lyman-α (Lyα) forest at z ≃ 1.5. Applying the curvature method on a sample of 60 Ultraviolet and Visual Echelle Spectrograph (UVES) spectra we investigated the thermal history of the IGM at z < 3 with precision comparable to the higher redshift results. We find that the temperature of the cosmic gas traced by the Lyα forest [T(Δ¯)] increases for increasing overdensity from T(Δ¯)∼22670 to 33740 K in the redshift range z ∼ 2.8–1.6. Under the assumption of two reasonable values for γ, the temperature at the mean density (T0) shows a tendency to flatten at z ≲ 2.8. In the case of γ ∼ 1.5, our results are consistent with previous ones which indicate a falling T0 for redshifts z ≲ 2.8. Finally, our T(Δ¯) values show reasonable agreement with moderate blazar heating models

Revealing reionization with the thermal history of the intergalactic medium: new constrainst from the Lyα flux power spectrum

We present a new investigation of the thermal history of the intergalactic medium (IGM) during and after reionization using the Lyman-α forest flux power spectrum at 4.0 z 5.2. Using a sample of 15 high– resolution spectra, we measure the flux power down to the smallest scales ever probed at these redshifts (−1 log(k/km −1 s) −0.7). These scales are highly sensitive to both the instantaneous temperature of the IGM and the total energy injected per unit mass during and after reionization. We measure temperatures at the mean density of T 0 ∼ 7000-8000 K, consistent with no significant temperature evolution for redshifts 4.2 z 5.0. We also present the first observational constraints on the integrated IGM thermal history, finding that the total energy input per unit mass increases from u 0 ∼ 4.6 eV m −1 p to 7.3 eV m −1 p from z ∼ 6 to 4.2 assuming a Λ-CDM cosmology. We show how these results can be used simultaneously to obtain information on the timing and the sources of the reionization process. Our first proof of concept using simplistic models of instantaneous reionization produces results comparable to and consistent with the recent Planck constraints, favoring models with z rei ∼ 8.5 +1.1 −0.8

Constraining the temperature–density relation of the intergalactic medium with the Lyman-alpha and beta forests

The post-reionization thermal state of the intergalactic medium is characterized by a power-law relationship between temperature and density, with a slope determined by the parameter γ. We describe a new method to measure γ using the ratio of flux curvature in the Lyman α and β forests. At a given redshift, this curvature ratio incorporates information from the different gas densities traced by Lyman α and β absorption. It is relatively simple and fast to compute and appears robust against several observational uncertainties. We apply this technique to a sample of 27 high-resolution quasar spectra from the Very Large Telescope. While promising statistical errors on γ appear to be achievable with these spectra, to reach its full potential, the dependence of the curvature ratio on the thermal state of the gas in the foreground Lyman α forest will require further, detailed forward modelling

Evidence for Large-Scale Fluctuations in the Metagalactic Ionizing Background Near Redshift Six

The observed scatter in intergalactic Lyman-$\alpha$ opacity at $z \lesssim 6$ requires large-scale fluctuations in the neutral fraction of the intergalactic medium (IGM) after the expected end of reionization. Post-reionization models that explain this scatter invoke fluctuations in either the ionizing ultraviolet background (UVB) or IGM temperature. These models make very different predictions, however, for the relationship between Lyman-$\alpha$ opacity and local density. Here we test these models using Lyman-$\alpha$ emitting galaxies (LAEs) to trace the density field surrounding the longest and most opaque known Lyman-$\alpha$ trough at $z < 6$. Using deep Subaru Hyper Suprime-Cam narrow-band imaging, we find a highly significant deficit of $z \simeq 5.7$ LAEs within 20 Mpc/$h$ of the trough. The results are consistent with a model in which the scatter in Lyman-$\alpha$ opacity near $z \sim 6$ is driven by large-scale UVB fluctuations, and disfavor a scenario in which the scatter is primarily driven by variations in IGM temperature. UVB fluctuations at this epoch present a boundary condition for reionization models, and may help shed light on the nature of the ionizing sources.Comment: 13 pages, 9 figure, submitted to ApJ, comments welcom

The relationship between IGM Lyman-alpha opacity and galaxy density near the end of reionization

Observed scatter in the Lyman-alpha opacity of quasar sightlines at $z<6$ has motivated measurements of the correlation between Ly$\alpha$ opacity and galaxy density, as models that predict this scatter make strong and sometimes opposite predictions for how they should be related. Our previous work associated two highly opaque Ly$\alpha$ troughs at $z\sim5.7$ with a deficit of Lyman-$\alpha$ emitting galaxies (LAEs). In this work, we survey two of the most highly transmissive lines of sight at this redshift, towards the $z=6.02$ quasar SDSS J1306+0356 and the $z=6.17$ quasar PSO J359-06. We find that both fields are underdense in LAEs within 10 $h^{-1}$ Mpc of the quasar sightline, somewhat less extensive than underdensities associated with Ly$\alpha$ troughs. We combine our observations with three additional fields from the literature, and find that while fields with extreme opacities are generally underdense, moderate opacities span a wider density range. The results at high opacities are consistent with models that invoke UV background fluctuations and/or late reionization to explain the observed scatter in IGM Ly$\alpha$ opacities. There is tension at low opacities, however, as the models tend to associate lower IGM Ly$\alpha$ opacities with higher densities. Although the number of fields surveyed is still small, the low-opacity results may support a scenario in which the ionizing background in low-density regions increases more rapidly than some models suggest after becoming ionized. Elevated gas temperatures from recent reionization may also be making these regions more transparent.Comment: 20 pages, 17 figures. Accepted to Ap

Unveiling Dark Matter free-streaming at the smallest scales with high redshift Lyman-alpha forest

This study introduces novel constraints on the free-streaming of thermal relic warm dark matter (WDM) from Lyman-$\alpha$ forest flux power spectra. Our analysis utilises a high-resolution, high-redshift sample of quasar spectra observed using the HIRES and UVES spectrographs ($z=4.2-5.0$). We employ a Bayesian inference framework and a simulation-based likelihood that encompasses various parameters including the free-streaming of dark matter, cosmological parameters, the thermal history of the intergalactic medium, and inhomogeneous reionization, to establish lower limits on the mass of a thermal relic WDM particle of $5.7\;\mathrm{keV}$ (at 95\% C.L.). This result surpasses previous limits from the Lyman-$\alpha$ forest through reduction of the measured uncertainties due to a larger statistical sample and by measuring clustering to smaller scales ($k_{\rm max}=0.2\;\mathrm{km^{-1}\,s}$). The approximately two-fold improvement due to the expanded statistical sample suggests that the effectiveness of Lyman-$\alpha$ forest constraints on WDM models at high redshifts are limited by the availability of high-quality quasar spectra. Restricting the analysis to comparable scales and thermal history priors as in prior studies ($k_{\rm max}<0.1\;\mathrm{km^{-1}\,s}$) lowers the bound on the WDM mass to $4.1\;\mathrm{keV}$. As the precision of the measurements increases, it becomes crucial to examine the instrumental and modelling systematics. On the modelling front, we argue that the impact of the thermal history uncertainty on the WDM particle mass constraint has diminished due to improved independent observations. At the smallest scales, the primary source of modeling systematic arises from the structure in the peculiar velocity of the intergalactic medium and inhomogeneous reionization.Comment: 22 pages, 14 figures, 2 tables; submitte

The Evolution of O i over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium

We present a survey for metal absorption systems traced by neutral oxygen over 3.2 0.05 Å, of which there are 49 nonproximate systems in our sample. We find that the number density does not monotonically increase with decreasing redshift, as would naively be expected from the buildup of metal-enriched circumgalactic gas with time. The number density over 4.9 < z < 5.7 is a factor of 1.7–4.1 lower (68% confidence) than that over 5.7 < z < 6.5, with a lower value at z < 5.7 favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at z ~ 6 than at lower redshifts. Absorption from highly ionized metals traced by C iv is also weaker in higher-redshift O i systems, supporting this picture. The evolution of O i absorbers implies that metal-enriched circumgalactic gas at z ~ 6 is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch

Unveiling dark matter free streaming at the smallest scales with the high redshift Lyman-alpha forest

This study introduces novel constraints on the free streaming of thermal relic warm dark matter (WDM) from Lyman-α forest flux power spectra. Our analysis utilizes a high resolution, high redshift sample of quasar spectra observed using the HIRES and UVES spectrographs (z=4.2-5.0). We employ a Bayesian inference framework and a simulation-based likelihood that encompasses various parameters including the free streaming of dark matter, cosmological parameters, the thermal history of the intergalactic medium, and inhomogeneous reionization to establish lower limits on the mass of a thermal relic WDM particle of 5.7 keV (at 95% CL). This result surpasses previous limits from the Lyman-α forest through reduction of the measured uncertainties due to a larger statistical sample and by measuring clustering to smaller scales (kmax=0.2 km-1 s). The approximately two-fold improvement due to the expanded statistical sample suggests that the effectiveness of Lyman-α forest constraints on WDM models at high redshifts are limited by the availability of high quality quasar spectra. Restricting the analysis to comparable scales and thermal history priors as in prior studies (kmax<0.1 km-1 s) lowers the bound on the WDM mass to 4.1 keV. As the precision of the measurements increases, it becomes crucial to examine the instrumental and modeling systematics. On the modeling front, we argue that the impact of the thermal history uncertainty on the WDM particle mass constraint has diminished due to improved independent observations. At the smallest scales, the primary source of modeling systematic arises from the structure in the peculiar velocity of the intergalactic medium and inhomogeneous reionization

The thermal state of the intergalactic medium 9-12 billion years ago

This work presents an investigation of the thermal state of the tenuous gas between galaxies, called the intergalactic medium. It is focused on measuring the temperature evolution of this cosmic gas during the second reionization phase of the Universe that happened between 9 and 12 billion years ago. During this event the radiation emitted by powerful objects, called quasars, profoundly changed the properties of the intergalactic medium that, in turn, determine the conditions for the formation and evolution of the structures that populate our Universe. Our measurements show evidence for this second reionization and track its final stages

The Relationship between IGM Lyα Opacity and Galaxy Density near the End of Reionization

Observed scatter in the Ly α opacity of quasar sightlines at z < 6 has motivated measurements of the correlation between Ly α opacity and galaxy density, as models that predict this scatter make strong and sometimes opposite predictions for how they should be related. Our previous work associated two highly opaque Ly α troughs at z ∼ 5.7 with a deficit of Ly α emitting galaxies (LAEs). In this work, we survey two of the most highly transmissive lines of sight at this redshift toward the z = 6.02 quasar SDSS J1306+0356 and the z = 6.17 quasar PSO J359-06. We find that both fields are underdense in LAEs within 10 h ^−1 Mpc of the quasar sightline, somewhat less extensive than underdensities associated with Ly α troughs. We combine our observations with three additional fields from the literature and find that while fields with extreme opacities are generally underdense, moderate opacities span a wider density range. The results at high opacities are consistent with models that invoke UV background fluctuations and/or late reionization to explain the observed scatter in intergalactic medium (IGM) Ly α opacities. There is tension at low opacities, however, as the models tend to associate lower IGM Ly α opacities with higher densities. Although the number of fields surveyed is still small, the low-opacity results may support a scenario in which the ionizing background in low-density regions increases more rapidly than some models suggest after becoming ionized. Elevated gas temperatures from recent reionization may also be making these regions more transparent