The amplitude of fluctuations in the Lyα forest on small spatial scales is sensitive to the temperature of the intergalactic medium (IGM) and its spatial fluctuations. The temperature of the IGM and its spatial variations contain important information about hydrogen and helium reionization. We present a new measurement of the small-scale structure in the Lyα forest from 40 high resolution, high signal-to-noise ratio, VLT spectra for absorbing gas at redshifts between 2.2 z 4.2. We convolve each Lyα forest spectrum with a suitably chosen Morlet wavelet filter, which allows us to extract the amount of small-scale structure in the forest as a function of position across each spectrum. We monitor contamination from metal line absorbers. We present a first comparison of these measurements with high-resolution hydrodynamic simulations of the Lyα forest that track more than 2 billion particles. This comparison suggests that the IGM temperature close to the cosmic mean density (T 0 ) peaks at a redshift near z = 3.4, at which point it is greater than 20,000 K at 2σ confidence. The temperature at lower redshift is consistent with the fall-off expected from adiabatic cooling (T 0 ∝ (1 + z) 2 ), after the peak temperature is reached near z = 3.4. In our highest redshift bin, centered around z = 4.2, the results favor a temperature of T 0 = 15-20,000 K. However, owing mostly to uncertainties in the mean transmitted flux at this redshift, a cooler IGM model with T 0 = 10,000 K is only disfavored at the 2σ level here, although such cool IGM models are strongly discrepant with the z ≈ 3-3.4 measurement. We do not detect large spatial fluctuations in the IGM temperature at any redshift covered by our data set. The simplest interpretation of our measurements is that He ii reionization completes sometime near z ≈ 3.4, although statistical uncertainties are still large. Our method can be fruitfully combined with future He ii Lyα forest measurements.

We investigated a set of high-resolution (R ∼ 45 000), high signal-to-noise (S /N ∼ 70) quasar spectra to search for the signature of the so-called proximity effect in the H i Lyα forest. The sample consists of 40 bright quasars with redshifts in the range 2.1 < z < 4.7. Using the flux transmission statistic, we determined the redshift evolution of the H i effective optical depth in the Lyman forest between 2 < ∼ z < ∼ 4.5, finding good agreement with previous measurements based on smaller samples. We also see the previously reported dip in τ eff (z) around redshift z ∼ 3.3, but as the significance of that feature is only 2.6σ, we consider this detection tentative. Comparing the flux transmission near each quasar with what was expected from the overall trend of τ eff (z), we clearly detect the proximity effect not only in the combined quasar sample, but also towards each individual line of sight at high significance, albeit with varying strength. We quantify this strength using a simple prescription based on a fiducial value for the intensity of the metagalactic UV background (UVB) radiation field at 1 Ryd, multiplied by a free parameter that varies from QSO to QSO. The observed proximity effect strength distribution (PESD) is asymmetric, with an extended tail towards values corresponding to a weak effect. We demonstrate that this is not simply an effect of gravitational clustering around quasars, as the same asymmetry is already present in the PESD predicted for purely Poissonian variance in the absorption lines. We present the results of running the same analysis on simulated quasar spectra generated by a simple Monte-Carlo code. Comparing the simulated PESD with observations, we argue that the standard method of determining the UVB intensity J ν 0 by averaging over several lines of sight is heavily biased towards high values of J ν 0 because of the PESD asymmetry. Using instead the mode of the PESD provides an estimate of J ν 0 that is unbiased with respect to his effect. For our sample we get a modal value for the UVB intensity of log J ν 0 = −21.51 ± 0.15 (in units of erg cm −2 s −1 Hz −1 sr −1 ) for a median quasar redshift of 2.73. With J ν 0 fixed we then corrected τ eff near each quasar for local ionisation and estimated the amount of excess H i absorption attributed to gravitational clustering. On scales of ∼3 Mpc, only a small minority of quasars show substantial overdensities of up to a factor of a few in τ eff ; these are exactly the objects with the weakest proximity effect signatures. After removing those quasars residing in overdense regions, we redetermined the UVB intensity using a hybrid approach of sample averaging and statistical correction for the PESD asymmetry bias, arriving at log J ν 0 = −21.46 +0.14 −0.21 . This is the most accurate measurement of J ν 0 to date. We present a new diagnostic based on the shape and width of the PESD that strongly supports our conclusion that there is no systematic overdensity bias for the proximity effect. This additional diagnostic breaks the oth...

We present a study of 15 long-duration γ-ray burst (GRB) host galaxies at z > 2. The GRBs are selected with available early-time afterglow spectra in order to compare interstellar medium (ISM) absorption-line properties with stellar properties of the host galaxies. In addition to five previously studied hosts, we consider new detections for the host galaxies of GRB 050820 and GRB 060206 and place 2-σ upper limits to the luminosities of the remaining unidentified hosts. We examine the nature of the host galaxy population and find that (1) the UV luminosity distribution of GRB host galaxies is consistent with expectations from a UV luminosity weighted random galaxy population with a median luminosity of L(UV ) = 0.1 L * ; (2) there exists a moderate correlation between UV luminosity and Si II λ 1526 absorption width, which together with the observed large line widths of W (1526) > 1.5 Å for a large fraction of the objects suggests a galactic outflow driven velocity field in the host galaxies; (3) there is tentative evidence for a trend of declining ISM metallicity with decreasing galaxy luminosity in the star-forming galaxy population at z = 2 − 4; (4) the interstellar UV radiation field is found ≈ 35 − 350× higher in GRB hosts than the Galactic mean value; and (5) additional galaxies are found at 2 ′′ from the GRB host in all fields with known presence of strong Mg II absorbers, but no additional faint galaxies are found at 2 ′′ in fields without strong Mg II absorbers. Our study confirms that the GRB host galaxies (with known optical afterglows) are representative of unobscured star-forming galaxies at z > 2, and demonstrates that high spatial resolution images are necessary for an accurate identification of GRB host galaxies in the presence of strong intervening absorbers.

We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z em ≃ 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from τ eff,HeII ≃ 1 at z ≃ 2.3 to τ eff,HeII 5 at z ≃ 3.2, but with a large scatter of 2 τ eff,HeII 5 at 2.7 < z < 3 on scales of ∼ 10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the co-eval H I forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z 2.7, probably indicating He II reionization was incomplete at z reion 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z reion ≃ 2.7 or even below, as suggested by a large τ eff,HeII 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 z 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized.

We report the discovery of 14 quasars in the vicinity of HE 2347−4342, one of the two quasars whose intergalactic He ii forest has been resolved with FUSE. By analysing the H i and the He ii opacity variations separately, no transverse proximity effect is detected near three foreground quasars of HE 2347−4342: QSO J23503−4328 (z = 2.282, ϑ = 3. 59), QSO J23500−4319 (z = 2.302, ϑ = 8. 77) and QSO J23495−4338 (z = 2.690, ϑ = 16. 28). This is primarily due to line contamination and overdensities probably created by large-scale structure. By comparing the H i absorption and the corresponding He ii absorption, we estimated the fluctuating spectral shape of the extragalactic UV radiation field along this line of sight. We find that the UV spectral shape near HE 2347−4342 and in the projected vicinity of the three foreground quasars is statistically harder than expected from UV background models dominated by quasars. In addition, we find three highly ionised metal line systems near the quasars. However, they do not yield further constraints on the shape of the ionising field. We conclude that the foreground quasars show a transverse proximity effect that is detectable as a local hardening of the UV radiation field, although the evidence is strongest for QSO J23495−4338. Thus, the relative spectral hardness traces the proximity effect also in overdense regions prohibiting the traditional detection in the H i forest. Furthermore, we emphasise that softening of quasar radiation by radiative transfer in the intergalactic medium is important to understand the observed spectral shape variations. From the transverse proximity effect of QSO J23495−4338 we obtain a lower limit on the quasar lifetime of ∼25 Myr.