Original article can be found at: http://www.nature.com/nature/index.html--Copyright Nature Publishing Group --DOI : 10.1038/nature0345

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 present a sample of 1777 bright (9 < B < 14) metal-poor candidates selected from the Hamburg/ESO Survey (HES). Despite saturation effects present in the red portion of the HES objectiveprism spectra, the data were recoverable and quantitative selection criteria could be applied to select the sample. Analyses of medium-resolution (∼ 2Å) follow-up spectroscopy of the entire sample, obtained with several 2 to 4 m class telescopes, yielded 145 new metal-poor stars with metallicity [Fe/H] < −2.0, of which 79 have [Fe/H] < −2.5, and 17 have [Fe/H] < −3.0. We also obtained C/Fe estimates for all these stars. From this, we find a frequency of C-enhanced ([C/Fe] > 1.0) metal-poor ([Fe/H] < −2.0) giants of 9% ± 2%, which is lower than previously reported. However, the frequency raises to similar (> 20%) and higher values with increasing distance from the Galactic plane. Although the numbers of stars at low metallicity are falling rapidly at the lowest metallicities, there is evidence that the fraction of carbon-enhanced metal-poor stars is increasing rapidly as a function of declining metallicity. For ∼ 60 objects, high-resolution data have already been obtained; one of these, HE 1327−2326, is the new record holder for the most iron-deficient star known.

We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely "best and brightest stars first". Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R ∼ 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. Although currently about ten stars at [Fe/H] < −3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] ∼ −5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < −3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.

Aims. We present the far-UV spectrum of the quasar HS 1700+6416 taken with FUSE. This QSO provides the second line of sight with the He ii absorption resolved into a Lyα forest structure. Since HS 1700+6416 is slightly less redshifted (z em = 2.72) than HE 2347-4342, we only probe the post-reionization phase of He ii, seen in the evolution of the He ii opacity, which is consistent with a simple power law.Methods. The He ii/H i ratio η is estimated using a line profile-fitting procedure and an apparent optical depth approach, respectively.The expected metal line absorption in the far-UV is taken into account as well as molecular absorption of galactic H 2 . About 27% of the η values are affected by metal line absorption. In order to investigate the applicability of the analysis methods, we create simple artificial spectra based on the statistical properties of the H i Lyα forest.Results. The analysis of the artificial data demonstrates that the apparent optical depth method as well as the line profile-fitting procedure lead to confident results for restricted data samples only (0.01 ≤ τ H i ≤ 0.1 and 12.0 ≤ log N H i ≤ 13.0, respectively). The reasons are saturation in the case of the apparent optical depth and thermal line widths in the case of the profile fits. Furthermore, applying the methods to the unrestricted data set may mimic a correlation between the He ii/H i ratio and the strength of the H i absorption.For the restricted data samples a scatter of 10−15% in η would be expected even if the underlying value is constant. The observed scatter is significantly larger than expected, indicating that the intergalactic radiation background is indeed fluctuating. In the redshift range 2.58 < z < 2.72, where the data quality is best, we find η ∼ 100, suggesting a contribution of soft sources like galaxies to the UV background.