The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of %10 6 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg 2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux-and wavelength-calibrated, with 4096 pixels from 3800 to 9200 Å at R % 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.

We use a sample of 332 Hubble Space Telescope spectra of 184 QSOs with z > 0.33 to study the typical ultraviolet spectral properties of QSOs, with emphasis on the ionizing continuum. Our sample is nearly twice as large as that of Zheng et al. (1997) and provides much better spectral coverage in the extreme ultraviolet (EUV). The overall composite continuum can be described by a power law with index α EUV = −1.76 ± 0.12 ( f ν ∝ ν α ) between 500 and 1200 Å. The corresponding results for subsamples of radio-quiet and radio-loud QSOs are α EUV = −1.57 ± 0.17 and α EUV = −1.96 ± 0.12, respectively. We also derive α EUV for as many individual objects in our sample as possible, totaling 39 radio-quiet and 40 radio-loud QSOs. The typical individually measured values of α EUV are in good agreement with the composites. We find no evidence for evolution of α EUV with redshift for either radio-loud or radio-quiet QSOs. However, we do find marginal evidence for a trend towards harder EUV spectra with increasing luminosity for radio-loud objects. An extrapolation of our radio-quiet QSO spectrum is consistent with existing X-ray data, suggesting that the ionizing continuum may be represented by a single power law. The resulting spectrum is roughly in agreement with models of the intergalactic medium photoionized by the integrated radiation from QSOs.

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We have performed a detailed statistical study of the evolution of structure in a photoionized intergalactic medium (IGM) using analytical simulations to extend the calculation into the mildly non-linear density regime found to prevail at z = 3. Our work is based on a simple fundamental conjecture: that the probability distribution function of the density of baryonic diffuse matter in the universe is described by a lognormal (LN) random field. The LN distribution has several attractive features and follows plausibly from the assumption of initial linear Gaussian density and velocity fluctuations at arbitrarily early times. Starting with a suitably normalized power spectrum of primordial fluctuations in a universe dominated by cold dark matter (CDM), we compute the behavior of the baryonic matter, which moves slowly toward minima in the dark matter potential on scales larger than the Jeans length. We have computed two models that succeed in matching observations. One is a non-standard CDM model with Ω = 1, h = 0.5 and Γ = 0.3, and the other is a low density flat model with a cosmological constant (LCDM), with Ω = 0.4, Ω Λ = 0.6 and h = 0.65. In both models, the variance of the density distribution function grows with time, reaching unity at about z = 4, where the simulation yields spectra that closely resemble the Lyα forest absorption seen in the spectra of high z quasars. The calculations also successfully predict the observed properties of the Lyα forest clouds and their evolution from z = 4 down to at least z = 2, assuming a constant intensity for the metagalactic UV background over this redshift range. However, in our model the forest is not due to discrete clouds, but rather to fluctuations in a continuous intergalactic medium. At z = 3, typical clouds with measured neutral hydrogen column densities N HI = 10 15.3 , 10 13.5 , and 10 11.5 cm −2 correspond to fluctuations with mean total densities approximately 10, 1, and 0.1 times the universal mean baryon density. Perhaps surprisingly, fluctuations whose amplitudes are less than or equal to the mean density still appear as "clouds" because in our model more than 70% of the volume of the IGM at z = 3 is filled with gas at densities below the mean value.We find that the column density distribution of Lyα forest lines can be fit to f (N HI ) ∝ N −β HI , with β = 1.46 in the range 12.5 < log N HI < 14.5, matching recent -2 -Keck results. At somewhat higher column densities the distribution steepens, giving β = 1.80 over the range 14.0 < log N HI < 15.5, and matching earlier observations for these stronger lines. The normalization of the line numbers in our model also agrees with observations if the total baryon density is Ω b = 0.015h −2 and the ionizing background intensity is J 21 = 0.18. Alternatively, if J 21 = 0.5 as recently estimated for the background due to observed quasars at z = 2.5, then Ω b = 0.025h −2 yields the observed number of Lyα lines and the observed mean opacity. The model predicts that about 80% of the baryons in the universe are associate...

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