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Kirkman, D, Tytler D, Suzuki N, Melis C, Hollywood S, James K, So G, Lubin D, Jena T, Norman ML, Paschos P.  2005.  The HI opacity of the intergalactic medium at redshifts 1.6 < z < 3.2. Monthly Notices of the Royal Astronomical Society. 360:1373-1380.   10.1111/j.1365-2966.2005.09126.x   AbstractWebsite

We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Ly alpha forest absorption (DA) over the redshift range 2.2 < z < 3.2. Our measurement of DA excludes absorption from metal lines or the Ly alpha lines of Lyman-limit systems and damped Ly alpha systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between 1.6 < z < 2.2, we find that the redshift evolution of DA is well described over f1.6 < z < 3.2 as A (1 +z)(gamma), where A = 0.0062 and gamma = 2.75. We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H i absorption than expected at z = 3, implying that the ultraviolet background is similar to 40 per cent higher than expected. Our data appears to be consistent with an H i ionization rate of Gamma similar to 1.4 x 10(-12) s(-1).

Suzuki, N, Tytler D, Kirkman D, O'Meara JM, Lubin D.  2005.  Predicting QSO continua in the Ly alpha forest. Astrophysical Journal. 618:592-600.   10.1086/426062   AbstractWebsite

We present a method to make predictions with sets of correlated data values, in this case QSO flux spectra. We predict the continuum in the Lyalpha forest of a QSO, from 1020 to 1216 8, using the spectrum of that QSO from 1216 to 1600 Angstrom. We find correlations between the unabsorbed flux in these two wavelength regions in the Hubble Space Telescope (HST) spectra of 50 QSOs. We use principal component analysis to summarize the variety of these spectra, relate the weights of the principal components for 1020-1600 Angstrom to the weights for 1216-1600 Angstrom, and apply this relation to make predictions. We test the method on the HST spectra and find an average absolute flux error of 9%, with a range of 3%-30%, where individual predictions are systematically too low or too high. We mention several ways in which the predictions might be improved.

Tytler, D, O'Meara JM, Suzuki N, Kirkman D, Lubin D, Orin A.  2004.  The Kast ground-based ultraviolet spectral survey of 79 quasi-stellar objects at redshift 2 for Ly alpha forest and metal absorption. Astronomical Journal. 128:1058-1077.   10.1086/423293   AbstractWebsite

We present spectra of 79 bright QSOs obtained with the Kast spectrograph on the 3 m Shane Telescope at Lick Observatory. The QSOs have emission redshifts 1.89 - 2.45, and most are near the mean value of z(em) = 2.17. Two of them, one a new discovery, are broad absorption line QSOs. The spectra have similar to250 km s(-1) resolution, and they cover 3175 - 5880 Angstrom with signal-to-noise ratios of 6 - 20 per 1.13 Angstrom pixel in the Lyalpha forest between Lyalpha and Lybeta. We show the continuum levels that we used in a previous paper to make a calibrated measurement of the amount of absorption in the Lyalpha forest at z = 1.9. We measure redshifts for 140 absorption-line systems, and we list the metal ions that we see in each system. We identify 526 emission lines and list their observed wavelengths, which we use to obtain new emission redshifts. We find that three emission lines, or line blends, in the forest have mean rest wavelengths of 1070.95 +/- 1.00, 1123.13 +/- 0.51, and 1175.88 +/- 0.30 Angstrom.