Publications

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2005
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.

2001
Kirkman, D, Tytler D, O'Meara JM, Burles S, Lubin D, Suzuki N, Carswell RF, Turner MS, Wampler EJ.  2001.  New Hubble Space Telescope spectra of QSO PG 1718+4807: No evidence for strong deuterium absorption. Astrophysical Journal. 559:23-28.   10.1086/322357   AbstractWebsite

The Z(abs) similar to 0.701 absorption system toward QSO PG 1718 + 4807 is the only example of a QSO absorption system which might have a deuterium/hydrogen ratio approximately 10 times the value found toward other QSOs. We have obtained new Space Telescope Imaging Spectrograph spectra from the Hubble Space Telescope of the Ly alpha and Lyman limit regions of the system. These spectra give the redshift and velocity dispersion of the neutral hydrogen, which produces most of the observed absorption. The Ly alpha line is too narrow to account for all of the observed absorption. It was previously known that extra absorption is needed on the blue side of the main H I near the expected position of deuterium. We do not find evidence in the current data that the extra absorption is entirely deuterium and find that it is more likely that some of the extra absorption is contaminating H. Until new data can be found that can independently constrain the line parameters of the potential contaminating H, it will not be possible to measure D/H in this system. Some uncertainty persists because we have a low signal-to-noise ratio and the extra absorption-be it deuterium or hydrogen-is heavily blended with the Ly alpha absorption from the main hydrogen absorption.

O'Meara, JM, Tytler D, Kirkman D, Suzuki N, Prochaska JX, Lubin D, Wolfe AM.  2001.  The deuterium to hydrogen abundance ratio toward a fourth QSO: HS 0105+1619. Astrophysical Journal. 552:718-730.   10.1086/320579   AbstractWebsite

We report the measurement of the primordial D/H abundance ratio toward QSO HS 0105+1619. The column density of the neutral hydrogen in the z similar or equal to 2.536 Lyman limit system is high, log N-HI = 19.422 +/- 0.009 cm(-2), allowing for the deuterium to be seen in five Lyman series transitions. The measured value of the D/H ratio toward QSO HS 0105 + 1619 is found to be D/H = 2.54 +/- 0.23 x 10(-5). The metallicity of the system showing D/H is found to be similar or equal to 0.01 solar, indicating that the measured D/H is the primordial D/H within the measurement errors. The gas that shows D/H is neutral, unlike previous D/H systems that were more highly ionized. Thus, the determination of the D/H ratio becomes more secure since we are measuring it in different astrophysical environments, but the error is larger because we now see more dispersion between measurements. Combined with prior measurements of D/H, the best D/H ratio is now D/H = 3.0 +/- 0.4 x 10(-5), which is 10% lower than the previous value. The new values for the baryon-to-photon ratio and baryonic matter density derived from D/H are eta = 5.6 +/- 0.5 x 10(-10) and Omega (b), h(2) = 0.0205 +/- 0.0018, respectively.

2000
Kirkman, D, Tytler D, Burles S, Lubin D, O'Meara JM.  2000.  QSO 0130-4021: A third QSO showing a low deuterium-to-hydrogen abundance ratio. Astrophysical Journal. 529:655-660.   10.1086/308317   AbstractWebsite

We have discovered a third quasar absorption system which is consistent with a low deuterium-to-hydrogen abundance ratio, D/H = 3.4 x 10(-5). The z(abs) similar to 2.8 partial Lyman limit system toward Q0130-4021 provides the strongest evidence to date against large D/H ratios because the H I absorption, which consists of a single high column density component with unsaturated high-order Lyman series lines, is readily modeled-a task which is more complex in other D/H systems. We have obtained 22 hr of spectra from the High-Resolution Echelle Spectrograph on the W. RI. Keek Telescope, which allow a detailed description of the hydrogen. We see excess absorption on the blue wing of the H I Ly alpha line, near the expected position of deuterium. However, we find that deuterium cannot explain all of the excess absorption, and hence there must be contamination by additional absorption, probably H I. This extra H I can account for most or all of the absorption at the D position, and hence D/H = 0 is allowed. We find an upper limit of D/H less than or equal to 6.7 x 10(-5) in this system, consistent with the value of D/H similar or equal to 3.4 x 10(-5) deduced toward QSO 1009 + 2956 and QSO 1937 - 1009 by Buries and Tytler. This absorption system shows only weak metal-line absorption, and we estimate [Si/H] less than or equal to -2.6, indicating that the D/H ratio of the system is likely primordial. All four of the known high-redshift absorption-line systems simple enough to provide useful limits on D are consistent with D/H = 3.4 +/- 0.25 x 10(-5). Conversely, this QSO provides the third case which is inconsistent with much larger values.