John M. O’Meara

The Deuterium to Hydrogen Abundance Ratio toward a Fourth QSO: HS 0105+1619

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 2.536 Lyman limit system is high, log N = 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 × 10-5. The metallicity of the system showing D/H is found to be 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 × 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 η = 5.6 ± 0.5 × 10-10 and Ωbh2 = 0.0205 ± 0.0018, respectively.

The UCSD HIRES/Keck I Damped Lyα Abundance Database. I. The Data

We present new chemical abundance measurements of 16 damped Lyα systems at z > 1.5 and update our previous abundance analyses. The entire database presented here was derived from HIRES observations on the Keck I telescope, reduced with the same software package, and analyzed with identical techniques. Altogether, we present a large, homogeneous database of chemical abundance measurements for protogalaxies in the early universe, ideal for studying a number of important aspects of galaxy formation. In addition, we have established an on-line directory for this database and will continuously update the results.

The Keck+Magellan Survey for Lyman Limit Absorption. I. The Frequency Distribution of Super Lyman Limit Systems*

We present the results of a survey for super Lyman limit systems (SLLSs; defined to be absorbers with 19.0 cm-2 ≤ log NH I ≤ 20.3 cm-2) from a large sample of high-resolution spectra acquired using the Keck and Magellan telescopes. Specifically, we present 47 new SLLSs from 113 QSO sight lines. We focus on the neutral hydrogen frequency distribution fH I(N, X) of the SLLS and its moments and compare these results with the Lyα forest and the damped Lyα systems (DLAs; absorbers with log NH I ≥ 20.3 cm-2). We find that the fH I(N, X) of the SLLSs can be reasonably described with a power law of index α = -1.43 or -1.19 depending on whether we set the lower NH I bound for the analysis at 1019.0 or 1019.3 cm-2, respectively. The results indicate a flattening in the slope of fH I(N, X) between the SLLSs and DLAs. We find little evidence for redshift evolution in the shape of fH I(N, X) for the SLLSs over the redshift range of the sample 1.68 < z < 4.47 and only tentative evidence for evolution in the zeroth moment of fH I(N, X), the line density lSLLS(X). We introduce the observable distribution function (N, X) and its moment, which elucidates comparisons of H I absorbers from the Lyα forest through to the DLA. We find that a simple three-parameter function can fit (N, X) over the range 17.0 cm-2 ≤ log NH I ≤ 22.0 cm-2. We use these results to predict that fH I(N, X) must show two additional inflections below the SLLS regime to match the observed fH I(N, X) distribution of the Lyα forest. Finally, we demonstrate that SLLSs contribute a minor fraction (≈15%) of the universes hydrogen atoms and, therefore, an even smaller fraction of the mass in predominantly neutral gas.

The UCSD HIRES/Keck I Damped Lyα Abundance Database. IV. Probing Galactic Enrichment Histories with Nitrogen*

We present 14 N^0 measurements from our HIRES/Keck database of damped Lya abundances. These data are combined with measurements from the recent and past literature to build an homogeneous, uniform set of observations. We examine photoionization diagnostics like Fe^++ and Ar^0 in the majority of the complete sample and assess the impact of ionization corrections on N/alpha and alpha/H values derived from observed ionic column densities of N^0, Si^+, H^0, and S^+. Our final sample of 19 N/alpha, alpha/H pairs appears bimodal; the majority of systems show N/alpha values consistent with metal-poor emission regions in the local universe but a small sub-sample exhibit significantly lower N/alpha ratios. Contrary to previous studies of N/alpha in the damped systems, our sample shows little scatter within each sub-sample. We consider various scenarios to explain the presence of the low N/alpha sightlines and account for the apparent bimodality. We favor a model where at least some galaxies undergo an initial burst of star formation with suppressed formation of intermediate-mass stars. We found a power-law IMF with slope 0.10 or a mass cut of ~5-8 Msolar would successfully reproduce the observed LN-DLA values. If the bimodal distribution is confirmed by a larger sample of measurements, this may present the first observational evidence for a top heavy initial mass function in some early stellar populations.

Cosmological Parameters σ8, the Baryon Density Ωb, the Vacuum Energy Density ΩΛ, the Hubble Constant and the UV Background Intensity from a Calibrated Measurement of H I Lyα Absorption at z = 1.9

We identify a concordant model for the intergalactic medium (IGM) at redshift z = 1.9 that uses popular values for cosmological and astrophysical parameters and accounts for all baryons with an uncertainty of 5%. The amount of absorption by H I in the IGM provides the best evidence on the physical conditions in the IGM, especially the combination of the mean gas density, the density fluctuations, the intensity of the ionizing flux, and the level of ionization. We have measured the amount of absorption, known as the flux decrement (DA), in the Lyα forest at redshift 1.9. We used spectra of 77 QSOs that we obtained with 250 km s-1 resolution from the Kast spectrograph on the Lick observatory 3 m telescope. We fitted unabsorbed continua to these spectra using b-splines. We also fitted equivalent continua to 77 artificial spectra that we made to match the real spectra in most obvious ways: redshift, resolution, signal-to-noise ratio (S/N), emission lines and absorption lines. The typical relative error in our continuum fits to the artificial spectra is 3.5%. Averaged over all 77 QSOs, the mean level is within 1%-2% of the correct value, except at S/N 17.2 cm-2 are responsible for a DA = 1.0% ± 0.4% at z = 1.9. These lines arise in higher density regions than the bulk of the IGM Lyα absorption, and hence they are harder to simulate in the huge boxes required to represent the large-scale variations in the IGM. If we subtract these lines, for comparison with simulations of the lower density bulk of the IGM, we are left with DA = 11.8% ± 1.0%. The mean DA in segments of individual spectra with Δz = 0.1, or 153 Mpc comoving at z = 1.9, has a large dispersion, σ = 6.1% ± 0.3% including Lyman limit systems (LLSs) and metal lines, and σ(Δz = 0.1) = 3.9% for the Lyα from the lower density IGM alone, excluding LLSs and metal lines. This is consistent with the usual description of large-scale structure and accounts for the large variations from QSO to QSO. Although the absorption at z = 1.9 is mostly from the lower density IGM, the Lyα of LLSs and the metal lines are both major contributors to the variation in the mean flux on 153 Mpc scales at z = 1.9, and they make the flux field significantly different from a random Gaussian field with an enhanced probability of a large amount of absorption. We find that a hydrodynamic simulation on a 10243 grid in a 75.7 Mpc box reproduces the observed DA from the low-density IGM alone when we use popular parameters values H0 = 71 km s-1 Mpc-1, Ωb = 0.044, Ωm = 0.27, ΩΛ = 0.73, σ8 = 0.9, and an ultraviolet background (UVB) that has an ionization rate per H I atom of Γ912 = (1.44 ± 0.11) × 10-12 s-1. This is 1.08 ± 0.08 times the prediction by Madau et al. with 61% from QSOs and 39% from stars. Our measurement of the DA gives a new joint constraint on these parameters, and the DA is very sensitive to each parameter. Given fixed values for all other parameters, and assuming the simulation has insignificant errors, the error of our DA measurement gives an error on H0 of 10%, ΩΛ of 6%, Ωb of 5%, and σ8 of 4%, comparable to the best measurements by other methods.We identify a concordant model for the intergalactic medium (IGM) at redshift z=1.9 that uses popular values for cosmological and astrophysical parameters and accounts for all baryons with an uncertainty of 6%. We have measured the amount of absorption, DA, in the Ly-alpha forest at redshift 1.9 in spectra of 77 QSO from the Kast spectrograph. We calibrated the continuum fits with realistic artificial spectra, and we found that averaged over all 77 QSOs the mean continuum level is within 1-2% of the correct value. Absorption from all lines in the Ly-alpha forest at z=1.9 removes DA=15.1 +/- 0.7% of the flux between 1070 and 1170 (rest) Angstroms. This is the first measurement using many QSOs at this z, and the first calibrated measurement at any redshift. Metal lines absorb 2.3 +/- 0.5%, and LLS absorb 1.0 +/- 0.4% leaving 11.8 +/- 1.0% from the lower density bulk of the IGM. Averaging over Delta z=0.1 or 154 Mpc, the dispersion is 6.1 +/- 0.3% including LLS and metal lines, or 3.9 (+0.5, -0.7)% for the lower density IGM alone, consistent with the usual description of large scale structure. LLS and metal lines are major contributors to the variation in the mean flux, and they make the flux field significantly non-Gaussian. We find that a hydrodynamic simulation on a 1024 cubed grid in a 75.7 Mpc box reproduces the observed DA from the low density IGM with parameters values H_o=71 km/s/Mpc, Omega_Lambda=0.73, Omega_m=0.27, Omega_b=0.044, sigma_8=0.9 and a UV background that has an ionization rate that is 1.08 +/- 0.08 times the prediction by Madau, Haardt & Rees (1999).

The UCSD HIRES/Keck I Damped Lyα Abundance Database.* III. An Empirical Study of Photoionization in the Damped Lyα System toward GB 1759+7539

We investigate the ionization state of the damped Lyα system at z = 2.62 toward GB 1759+7539 through an analysis of ionic ratios sensitive to photoionization: Ar0/S+, Fe++/Fe+, N+/N0, and Al++/Al+. Approximately half of the metals arise in a mostly neutral velocity component with H I/H > 0.9, based on Fe++/Fe+ < 0.013. In contrast, the remaining half exhibit Fe++/Fe+ ≈ 0.3, indicative of a partially ionized medium with H I/H ≈ 0.5. These conclusions are supported by the observed N+/N0, Al++/Al+, and Ar0/Si+ ratios. We assess ionization corrections for the observed column densities through photoionization models derived from the CLOUDY software package. In the neutral gas, the ionization corrections are negligible, except for Ar0. However, for the partially ionized gas, element abundance ratios differ from the ionic ratios by 0.1-0.3 dex for (Si+, S+, Ni+, Al+)/Fe+ ratios and more for (N0, Ar0)/Fe+. Independent of the shape of the photoionizing spectrum and assumptions of the number of ionization phases, these ionization corrections have minimal impact (0.1 dex) on the total metallicity inferred for this damped Lyα system. Measurements of the relative elemental abundances of the partially ionized gas, however, have a greater than ≈0.15 dex uncertainty, which hides the effects of nucleosynthesis and differential dust depletion. We caution the reader that this damped system is unusual for a number of reasons (e.g., a very low Ar0/S+ ratio), and we believe its ionization properties are special but not unique. Nevertheless, it clearly shows the value of examining photoionization diagnostics such as Fe++/Fe+ in a larger sample of damped systems.

Constraints on the universal C IV mass density at z ∼ 6 from early infrared spectra obtained with the Magellan fire spectrograph

We present a new determination of the intergalactic C IV mass density at 4.3 5.8 obtained with the newly commissioned Folded-Port Infrared Echellette (FIRE) spectrograph on the Magellan Baade telescope, coupled with six observations of northern objects taken from the literature. We confirm the presence of a downturn in the C IV abundance at (z) = 5.66 by a factor of 4.1 relative to its value at (z) = 4.96, as measured in the same sight lines. In the FIRE sample, a strong system previously reported in the literature as C IV at z = 5.82 is re-identified as Mg II at z = 2.78, leading to a substantial downward revision in {Omega}{sub Civ} for these prior studies. Additionally, we confirm the presence of at least two systems with low-ionization C II, Si II, and O I absorption but relatively weak signal from C IV. The latter systems may be of interest if the downward trend in {Omega}{sub Civ} at high redshift is driven in part by ionization effects.

Predicting QSO continua in the Lyα forest

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 Lyα forest of a QSO, from 1020 to 1216 A, using the spectrum of that QSO from 1216 to 1600 A. 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 A to the weights for 1216-1600 A, 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.

QSO 0130–4021: A Third QSO Showing a Low Deuterium-to-Hydrogen Abundance Ratio

We have discovered a third quasar absorption system which is consistent with a low deuterium-to-hydrogen abundance ratio, D/H = 3.4 × 10-5. The zabs ~ 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. M. Keck Telescope, which allow a detailed description of the hydrogen. We see excess absorption on the blue wing of the H I Lyα 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 ≤ 6.7 × 10-5 in this system, consistent with the value of D/H 3.4 × 10-5 deduced toward QSO 1009+2956 and QSO 1937-1009 by Burles and Tytler. This absorption system shows only weak metal-line absorption, and we estimate [Si/H] ≤ -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 × 10-5. Conversely, this QSO provides the third case which is inconsistent with much larger values.

The Interstellar Medium of Gamma-Ray Burst Host Galaxies. I. Echelle Spectra of Swift GRB Afterglows

We present optical echelle spectra of four gamma-ray burst (GRB) afterglows (GRB 050730, GRB 050820, GRB 051111, and GRB 060418) discovered during the first 1.5 yr of operation of the Swift satellite and localized by either the Swift telescope or follow-up ground-based imaging. We analyze the spectra to derive accurate column density measurements for the transitions arising in the interstellar medium (ISM) of the GRB host galaxies. These measurements can be used to constrain the physical properties of the ISM, including the metallicity, dust-to-gas ratio, ionization state, and chemical abundances of the gas. We also present measurements of the strong Mg II systems in the GRB afterglow spectra. With the publication of this paper, we provide the first data release of echelle afterglow spectra by the GRAASP collaboration to the general community.