David Tytler

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.

Cosmological baryon density derived from the deuterium abundance at redshift z = 3.57

THE primordial ratio of deuterium to hydrogen nuclei (D/H), created as a result of the Big Bang, provides the most sensitive measure of the cosmological density of baryons1–5. Measurements of the D/H ratio in the interstellar medium of our Galaxy place a strict lower limit on the primordial ratio6, because processing of gas by stars reduces the abundance of deuterium relative to hydrogen. Absorption of radiation from distant quasars by intervening clouds of gas offers a means of probing D/H ratios at large redshifts, where the effects of stellar processing should be negligible. Measurements7,8 on one absorption system have indicated an extremely high primordial abundance ratio of 24 × 10–5. Here we report a measurement of the D/H ratio in another high-redshift absorption system, and obtain a value that is an order of magnitude lower than that reported previously7,8. The measured ratio of 2.3 × 10–5 is consistent with that in the interstellar medium (after allowing for Galactic chemical evolution9,10), and indicates that the absorption spectra on which the earlier estimates are based may have been subject to strong contamination. We calculate a baryon density that is 5% of the critical density required to close the Universe.

Systematic QSO emission-line velocity shifts and new unbiased redshifts

A novel method for determining mean line velocities which is an order of magnitude more accurate than past work is presented. Using a large sample of 518 lines from 160 QSOs, it is found that each UV emission line has a well-determined mean velocity, with a surprisingly small QSO-to-QSO dispersion of under 200 km/s. All correlations between emission-line velocities and QSO properties are found to be explained by just three basic correlations. Both N V and Mg II tend to be at less negative velocities in radio-quiet vs radio-loud QSOs, and C IV lines with small equivalent widths are at more negative velocities, probably because of the line asymmetry. The finding of Sargent et al. (1989) that the UV spectral index increases with UV luminosity is confirmed, but their claim that radio-loud and radio-quiet QSOs have different indices is not confirmed.

The Distribution of Lyman-alpha absorption lines in the spectra of six QSOs - Evidence for an intergalactic origin

We have investigated the properties of the L..cap alpha.. absorption lines in high-resolution spectra of six QSOs, namely, PKS 2126-158, Q0002-422, PHL 957, Q0453-423, B2 1225+31.7, and PKS 0237-233. The first five objects form a homogeneous sample with 2.2

Intrinsic Properties of the ⟨z⟩ = 2.7 Lyα Forest from Keck Spectra of Quasar HS 1946+7658

We present the highest quality Ly? forest spectra published to date. We have complete 7.9 km s-1 FWHM spectra between the Ly? and Ly? emission lines of the bright, high-redshift (V = 15.9, z = 3.05) QSO HS 1946+7658. The mean redshift of observed Ly? forest clouds is z = 2.7. The spectrum has a signal-to-noise ratio per pixel of 2 km s-1 that varies from 15 at 4190 ? to 100 at 4925 ?. The absorption lines in the spectrum have been fitted with Voigt profiles, and the distribution of Voigt parameters has been analyzed. We show that fitting Voigt profiles to high quality data does not give unique results. We have performed simulations to differentiate between true features of the line distributions and artifacts of line blending and the fitting process. We show that the distribution of H I column densities is a power law of slope -1.5 from N(H I) = 1014 cm-2 to N(H I) 1013 cm-2.

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.

Review of Big Bang Nucleosynthesis and Primordial Abundances

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or 2H), 3He, 4He and 7Li during the first few minutes of the universe. This review concentrates on recent improvements in the measurement of the primordial (after BBN, and prior to modification) abundances of these nuclei. We mention improvement in the standard theory, and the non-standard extensions which are limited by the data. We have achieved an order of magnitude improvement in the precision of the measurement of primordial D/H, using the HIRES spectrograph on the W. M. Keck telescope to measure D in gas with very nearly primordial abundances towards quasars. From 1994 – 1996, it appeared that there could be a factor of ten range in primordial D/H, but today four examples of low D are secure. High D/H should be much easier to detect, and since there are no convincing examples, it must be extremely rare or non-existent. All data are consistent with a single low value for D/H, and the examples which are consistent with high D/H are readily interpreted as H contamination near the position of D. The new D/H measurements give the most accurate value for the baryon to photon ratio, η, and hence the cosmological baryon density. A similar density is required to explain the amount of Lyα absorption from neutral Hydrogen in the intergalactic medium (IGM) at redshift z 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. Instead, the agreement between the abundances is used to limit the non-standard physics. New measurements are giving improved understanding of the difficulties in estimating the abundances of all the light nuclei, but unfortunately in most cases we are not yet seeing much improvement in the accuracy of the primordial abundances. Since we are now interested in the highest accuracy and reliability for all nuclei, the few objects with the most extensive observations give by far the most convincing results. Earlier measurements of 4He may have obtained too low a value because the He emission line strengths were reduced by undetected stellar absorption lines. The systematic errors associated with the 4He abundance have frequently been underestimated in the past, and this problem persists. When two groups use the same data and different ways to estimate the electron density and 4He abundance, the results differ by more than the quoted systematic errors. While the methods used by Izotov and Thuan seem to be an advance on those used before, the other method is reasonable, and hence the systematic error should encompass the range in results. The abundance of 7Li is measured to high accuracy, but we do not know how much was produced prior to the formation of the stars, and how much was destroyed (depleted) in the stars. 6Li helps limit the amount of depletion of 7Li, but by an uncertain amount since it too has been depleted. BBN is successful because it uses known physics and measured cross-sections for the nuclear reactions. It gives accurate predictions for the abundances of five light nuclei as a function of the one free parameter η. The other initial conditions seem natural: the universe began homogeneous and hotter than T > 1011 K (30 Mev). The predicted abundances agree with most observations, and the required η is consistent with other, less accurate, measurements of the baryon density. Abundance measurements of the baryon density, from the CMB, clusters of galaxies and the Lyα forest, will give η. Although the accuracy might not exceed that obtained from D/H, this is an important advance because BBN then gives abundance predictions with no adjustable parameters. New measurement in the coming years will give improved accuracy. Measurement of D/H in many more quasar spectra would improve the accuracy of D/H by a factor of a few, to a few percent, but even with improved methods of selecting the target quasars, this would need much more time on the largest telescopes. More reliable 4He abundances might be obtained from spectra which have higher spectral and spatial resolution, to help correct for stellar absorption, higher signal to noise to show weaker emission lines, and more galaxies with low metal abundances, to minimize the extrapolation to primordial abundances. Measurements of 6Li, Be and Boron in the same stars and observations of a variety of stars should give improved models for the depletion of 7Li in halo stars, and hence tighter constraints on the primordial abundance. However, in general, it is hard to think of any new methods which could give any primordial abundances with an order of magnitude higher accuracy than those used today. This is a major unexploited opportunity, because it means that we can not yet test BBN to the accuracy of the predictions.

The distribution of QSO absorption system column densities - Evidence for a single population

The distribution of H I column densities in a representative sample of all cosmologically distributed QSO absorption-line systems is examined. Remarkably, this distribution function f(N) is a featureless power law extending over about seven orders of magnitude in column density. It is argued that this finding is in conflict with the standard interpretation of absorption systems, which invokes separate and unrelated intergalactic (Ly-alpha) and galactic halo (metal-line) absorption systems. A more natural explanation is that both the Ly-alpha and metal-line systems belong to the same population. They would have the same origins and the same spatial locations. The simplest possibility, that they all have the same high metal abundances, is considered to be unlikely, even though it does offer straightforward explanations for four different observational enigmas. However, it is entirely possible that the majority of Ly-alpha systems (those with low H I column densities) contain metals. Possible physical origins for the power-law form of f(N) are also discussed. 108 references.

The Type IC supernova 1994I in M51: detection of helium and spectral evolution

We present a series of spectra of SN 1994I in M51, starting 1 week prior to maximum brightness. The nebular phase began about 2 months after the explosion; together with the rapid decline of the optical light, this suggests that the ejected mass was small. Although lines of He I in the optical region are weak or absent, consistent with the Type Ic classification, we detect strong He I λ10830 absorption during the first month past maximum. Thus, if SN 1994I is a typical Type Ic supernova, the atmospheres of these objects cannot be completely devoid of helium. The emission-line widths are smaller than predicted by the model of Nomoto and coworkers, in which the iron core of a low-mass carbon-oxygen star collapses. They are, however, larger than in Type Ib supernovae.

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.