arXiv subjects
Publications and source records attributed to David Tytler.
We present our measurements of the deuterium to hydrogen ratio (D/H) in QSO absorption systems, which give D/H = 3.40 $\pm 0.25 \times 10^{-5}$ based on analysis of four independent systems. We discuss the properties of two systems which provide the strongest constraints on D/H. We outline the systematic effects involved in measurements of D/H and introduce a sophisticated method of analysis which properly accounts for these effects.
We present a new measurement of the deuterium-to-hydrogen ratio (D/H) in the Lyman limit absorption system at $z = 3.572$ towards Q1937--1009. We use an improved method to measure D/H in QSO absorption systems, which includes extra parameters to treat the continuum uncertainties, a variety of new absorption models which allow for undetected velocity structure, and the improved measurement of the total hydrogen column density by Burles & Tytler (1997a). We find that all models, including contamination, give an upper limit D/H $< 3.9 \times 10^{-5}$ (95 % confidence). Both this and previous analyses find contamination to be unlikely in this absorption system, A $χ^2$ analysis in models without contamination gives D/H $= 3.3 \pm 0.3 \times 10^{-5}$ (67% confidence), which is higher but consistent with the earlier results of Tytler et al. (1996), and a second measurement of D/H towards Q1009+2956 (Burles & Tytler 1997). With calculations of standard big bang nucleosynthesis (SBBN) and the assumption that this measurement of D/H is representative of the primordial value, we find a high baryon-to-photon ratio, $η= 5.3 \pm 0.4 \times 10^{-10}$. This is consistent with primordial abundance determinations of $^4$He in H II regions (Izotov et al. 1997) and $^7$Li in the atmospheres of warm metal-poor population II stars (Bonifacio & Molaro 1997). We find a high value for the present-day baryon density, $Ω_b h^2 = 0.0193 \pm 0.0014$, which is consistent with other inventories of baryonic matter, from low to high redshift: clusters of galaxies, the Lyman alpha forest & the Cosmic Microwave Background.
We present a measurement of the deuterium to hydrogen ratio (D/H) in a metal-poor absorption system at redshift $z=2.504$ towards the QSO 1009+2956. We apply the new method of Burles & Tytler (1997) to robustly determine D/H in high resolution \Lya forest spectra, and include a constraint on the neutral hydrogen column density determined from the Lyman continuum optical depth in low resolution spectra. We introduce six separate models to measure D/H and to assess the systematic dependence on the assumed underlying parameters. We find that the deuterium absorption feature contains a small amount of contamination from unrelated \ion{H}{1}. Including the effects of the contamination, we calculate the 67% confidence interval of D/H in this absorption system, log (D/H) $= -4.40 ^{+0.06}_{-0.08}$. This measurement agrees with the low measurement by Burles & Tytler (1997) towards Q1937--1009, and the combined value gives the best determination of primordial D/H, log (D/H)$_p = -4.47 ^{+0.030}_{-0.035}$ or D/H $= 3.39 \pm 0.25 \times 10^{-5}$. Predictions from standard big bang nucleosynthesis (SBBN) give the cosmological baryon to photon ratio, $η= 5.1 \pm 0.3 \times 10^{-10}$, and the baryon density in units of the critical density, $Ω_b h^2 = 0.019 \pm 0.001$, where $H_0 = 100 h$ \kms Mpc$^{-1}$. The measured value of (D/H)$_p$ implies that the primordial abundances of both $^4$He and $^7$Li are high, and consistent with some recent studies. Our two low measurements of primordial D/H also place strong constraints on inhomogeneous models of big bang nucleosynthesis.
We observe with Keck/HIRES the z ~ 2.5 QSO triplet 1623+27 in order to explore on the scale of a Megaparsec the spatial clustering of C IV absorbers between adjacent sightlines. We find this signal to be significantly weaker than the clustering in velocity on corresponding scales along single sightlines, assuming that the relative velocity of absorbers is dominated by the Hubble flow. This indicates that small-scale clustering (200 km/s < Delta v < 600 km/s) of the C IV absorbers cannot be interpreted in terms of the positions of the absorbers in space, but must be considered as internal motions within individual absorbers, or within clusters of absorbers whose internal velocities dominate over Hubble expansion across the cluster scale. If the single- sightline signal is due to spatial clustering, it is caused by absorber clusters smaller than would be implied by their velocities if a Hubble flow is assumed. The spatial clustering of C IV absorbers at z ~ 2 is consistent with data on Ly alpha forest clustering measured in the same way at the same redshifts. However, present-day galaxy clustering, evolved back to z ~ 2, is consistent with C IV spatial clustering but perhaps not with that of the Ly alpha forest. Even so, one cannot as yet distinguish the two absorber populations on the basis of spatial clustering on these small scales.
The absorption system at $z=3.572$ towards Q1937--1009 provides the best extragalactic measurement of the atomic deuterium to hydrogen ratio, D/H. We have obtained a new low-resolution, high signal-to-noise ratio (SNR) Keck spectrum to re-measure the total neutral hydrogen column density N(H I) using the amount of Lyman continuum absorption. We develop a new method to directly determine the intrinsic unabsorbed quasar continuum from low-resolution spectra of the \Lya forest for the first time. We use three types of spectra to measure N(H I): (1) A wide slit spectrum for flux calibration, (2) a high-resolution spectrum to determine the unabsorbed continuum between \Lya forest lines, and (3) a high SNR spectrum to measure the residual flux below the Lyman limit. We measure Log [N(H I)] = 17.86 $\pm$ 0.02 \cm2, which is reliable because N(H I) is fully specified by the data. This result is consistent with the N(H I) measured by Tytler, Fan & Burles (1996) from the Lyman series absorption lines, but not with absorption models proposed by Wampler (1996) nor estimates of the total N(H I) by Songaila et al. (1997), both of which suggested lower N(H I) and higher D/H. The Wampler models predict abundant flux below the Lyman limit which is absent from both our old and new spectra, taken with different instruments. The new Keck data is consistent with the data presented by Songaila et al. (1997). The results differ due to the different methods of analysis, and our measurement of the QSO continuum does not agree with the continuum models assumed by Songaila et al. (1997).
Cosmology with large interferometric telescopes is a rich and largely unexplored subject, involving three types of measurement: astrometric measurement of absolute distances and proper motions, dispersions of relative proper motions, and images. The ground based interferometers can have huge apertures, which are necessary for faint cosmological targets. But, alone, they are limited to astrometry within the isoplanatic patch, and hence to relative positions, which are of little use for parallaxes and proper motions because reference stars have unknown parallaxes and huge (500 μarcsec) unknown motions. We propose that space missions should measure global astrometric parallaxes and proper motions for (V > 16) reference stars within the isoplanatic patches of important cosmological and Galactic targets. Ground based interferometers can then measure absolute distances (parallaxes) and proper motions to 10 μarcsec, tied to these reference stars. But cosmological observations stretch the VLTI technically. To observe the few best targets, we need to be able to measure positions to < 10 μarcsec over a large portion of the sky. Since natural guide stars are too far apart, or too faint, laser guide stars are needed to correct the wavefronts of the individual 8-m unit telescopes, and the fringe tracking system must have an extremely high throughput to work on the brightest stars (V > 16) near to important targets. Most of the science is at 1 --2 microns, where excellent adaptive optics will be needed on the 8-m telescopes.
The spectrum of quasar 0014+813 was the first in which deuterium was claimed to be observed, at first as a probable detection, and later ``confirmed''. It is important because it is the only case in which there is a serious likelihood of a high D/H ratio, since the others are more ambiguous and admit a wide range of D/H including zero. We present new high-resolution spectra with higher quality than those published. Deuterium has been reported in two Lyman limit systems (\zabs = 3.32, 2.80) along the line of sight. In both we find that the absorption is readily explained by H alone and that deuterium absorption is neither required nor suggested by the spectra. The three previous models with high D/H values (D/H $\approx 20 \times 10^{-5}$) are not compatible with the new data, and in some cases the differences are surprisingly large. Since the spectrum is compatible with all values of D/H from zero up to large upper limits, neither absorption system can be used to measure D/H, and previously cited values should be disregarded. There is now no case in which a high value of D/H has been established, and all data are compatible with low D/H. Deuterium has been seen and measured in only two QSOs (Q1937--1009 and Q1009+2956), both of which give low D/H. Several other QSOs give upper limits which are about ten times less sensitive.
To expand the known number of low-redshift galaxies which lie close to bright ($V < 17.2$) QSO lines of sight, we have identified 24 galaxies within 11 arcmins of nine QSOs which have been observed with the Hubble Space Telescope (HST). Galaxies are found between redshifts of $0.0114-0.145$ and lie between $39-749$ $h^{-1}$ kpc from QSO sightlines. Knowing the redshifts of these galaxies has already proved important in understanding results from HST programmes designed to search for UV absorption lines from low-redshift galaxies, and will enable future observations to probe the halos of these galaxies in detail.
The next decade promises an observational revolution which will change cosmology forever. The precise measurement of the angular anisotropy of the cosmic microwave background should specify to a few percent all of the parameters of the cosmological model which effect astrophysics. The growth of structure will then be determined (but not yet observed) until gravitational collapse becomes highly non-linear and stars, galaxies and active galactic nuclei (AGN) form. These processes are hard to model with basic physics because they are complex and allow a rich variety of expression. Instead observations will determine when the first stars and quasars formed, and how and when galaxies assembled. If we can reconcile the numerous contradictions which characterize the subject today, cosmology will become a mature subject, founded on the agreement between detailed, inclusive and realistic models, which make precise predictions, and the wealth of new data which will come from a wide variety of observations, at all wavelengths. This is an ambitious schedule, but nothing less is worthy of the outstanding capabilities of the 8 -- 10 m telescopes, the next generation space telescope, the opportunities at millimeterto sub-millimeter wavelengths and advanced computer modeling. The ESO Very Large Telescope (VLT) should play a major role in this revolution.
The primordial D/H ratio now provides the best measure of the cosmological density of baryons. We describe in detail how we deduce the D/H ratio from absorption lines in the spectra of quasars, and we present our first two measurements of D/H in different QSOs, which agree to within the random errors. We describe how we correct D/H for blending with Lyman alpha forest H lines. For our two QSOs these corrections are small, because the D lines are narrow, deep and constrained by H and metal lines. The similarity of the two measurements rules out ad hoc astrophysical effects, such as the destruction of D without production of metals, or unusual data problems. We are confident that we have measured the primordial abundance of D (log D/H $= -4.62 \pm 0.05$), which is the first such measurement because other values in the literature are much larger, and apparently strongly contaminated.
Standard big bang nucleosynthesis (BBNS) promises accurate predictions of the primordial abundances of deuterium, helium-3, helium-4 and lithium-7 as a function of a single parameter. Previous measurements have nearly always been interpreted as confirmation of the model (Copi, Schramm & Turner 1995). Here we present a measurement of the deuterium to hydrogen ratio (D/H) in a newly discovered high redshift metal-poor gas cloud at redshift $z=2.504$. This confirms our earlier measurement of D/H (Tytler, Fan & Burles 1996), and together they give the first accurate measurement of the primordial D abundance, and a ten fold improvement in the accuracy of the cosmological density of ordinary matter.This is a high density, with most ordinary matter unaccounted or dark, which is too high to agree with measurements of the primordial abundances of helium-4 and lithium-7. Since the D/H measurement is apparently simple, direct, accurate and highly sensitive, we propose that helium requires a systematic correction, and that population II stars have less than the primordial abundance of $^7$Li. Alternatively, there is no concordance between the light element abundances, and the simple model of the big bang must be incomplete and lacking physics, or wrong.
We present a measurement of the deuterium to hydrogen ratio in a quasar absorption system at redshift z = 3.57 towards QSO 1937-1009. We use a two component fit, with redshifts determined from unsaturated metal lines, to fit the hydrogen and deuterium features simultaneously. We find a low value of D/H = 2.3 \pm 0.6 \times 10^{-5}, which does not agree with other measurements of high D/H (Songaila et al. 1994, Carswell et al. 1994). The absorption system is very metal poor, with metallicities less than 1/100 solar. Standard models of chemical evolution show the astration of deuterium is limited to a few percent from primordial for systems this metal-poor, so we believe our value represents the primordial one. Using predictions of standard big-bang nucleosynthesis and measurements of the cosmic microwave background, our measurement gives the density of baryons in units of the critical density, $Ω_b h^2 = 0.024 \pm 0.006$, where $H_0 = 100 h km s^{-1] Mpc^{-1}$.
We have conducted the first survey for Oxygen VI $λλ1032,1038$ absorption lines in QSO spectra. We used medium resolution ($R \approx 1300$) high signal-to-noise ($\approx 20$) Faint Object Spectrograph spectra of 11 QSOs ($0.53\leq$\zem$\leq2.08$) from the Hubble Space Telescope Archive. We use simulated spectra to determine the significance of the line identifications, which lie exclusively in the \Lya forest. We found 12 O~VI doublets of which 9 are expected to be real and 6 constitute a uniform sample with both lines exceeding a rest equivalent width of $W_r =0.21$ Å. The number of O~VI doublets per unit redshift at a mean absorption redshift of $z_{ave} = 0.9$ is $\langle N(z)\rangle = 1.0 \pm0.6$, which is similar to the density of C~IV and Mg~II absorbers.