arXiv subjects

Scott Burles

Publications and source records attributed to Scott Burles.

70 records · Page 4Linked to original sources

arXiv1999

Cosmological Magnetic Fields Limits in an Inhomogeneous Universe

We study the effect of inhomogeneities in the matter distribution of the universe on the Faraday rotation of light from distant QSOs and derive new limits on the cosmological magnetic field. The matter distribution in the Universe is far from being homogeneous and, for the redshifts of interest to rotation measures (RM), it is well described by the observed Ly-$α$ forest. We use a log-normal distribution to model the Ly-$α$ forest and assume that a cosmological magnetic field is frozen into the plasma and is therefore a function of the density inhomogeneities. The Ly-$α$ forest results are much less sensitive to the cosmological magnetic field coherence length than those for a homogeneous universe and show an increase in the magnitude of the expected RM for a given field by over an order of magnitude. The forest also introduces a large scatter in RM for different lines-of-sight with a highly non-gaussian tail that renders the variance and the mean RM impractical for setting limits. The $median|{\rm RM}|$ is a better statistical indicator which we use to derive the following limits using the observed RM for QSOs between $z = 0$ and $z = 2.5$. We set $Ω_b h^2 = 0.02$ and get for cosmological fields coherent accross the present horizon, $B_{H_0^{-1}} \la 10^{-9}$ G in the case of a Ly-$α$ forest which is stronger than the limit for a homogeneous universe, $B_{H_0^{-1}}^h \la 2 \times 10^{-8}$ G; while for 50 Mpc coherence length, the inhomogeneous case gives $B_{50 {\rm Mpc}} \la 6 \times 10^{-9}$ G while the homogeneous limit is $B_{50 {\rm Mpc}}^h \la 10^{-7}$ G and for coherence length equal to the Jeans length, $B_{λ_J} \la 10^{-8}$ G for the Ly-$α$ case while $B_{λ_J}^h \la 10^{-6}$ G.

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arXiv1998

Deuterium to hydrogen towards QSO 1009+2956 from a mesoturbulent model

We present a new analysis of the deuterium absorption at z = 2.504 towards the quasar Q1009+2956 using the mesoturbulent model which accounts for possible correlations in the large scale velocity field. We obtain a slightly higher deuterium-to-hydrogen ratio D/H = (3.5 - 5.0)*10^{-5} as compared with the recent measurement D/H = (3.3 - 4.5)*10^{-5} (68% C.L.) performed by Burles & Tytler (1998b) using the usual microturbulent approximation which assumes that the velocity field is uncorrelated. Other mesoturbulent calculations of the D-abundances at z = 3.572 towards Q1937--1009 and at z = 0.701 towards Q1718+4807 (the systems showing `low' and possibly `high' D/H values, respectively, in the microturbulent approaches) agree with the present one within the errors of measurements. Thus, the mesoturbulent analysis does not reveal any spatial variations of D/H and supports the standard homogeneous model of big bang nucleosynthesis.

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arXiv1998

The Deuterium Abundance at z=0.701 towards QSO 1718+4807

We present constraints on the deuterium to hydrogen ratio (D/H) in the metal-poor gas cloud at redshift $z=0.701$ towards QSO 1718+4807. We use new Keck spectra in addition to Hubble Space Telescope (HST) and International Ultraviolet Explorer (IUE) spectra. We use an improved redshift and a lower \HI column density to model the absorption. The HST spectrum shows an asymmetric Lyman-$α$ (\lya) feature which is produced by either \HI at a second velocity, or a high abundance of D. Three models with a single simple H+D component give $8 \times 10^{-5} < D/H < 57 \times 10^{-5}$ (95%), a much larger range than reported by Webb et al (1997a,b). A more sophisticated velocity distribution, or a second component is necessary for lower D/H. With two components, which could be a part of one absorbing structure, or separate clouds in a galaxy halo, we find $D/H < 50 \times 10^{-5}$. We do not know if this second component is present, but it is reasonable because 40 -- 100% of absorption systems with similar redshifts and \HI column densities have more than one component.

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arXiv1998

Cosmology with the Lyman-alpha Forest

We outline the physical picture of the high-redshift Ly-alpha forest that has emerged from cosmological simulations, describe statistical characteristics of the forest that can be used to test theories of structure formation, present a preliminary comparison between simulation results and measurements from Keck HIRES spectra, and discuss a recent determination of the slope and amplitude of the linear mass power spectrum P(k) at z=2.5 from moderate resolution spectra. The physical picture is simple if each QSO spectrum is viewed as a continuous non-linear map of the line-of-sight density field rather than a collection of discrete absorption lines. The distribution of flux decrements depends mainly on the amplitude and PDF (Gaussian vs. non-Gaussian) of the primordial density fluctuations. The threshold crossing frequency, analogous to the 3-d genus curve, responds to the shape and amplitude of P(k) and to the values of Omega and Lambda. Open and Lambda-CDM models agree well with the measured flux decrement distribution at smoothing lengths of 25 km/s and 100 km/s and with the threshold crossing frequency at 100 km/s. Discrepancy with the observed threshold crossing frequency at 25 km/s may reflect the combined effects of noise in the data and limited mass resolution of the simulations. The slope of the measured P(k) agrees with the predictions of inflation+CDM models. Combining the amplitude with COBE normalization imposes a constraint on these models of the form Omega h^x n^y Omega_b^z = constant. Assuming Gaussian primordial fluctuations and a power spectrum shape parameter Gamma~0.2, consistency of the measured P(k) with the observed cluster mass function at z=0 requires Omega=0.46^{+0.12}_{-0.10} for an open universe and Omega=0.34^{+0.13}_{-0.09} for a flat universe (1-sigma errors). (Shortened)

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arXiv1998

A Geometrical Test of the Cosmological Energy Contents Using the Lyman-alpha Forest

In this Letter we explore a version of the test of cosmological geometry proposed by Alcock and Paczynski (1979), using observations of the Lyman-alpha forest in the spectra of close quasar pairs. By comparing the correlations in absorption in one quasar spectrum with correlations between the spectra of neighboring quasars one can determine the relation of the redshift distance scale to the angle distance scale at the redshift of the absorbers, $z \sim 2 - 4$. Since this relationship depends on the parameters of the cosmological model, these parameters may be determined using the Lyman-alpha forest. While this test is relatively insensitive to the density parameter $Ω_m$ in a dust-dominated universe, it is more sensitive to the presence of a matter component with large negative pressure (such as a cosmological constant $Λ$) and its equation of state. With only 25 pairs of quasar spectra at angular separations $0.5' - 2'$, one can discriminate between an $Ω_m = 0.3$ open universe ($Λ=0$) and an $Ω_m = 0.3$ flat ($Λ$-dominated) universe at the $4-σ$ level. The S/N can be enhanced by considering quasar pairs at smaller angular separations, but requires proper modeling of nonlinear redshift space distortions. Here the correlations and redshift space distortions are modeled using linear theory.

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arXiv1998

Measurements of the deuterium abundance in quasar absorption systems

Observational constraints on the primordial deuterium-to-hydrogen ratio (D/H) can test theories of the early universe and provide constraints on models of big bang nucleosynthesis (BBN). We measure deuterium absorption in high-redshift, metal-poor QSO absorption systems and directly infer the value of primordial D/H. We present two measurements of D/H, and find D/H = 3.3 $\pm 0.3 \times 10^{-5}$ at $z=3.572$ towards QSO 1937-1009 and D/H = 4.0 $\pm 0.7 \times 10^{-5}$ at $z=2.504$ towards QSO 1009+2956. Both measurements use multiple-component Voigt profile analysis of high resolution, high signal-to-noise spectra and determinations of the Lyman continuum optical depth in low resolution spectra to constrain the column densities of deuterium and hydrogen. The measurements are consistent with a single primordial value of D/H = $3.4 \pm 0.3 \times 10^{-5}$. This is a relatively low value, which supports homogeneous models of BBN and standard models of galactic chemical evolution. With standard BBN, we find a cosmological baryon-to-photon ratio, $η= 5.1 \pm 0.3 \times 10^{-10}$, and a present-day baryon density in units of the critical density, $Ω_b h_{100}^2 = 0.019 \pm 0.001$.

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arXiv1997

Closing in on the Primordial Abundance of Deuterium

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.

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arXiv1997

The Deuterium Abundance Towards Q1937-1009

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.

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arXiv1997

The Deuterium Abundance Towards Q1009+2956

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.

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arXiv1997

The Weakness of C IV Absorber Clustering in Keck HIRES Spectra of Adjacent QSO Sightlines

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.

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arXiv1997

The Neutral Hydrogen Column Density towards Q1937--1009 from the Unabsorbed Intrinsic Continuum in the Lyman-$α$ Forest

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).

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arXiv1996

New Keck Spectra of Q0014+813: annulling the case for high deuterium abundances

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.

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arXiv1996

The Cosmological Baryon Density from Deuterium in QSO Spectra

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.

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arXiv1996

Cosmological Deuterium Abundance and the Baryon Density of the Universe

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.

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arXiv1996

The Cosmological Baryon Density from the Deuterium Abundance at a redshift z = 3.57

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 $H_0 = 100 h km s^{-1] Mpc^{-1}$.

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arXiv1995

The Cosmological Density and Ionization of Hot Gas: Oxygen VI Absorption in Quasar Spectra

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.

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