Pascal Paschos

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 H i opacity of the intergalactic medium at redshifts 1.6 < z < 3.2

We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range 2.2 < z < 3.2. Our measurement of DA excludes absorption from metal lines or the Lya lines of Lyman-limit systems and damped Lya 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 1.6 < z < 3.2 as A (1 +z) γ , where A = 0.0062 and y = 2.75. We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less HI absorption than expected at z = 3, implying that the ultraviolet background is ∼40 per cent higher than expected. Our data appears to be consistent with an H I ionization rate of r ∼ 1.4 × 10 -12 s -1 .

A concordance model of the Lyman α forest at z = 1.95

We present 40 fully hydrodynamical numerical simulations of the intergalactic gas that gives rise to the Lyforest. The simulation code, input and output files are available at http://www.cosmos.ucsd.edu/˜gso/index.html. For each simula- tion we predict the observable properties of the H I absorption in QSO spectra. We then find the sets of cosmological and astrophysical parameters that match the QSO spectra. We present our results as scaling relationships between input and output parameters. The input parameters include the main cosmological parameters b, m, �, H0 and �8; and two astrophysical parameters 912 and X228. The parameter 912 controls the rate of ionization of H I, He I and He II and is equivalent to the intensity of the UV background. The second parameter X228 controls the rate of heating from the photoionization of He II and can be related to the shape of the UVB at � < 228 u We show how these input param- eters; especially �8, 912 and X228; effect the output parameters that we measure in simulated spectra. These parameters are the mean flux ¯ F, a measure of the most common Lyline width (b-value) b�, and the 1D power spectrum of the flux on scales from 0.01 - 0.1 s/km. We compare the simulation output to data from Kim et al. (2004) and Tytler et al. (2004) and we give a new measurement of the flux power from HIRES and UVES spectra for the low density IGM alone at z = 1.95. We find that simulations with a wide variety of �8 values, from at least 0.8 - 1.1, can fit the small scale flux power and b-values when we adjust X228 to compensate for the �8 change. We can also use 912 to adjust the H I ionization rate to simultaneously match the mean flux. When we examine only the mean flux, b-values and small scale flux power we can not readily break the strong degeneracy between �8 and X228.

Self-consistent solution of cosmological radiation-hydrodynamics and chemical ionization

We consider a PDE system comprising compressible hydrodynamics, flux-limited diffusion radiation transport and chemical ionization kinetics in a cosmologically-expanding universe. Under an operator-split framework, the cosmological hydrodynamics equations are solved through the piecewise parabolic method, as implemented in the Enzo community hydrodynamics code. The remainder of the model, including radiation transport, chemical ionization kinetics, and gas energy feedback, form a stiff coupled PDE system, which we solve using a fully-implicit inexact Newton approach, and which forms the crux of this paper. The inner linear Newton systems are solved using a Schur complement formulation, and employ a multigrid-preconditioned conjugate gradient solver for the inner Schur systems. We describe this approach and provide results on a suite of test problems, demonstrating its accuracy, robustness, and scalability to very large problems.

The Lyα forest at redshifts 0.1–1.6: good agreement between a large hydrodynamic simulation and HST spectra

We give a comprehensive statistical description of the Lyα absorption from the intergalactic medium in a hydrodynamic simulation at redshifts 0.1-1.6, the range of redshifts covered by spectra of quasi-stellar objects obtained with the Hubble Space Telescope (HST). We use the ENZO code to make a simulation in a 1024 3 cube with side length 76.8 Mpc comoving using 75-kpc cells, for a Hubble constant of 71 km s -1 Mpc -1 . The best prior work, by Dave et al., used a smoothed particle hydrodynamics simulation in a 15.6-Mpc box with an effective resolution of 245 kpc at the cosmic mean density and slightly different cosmological parameters. We use a popular cosmological model and astrophysical parameters that describe the ultraviolet background that photoionizes and heats the gas. At redshift z = 2 this simulation is different from data. Tytler et al. found that the simulated spectra at z = 2 have too little power on large scales, Lyα lines are too wide, there is a lack of high column density lines and there is a lack of pixels with low flux. Here we present statistics at z < 1.6, including the flux distribution, the mean flux, the effective opacity and the power and correlation of the flux. We also give statistics of the Lyα lines including the linewidth distribution, the column density distribution, the number of lines per unit equivalent width and redshift and the correlation between the linewidth and column density. We find that the mean amount of absorption in the simulated spectra changes smoothly with redshift as DA(z) = 0.0102(1 + z) 2.252 . Both the trend and absolute values are close to measurements of HST spectra by Kirkman et al. The column density distribution is the same as data at redshift 1.6, but at lower redshifts the simulation has fewer lines than data, perhaps because of issues making measurements in low signal-to-noise ratio spectra. The lines in data have smaller widths than in the simulation but again the data are not very reliable. We argue that these differences are probably not caused by lack of numerical resolution. Although these possible differences are larger than those that we saw at z = 2, overall, the simulation gives a good description of HST spectra at 0.1 < z < 1.6 given that we did not adjust either the parameters that we input to specify the simulations or the output from the simulation.

H I Gas in Higher Density Regions of the Intergalactic Medium

Using H I absorption alone, we attempt to separate H I absorption lines in quasar spectra into two categories: higher density lines (HDLs) and lower density lines (LDLs), and we discuss the difference in their physical properties. We deblend and fit all H I lines with Voigt profiles and make an unbiased sample of H I lines covering a wide column density range (12 15 and LDLs as all others with 12 < log NH I < 15. We found that the HDLs have smaller minimum b-values for a given column density than the LDLs. This difference is successfully reproduced by our hydrodynamic simulation. The LDLs seem to be cool or shock-heated diffuse intergalactic medium gas, whereas the HDLs are likely to be cooler dense gas near to galaxies.

Baryon acoustic oscillations in the Lyman alpha forest

We use hydrodynamic cosmological simulations in a (600 Mpc)3 volume to study the observability of baryon acoustic oscillations (BAO) in the intergalactic medium as probed by Lyman alpha forest (LAF) absorption. The large scale separation between the wavelength of the BAO mode (~150 Mpc) and the size of LAF absorbers (~100 kpc) makes this a numerically challenging problem. We report on several 20483 simulations of the LAF using the ENZO code. We adopt WMAP5 concordance cosmological parameters and power spectrum including BAO perturbations. 5000 synthetic HI absorption line spectra are generated randomly piercing the box face. We calculate the cross-correlation function between widely separated pairs. We detect the BAO signal at z=3 where theory predicts to moderate statistical significance.

H I gas in higher density regions of the IGM

Using H I absorption alone, we attempt to separate H I absorption lines in quasar spectra into two categories; HDLs (Higher Density Lines) and LDLs (Lower Density Lines), and we discuss the difference in their physical properties. We deblend and fit all H I lines with Voigt profiles, and make an unbiased sample of H I lines covering a wide column density range (

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 = 1.9Based on data obtained with the Kast spectrograph on the Lick Observatory 3 m Shane Telescope.

12 cm

Cosmological parameters sigma(8), the baryon density omega(b), and the UV background intensity from a calibrated measurement of HI Lyman-alpha absorption at z = 1.9

^{-2}