Renyue Cen

WHERE ARE THE BARYONS

New high-resolution, large-scale cosmological hydrodynamic galaxy formation simulations of a standard cold dark matter model (with a cosmological constant) are utilized to predict the distribution of baryons at the present and at moderate redshift. It is found that the average temperature of baryons is an increasing function of time, with most of the baryons at the present time having a temperature in the range of 105-107 K. Thus not only is the universe dominated by dark matter, but more than one-half of the normal matter is yet to be detected. Detection of this warm/hot gas poses an observational challenge, which requires sensitive EUV and X-ray satellites. Signatures include a soft cosmic X-ray background, apparent warm components in hot clusters due to both intrinsic warm intracluster and intercluster gas projected onto clusters along the line of sight, absorption lines in X-ray and UV quasar spectra [e.g., O VI (1032, 1038) A lines, O VII 574 eV line], strong emission lines (e.g., O VIII 653 eV line), and low-redshift, broad, low column density Lyα absorption lines. We estimate that approximately one-fourth of the extragalactic soft X-ray background (at 0.7 keV) arises from the warm/hot gas, half of it coming from z<0.65, and three-quarters coming from z<1.00, so the source regions should be identifiable on deep optical images.

Cosmological parameter analysis including SDSS lyα forest and galaxy bias : Constraints on the primordial spectrum of fluctuations, neutrino mass, and dark energy

We combine the constraints from the recent Lyα forest analysis of the Sloan Digital Sky Survey (SDSS) and the SDSS galaxy bias analysis with previous constraints from SDSS galaxy clustering, the latest supernovae, and 1st year WMAP cosmic microwave background anisotropies. We find significant improvements on all of the cosmological parameters compared to previous constraints, which highlights the importance of combining Lyα forest constraints with other probes. combining WMAP and the Lyα forest we find for the primordial slope ns = 0:98±0:02. We see no evidence of running, dn/=d lnk 0:003±0:010, a factor of 3 improvement over previous constraints. We also find no evidence of tensors, r < 0:36 (95% c.l.). Inflationary models predict the absence of running and many among them satisfy these constraints, particularly negative curvature models such as those based on spontaneous symmetry breaking. A positive correlation between tensors and primordial slope disfavors chaotic inflation-type models with steep slopes: while the V αo 2 model is within the 2-sigma contour, V αo4 is outside the 3- sigma contour. For the amplitude we find σ8 = 0:90 ± 0:03 from the Lyα forest and WMAP alone. We find no evidence of neutrino mass: for the case of 3 massive neutrino families with an inflationary prior, Σmv < 0:42 eV and the mass of lightest neutrino is m1 < 0:13 eV at 95% c.l. For the 3 massless +1 massive neutrino case we find mv < 0:79 eV for the massive neutrino, excluding at 95% c.l. all neutrino mass solutions compatible with the LSND results. We explore dark energy constraints in models with a fairly general time dependence of dark energy equation of state, finding Ωλ =0:72± 0:02, w(z = 0:3) = 0:98+0.10 -0.12,the latter changing to w(z = 0:3) = -0.92+0.09-0.10 if tensors are allowed. We find no evidence for variation of the equation of state with redshift, w(z = 1) = -1.03+0.21-0.28. These results rely on the current understanding of the Lyα forest and other probes, which need to be explored further both observationally and theoretically, but extensive tests reveal no evidence of inconsistency among different data sets used here.

Baryons in the Warm-Hot Intergalactic Medium

Approximately 30%-40% of all baryons in the present-day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures in the range 105 < T < 107 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, nonequilibrium structures such as filaments. Supernova feedback energy and radiative cooling play lesser roles in its evolution. WHIM gas may be consistent with observations of the 0.25 keV X-ray background without being significantly heated by nongravitational processes because the emitting gas is very diffuse. Our results confirm and extend previous work by Cen & Ostriker and Dave et al.

The Santa Barbara Cluster Comparison Project: A Comparison of Cosmological Hydrodynamics Solutions

We have simulated the formation of an X-ray cluster in a cold dark matter universe using 12 different codes. The codes span the range of numerical techniques and implementations currently in use, including smoothed particle hydrodynamics (SPH) and grid methods with fixed, deformable, or multilevel meshes. The goal of this comparison is to assess the reliability of cosmological gasdynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be nonradiative. We compare images of the cluster at different epochs, global properties such as mass, temperature and X-ray luminosity, and radial profiles of various dynamical and thermodynamical quantities. On the whole, the agreement among the various simulations is gratifying, although a number of discrepancies exist. Agreement is best for properties of the dark matter and worst for the total X-ray luminosity. Even in this case, simulations that adequately resolve the core radius of the gas distribution predict total X-ray luminosities that agree to within a factor of 2. Other quantities are reproduced to much higher accuracy. For example, the temperature and gas mass fraction within the virial radius agree to within about 10%, and the ratio of specific dark matter kinetic to gas thermal energies agree to within about 5%. Various factors, including differences in the internal timing of the simulations, contribute to the spread in calculated cluster properties. Based on the overall consistency of results, we discuss a number of general properties of the cluster we have modeled.

The Lyα Forest from Gravitational Collapse in the Cold Dark Matter + Λ Model

We use an Eulerian hydrodynamic cosmological simulation to model the Ly? forest in a spatially flat, COBE-normalized, cold dark matter model with) ? = 0.4. We find that the intergalactic, photoionized gas is predicted to collapse into sheetlike and filamentary structures which give rise to absorption lines having characteristics similar to the observed Ly? forest. A typical filament is ~500 h?1 kpc long with thickness ~50 h?1 kpc (in proper units), and baryonic mass ~ 1010 h?1 M. In comparison our cell size is (2.5, 9) h?1 kpc in the two simulations we perform, with true resolution perhaps a factor of 2.5 worse than this. The gas temperature is in the range 104-105 K, and it increases with time as structures with larger velocities collapse gravitationally. We show that the predicted distributions of column densities, b-parameters, and equivalent widths of the Ly? forest clouds agree reasonably with observations, and that their evolution is consistent with the observed evolution, if the ionizing background has an approximately constant intensity between z = 2 and z = 4. A new method of identifying lines as contiguous regions in the spectrum below a fixed flux threshold is suggested to analyze the absorption lines, given that the Ly? spectra arise from a continuous density field of neutral hydrogen rather than discrete clouds. We also predict the distribution of transmitted flux and its correlation along a spectrum and on parallel spectra, and the He ii flux decrement as a function of redshift. We predict a correlation length of ~80 h?1 kpc perpendicular to the line of sight for features in the Ly? forest. In order to reproduce the observed number of lines and average flux transmission, the baryon content of the clouds may need to be significantly higher than in previous models because of the low densities and large volume-filling factors we predict. If the background intensity JH I is at least that predicted from the observed quasars, ?b needs to be as high as ~0.25 h?2. The model also predicts that most of the baryons at z > 2 are in Ly? clouds, and that the rate at which the baryons move to more overdense regions is slow. A large fraction of the baryons which are not observed at present in galaxies might be intergalactic gas in the currently collapsing structures, with T ~ 105?106 K. All our results on the statistical properties of the simulated spectra are predictions that can be directly tested by applying the same methods to observed spectra. We are making the simulated spectra electronically available.

The Opacity of the Lyα Forest and Implications for Ωb and the Ionizing Background

We have measured the distribution function of the flux decrement D = 1 - e-τ caused by Lyα forest absorption from intervening gas in the lines of sight to high-redshift QSOs from a sample of seven high-resolution QSO spectra obtained with the Keck telescope. The observed flux decrement distribution function (FDDF) is compared with the FDDF from two simulations of the Lyα forest: a ΛCDM model (with Ω = 0.4, Λ = 0.6), computed with the Eulerian code of Cen & Ostriker, and a standard cold dark matter (SCDM) model (with Ω = 1), computed with the smoothed particle hydrodynamics code of Hernquist et al. Good agreement is obtained between the shapes of the simulated and observed FDDFs for both simulations after fitting only one free parameter, which controls the mean flux decrement. The difference between the predicted FDDFs from the two simulations is small, and we show that it arises mostly from a different temperature in the low-density gas (caused by different assumptions that were made about the reionization history in the two simulations), rather than differences between the two cosmological models or numerical effects in the two codes, which use very different computational methods. A measurement of the parameter μΩ -->2b h -->3/Γ (where Γ is the H I ionization rate due to the ionizing background) is obtained by requiring the mean flux decrement in the simulations to agree with the observed one. Estimating the lower limit Γ > 7 × 10-13 s-1 from the abundance of known QSOs, we derive a lower limit on the baryonic matter density, Ωbh2 > 0.021 (0.017) for the ΛCDM (SCDM) model. The difference between the lower limits inferred from the two models is again due to different temperatures in the low-density gas. We give general analytical arguments for why this lower limit is unlikely to be reduced for any other models of structure formation by gravitational collapse that can explain the observed Lyα forest. When combined with constraints from big bang nucleosynthesis, the large Ωb we infer is inconsistent with some recent D/H determinations (Rugers & Hogan), favoring a low deuterium abundance as reported by Tytler, Fan & Burles. Adopting a fixed Ωb, the measurement of μ(z) allows a determination of the evolution of the ionizing radiation field with redshift. Our models predict an intensity that is approximately constant with redshift, which is in agreement with the assumption that the ionizing background is produced by known quasars for z < 3, but requires additional sources of ionizing photons at higher redshift given the observed rapid decline of the quasar abundance.

The Protogalactic origin for cosmic magnetic fields

It is demonstrated that strong magnetic fields are produced from a zero initial magnetic field during the pregalactic era, when the galaxy is first forming. Their development proceeds in three phases. In the first phase, weak magnetic fields are created by the Biermann battery mechanism. During the second phase, results from a numerical simulation make it appear likely that homogenous isotropic Kolmogorov turbulence develops that is associated with gravitational structure formation of galaxies. Assuming that this turbulence is real, then these weak magnetic fields will be amplified to strong magnetic fields by this Kolmogorov turbulence. During this second phase, the magnetic fields reach saturation with the turbulent power, but they are coherent only on the scale of the smallest eddy. During the third phase, which follows this saturation, it is expected that the magnetic field strength will increase to equipartition with the turbulent energy and that the coherence length of the magnetic fields will increase to the scale of the largest turbulent eddy, comparable to the scale of the entire galaxy. The resulting magnetic field represents a galactic magnetic field of primordial origin. No further dynamo action after the galaxy forms is necessary to explain the origin of magnetic fields. However, the magnetic field will certainly be altered by dynamo action once the galaxy and the galactic disk have formed. It is first shown by direct numerical simulations that thermoelectric currents associated with the Biermann battery build the field up from zero to 10-21 G in the regions about to collapse into galaxies, by z ~ 3. For weak fields, in the absence of dissipation, the cyclotron frequency -ωcyc = eB/mH c and ω/(1 + χ), where ∇ × v is the vorticity and χ is the degree of ionization, satisfy the same equations, and initial conditions ωcyc = ω = 0, so that, globally, -ωcyc(r, t) = ω(r, t)/(1 + χ). The vorticity grows rapidly after caustics (extreme nonlinearities) develop in the cosmic fluid. At this time, it is made plausible that turbulence has developed into Kolmogorov turbulence. Numerical simulations do not yet have the resolution to demonstrate that, during the second phase, the magnetic fields are amplified by the dynamo action of the turbulence. Instead, an analytic theory of the turbulent amplification of magnetic fields is employed to explore this phase of the magnetic field development. From this theory, it is shown that, assuming the turbulence is really Kolmogorov turbulence, the dynamo action of this protogalactic turbulence is able to amplify the magnetic fields by such a large factor during the collapse of the protogalaxy that the power into the magnetic field must reach saturation with the turbulent power. For the third phase, there is as yet no analytic theory capable of describing this phase. However, preliminary turbulence calculations currently in progress seem to confirm that the magnetic fields may proceed to equipartition with the turbulent energy, and that the coherence length may increase to the largest scales. Simple physical arguments are presented that show that this may be the case. Such an equipartition field is actually too strong to allow immediate collapse to a disk. Possible ways around this difficulty are discussed.

ENZO: AN ADAPTIVE MESH REFINEMENT CODE FOR ASTROPHYSICS

This paper describes the open-source code Enzo, which uses block-structured adaptive mesh refinement to provide high spatial and temporal resolution for modeling astrophysical fluid flows. The code is Cartesian, can be run in one, two, and three dimensions, and supports a wide variety of physics including hydrodynamics, ideal and non-ideal magnetohydrodynamics, N-body dynamics (and, more broadly, self-gravity of fluids and particles), primordial gas chemistry, optically thin radiative cooling of primordial and metal-enriched plasmas (as well as some optically-thick cooling models), radiation transport, cosmological expansion, and models for star formation and feedback in a cosmological context. In addition to explaining the algorithms implemented, we present solutions for a wide range of test problems, demonstrate the codes parallel performance, and discuss the Enzo collaborations code development methodology.

The Universe Was Reionized Twice

We show that the universe was reionized twice, first at z ~ 15-16 and again at z ~ 6. Such an outcome appears inevitable when normalizing to two well-determined observational measurements, namely, the epoch of the final cosmological reionization at z ~ 6 and the density fluctuations at z ~ 6, which in turn are tightly constrained by Lyα forest observations at z ~ 3. These two observations most importantly fix the product of star formation efficiency and the ionizing photon escape fraction from galaxies at high redshift. The only major assumption made is that the initial mass function of metal-free, Population III stars is top-heavy. To the extent that the relative star formation efficiencies in gaseous minihalos with H2 cooling and large halos with atomic cooling at high redshift are unknown, the primary source for the first reionization is still uncertain. If star formation efficiency in minihalos is at least 10% of that in large halos, then Population III stars in the minihalos may be largely responsible for the first reionization; otherwise, the first reionization will be attributable largely to Population III stars in large halos. In the former case, H2 cooling in minihalos is necessarily efficient. We show that gas in minihalos can be cooled efficiently by H2 molecules and that star formation can continue to take place largely unimpeded throughout the first reionization period, as long as gas is able to accumulate in them. This comes about thanks to two new mechanisms for generating a high X-ray background during the Population III era put forth here, namely, X-ray emission from the cooling energy of Population III supernova blast waves and that from miniquasars powered by Population III black holes. Consequently, H2 formation in the cores of minihalos is significantly induced to be able to counteract the destruction by Lyman-Werner photons produced by the same Population III stars. In addition, an important process for producing a large number of H2 molecules in relic H II regions of high-redshift galaxies, first pointed out by Ricotti, Gnedin, & Shull in 2001, is quantified here for Population III galaxies. It is shown that H2 molecules produced by this process may overwhelm the dissociating effects of the Lyman-Werner photons produced by stars in the same Population III galaxies. As a result, the Lyman-Werner background may not build up in the first place during the Population III era. The long cosmological reionization and reheating history is complex. From z ~ 30, Population III stars gradually heat up and ionize the intergalactic medium, completing the first reionization at z ~ 15-16, followed by a brief period of Δz ~ 1, during which the intergalactic medium stays completely ionized because of sustained ionizing photon emission from concomitant Population III galaxies. The transition from Population III stars to Population II stars at z ~ 13 suddenly reduces, by a factor of ~10, the ionizing photon emission rate, causing hydrogen to rapidly recombine, marking the second cosmological recombination. From z ~ 13 to 6, Compton cooling by the cosmic microwave background and photoheating by the stars self-regulate the Jeans mass and the star formation rate, giving rise to a mean temperature of the intergalactic medium maintained nearly at a constant of ~104 K. Meanwhile, recombination and photoionization balance one another such that the intergalactic medium stays largely ionized during this stage, with n/nH ≥ 0.6. Most of the star formation in this period occurs in large halos with dominant atomic line cooling. We discuss a wide range of implications and possible tests for this new reionization picture. In particular, the Thomson scattering optical depth is increased to 0.10 ± 0.03, compared to 0.027 for the case of only one rapid reionization at z = 6. Upcoming Wilkinson Microwave Anisotropy Probe observations of the polarization of the cosmic microwave background should be able to distinguish between these two scenarios. In addition, properties of minihalos at high redshift (z ≥ 6) will be very different from previous expectations; in particular, they will be largely deprived of gas, perhaps alleviating the cosmological overcooling problem.

The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyα Forest*

A sample of eight quasars observed at high resolution and signal-to-noise ratio is used to determine the transmitted flux probability distribution function (TFPDF), and the power spectrum and correlation function of the transmitted flux in the Lyα forest, in three redshift bins centered at z = 2.41, 3.00, and 3.89. All the results are presented in tabular form, with full error covariance matrices, to allow for comparisons with any numerical simulations and with other data sets. The observations are compared with a numerical simulation of the Lyα forest of a ΛCDM model with Ω = 0.4, known to agree with other large-scale structure observational constraints. There is excellent agreement for the TFPDF if the mean transmitted flux is adjusted to match the observations. A small difference between the observed and predicted TFPDF is found at high fluxes and low redshift, which may be due to the uncertain effects of fitting the spectral continuum. Using the numerical simulation, we show how the flux power spectrum can be used to recover the initial power spectrum of density fluctuations. From our sample of eight quasars, we measure the amplitude of the mass power spectrum to correspond to a linear variance per unit ln k of Δ(k) = 0.72 ± 0.09 at k = 0.04(km s-1)-1 and z = 3, and the slope of the power spectrum near the same k to be np = -2.55 ± 0.10 (statistical error bars). The results are statistically consistent with those of Croft et al., although our value for the rms fluctuation is lower by a factor of 0.75. For the ΛCDM model we use, the implied primordial slope is n = 0.93 ± 0.10, and the normalization is σ8 = 0.68 + 1.16(0.95 - n) ± 0.04.