arXiv subjects

Long-Long Feng

Publications and source records attributed to Long-Long Feng.

At least 55 records · Page 3Linked to original sources

arXiv2007

21 cm Signals from Early Ionizing Sources

We investigate the 21 cm signals from the UV ionizing sources in the reionization epoch. The formation and evolution of 21 cm emission and absorption regions depend essentially on the kinetics of photons in the physical and frequency spaces. To solve the radiative transfer equation, we use the WENO algorithm, which is effective to capture the sharp ionization profile and the cut-off at the front of light $(r=ct)$ and to handle the small fraction of neutral hydrogen and helium in the ionized sphere. We show that a spherical shell of 21 cm emission and absorption will develop around a point source once the speed of the ionization front (I-front) is significantly lower than the speed of light. The 21 cm shell extends from the I-front to the front of light; its inner part is the emission region and its outer part is the absorption region. The 21 cm emission region depends strongly on the intensity, frequency-spectrum and life-time of the UV ionizing source. For a source of short life-time, no 21 cm emission region can be formed if the source dies out before the I-front speed is significantly lower than the speed of light. Yet, a 21 cm absorption region can form and develop even after the emission of the source ceases.

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arXiv2006

A WENO Algorithm of the Temperature and Ionization Profiles around a Point Source

We develop a numerical solver for radiative transfer problems based on the weighted essentially nonoscillatory (WENO) scheme modified with anti-diffusive flux corrections, in order to solve the temperature and ionization profiles around a point source of photons in the reionization epoch. Algorithms for such simulation must be able to handle the following two features: 1. the sharp profiles of ionization and temperature at the ionizing front (I-front) and the heating front (T-front), and 2. the fraction of neutral hydrogen within the ionized sphere is extremely small due to the stiffness of the rate equations of atom processes. The WENO scheme can properly handle these two features, as it has been shown to have high order of accuracy and good convergence in capturing discontinuities and complicated structures in fluid as well as to be significantly superior over piecewise smooth solutions containing discontinuities. With this algorithm, we show the time-dependence of the preheated shell around a UV photon source. In the first stage the I-front and T-front are coincident, and propagate with almost the speed of light. In later stage, when the frequency spectrum of UV photons is hardened, the speeds of propagation of the ionizing and heating fronts are both significantly less than the speed of light, and the heating front is always beyond the ionizing front. In the spherical shell between the I- and T-fronts, the IGM is heated, while atoms keep almost neutral. The time scale of the preheated shell evolution is dependent on the intensity of the photon source. We also find that the details of the pre-heated shell and the distribution of neutral hydrogen remained in the ionized sphere are actually sensitive to the parameters used. The WENO algorithm can provide stable and robust solutions to study these details.

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arXiv2006

The DWT Power Spectrum of the two-degree Field Galaxy Redshift Survey

The power spectrum of the two-degree Field Galaxy Redshift Survey (2dFGRS) sample is estimated with the discrete wavelet transform (DWT) method. The DWT power spectra within $0.04 <k< 2.3 h$Mpc$^{-1}$ are measured for three volume-limited samples defined in connective absolute magnitude bins $-19 \sim -18$, $-20 \sim -19$ and $-21 \sim -20$. We show that the DWT power spectrum can effectively distinguish $Λ$CDM models of $σ_8=0.84$ and $σ_8=0.74$. We adopt maximum likelihood method to perform three-parameter fitting with bias parameter $b$, pairwise velocity dispersion $σ_{pv}$ and redshift distortion parameter $β=Ω_m^{0.6}/b$ to the measured DWT power spectrum. Fitting results denotes that in a $σ_8=0.84$ universe the best fitted $Ω_m$ given by the three samples are consistent in the range $0.28 \sim 0.36$, and the best fitted $σ_{pv}$ are $398^{+35}_{-27}$, $475^{+37}_{-29}$ and $550 \pm 20$km/s for the three samples, respectively. However in the model of $σ_8=0.74$, our three samples give very different values of $Ω_m$. We repeat the fitting by using empirical formula of redshift distortion. The result of the model of low $σ_8$ is still poor, especially, one of the best value $σ_{pv}$ is as large as $10^3$km/s. The power spectrum of 2dFGRS seems in disfavor of models with low amplitude of density fluctuations.

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arXiv2006

Is the cosmic UV background fluctuating at redshift z ~ 6 ?

We study the Gunn-Peterson effect of the photo-ionized intergalactic medium(IGM) in the redshift range 5< z <6.4 using semi-analytic simulations based on the lognormal model. Assuming a rapidly evolved and spatially uniform ionizing background, the simulation can produce all the observed abnormal statistical features near redshift z ~ 6. They include: 1) rapidly increase of absorption depths; 2) large scatter in the optical depths; 3) long-tailed distributions of transmitted flux and 4) long dark gaps in spectra. These abnormal features are mainly due to rare events, which correspond to the long-tailed probability distribution of the IGM density field, and therefore, they may not imply significantly spatial fluctuations in the UV ionizing background at z ~ 6.

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arXiv2006

A WENO Algorithm for the Radiative Transfer and Ionized Sphere at Reionization

We show that the algorithm based on the weighted essentially nonoscillatory (WENO) scheme with anti-diffusive flux corrections can be used as a solver of the radiative transfer equations. This algorithm is highly stable and robust for solving problems with both discontinuities and smooth solution structures. We test this code with the ionized sphere around point sources. It shows that the WENO scheme can reveal the discontinuity of the radiative or ionizing fronts as well as the evolution of photon frequency spectrum with high accuracy on coarse meshes and for a very wide parameter space. This method would be useful to study the details of the ionized patch given by individual source in the epoch of reionization. We demonstrate this method by calculating the evolution of the ionized sphere around point sources in physical and frequency spaces. It shows that the profile of the fraction of neutral hydrogen and the ionized radius are sensitively dependent on the intensity of the source.

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arXiv2006

A Unified Fitting of HI and HeII LyαTransmitted Flux of QSO HE2347 with LCDM Hydrodynamic Simulations

Using cosmological hydrodynamic simulations of the LCDM model, we present a comparison between the simulation sample and real data sample of HI and HeII Lyαtransmitted flux in the absorption spectra of the QSO HE2347-4342. The LCDM model is successful in simultaneously explaining the statistical features of both HI and HeII Lyαtransmitted flux. It includes: 1.) the power spectra of the transmitted flux of HI and HeII can be well fitted on all scales > 0.28h^{-1} Mpc for H, and > 1.1h^{-1} Mpc for He; 2.) the Doppler parameters of absorption features of HeII and HI are found to be turbulent-broadening; 3.) the ratio of HeII to HI optical depths are substantially scattered, due to the significant effect of noise. A large part of the η-scatter is due to the noise in the HeII flux. However, the real data contain more low-ηevents than simulation sample. This discrepancy may indicate that the mechanism leading extra fluctuations upon the simulation data, such as a fluctuating UV radiation background, is needed. Yet, models of these extra fluctuations should satisfy the constraints: 1.) if the fluctuations are Gaussian, they should be limited by the power spectra of observed HI and HeII flux; 2.) if the fluctuations are non-Gaussian, they should be limited by the observed non-Gaussian features of the HI and HeII flux.

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arXiv2006

Low-Redshift Cosmic Baryon Fluid on Large Scales and She-Leveque Universal Scaling

We investigate the statistical properties of cosmic baryon fluid in the nonlinear regime, which is crucial for understanding the large-scale structure formation of the universe. With the hydrodynamic simulation sample of the Universe in the cold dark matter model with a cosmological constant, we show that the intermittency of the velocity field of cosmic baryon fluid at redshift z=0 in the scale range from the Jeans length to about 16 Mpc/h can be extremely well described by She-Leveque's universal scaling formula. The baryon fluid also possesses the following features: (1) for volume weight statistics, the dissipative structures are dominated by sheets, and (2) the relation between the intensities of fluctuations is hierarchical. These results imply that the evolution of highly evolved cosmic baryon fluid is similar to a fully developed turbulence.

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arXiv2005

The velocity field of baryonic gas in the universe

The dynamic evolution of the baryonic intergalactic medium (IGM) caused by the underlying dark matter gravity is governed by the Navier-Stokes equations in which many cooling and heating processes are involved. However, it has long been recognized that the growth mode dynamics of cosmic matter clustering can be sketched by a random force driven Burgers' equation if cooling and heating are ignored. Just how well the dynamics of the IGM can be described as a Burgers fluid has not been fully investigated probably because cooling and heating are essential for a detailed understanding of the IGM. Using IGM samples produced by a cosmological hydrodynamic simulation in which heating and cooling processes are properly accounted for, we show that the IGM velocity field in the nonlinear regime shows the features of a Burgers fluid, that is, when the Reynolds number is high, the velocity field consists of an ensemble of shocks. Consequently, (1) the IGM velocity $v$ is generally smaller than that of dark matter; (2) for the smoothed field, the IGM velocity shows tight correlation with dark matter given by $v \simeq s v_{dm}$, with $s<1$, such that the lower the redshift, the smaller $s$; (3) the velocity PDFs are asymmetric between acceleration and deceleration events; (4) the PDF of velocity difference $Δv=v(x+r)-v(x)$ satisfies the scaling relation for a Burgers fluid, i.e., $P(Δv)=(1 r^y)F(Δv/r^y)$. We find the scaling function and parameters for the IGM which are applicable to the entire scale range of the samples (0.26 - 8 h$^{-1}$ Mpc). These properties show that the similarity mapping between the IGM and dark matter is violated on scales much larger than the Jeans length of the IGM.

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arXiv2005

Power Spectrum and Intermittency of Ly$α$ Transmitted Flux of QSO He2347-4342

We have studied the power spectrum and the intermittent behavior of the fluctuations in the transmitted flux of HE2347-4342 ${\rm Ly}α$ absorption in order to investigate if there is any discrepancy between the LCDM model with parameters given by the WMAP and observations on small scales. If the non-Gaussianity of cosmic mass field is assumed to come only from halos with an universal mass profile of the LCDM model, the non-Gaussian behavior of mass field would be effectively measured by its intermittency, because intermittency is a basic statistical feature of the cuspy structures. We have shown that the Ly$α$ transmitted flux field of HE2347-4342 is significantly intermittent on small scales. With the hydrodynamic simulation, we demonstrate that the LCDM model is successful in explaining the power spectrum and intermittency of ${\rm Ly}α$ transmitted flux. Using statistics ranging from the second to eighth order, we find no discrepancy between the LCDM model and the observed transmitted flux field, and no evidence to support the necessity of reducing the power of density perturbations relative to the standard LCDM model up to comoving scales as small as about $0.08 {\rm h}^{-1} {\rm Mpc}$. Moreover, our simulation samples show that the intermittent exponent of the Ly$α$ transmitted flux field is probably scale-dependent. This result is different from the prediction of universal mass profile with a constant index of the central cusp. The scale-dependence of the intermittent exponent indicates that the distribution of baryonic gas is decoupled from the underlying dark matter.

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arXiv2005

A Parameter-free Statistical Measurement of Halos with Power Spectra

We show that, in the halo model of large-scale structure formation, the difference between the Fourier and the DWT (discrete wavelet transform) power spectra provides a statistical measurement of the halos. This statistical quantity is free from parameters related to the shape of the mass profile and the identification scheme of halos. That is, the statistical measurement is invariant in the sense that models with reasonably defined and selected parameters of the halo models should yield the same difference of the Fourier and DWT spectra. This feature is useful to extract ensemble averaged properties of halos, which cannot be obtained with the identification of individual halo. To demonstrate this point, we show with WIGEON hydrodynamical simulation samples that the spectrum difference provides a quantitative measurement of the discrepancy of the distribution of baryonic gas from that of the underlying dark matter field within halos. We also show that the mass density profile of halos in physical space can be reconstructed with this statistical measurement. This profile essentially is the average over an ensemble of halos, including well virialized halos as well as halos with significant internal substructures. Moreover, this reconstruction is sensitive to the tail of the mass density profile. We showed that the profile with $1/r^3$ tail gives very different result from that of $1/r^2$. Other possible applications of this method are discussed as well.

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arXiv2005

Distributions of the Baryon Fraction on Large Scales in the Universe

(Abridged) The nonlinear evolution of a system consisting of baryons and dark matter is generally characterized by strong shocks and discontinuities. The baryons slow down significantly at postshock areas of gravitational strong shocks, which can occur in high overdense as well as low overdense regions. Consequently, the baryon fraction would be nonuniform on large scales. We studied these phenomena with simulation samples produced by the WENO hybrid cosmological hydrodynamic/N-body code. We find that the baryon fraction in high mass density regions is lower on average than the cosmic baryon fraction, and many baryons accumulate in the regions with moderate mass density to form a high baryon fraction phase (HBFP). In dense regions with rho>100, which are the possible hosts for galaxy clusters, the baryon fraction can be lower than the cosmic baryon fraction by about 10%--20% at z ~ 0. Our simulation samples show that about 3% of the cosmic baryon budget was hidden in the HBFP at redshift z=3, while this percentage increases to about 14% at the present day. The gas in the HBFP cannot be detected either by Ly-alpha forests of QSO absorption spectra or by soft X-ray background. That is, the HBFP would be missed in the baryon budget given by current observations.

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arXiv2004

Statistical features of 21-cm emission from the epoch between reionization and Gunn-Peterson transparency

We investigate the 21 cm emission from the epoch between reionization z_r and Gunn-Peterson transparency z_{gp}. According to the lognormal model of the thermal history around reionization, hydrogen clouds in z_r > z > z_{gp} are hot and a predominant part of baryonic gas is ionized, but still opaque to Ly-alpha photons. Therefore, 21 cm emission is a distinctive characteristic of this epoch. We show that the 21 cm emission comes from both uncollapsed and collapsing hydrogen clouds. The spatial distribution of the brightness temperature excess $δT_b$ is highly non-Gaussian. It consists of spikes with high $δT_b$ and a low $δT_b$ area between the spikes. The field has the following statistical features: (1) the one-point distributions of $δT_b$ are described approximately by power-law tailed probability distribution functions; (2) the nth-order moment of $δT_b$ is increasing much faster with n than that of a Gaussian field, but slower than that of a lognormal field; (3) the scale-scale correlation of the $δT_b$ field is significant for all scales larger than the Jeans length of the gas. These features would be useful for distinguishing the 21 cm emission of the early clustering from the noise of foreground contamination.

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arXiv2004

Temperature and Entropy Fields of Baryonic Gas in the Universe

(Abridged)The temperature (T) and entropy (S) fields of baryonic gas/IGM are analyzed using simulation samples by a cosmological hydro/N-body code. We show that in the nonlinear regime the dynamical similarity between the IGM and dark matter will be broken by strong shocks. The heating and entropy production by the shocks breaks the IGM into multiple phases. The mean entropy, or the cosmological entropy floor, is found to be more than 100 h^(-1/3) keV cm^2 in all regions when z < 1. At redshift z ~ 2 - 3, high-entropy gas (S > 50 h^(-1/3) keV cm^2) mostly resides in areas on scales larger than 1 h^(-1) Mpc and with density $ρ_{dm}>10^2$. Therefore, gravitational shocks are an effective preheating mechanism of the IGM, and probably enough to provide the entropy excess of clusters and groups if the epoch of the gas falling in cluster cores is not earlier than z ~ 2 - 3. On the other hand, at redshifts z < 4, there is always a more than 90% volume of the low dark matter mass density regions filled by the IGM with T < 10^4.5 K. Therefore, the multiphased character and non-Gaussianity of the IGM field would explain the high-temperature and high-entropy gas observed in groups and clusters.

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arXiv2004

The Statistical Discrepancy between the IGM and Dark Matter Fields: One-Point Statistics

We investigate the relationship between the mass and velocity fields of the intergalactic medium (IGM) and dark matter. Although the evolution of the IGM is dynamically governed by the gravity of the underlying dark matter field, some statistical properties of the IGM inevitably decouple from those of the dark matter once the nonlinearity of the dynamical equations and the stochastic nature of the field is considered. With simulation samples produced by a hybrid cosmological hydrodynamic/N-body code, which is effective in capturing shocks and complicated structures with high precision, we find that the one-point distributions of the IGM field are systematically different from that of dark matter as follows: 1.) the one-point distribution of the IGM peculiar velocity field is exponential at least at redshifts less than 2, while the dark matter velocity field is close to a Gaussian field; 2.) although the one-point distributions of the IGM and dark matter are similar, the point-by-point correlation between the IGM and dark matter density fields significantly differs on all scales and redshifts analyzed; 3.) the one-point density distributions of the difference between IGM and dark matter fields are highly non-Gaussian and long tailed. These discrepancies violate the similarity between the IGM and dark matter and cannot be explained simply as Jeans smoothing of the IGM. However, these statistical discrepancies are consistent with the fluids described by stochastic-force driven nonlinear dynamics.

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arXiv2004

A Hybrid Cosmological Hydrodynamic/N-body Code Based on a Weighted Essentially Non-Oscillatory Scheme

We present a newly developed cosmological hydrodynamics code based on weighted essentially non-oscillatory (WENO) schemes for hyperbolic conservation laws. WENO is a higher order accurate finite difference scheme designed for problems with piecewise smooth solutions containing discontinuities, and has been successfully applied for problems involving both shocks and complicated smooth solution structures. We couple hydrodynamics based on the WENO scheme with standard Poisson solver - particle-mesh (PM) algorithm for evolving the self-gravitating system. The third order low storage total variation diminishing (TVD) Runge-Kutta scheme has been used for the time integration of the system. To test accuracy and convergence rate of the code, we subject it to a number of typical tests including the Sod shock tube in multidimensions, the Sedov blast wave and formation of the Zeldovich pancake. These tests validate the WENO hydrodynamics with fast convergence rate and high accuracy. We also evolve a low density flat cosmological model ($Λ$CDM) to explore the validity of the code in practical simulations.

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arXiv2003

The Reionization History in the Lognormal Model

We study the evolution of baryonic gas before the reionization in the lognormal (LN) model of cosmic clustering. We show that the thermal history of the universe around the reionization can roughly be divided into three epochs: 1) cold dark age $z>z_r$, in which baryon gas is neutral, and opaque to Ly$α$ photons; 2) hot dark age $z_r > z> z_{gp}$, in which a predominant part of baryon gas is ionized and hot, but it is still opaque to Ly$α$ photons; 3) bright age $z<z_{gp}$, in which the universe is ionized highly enough to be transparent to Ly$α$ photons. In the flat cold dark matter cosmological models given by WMAP and COBE, the difference of the two redshifts $z_r - z_{gp}$ is found to be as large as $\sim 10$ with $z_r\sim 17$ and $z_{gp}\sim 7$. This reionization history naturally yields a high optical depth to the CMB $τ_e \simeq 0.12 - 0.19$ observed by the TE polarization of the WMAP, and a low redshift $z_{gp}$ of the appearance of the Ly$α$ Gunn-Peterson trough $z_{gp} \simeq 6 - 8$ in QSO's absorption spectra. The reason why the universe stays long in an ionized, yet Ly$α$ opaque, stage is because the first photo-ionization heats the intergalactic gas effectively and has balanced the gravitational clustering a long period of time. Therefore, the result of a high $τ_e$ and low $z_{gp}$ is a common feature of all the models considered. Besides the cosmological parameters, the only free parameter we used in the calculation is $N_{ion}$, the mean ionization photons produced by each baryon in collapsed objects. We take it to be 40 - 80 in the calculation.

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arXiv2003

Quasi-Local Evolution of the Cosmic Gravitational Clustering in Halo Model

We show that the nonlinear evolution of the cosmic gravitational clustering is approximately spatial local in the $x$-$k$ (position-scale) phase space if the initial perturbations are Gaussian. That is, if viewing the mass field with modes in the phase space, the nonlinear evolution will cause strong coupling among modes with different scale $k$, but at the same spatial area $x$, while the modes at different area $x$ remain uncorrelated, or very weakly correlated. We first study the quasi-local clustering behavior with the halo model, and demonstrate that the quasi-local evolution in the phase space is essentially due to the self-similar and hierarchical features of the cosmic gravitational clustering. The scaling of mass density profile of halos insures that the coupling between $(x-k)$ modes at different physical positions is substantially suppressed. Using high resolution N-body simulation samples in the LCDM model, we justify the quasi-locality with the correlation function between the DWT (discrete wavelet transform) variables of the cosmic mass field. Although the mass field underwent a highly non-linear evolution, and the DWT variables display significantly non-Gaussian features, there are almost no correlations among the DWT variables at different spatial positions. Possible applications of the quasi-locality have been discussed.

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arXiv2003

Power spectrum and intermittency of the transmitted flux of QSOs Ly-alpha absorption spectra

Using a set of 28 high resolution, high signal to noise ratio (S/N) QSO Ly-alpha absorption spectra, we investigate the non-Gaussian features of the transmitted flux fluctuations, and their effect upon the power spectrum of this field. We find that the spatial distribution of the local power of the transmitted flux on scales k >= 0.05 s/km is highly spiky or intermittent. The probability distribution functions (PDFs) of the local power are long-tailed. The power on small scales is dominated by small probability events, and consequently, the uncertainty in the power spectrum of the transmitted flux field is generally large. This uncertainty arises due to the slow convergence of an intermittent field to a Gaussian limit required by the central limit theorem (CLT). To reduce this uncertainty, it is common to estimate the error of the power spectrum by selecting subsamples with an "optimal" size. We show that this conventional method actually does not calculate the variance of the original intermittent field but of a Gaussian field. Based on the analysis of intermittency, we propose an algorithm to calculate the error. It is based on a bootstrap re-sampling among all independent local power modes. This estimation doesn't require any extra parameter like the size of the subsamples, and is sensitive to the intermittency of the fields. This method effectively reduces the uncertainty in the power spectrum when the number of independent modes matches the condition of the CLT convergence.

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