Hongguang Bi

Johns Hopkins University, Baltimore, MD, United States

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Publications (14)59.04 Total impact

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    Hongguang Bi · F. Davidsen
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    ABSTRACT: We have performed a detailed statistical study of the evolution of structure in a photoionized intergalactic medium (IGM) using analytical simulations to extend the calculation into the mildly nonlinear density regime found to prevail at z = 3. Our work is based on a simple fundamental conjecture: that the probability distribution function of the density of baryonic diffuse matter in the universe is described by a lognormal (LN) random field. The LN distribution has several attractive features and follows plausibly from the assumption of initial linear Gaussian density and velocity fluctuations at arbitrarily early times. Starting with a suitably normalized power spectrum of primordial fluctuations in a universe dominated by cold dark matter (CDM), we compute the behavior of the baryonic matter, which moves slowly toward minima in the dark matter potential on scales larger than the Jeans length. We have computed two models that succeed in matching observations. One is a nonstandard CDM model with Ω = 1, h = 0.5, and Γ = 0.3, and the other is a low-density flat model with a cosmological constant (LCDM), with Ω = 0.4, ΩΛ = 0.6, and h = 0.65. In both models, the variance of the density distribution function grows with time, reaching unity at about z = 4, where the simulation yields spectra that closely resemble the Lyα forest absorption seen in the spectra of high-z quasars. The calculations also successfully predict the observed properties of the Lyα forest clouds and their evolution from z = 4 down to at least z = 2, assuming a constant intensity for the metagalactic UV background over this redshift range. However, in our model the forest is not due to discrete clouds, but rather to fluctuations in a continuous intergalactic medium. At z = 3, typical clouds with measured neutral hydrogen column densities NH I = 1015.3, 1013.5, and 1011.5 cm-2 correspond to fluctuations with mean total densities approximately 10, 1, and 0.1 times the universal mean baryon density. Perhaps surprisingly, fluctuations whose amplitudes are less than or equal to the mean density still appear as "clouds" because in our model more than 70% of the volume of the IGM at z = 3 is filled with gas at densities below the mean value. We find that the column density distribution of Lyα forest lines can be fitted to f(NH I)∝NH I, with β = 1.46 in the range 12.5 < log NH I < 14.5, matching recent Keck results. At somewhat higher column densities the distribution steepens, giving β = 1.80 over the range 14.0 < log NH I < 15.5, matching earlier observations for these stronger lines. The normalization of the line numbers in our model also agrees with observations if the total baryon density is Ωb = 0.015 h-2 and the ionizing background intensity is J21 = 0.18. Alternatively, if J21 = 0.5 as recently estimated for the background due to observed quasars at z = 2.5, then Ωb = 0.025 h-2 yields the observed number of Lyα lines and the observed mean opacity. The model predicts that about 80% of the baryons in the universe are associated with Lyα forest features with 13 < log NH I < 15 at z = 3, while 10% are in more diffuse gas with smaller column densities and 10% are in higher column density clouds and in collapsed structures, such as galaxies and quasars. Our model requires that absorbers at z = 3 with column densities higher than about 1016 cm-2—Lyman limit systems and damped Lyα systems—represent a separate population that has collapsed out of the IGM. We find the number density of forest lines is dN/dz = 75[(1 + z)/4]2.5 for lines with EW > 0.32 Å, also in good agreement with observations. We fit Voigt profiles to our simulated lines and find a distribution of b parameters that matches that obtained from similar fits to real spectra. The effective opacity in the Lyα forest is found from the model to be τeff = 0.26[(1 + z)/4]3.1, again in good agreement with observations. The exponents for the evolution of the number of lines and the effective opacity do not simply differ by 1.0, as in the standard cloud picture, owing to saturation effects for the stronger lines. This also explains why the effective opacity evolves more slowly than (1 + z)4.5, which is expected for the Gunn-Peterson effect in a uniform medium with J = constant (and Ω = 1). We compare the spectral filling factor for our CDM and LCDM models with observations of the Lyα forest in HS 1700+64 and find good agreement. Similar calculations for a standard CDM model and a cold plus hot dark matter model fail to match the observed spectral filling factor function. We have also compared a number of predictions of our analytical model with results of numerical hydrodynamical calculations (Miralda-Escudé et al. 1996) and again find them to be in good agreement. The lognormal hypothesis, coupled with otherwise attractive CDM-dominated cosmological models, appears to provide a plausible and useful description of the distribution of photoionized intergalactic gas and provides a new estimate of the baryonic density of the universe.
    Preview · Article · Jan 2009 · The Astrophysical Journal
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    Priya Jamkhedkar · Hongguang Bi · and Li-Zhi Fang
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    ABSTRACT: The calculation of the transmission power spectrum of QSO Lyα absorption requires two parameters for the normalization: the continuum Fc and mean transmission -τ. Traditionally, the continuum is obtained by a polynomial fitting truncating it at a lower order, and the mean transmission is calculated over the entire wavelength range considered. The flux F is then normalized by Fc-τ. However, the fluctuations in the transmitted flux are significantly correlated with the local background flux on scales for which the field is intermittent. As a consequence, the normalization of the entire power spectrum by an overall mean transmission -τ will overlook the effect of the fluctuation-background correlation upon the powers. In this paper we develop a self-normalization algorithm of the transmission power spectrum based on a multiresolution analysis. This self-normalized power spectrum estimator needs neither a continuum fitting nor a predetermining of the mean transmission. With simulated samples, we show that the self-normalization algorithm can perfectly recover the transmission power spectrum from the flux regardless of how the continuum varies with wavelength. We also show that the self-normalized power spectrum is also properly normalized by the mean transmission. Moreover, this power spectrum estimator is sensitive to the nonlinear behavior of the field. That is, the self-normalized power spectrum estimator can distinguish between fields with or without the fluctuation-background correlation. This cannot be accomplished by the power spectrum with the normalization by an overall mean transmission. Applying this analysis to a real data set of Q1700+642 Lyα forest, we demonstrate that the proposed power spectrum estimator can perform correct normalization and effectively reveal the correlation between the fluctuations and background of the transmitted flux on small scales. Therefore, the self-normalized power spectrum would be useful for the discrimination among models without the uncertainties caused by free (or fitting) parameters.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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    Jiren Liu · Hongguang Bi · and Li-Zhi Fang
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    ABSTRACT: Spectra of high-redshift QSOs show deep Gunn-Peterson absorptions on the blue sides of the Lyα emissions lines. They can be decomposed into components called Lyα leaks, defined to be emissive regions in complementary to otherwise zero-flux absorption gaps. Just like Lyα absorption forests at low redshifts, Lyα leaks are easy to find in observations and contain rich sets of statistical properties that can be used to study the early evolution of the intergalactic medium (IGM). Among all properties of a leak profile, we investigate its equivalent width in this paper, since it is weakly affected by instrumental resolution and noise. Using 10 Keck QSO spectra at z ~ 6, we have measured the number density distribution function n(W, z) , defined to be the number of leaks per equivalent width W and per redshift z in the redshift range 5.4-6.0. These new observational statistics, in both the differential and cumulative forms, fit well to hydrodynamic simulations of uniform ionizing background in the ΛCDM cosmology. In this model, Lyα leaks are mainly due to low-density voids. It supports the early studies that the IGM at z 6 would still be in a highly ionized state with a neutral hydrogen fraction 10−4. Measurements of n(W, z) at z > 6 would be effective to probe the reionization of the IGM.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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    Jiren Liu · Hongguang Bi · Long-Long Feng · and Li-Zhi Fang
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    ABSTRACT: We study the Gunn-Peterson effect of the photoionized intergalactic medium (IGM) in the redshift range 5 < z < 6.4 using semianalytic 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) a rapid 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 significant spatial fluctuations in the UV ionizing background at z 6.
    Preview · Article · Dec 2008 · The Astrophysical Journal
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    Longlong Feng · Hongguang Bi · Jiren Liu · Li-Zhi Fang
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    ABSTRACT: Lyα absorption spectra of QSOs at redshifts z ≃ 6 show complete Gunn-Peterson absorption troughs (dark gaps) separated by tiny leaks. The dark gaps are from the intergalactic medium (IGM) where the density of neutral hydrogen are high enough to produce almost saturated absorptions, however, where the transmitted leaks come from is still unclear so far. We demonstrate that leaking can originate from the lowest density voids in the IGM as well as the ionized patches around ionizing sources using semi-analytical simulations. If leaks were produced in lowest density voids, the IGM might already be highly ionized, and the ionizing background should be almost uniform; in contrast, if leaks come from ionized patches, the neutral fraction of IGM would be still high, and the ionizing background is significantly inhomogeneous. Therefore, the origin of leaking is crucial to determining the epoch of inhomogeneous-to-uniform transition of the the ionizing photon background. We show that the origin could be studied with the statistical features of leaks. Actually, Lyα leaks can be well defined and described by the equivalent width W and the full width of half area WH, both
    Preview · Article · Jan 2008 · Monthly Notices of the Royal Astronomical Society
  • Jiren Liu · H. Bi · L. Fang
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    ABSTRACT: Spectra of high redshift QSOs show deep Gunn-Peterson absorptions on the blue sides of the Lya emissions lines. They can be decomposed into components called Lya leaks, defined to be emissive regions in complementary to otherwise zero-fluxed absorption gaps. Just like Lya absorption forests at low redshifts, Lya leaks are both easy to find in observations and containing rich sets of statistical properties that can be used to study the early evolution of the IGM. Among all properties of a leak profile, we investigate its equivalent width in this paper, since it is weakly affected by instrumental resolution and noise. Using 10 Keck QSO spectra at z 6, we have measured the number density distribution function n(W,z), defined to be the number of leaks per equivalent width W and per redshift z, in the redshift range 5.4 - 6.0. These new observational statistics, in both the differential and cumulative forms, fit well to hydro numerical simulations of uniform ionizing background in the LCDM cosmology.In this model, Lya leaks are mainly due to low density voids.It supports the early studies that the IGM at z 6 would still be in a highly ionized state with neutral hydrogen fraction 10-4. Measurements of n(W,z) at z>6; would be effective to probe the reionization of the IGM.
    No preview · Article · Dec 2007
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    Hongguang Bi · Li-Zhi Fang · Longlong Feng · Yipeng Jing
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    ABSTRACT: We study the baryonic gas clouds (the IGM) in the universe before the reionization with the lognormal model which is shown to be dynamcially legitimate in describing the fluctuation evolution in quasilinear as well as nonlinear regimes in recent years. The probability distribution function of the mass field in the LN model is long tailed and so plays an important role in rare events, such as the formation of the first generation of baryonic objects. We calculate density and velocity distributions of the IGM at very high spatial resolutions, and simulate the distributions at resolution of 0.15 kpc from z=7 to 15 in the LCDM cosmological model. We performed a statistics of the hydrogen clouds including column densities, clumping factors, sizes, masses, and spatial number density etc. One of our goals is to identify which hydrogen clouds are going to collapse. By inspecting the mass density profile and the velocity profile of clouds, we found that the velocity outflow significantly postpones the collapsing process in less massive clouds, in spite of their masses are larger than the Jeans mass. Consequently, only massive (> 10^5 M_sun) clouds can form objects at higher redshift, and less massive (10^4-10^5) collapsed objects are formed later. For example, although the mass fraction in clouds with sizes larger than the Jeans length is already larger than 1 at z=15, there is only a tiny fraction of mass (10^{-8}) in the clouds which are collapsed at that time. If all the ionizing photons, and the 10^{-2} metallicity observed at low redshift are produced by the first 1% mass of collapsed baryonic clouds, the majority of those first generation objects would not happen until z=10.
    Preview · Article · Sep 2003 · The Astrophysical Journal
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    Priya Jamkhedkar · Hongguang Bi · Li-Zhi Fang

    Preview · Article · Jul 2001
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    Hongguang Bi · Li-Zhi Fang
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    ABSTRACT: We have developed a method for calculating the two-point correlation function of nonlinearly evolved mass and collapsed halos in the Press-Schechter formalism. The nonlinear gravitational interaction is treated as the sum of various individual spherical top-hat clustering. Because no collapsed halo of mass M can exist in initial regions (or top-hat spheres) of mass less than M, the bias that massive halos have stronger correlation than the background mass can be naturally introduced. We apply this method to derive constraints on popular dark-matter models from the spatial number density and the correlation function of C IV absorption systems in QSO spectra. Considering C IV systems should behosted by collapsed halos, one can obtain an upper limit to the threshold mass of the collapsed halos by requiring their number density to be larger than that of observed C IV systems. On the other hand, in order to explain the observed clustering of C IV systems, a lower limit to the threshold mass will be set for the hosting halos. The cold dark matter model, the Lamda CDM model and the cold-plus-hot dark matter model are tested using the C IV observations. Comment: 23 pages, LaTeX, plus 11 figures in the PS format to be published in ApJ
    Preview · Article · Feb 1996 · The Astrophysical Journal
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    Hongguang Bi · Jian Ge · Li-Zhi Fang
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    ABSTRACT: Our goal in this paper is to test some popular dark matter models by means of the Lyalpha forest in QSO spectra. Recent observations of the size and velocity of Lyalpha forest clouds have indicated that the Lyalpha absorption is probably not given by collapsed objects but, rather, by precollapsed regions in the baryonic density field. Therefore, a linear approximation description would be able to provide valuable information. We developed a technique to simulate the Lyalpha forest as the absorption of such precollapsed regions under a linear approximation regime. The simulated Lyalpha forests in the standard cold dark matter (SCDM) model, the cold plus hot dark matter (CHDM) model, and the low-density flat cold dark matter (LCDM) model have been confronted with observational features including (1) the number density of Lyalpha lines and its dependencies on redshift and equivalent width, (2) the distribution of equivalent widths and its redshift dependence, (3) clustering, and (4) the Gunn-Peterson effect. We find that the "standard" CHDM model, i.e., 60% cold dark matter, 30% hot dark matter, and 10% baryons, does not pass the Lyalpha forest test, probably because it produces structures too late and favors to forming structures on large scales instead of small-scale objects such as Lyalpha clouds. Within a reasonable range of Jv, the UV background radiation at high redshift, and deltath, the threshold of the onset of gravitational collapse of the baryonic matter, the LCDM model is consistent with observational data in all four aspects mentioned above. The SCDM model can also fit with observations, but it requires a smaller Jv and a higher deltath This suggests that whether or not a significant part of the Lyalpha forest lines is located in the halos of collapsed objects would be crucial to the success of the SCDM model.
    Preview · Article · Oct 1995 · The Astrophysical Journal
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    Hongguang Bi · Gerhard Boerner
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    ABSTRACT: (\rl = Richardson-Lucy) We propose a simulation-based bootstrap method to access global significance levels of deconvolution models in the \rl and other iterative restoration algorithms that converge locally. These significance levels allow one to check at each iterative step how good the model is and when iterations can be stopped. Adding more iterations in the deconvolution improves the fitting but is very slow at later time; while too much entropy or smoothness will be lost in the models. A good deconvolution model should firstly have a significance level as high as possible ($\ge$ 20\%), and secondly, be as smooth as possible. We have used two examples to illustrate how such models can be derived in practice. We point out that maximizing the sum of the likelihood of fitting and {\em a priori} entropy does not guarantee an acceptable significance level for the resulting model. If one's {\em a priori} knowledge is too poor, the model may not be able to fit the data at a reasonable significance level. Instead, a maximum-entropy-like iterative restoration algorithm can be performed later by acquiring {\em a priori} knowledge from the \rl restoration. However, this is necessary only when it does increase the levels significantly.
    Preview · Article · Oct 1994
  • Li-Zhi Fang · Hongguang Bi · Jian Ge
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    ABSTRACT: We have developed a technique of simulating Lyalpha absorption in QSO spectra in a linear approximation regime, in which the collapse of baryonic matter is treated by a Press-Schecter-like criterion. The simulated Lyalpha forests in models of the standard cold dark matter (SCDM), the cold plus hot dark matter (CHDM), and the low-density flat cold dark matter (LCDM) have been confronted with observational features, including 1) the number density of Lyalpha lines and its dependencies on redshift and equivalent width; 2) the distribution of equivalent widths and its redshift dependence; 3) clustering; and 4) the Gunn-Peterson effect. Within a reasonable range of J_nu , the UV background radiation at high redshift, and delta_ {th}, the linear threshold of the onset of gravitational collapse of the baryonic matter, the LCDM model is generally in agreement with observational data in all above-mentioned aspects. The model of SCDM seems to show some lack of Lyalpha lines at redshifts less than 2.5, but it can survive if we take a smaller J_nu and a higher delta_ {th}. This suggests that whether a significant part of the Lyalpha forest lines is given by halos of galaxies or galaxy-like objects would be crucial to the success of SCDM. The ``standard" CHDM model, i.e. 60% cold and 30% hot dark matters and 10% baryons, is found to be unable to survive under the Lyalpha forest tests, because it produces structures too late and favors to form structures on large scales instead of small scale objects like Lyalpha clouds.
    No preview · Article · Jan 1994
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    ABSTRACT: The evolution of a diffuse baryonic medium in a flat Friedmann universe containing cold or hybrid (cold plus hot) dark matters has been studied in the linear regime. Numerical and analytical calculations showed that for all models considered, the spectrum of density fluctuations of the IGM can be expressed on average as a proportion of those of dark matters by a factor less than one, in which the only parameter is the Jeans length of the medium. This solution can provide a coherent explanation of cosmological properties of the IGM, including the cosmic Mach number of IGM, Comptonization parameter, and soft X-ray emission of the IGM. This model showed that the IGM temperature underwent an increase with cosmic time. It implies that (1) clusters may have a halo consisting of hot and thin diffuse baryonic medium, and (2) the temperature of the X-ray gas of clusters increases with the distance from the center of the cluster.
    Preview · Article · Jul 1993 · The Astrophysical Journal
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    Hongguang Bi
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    ABSTRACT: The Ly-alpha absorption by four intergalactic medium (IGM) distribution models in the cold dark matter cosmology is simulated, taking the role of the Doppler effect of the peculiar velocity into account. It is found that the medium can produce forestlike Ly-alpha absorption spectra compatible with those observed by fitting statistically the probability distribution function of the line equivalent width and the two-point correlation function. The peculiar velocity shifts the line positions slightly, but does not affect the line profiles when compared to the thermal broadening of atoms. It is considered that some fundamental physical process of the IGM neglected here should follow the same dynamical behavior as in traditional cloud models.
    Preview · Article · Feb 1993 · The Astrophysical Journal

Publication Stats

314 Citations
59.04 Total Impact Points

Institutions

  • 2009
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
  • 2008
    • Government of the People's Republic of China
      Peping, Beijing, China
    • The University of Arizona
      • Department of Physics
      Tucson, Arizona, United States