arXiv subjects

Hong-Guang Bi

Publications and source records attributed to Hong-Guang Bi.

4 recordsLinked to original sources

arXiv2007

Ly$α$ Leaks and Reionization

Ly$α$ absorption spectra of QSOs at redshifts $z\simeq6$ 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, Ly$α$ leaks can be well defined and described by the equivalent width $W$ and the full width of half area $W_{\rm H}$, both of which are less contaminated by instrumental resolution and noise. It is found that the distribution of $W$ and $W_{\rm H}$ of Ly$α$ leaks are sensitive to the modeling of the ionizing background. We consider four representative reionization models. It is concluded that the leak statistics provides an effective tool to probe the evolutionary history of reionization at $z\simeq5-6.5$. Similar statistics would also be applicable to the reionization of He II at $z \simeq 3$(Abridged)

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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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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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arXiv1997

Abundance and Clustering of QSOs in Cosmic Structure Formation Models

Combining the observations of spatial abundance and two-point correlation function of QSOs, we can effectively set constraints on models of cosmic structure formation. Both the abundance of gravitationally confined halos and their two-point correlation functions can be calculated in the conventional Press-Schechter formalism. We apply this method to examine the properties of possible host halos of QSOs in three popular models: the standard cold dark matter (SCDM) model, the low density flat cold dark matter (LCDM) model and the cold-plus-hot dark matter (CHDM) model. The LCDM and CHDM models are normalized to the COBE-DMR observations, and the SCDM is normalized to $σ= 0.58$. We find that the SCDM and LCDM models can pass the abundance-plus-correlation test for QSOs. However, the CHDM are difficult to produce host halos to fit with the number density of high redshift QSOs and their clustering on large scales (10 h^{-1} Mpc) simultaneously. We studied various mechanisms, originated both gravitationally and non-gravitationally, which may lead to a biasing of the halo clustering. We conclude that these effects are too weak in order to release the trouble of the CHDM models.

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