15．Zhang, Tong-Jie  (张同杰)

       Department of Astronomy

       Beijing Normal University

       Beijing, 100875

       P.R.China

       Email: tjzhang@bnu.edu.cn

     Title of talk: X-ray emission of baryonic gas in the universe: luminosity - temperature relationship and soft-band background

     Abstract: We study the X-ray emission of baryon fluid in the universe using the WIGEON cosmological hydrodynamic simulations. It has been revealed that cosmic baryon fluid in the nonlinear regime behaves like Burgers turbulence, i.e. the fluid field consists of shocks. Like turbulence in incompressible fluid, the Burgers turbulence plays an important role in converting the kinetic energy of the fluid to thermal energy and heats the gas. We show that the simulation sample of the $\Lambda$CDM model without adding extra heating sources can fit well the observed distributions of X-ray luminosity versus temperature ($L_{\rm x}$ vs. $T$) of galaxy groups and is also consistent with the distributions of X-ray luminosity versus velocity dispersion ($L_{\rm x}$ vs. $\sigma$). Because the baryonic gas is multiphase, the $L_{\rm x}-T$ and $L_{\rm x}-\sigma$ distributions are significantly scattered. If we describe the relationships by power laws $L_{\rm x}\propto T^{\alpha_{LT}}$ and $L_{\rm x}\propto \sigma^{\alpha_{LV}}$, we find $\alpha_{LT}>2.5$ and $\alpha_{LV}>2.1$. The X-ray background in the soft $0.5-2$ keV band emitted by the baryonic gas in the temperature range $10^5<T<10^7$ K has also been calculated. We show that of the total background, (1) no more than 2\% comes from the region with temperature less than $10^{6.5}$ K, and (2) no more than 7\% is from the region of dark matter with mass density $\rho_{\rm dm}<50 \bar{\rho}_{\rm dm}$. The region of $\rho_{\rm dm}>50\bar{\rho}_{\rm dm}$ is generally clustered and discretely distributed. Therefore, almost all of the soft X-ray background comes from clustered sources, and the contribution from truly diffuse gas is probably negligible. This point agrees with current X-ray observations.

 16.  Zheng, Wei  (鄭瑋)  (May be)

       Division of Astronomical Sciences

       National Science Foundation

       Arlington, VA 22230

       Phone (703)292-4899

       Email:  wzheng@nsf.gov

       Fax: 1 703 292 9034

     Title of talk: In Search for Extremely Young Galaxies with "Cosmic Lens"

     Abstract: Significant progress has been made in our understanding of the universe at redshift of ~6, as the results of GOODS, UDF and other studies. We have carried out a search of z-band dropouts in the clusters that have been observed by the HST/ACS. The lensing effect of the clusters enhances our search power and enables us to find more than a dozen candidates of galaxies at redshift of 7-8. Some of these objects are very bright, making it possible for spectroscopic followup observations. I will describe the implications to the reionization history of the intergalactic medium, review other search efforts and discuss the prospect of using gamma-ray bursts to find objects at even higher redshift.