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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.
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