2,904 research outputs found
An Application of Kerr Blackhole Fly-Wheel Model to Statistical Properties of QSOs/AGNs
The aim of this work is to demonstrate the properties of the magnetospheric
model around Kerr blackholes (BHs), so-called the fly-wheel (rotation driven)
model. The fly-wheel engine of the BH-accretion disk system is applied to the
statistics of QSOs/AGNs. In the model, the central BH is assumed to be formed
at and obtains nearly maximum but finite rotation energy (
extreme Kerr BH) at the formation stage. The inherently obtained rotation
energy of the Kerr BH is released through an magnetohydrodynamic process. This
model naturally leads finite lifetime of AGN activity.
Nitta et al. (1991) clarified individual evolution of Kerr BH fly-wheel
engine which is parametrized by BH mass, initial Kerr parameter, magnetic field
near the horizon and a dimension-less small parameter. We impose a statistical
model for the initial mass function (IMF) of ensemble of BHs by the
Press-Schechter formalism. By the help of additional assumptions, we can
discuss the evolution of the luminosity function and the spatial number density
of QSOs/AGNs.Comment: 12 pages, 7 figures Fig.7 has been replace
The simulated 21 cm signal during the epoch of reionization : full modeling of the Ly-alpha pumping
The 21 cm emission of neutral hydrogen is the most promising probe of the
epoch of reionization(EoR). In the next few years, the SKA pathfinders will
provide statistical measurements of this signal. Numerical simulations
predicting these observations are necessary to optimize the design of the
instruments. The main difficulty is the computation of the spin temperature of
neutral hydrogen which depends on the gas kinetic temperature and on the level
of the local Lyman-alpha flux. A T_s >> T_cmb assumption is usual. However,
this assumption does not apply early in the reionization history, or even later
in the history as long as the sources of X-rays are too weak to heat the
intergalactic medium significantly. This work presents the first EoR numerical
simulations including, beside dynamics and ionizing continuum radiative
transfer, a self-consistent treatment of the Ly-alpha radiative transfer. This
allows us to compute the spin temperature more accurately. We use two different
box sizes, 20 Mpc/h and 100 Mpc/h, and a star source model. Using the redshift
dependence of average quantities, maps, and power spectra, we quantify the
effect of using different assumptions to compute the spin temperature and the
influence of the box size. The first effect comes from allowing for a signal in
absorption. The magnitude of this effect depends on the amount of heating by
hydrodynamic shocks and X-rays in the intergalactic medium(IGM). The second
effects comes from using the real, local, Lyman-alpha flux. This effect is
important for an average ionization fraction of less than 10%: it changes the
overall amplitude of the 21 cm signal, and adds its own fluctuations to the
power spectrum.Comment: 20 pages, 16 figures, 2 tables, To be publish A&A. High resolution
version available at http://aramis.obspm.fr/~baek/21cm_Lya.pd
Cosmological Radiative Transfer Codes Comparison Project I: The Static Density Field Tests
Radiative transfer simulations are now at the forefront of numerical
astrophysics. They are becoming crucial for an increasing number of
astrophysical and cosmological problems; at the same time their computational
cost has come to the reach of currently available computational power. Further
progress is retarded by the considerable number of different algorithms
(including various flavours of ray-tracing and moment schemes) developed, which
makes the selection of the most suitable technique for a given problem a
non-trivial task. Assessing the validity ranges, accuracy and performances of
these schemes is the main aim of this paper, for which we have compared 11
independent RT codes on 5 test problems: (0) basic physics, (1) isothermal H II
region expansion and (2) H II region expansion with evolving temperature, (3)
I-front trapping and shadowing by a dense clump, (4) multiple sources in a
cosmological density field. The outputs of these tests have been compared and
differences analyzed. The agreement between the various codes is satisfactory
although not perfect. The main source of discrepancy appears to reside in the
multi-frequency treatment approach, resulting in different thicknesses of the
ionized-neutral transition regions and different temperature structure. The
present results and tests represent the most complete benchmark available for
the development of new codes and improvement of existing ones. To this aim all
test inputs and outputs are made publicly available in digital form.Comment: 32 pages, 39 figures (all color), comments welcom
Cosmological radiative transfer comparison project - II. The radiation-hydrodynamic tests
The development of radiation hydrodynamical methods that are able to follow gas dynamics and radiative transfer (RT) self-consistently is key to the solution of many problems in numerical astrophysics. Such fluid flows are highly complex, rarely allowing even for approximate analytical solutions against which numerical codes can be tested. An alternative validation procedure is to compare different methods against each other on common problems, in order to assess the robustness of the results and establish a range of validity for the methods. Previously, we presented such a comparison for a set of pure RT tests (i.e. for fixed, non-evolving density fields). This is the second paper of the Cosmological Radiative Transfer Comparison Project, in which we compare nine independent RT codes directly coupled to gas dynamics on three relatively simple astrophysical hydrodynamics problems: (i) the expansion of an H ii region in a uniform medium, (ii) an ionization front in a 1/r2 density profile with a flat core and (iii) the photoevaporation of a uniform dense clump. Results show a broad agreement between the different methods and no big failures, indicating that the participating codes have reached a certain level of maturity and reliability. However, many details still do differ, and virtually every code has showed some shortcomings and has disagreed, in one respect or another, with the majority of the results. This underscores the fact that no method is universal and all require careful testing of the particular features which are most relevant to the specific problem at han
Fragmentation of the Primordial Gas Clouds and the Lower Limit on the Mass of the First Stars
We discuss the fragmentation of primordial gas clouds in the universe after
decoupling. Comparing the time scale of collapse with that of fragmentation, we
obtain the typical mass of a fragment both numerically and analytically. It is
shown that the estimated mass gives the minimum mass of a fragment which is
formed from the primordial gas cloud and is essentially determined by the
Chandrasekhar mass.Comment: Astrophysical Journal Letters, in press (accepted 4 Oct 1996), 10
pages, Latex, 1 fi
Relativistic Heavy-Ion Physics: Experimental Overview
The field of relativistic heavy-ion physics is reviewed with emphasis on new
results and highlights from the first run of the Relativistic Heavy-Ion
Collider at BNL and the 15 year research programme at the SPS at CERN and the
AGS at BNL.Comment: 15 pages, 9 figures. Invited Talk at the Fourth International
Conference on Physics and Astrophysics of Quark Gluon Plasma, (ICPAQGP-2001)
Jaipur, India, November 26-30, 200
Parton coalescence at RHIC
Using a covariant coalescence model, we study hadron production in
relativistic heavy ion collisions from both soft partons in the quark-gluon
plasma and hard partons in minijets. Including transverse flow of soft partons
and independent fragmentation of minijet partons, the model is able to describe
available experimental data on pion, kaon, and antiproton spectra. The
resulting antiproton to pion ratio is seen to increase at low transverse
momenta and reaches a value of about one at intermediate transverse momenta, as
observed in experimental data at RHIC. A similar dependence of the antikaon to
pion ratio on transverse momentum is obtained, but it reaches a smaller value
at intermediate transverse momenta. At high transverse momenta, the model
predicts that both the antiproton to pion and the antikaon to pion ratio
decrease and approach those given by the perturbative QCD. Both collective flow
effect and coalescence of minijet partons with partons in the quark-gluon
plasma affect significantly the spectra of hadrons with intermediate transverse
momenta. Elliptic flows of protons, Lambdas, and Omegas have also been
evaluated from partons with elliptic flows extracted from fitting measured pion
and kaon elliptic flows, and they are found to be consistent with available
experimental data.Comment: 12 pages, 11 figure
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