360 research outputs found
Effect of start-stop motion on contact replenishment in a grease lubricated deep groove ball bearing
Star Formation Feedback and Metal Enrichment History Of The Intergalactic Medium
Using hydrodynamic simulations we compute the metal enrichment history of the
intergalactic medium (IGM). We show that galactic superwind (GSW) feedback can
transport metals to the IGM and that the properties of simulated metal
absorbers match observations. The distance of influence of GSW is typically
limited to >0.5Mpc and within regions of overdensity >10. Most CIV and OVI
absorbers are located within shocked regions of elevated temperature
(T>2x10^4K), overdensity (>10), and metallicity ([-2.5,-0.5]). OVI absorbers
have typically higher metallicity, lower density and higher temperature than
CIV absorbers. For OVI absorbers collisional ionization dominates over the
entire redshift range z=0-6, whereas for CIV absorbers the transition occurs at
moderate redshift z~3 from collisionally dominated to photoionization
dominated. We find that the observed column density distributions for CIV and
OVI in the range log N cm^2=12-15 are reasonably reproduced by the simulations.
The evolution of mass densities contained in CIV and OVI lines, Omega_CIV and
Omega_OVI, is also in good agreement with observations, which shows a near
constancy at low redshifts and an exponential drop beyond redshift z=3-4. For
both CIV and OVI, most absorbers are transient and the amount of metals probed
by CIV and OVI lines of column log N cm^2=12-15 is only ~2% of total metal
density at any epoch. While gravitational shocks from large-scale structure
formation dominate the energy budget (80-90%) for turning about 50% of IGM to
the warm-hot intergalactic medium (WHIM) by z=0, GSW feedback shocks are
energetically dominant over gravitational shocks at z > 1-2. Most of the
so-called "missing metals" at z=2-3 are hidden in a warm-hot (T=10^{4.5-7}K)
gaseous phase, heated up by GSW feedback shocks. Their mass distribution is
broadly peaked at in the IGM, outside virialized halos.Comment: 52 pages, 26 figures, published in the Astrophysical Journal, 2011,
ApJ, 731, 1
Comparisons of Cosmological MHD Galaxy Cluster Simulations to Radio Observations
Radio observations of galaxy clusters show that there are G magnetic
fields permeating the intra-cluster medium (ICM), but it is hard to accurately
constrain the strength and structure of the magnetic fields without the help of
advanced computer simulations. We present qualitative comparisons of synthetic
VLA observations of simulated galaxy clusters to radio observations of Faraday
Rotation Measure (RM) and radio halos. The cluster formation is modeled using
adaptive mesh refinement (AMR) magneto-hydrodynamic (MHD) simulations with the
assumption that the initial magnetic fields are injected into the ICM by active
galactic nuclei (AGNs) at high redshift. In addition to simulated clusters in
Xu et al. (2010, 2011), we present a new simulation with magnetic field
injections from multiple AGNs. We find that the cluster with multiple injection
sources is magnetized to a similar level as in previous simulations with a
single AGN. The RM profiles from simulated clusters, both and the
dispersion of RM (), are consistent at a first-order with the
radial distribution from observations. The correlations between the
and X-ray surface brightness from simulations are in a broad
agreement with the observations, although there is an indication that the
simulated clusters could be slightly over-dense and less magnetized with
respect to those in the observed sample. In addition, the simulated radio halos
agree with the observed correlations between the radio power versus the cluster
X-ray luminosity and between the radio power versus the radio halo size. These
studies show that the cluster wide magnetic fields that originate from AGNs and
are then amplified by the ICM turbulence (Xu et al. 2010) match observations of
magnetic fields in galaxy clusters.Comment: Accepted for publication in Ap
Detecting the Transition From Pop III to Pop II Stars
We discuss the cosmological significance of the transition from the Pop III
to Pop II mode of star formation in the early universe, and when and how it may
occur in primordial galaxies. Observations that could detect this transition
include those of element abundances in metal-poor Galactic halo stars, and of
the helium reionization and associated heating of the intergalactic medium. We
suggest that gamma-ray bursts may be a better probe of the end of the
first-stars epoch than of Pop III stars.Comment: 10 pages, 3 figures; to appear in New Astronomy Reviews as
proceedings of "First Light and Reionization Workshop", eds. A. Cooray & E.
Barton, Irvine, CA, May 19-21, 200
Systematic Continuum Errors in the Lyman-Alpha Forest and The Measured Temperature-Density Relation
Continuum fitting uncertainties are a major source of error in estimates of
the temperature-density relation (usually parametrized as a power-law, ) of the inter-galactic medium (IGM) through the
flux probability distribution function (PDF) of the Lyman- forest.
Using a simple order-of-magnitude calculation, we show that few percent-level
systematic errors in the placement of the quasar continuum due to e.g. a
uniform low-absorption Gunn-Peterson component, could lead to errors in
of order unity. This is quantified further using a simple
semi-analytic model of the Lya forest flux PDF. We find that
under-(over-)estimates in the continuum level can lead to a lower (higher)
measured value of . Within current observational uncertainties,
continuum biases double the error in from to within our model. We argue that steps
need to be taken to directly estimate the level of continuum bias in order to
make recent claims of an inverted \tdr\ more robust.Comment: 8 pages, 8 figures. Accepted by Ap
The HeII Lyman alpha forest and the thermal state of the IGM
Recent analyses of the intergalactic UV background by means of the HeII Lyman
alpha forest assume that HeII and HI absorption features have the same line
widths. We omit this assumption to investigate possible effects of thermal line
broadening on the inferred HeII/HI ratio eta and to explore the potential of
intergalactic HeII observations to constrain the thermal state of the IGM.
Deriving a simple relation between the column density and the temperature of an
absorber we develop a procedure to fit the parameters of a power law
temperature-density relation and eta simultaneously. In an alternative approach
the temperature of an absorber, eta, and the redshift scale of eta variations
are estimated simultaneously. Tests with artificial data show that
well-constrained results can be obtained only if the signal-to-noise ratio in
the HeII forest is S/N > 20. Thus, it is impossible to give an estimate of the
temperature-density relation with the HeII data available at present (S/N ~5).
However, we find that only 45% of the lines in our sample favor turbulent line
widths. Furthermore, the inferred eta values are on average about 0.05 dex
larger if a thermal component is taken into account, and their distribution is
46% narrower in comparison to a purely turbulent fit. Therefore, variations of
eta on a 10% level may be related to the presence of thermal line broadening.
The apparent correlation between the strength of the HI absorption and the eta
value, which has been found in former studies, essentially disappears if
thermal broadening is taken into account. In the redshift range 2.58 < z < 2.74
towards the quasars HE2347-4342 and HS1700+6416 we obtain eta ~ 100. (abridged)Comment: accepted for publication by A&A, 11 pages, 13 figure
A Measurement of the Temperature-Density Relation in the Intergalactic Medium Using a New Lyman-alpha Absorption Line Fitting Method
The evolution of the temperature in the intergalactic medium is related to
the reionization of hydrogen and helium, and has important consequences for our
understanding of the Lya forest and of galaxy formation in gravitational models
of large-scale structure. We measure the temperature-density relation of
intergalactic gas from Lya forest observations of eight quasar spectra with
high resolution and signal-to-noise ratio, using a new line fitting technique
to obtain a lower cutoff of the distribution of line widths from which the
temperature is derived. We carefully test the accuracy of this technique to
recover the gas temperature with a hydrodynamic simulation. The temperature at
redshift z=(3.9, 3.0, 2.4) is best determined at densities slightly above the
mean: T_star=(20200\pm2700, 20200\pm1300, 22600\pm1900)K (statistical error
bars) for gas density (in units of the mean density) Delta_star=(1.42\pm0.08,
1.37\pm0.11, 1.66\pm0.11). The power-law index of the temperature-density
relation, defined by T=T_star(Delta/Delta_star)^{gamma-1}, is gamma-1=
(0.43\pm0.45, 0.29\pm0.30, 0.52\pm0.14) for the same three redshifts. The
temperature at the fixed over-density Delta=1.4 is T_1.4=(20100\pm2800,
20300\pm1400, 20700\pm1900)K. These temperatures are higher than expected for
photoionized gas in ionization equilibrium with a cosmic background, and can be
explained by a gradual additional heating due to on-going HeII reionization.
The measurement of the temperature reduces one source of uncertainty in the
lower limit to the baryon density implied by the observed mean flux decrement.
We find that the temperature cannot be reliably measured for under-dense gas,
because the velocities due to expansion always dominate the widths of the
corresponding weak lines.Comment: submitted to Ap
Quantum Interference Effects in Molecular Y- and Rhomb-Type Systems
In this paper we report the first observation of molecular population trapping in four level systems. Constructive and destructive quantum interferences between two sum-frequncy two-photon transitions in Y- and rhomb-type four-level systems, respectively, im sodium molecules have been experimentally achieved by using only one laser source. Their energy level schemes are featured by the extremely near-resonant enhancement of the equal-frequency two-photon transitions, sharing both the initial and the intermediate levels for the Y-type, and sharing both the initial and the final levels for the rhomb-type systems. Their novel spectral effects are to show seriously restrained Doppler-free UV peak at the nominal location of the induced two-photon transition with visible fluorescence in rhomb-type schems, and to show a strong extra UV peak but null visible fluorescence in the middle between the two dipole allowed two-photon transitions
Metal Enrichment and Temperature of the Intergalactic Medium
Hydrodynamic simulations of Lyman alpha clouds based on ab inito cosmological
models have produced results that are in broad agreement with observations.
However, further analyses have revealed that, with progressively higher
numerical resolution, the median or cutoff line width of the simulated Lyman
alpha clouds (i.e. the Doppler parameter) appears to converge to a value
significantly below what is observed at z~3 (by about a factor of 1.5). These
convergence test simulations do not include feedback from star formation. Given
the observed metallicity in the Lyman alpha clouds we suggest that supernovae,
which presumably polluted the IGM with metals, may have deposited a sufficient
amount of energy in the IGM to reconcile the theory with observations. Simple
arguments immediately narrow the redshift range of pollution down to
4<z_{dep}<9. It seems quite certain that dwarf and sub-dwarf galaxies with
total masses in the range 10^{6.5-9.0}Msun have to be largely responsible for
the pollution. Furthermore, it is implied that either star formation is very
efficient or metal yield is very high for these early dwarf galaxies, if the
mean metallicity in the universe at z=3 is as high as =0.01Zsun. Finally,
assuming the specific supernova heating energy is proportional to the
metallicity of a gas, we note that the picture proposed here would be
consistent with supernovae being the apparently needed heating source for the
intra-cluster gas, if the required heating of the intra-cluster gas is no
greater than 1 keV per particle.Comment: accepted to ApJ Letter
CosmoMHD: A Cosmological Magnetohydrodynamics Code
In this era of precision cosmology, a detailed physical understanding on the
evolution of cosmic baryons is required. Cosmic magnetic fields, though still
poorly understood, may represent an important component in the global cosmic
energy flow that affects the baryon dynamics. We have developed an
Eulerian-based cosmological magnetohydrodynamics code (CosmoMHD) with modern
shock capturing schemes to study the formation and evolution of cosmic
structures in the presence of magnetic fields. The code solves the ideal MHD
equations as well as the non-equilibrium rate equations for multiple species,
the Vlasov equation for dynamics of collisionless particles, the Poisson's
equation for the gravitational potential field and the equation for the
evolution of the intergalactic ionizing radiation field. In addition, a
detailed star formation prescription and feedback processes are implemented.
Several methods for solving the MHD by high-resolution schemes with
finite-volume and finite-difference methods are implemented. The
divergence-free condition of the magnetic fields is preserved at a level of
computer roundoff error via the constraint transport method. We have also
implemented a high-resolution method via dual-equation formulations to track
the thermal energy accurately in very high Mach number or high Alfven-Mach
number regions. Several numerical tests have demonstrated the efficacy of the
proposed schemes.Comment: 10 figures, submitted to ApJ
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